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Application of Hydrologic Simulation Program—FORTRAN (HSPF) as Part of an Integrated Hydrologic Model for the Salinas Valley, California

Joseph A. Hevesi, Wesley Henson, Randall T. Hanson, Elizabeth R. Jachens, Sandra Bond, Marisa M. Earll · U.S. Geological Survey

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Water Resource Mission Area—Water Availability and Use Science Program Prepared in cooperation with the Monterey County Water Resources Agency

Application of Hydrologic Simulation Program—FORTRAN (HSPF) as Part of an Integrated Hydrologic Model for the Salinas Valley, California

Scientific Investigations Report 2025–5009

U.S. Department of the Interior U.S. Geological Survey

Cover. Crucifer fields in Salinas, California. Photograph taken by Wesley Henson July 13, 2023.

Application of Hydrologic Simulation Program—FORTRAN (HSPF) as Part of an Integrated Hydrologic Model for the Salinas Valley, California By Joseph A. Hevesi, Wesley Henson, Randall T. Hanson, Elizabeth R. Jachens, Sandra Bond, Marisa M. Earll, and Deidre Herbert

Water Resource Mission Area—Water Availability and Use Science Program Prepared in cooperation with the Monterey County Water Resources Agency

Scientific Investigations Report 2025–5009

U.S. Department of the Interior U.S. Geological Survey

U.S. Geological Survey, Reston, Virginia: 2025

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–392–8545. For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov/ or contact the store at 1–888–275–8747. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Hevesi, J.A., Henson, W., Hanson, R.T., Jachens, E.R., Bond, S., Earll, M.M., and Herbert, D., 2025, Application of Hydrologic Simulation Program—FORTRAN (HSPF) as part of an integrated hydrologic model for the Salinas Valley, California: U.S. Geological Survey Scientific Investigations Report 2025–5009, 148 p., https://doi.org/10.3133/sir20255009. Associated data for this publication: Hevesi, J., Henson, W., Hanson, R.T., Earll, M.M., Herbert, D.M., and Jachens, E.R., 2025, Salinas Valley watershed model—Application of Hydrologic Simulation Program—FORTRAN (HSPF): U.S. Geological Survey data release, https://doi.org/10.5066/P9FJAWC4. ISSN 2328-0328 (online)

iii

Acknowledgments This report documents a cooperative project between the Monterey County Water Resources Agency (MCWRA) and the U.S. Geological Survey (USGS). The project was funded by MCWRA and USGS cooperative matching funds.

Contents Acknowledgments��iii Abstract��1 Introduction��2 Purpose and Scope��4 Study Area��4 Water Use and Management��5 Climate��5 Hydrography��5 Physiography��11 Streamflow��11 Groundwater��15 Land Cover��15 Soils��21 Generalized Surface Geology��21 Model Development��28 Model Domain��28 Conceptual Model��28 Simulation Period and Initial Conditions��31 Model Layout and Discretization��31 Basin Characterization Model Climate Inputs��34 Model Parameters��46 Model Calibration��49 Calibration Procedure��49 Calibration Statistics��51 Parameter Sensitivity��52 Calibrated Parameters��57 Calibration Results��60 Model Validation��83 Model Limitations��87 Simulation Results, Water Years 1949–2018��88 Evapotranspiration��88 Runoff��96 Recharge��101 Streamflow��105 Annual Results��111 Mean Monthly Results��111 Multi-Year Variability��126 Summary and Conclusions��128 References Cited��131 Appendix 1. Climate Stations with Records of Daily Climate Used to Develop Climate Inputs for the Salinas Valley Watershed Model��135

Figures 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

15. 16.

17.

18. 19.

20. 21. 22. 23.

Map showing the Salinas Valley study area, with land-surface elevation, major rivers and streams, reservoirs, and U.S. Geological Survey streamgages��3 Map showing surface-water subbasins in the Salinas Valley study area��6 Map showing surface-water subdrainages in the Salinas Valley study area��7 Map showing land-surface slope, calculated as rise over run, in the Salinas Valley study area��12 Bar graphs showing monthly streamflow at selected streamgages in the Salinas Valley study area��13 Map showing National Land Cover Data 2011 land cover types in the Salinas Valley study area��16 Maps showing National Land Cover Database 2011 percentage of forest canopy cover and impervious land cover in the Salinas Valley study area��18 Map showing Soil Survey Geographic database information on soil texture classes in the Salinas Valley study area��22 Map showing Soil Survey Geographic database soil available water storage capacity up to 150 centimeters soil depth, Salinas Valley study area��24 Map showing generalized California surficial geology for the Salinas Valley Watershed Model study area��25 Diagrams showing water flow and storages simulated by the Hydrologic Simulation Program—Fortran��29 Map showing mean land-surface elevation for 690 hydrologic response units used in the Salinas Valley Watershed Model��32 Map showing the mean percentage of impervious land cover for 690 hydrologic response units used in the Salinas Valley Watershed Model��33 Map showing location and size of total contributing drainage areas for 690 stream reaches or reservoirs used to discretize the Salinas Valley Watershed Model��35 Map showing the lower Salinas Valley study area tributary drainages and locations of 148 surface-water inflows��36 Map showing the locations of climate stations having records of daily precipitation used in the Basin Characterization Model to develop daily climate input for the Salinas Valley Watershed Model��38 Map showing the locations of climate stations having records of daily maximum and minimum air temperature used in the Basin Characterization Model to simulate daily potential evapotranspiration for the Salinas Valley Watershed Model��39 Graphs showing precipitation for water years 1948–2018 estimated using the Basin Characterization Model��41 Map showing mean annual precipitation estimated using the Basin Characterization Model for 690 hydrologic response units used in the Salinas Valley Watershed Model��42 Graphs showing potential evapotranspiration simulated using the Basin Characterization Model��43 Map showing mean annual potential evapotranspiration for water years 1949–2018 simulated using the Basin Characterization Model��44 Graphs showing climate inputs developed using the Basin Characterization Model��45 Graphs showing comparison of observed versus simulated daily streamflow at selected U.S. Geological Survey streamgages��65

vii

24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43.

44. 45.

46.

Graphs showing comparison of observed versus simulated monthly streamflow at selected U.S. Geological Survey streamgages��69 Graphs showing comparison of observed versus simulated annual and monthly streamflow at selected U.S. Geological Survey streamgages��72 Graphs showing summarized comparison of observed versus simulated streamflow for 29 U.S. Geological Survey streamgages and 2 reservoir inflows��75 Graphs showing summarized comparison of simulated versus observed streamflow for all U.S. Geological Survey streamgages and reservoir inflows��82 Graphs showing model validation using comparisons of observed and simulated streamflow��84 Map showing Salinas Valley Watershed Model simulation results for mean evapotranspiration, water years 1949–2018��95 Map showing simulated mean total runoff to stream channels using the Salinas Valley Watershed Model, water years 1949–2018��97 Map showing Salinas Valley Watershed Model simulation results for mean overland runoff to stream channels, water years 1949–2018��98 Map showing Salinas Valley Watershed Model simulation results for mean interflow runoff to stream channels, water years 1949–2018��99 Map showing Salinas Valley Watershed Model simulation results for mean groundwater discharge to streams, water years 1949–2018��100 Map showing the Salinas Valley Watershed Model simulation results for mean groundwater losses, water years 1949–2018��102 Graphs showing a comparison of observed and simulated differences in annual streamflow at selected U.S. Geological Survey streamgages��103 Map showing Salinas Valley Watershed Model simulation results for mean streamflow seepage losses, in cubic feet per second, water years 1949–2018��106 Map showing Salinas Valley Watershed Model simulation results for mean streamflow, in cubic feet per second, water years 1949–2018��107 Map showing Salinas Valley Watershed Model simulation results for mean streamflow runoff equivalent in inches per year, water years 1949–2018��108 Map showing Salinas Valley Watershed Model simulation results for maximum daily streamflow seepage losses, in cubic feet per second, water years 1949–2018��109 Graphs showing simulated total daily surface-water inflows to the lower Salinas Valley study area��110 Graphs showing annual simulation results for the Salinas Valley study area using the Salinas Valley Watershed Model��112 Graphs showing simulated annual inflows and outflows for subbasins��115 Graphs showing a comparison of annual simulation results for 10 subbasins in the Salinas Valley study area using the Salinas Valley Watershed Model, water years 1949–2018��120 Graphs showing Salinas Valley Watershed Model mean monthly simulation results during water years 1949–2018��123 Graphs showing Salinas Valley Watershed Model multi-year variability in basinwide annual precipitation, evapotranspiration, recharge, and surface-water outflow��127 Graph showing Salinas Valley Watershed Model variability in basinwide average annual precipitation, evapotranspiration, recharge, and surface-water outflow for multi-year wet and dry periods��128

viii

Tables 1. 2. 3. 4. 5.

6. 7. 8. 9.

10.

11. 12.

13. 14. 15. 16. 17. 18. 19. 20.

Subbasin areas and topographic characteristics of subbasins in the Salinas Valley study area��8 U.S. Geological Survey streamgages in the Salinas Valley study area with daily streamflow records between water years 1948 and 2018��8 Subdrainage areas and topographic characteristics of subdrainages in the Salinas Valley study area��10 National Land Cover Data 2011 land cover types, as a percentage of subdrainage and sub-model areas, for the Salinas Valley study area��17 National Land Cover Database 2011, percentage of forest canopy and percentage of impervious area for subdrainage and sub-model areas, Salinas Valley study area��20 Soil Survey Geographic soil texture classes for subdrainage areas in the Salinas Valley study area��23 Generalized surficial geology for subdrainage areas in the Salinas Valley study area��26 Drainage basin areas and topographic characteristics of tributary drainages to the lower Salinas Valley study area��37 Hydrologic Simulation Program—Fortran pervious land-area parameters used in the Salinas Valley Watershed Model to represent basin characteristics for the Salinas Valley study area��47 U.S. Geological Survey streamgages and reservoirs with estimated inflows along with calibration periods used for calibrating the Salinas Valley Watershed Model��50 Calibration criteria applied to goodness-of-fit results for daily, monthly, and annual streamflow for calibrating the Salinas Valley Watershed Model��52 Goodness-of-fit results comparing observed streamflow with six baseline models developed using documented ranges in the Hydrologic Simulation Program—Fortran parameter values��53 Goodness-of-fit results used to evaluate model sensitivity to adjustments in values of pervious land parameters used in the Salinas Valley Watershed Model��53 Summary statistics for parameters with constant monthly values calibrated to 690 hydrologic response units used in the Salinas Valley Watershed Model��57 Summary statistics for monthly varying parameters calibrated to 690 hydrologic response units used in the Salinas Valley Watershed Model��58 Salinas Valley Watershed Model calibration results showing goodness-of-fit statistics using daily, monthly, and annual streamflow��61 Salinas Valley Watershed Model calibration results showing goodness-of-fit statistics using mean monthly streamflow��78 Model validation results for five selected U.S. Geological Survey streamgages��86 Simulation results for selected water balance components calculated as mean annual inches of water inflows and outflows��89 Simulation results for selected water balance components calculated as mean annual inflow and outflow volumes��92

Conversion Factors U.S. customary units to International System of Units

Multiply

To obtain

Length inch (in.)

2.54

centimeter (cm)

inch (in.)

25.4

millimeter (mm)

foot (ft)

0.3048

meter (m)

mile (mi)

1.609

kilometer (km)

Area acre

4,047

square meter (m2)

acre

0.4047

hectare (ha)

acre

0.004047

square kilometer (km2)

square mile (mi2) square mile (mi2)

259.0 2.590

hectare (ha) square kilometer (km2)

Volume acre-foot (acre-ft)

1,233

cubic meter (m3)

Flow rate acre-foot per year (acre-ft/yr) cubic foot per second (ft3/s)

1,233

cubic meter per year (m3/yr)

0.02832

cubic meter per second (m3/s)

inch per hour (in/h)

0.0254

meter per hour (m/h)

inch per year (in/yr)

25.4

millimeter per year (mm/yr)

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C = (°F – 32) / 1.8.

Datums Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88).

Supplemental Information A water year is the 12-month period from October 1 through September 30 and is designated by the calendar year in which it ends.

Abbreviations AET

actual evapotranspiration

BCM

Basin Characterization Model

D-8

eight-direction routing

DEM

digital elevation model

evapotranspiration

Ftable

Flow-table

GIS

Geographic Information System

HRU

hydrologic response unit

HSPF

Hydrologic Simulation Program—FORTRAN

HUC

hydrologic unit code

IMPLND

impervious land area

LSVS

lower Salinas Valley sub-model

MCWRA

Monterey County Water Resources Agency

NHD

National Hydrography Dataset

NLCD

National Land Cover Data

NSME

Nash–Sutcliffe model efficiency

PAE

percent-average estimation error

percent estimation error

PERLND

pervious land area

PET

potential evapotranspiration

PRISM

Parameter-elevation Regression on Independent Slopes Model

coefficient of determination

RCHRES

stream reach or reservoir in HSPF

RETSC

retention storage capacity

SRW

Salinas River watershed

SSURGO

Soil Survey Geographic database

SVIHM

Salinas Valley Integrated Hydrologic Flow Model

SVU

Salinas Valley upland area

SVWM

Salinas Valley Watershed Model

UCI

user control input

USGS

U.S. Geological Survey

USVS

upper Salinas Valley sub-model

WDM

Watershed Data Management

Subbasins ARR

Arroyo Seco

EST

Estrella River

LOR

San Lorenzo Creek

LSR

lower Salinas River

MCB

Monterey Coastal Basins

MSR

middle Salinas River

NAC

Nacimiento River

SAN

San Antonio River

SRH

Salinas River headwaters

USR

upper Salinas River

Subdrainages BSC

Big Sandy Creek

CHO

Cholame Creek

CSC

Chalone–Stonewall Creek

ELK

Elkhorn Slough

ETC

El Toro Creek

HUE

Huerhuero Creek

LAS

lower Arroyo Seco

LCS

Limekiln Creek–Salinas River

LER

lower Estrella River

LNR

lower Nacimiento River

LSA

lower San Antonio River

LSJ

lower San Juan Creek

MCS

Monroe Creek–Salinas River

MSB

Monterey–Seaside basin

PCD

Pine Creek drainages

PRC

Paso Robles Creek

xii

QCC

Quail–Chualar Creek

SLC

San Lorenzo Creek

SML

Santa Margarita Lake

SNB

Salinas River near Bradley

SPR

Sargent–Pancho Rico Creeks

SRO

Salinas River outflow

SRP

Salinas River near Paso Robles

UAS

upper Arroyo Seco

USA

upper San Antonio River

UNR

upper Nacimiento River

USJ

upper San Juan Creek

USV

upper Salinas Valley

Tributary Drainages ALIS

Alisal Creek

BIGS

Big Sandy Creek

CHAL

Chalone Creek

CHER

Cherry Canyon

CHUA

Chualar Creek

GABI

Gabilan Creek

HAME

Hames Creek

HARE

Hare Canyon

LSRW

lower Salinas River West

MCOY

McCoy Creek

MONR

Monroe Creek

MSRW

middle Salinas River West

NACI

Nacimiento River

PANR

Pancho Rico Creek

PINE

Pine Creek

QUAI

Quail Creek

SALI

Salinas River

SANL

San Lorenzo Creek

SANR

San Antonio River

SARG

Sargent Creek

SECO

Arroyo Seco

STON

Stonewall Creek

TORO

El Toro Creek

VINE

Vineyard Canyon

WILD

Wildhorse Canyon

Abstract The U.S. Geological Survey (USGS), in cooperation with the Monterey County Water Resources Agency, completed studies to help evaluate the surface-water and groundwater resources of the Salinas Valley study area, consisting of the entire Salinas River watershed and several smaller, adjacent coastal watersheds draining into Monterey Bay. The Salinas Valley study area is a highly productive agricultural region that depends on the coordinated use of surface water and groundwater to meet demand for irrigation and public water supply. To continue to meet these demands, a better understanding of the historical water balance and the effects of water-resource development on the long-term sustainability of water resources in the Salinas Valley study area is needed. This report documents the development and application of the Salinas Valley Watershed Model (SVWM) to simulate the daily historical water balance and hydrologic conditions of the Salinas Valley study area for water years 1949–2018, including the many ungaged tributary subdrainages in the rugged and mountainous upland areas that surround flat-lying valley lowlands, which coincide with developed areas and croplands irrigated with groundwater. The SVWM simulates the natural hydrologic system for the entire Salinas Valley watershed and adjacent coastal basins, excluding anthropogenic components such as pumping, diversions, irrigation, and reservoir operations, for the 70-year period beginning October 1, 1948, and ending September 30, 2018. The SVWM uses two modeling applications: the Hydrologic Simulation Program—Fortran (HSPF) to simulate the natural hydrologic system and the Basin Characterization Model (BCM) to develop spatially distributed, historical climate inputs for HSPF. The HSPF application simulates the daily surface-water and shallow subsurface-water storage and flow processes, including interception storage and evaporation on vegetation, surface retention storage and evaporation,

pervious land soil water storage and evapotranspiration, runoff from impervious and pervious land areas, streamflow, recharge from pervious land areas, and recharge from streamflow seepage. Climate inputs developed using the BCM are daily precipitation, daily maximum and minimum air temperature, and daily potential evapotranspiration (PET). Salinas Valley Watershed Model parameters were estimated using geospatial data and then adjusted by trial-and-error fitting of simulated daily streamflow to long-term records of observed streamflow at 29 USGS streamgages and to estimated daily surface-water inflows to two reservoirs in the Salinas Valley study area, Lakes Nacimiento and San Antonio. The trial-and-error calibration provided a good match between simulated and observed daily, monthly, mean-monthly, and annual streamflow. The overall goodness-of-fit statistics for the calibrated model included a weighted mean percent-average estimation error of 2.0 percent for daily and monthly streamflow; 2.1 percent for annual streamflow; and Nash–Sutcliffe model efficiency values of 0.64 for daily mean streamflow, 0.84 for monthly streamflow, and 0.88 for annual streamflow. Spatially averaged, 70-year mean simulation results for the Salinas Valley study area included precipitation of 18.5 inches per year (in/yr), evapotranspiration of 14.9 in/yr, net recharge of 2.7 in/yr, and surface-water outflow to Monterey Bay of about 0.96 in/yr. Net recharge consisted of two components, about 2.6 in/yr stream seepage recharge and 0.08 in/yr inter-channel net land-area recharge. Total recharge for the Salinas Valley study area was 4.4 in/yr, about 24 percent of precipitation, with about 1.7 in/yr becoming groundwater discharge from pervious land areas. The 70-year mean runoff was 3.5 in/yr, about 19 percent of precipitation; however, most of the runoff, about 74 percent, became stream seepage recharge rather than surface-water outflow to Monterey Bay. About 48 percent of the runoff was from groundwater discharge, with overland runoff contributing 28 percent and interflow runoff contributing 24 percent.

2 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley Evapotranspiration varied spatially in response to variability in precipitation, PET, land cover, and the root zone’s water-holding capacity. The relative contributions to runoff from overland runoff, interflow runoff, and groundwater discharge also varied spatially, with the highest percentages of overland and interflow runoff occurring for the more rugged and wetter, high-elevation locations along the western crest of the valley. Results indicated mostly ephemeral streamflow with a lack of sustained baseflow during summer months for most locations and a high degree of spatial and temporal variability in streamflow characterized by the rapid onset of peak flows in response to precipitation. The lack of sustained groundwater discharge to streams (baseflow) caused streamflow to be highly sensitive to the temporal variability in precipitation, especially in response to drier-than-average winters, resulting in no-flow conditions along the main channel of the Salinas River. The Nacimiento River subdrainage was the largest source of surface-water inflow to the lower Salinas River valley, with a 70-year mean discharge of 259 cubic feet per second (ft3/s), and the Arroyo Seco subdrainage was the second largest inflow with a mean discharge of 163 ft3/s. Compared to tributary drainages in the hotter and drier eastern and southern parts of the Salinas Valley with lower mean discharge rates, the Nacimiento River and Arroyo Seco subdrainages are located closer to the Pacific Ocean moisture source on the west side of the valley and include higher elevation, steeper terrain with thinner soil cover, higher precipitation, and lower PET, all characteristics that are conducive to runoff generation. The total 70-year mean surface-water inflow from all tributaries to the lower Salinas Valley was 558,000 acre-feet per year (acre-ft/yr), or about 770 ft3/s. The mean surface-water outflow to Monterey Bay from the Salinas Valley study area was only 232,000 acre-ft/yr (320 ft3/s), including 201,000 acre-ft/yr (278 ft3/s) outflow from the Salinas River, indicating that 67 percent of the tributary inflows, 374,000 acre-ft/yr, became stream seepage recharge. The final 20 years of the simulation period (water years 1999–2018) was the driest 20-year period within the 70-year simulation period. The mean recharge for water years 1999–2018 was about 2.1 in/yr, or 20 percent less than the 70-year mean. The mean surface-water outflow to Monterey Bay for water years 1999–2018 was about 0.65 in/yr, or 32 percent less than the 70-year mean. Water years 1999–2018 also included the driest 10-year period, water years 2007–16, with a mean recharge of 1.5 in/yr (44 percent less than the 70-year mean) and a mean surface-water outflow of 0.59 in/yr (39 percent less than the 70-year mean).

Introduction California’s Salinas Valley is one of the most productive agricultural basins in the world (California Department of Food and Agriculture, 2022) due to the fertile valley soil, temperate climate, and availability of water for irrigation (Lapham and Heileman, 1901; Cook, 1978). The groundwater resources of the basin are used heavily to meet water supply needs, including crop irrigation and municipal water supply. To better understand impacts on groundwater resources, the lower Salinas Valley study area was defined as an area of interest for the development and application of a groundwater flow model, referred to as the Salinas Valley Integrated Hydrologic Flow Model (SVIHM; fig. 1; Henson and Jachens, 2022). The lower Salinas Valley study area includes several smaller watersheds draining the coastal region adjacent to the mouth of the Salinas River. Most of the area within the lower Salinas Valley study area consists of extensively farmed alluvial lowlands at elevations approximately less than 500 feet (ft), surrounded by mountainous tributary drainages ranging in elevation from 500 to 5,872 ft (fig. 1). The Salinas River watershed (SRW) and adjacent coastal drainages including the areas of agricultural and groundwater development comprise a total area of 4,529 square miles (mi2), herein referred to as the Salinas Valley study area (fig. 1). The Salinas Valley study area has been experiencing insufficient water supplies, and stakeholders are facing legal and regulatory restrictions on water use. The historical imbalances between supply and demand have resulted in declining groundwater levels (California Department of Public Works, 1946; Monterey County Water Resources Agency, 1995; Baillie and others, 2015), seawater intrusion (California Department of Public Works, 1946; Leedshill-Herkenhoff, Inc., 1985; Monterey County Water Resources Agency, 1995, 1996), impaired water supplies (California Department of Water Resources, 1971; Kulongoski and Belitz, 2007; Moran and others, 2011; Harter and others, 2012), regulatory actions on pumping, adjudication (Monterey County Water Resources Agency, 1995; Baillie and others, 2015), and requirements for minimum in-stream fish flows (Monterey County Water Resources Agency, 2018). Water imbalances are likely to be further exacerbated by potential future climate change and variability, such as longer and more severe drought periods followed by periods with extreme precipitation events (Brown and Caldwell, 2014). Finding replacement water supplies and improving watershed management could help stakeholders comply with legal mandates, adapt to future climate variability and changing land use, and improve environmental and ecohydrological conditions.

Introduction 3 121°30'

122°

River aro aj

121°

120°30'

120°

101

MERCED COUNTY Sacramento

39 38

SAN BENITO COUNTY

21 7

en it

PACIFIC OCEAN

L IN

Ar r o y

101

Lake San Antonio

1,501 to 2,000

4,001 to 4,500

2,001 to 2,500

4,501 to 5,872

Estrel la R i

32 Upper Salinas Valley sub-model boundary

Base from U.S. Geological Survey, The National Map (2023) and other Federal digital data, various scales; California IV State Plane Coordinate System (FIPS 0404); North American Datum of 1983

101

24 Santa Margarita Lake

RA E

PACIFIC OCEAN

San Luis Obispo

KERN COUNTY

U.S. Geological Survey streamgage at Bradley (see table 2)

36 Paso Robles

Lower Salinas Valley study area and SVWM boundary U.S. Geological Survey streamgage and identifier (see table 2)

Lake Nacimiento

3,501 to 4,000

1,001 to 1,500

KINGS COUNTY

3,001 to 3,500

501 to 1,000

r ve

2,501 to 3,000

SVWM sub-model boundary

re e

r ve Ri

ive r

0 to 500

Salinas Valley Watershed Model (SVWM) boundary

Lo re n

Elevation, in feet above North American Vertical Datum of 1988

35° 30'

as in

3 EXPLANATION

Sa n

Sa l

io on

Na cim ien to

AL NT

San Ant

King City

Junipero Serra Peak

36°

FRESNO COUNTY

Lower Salinas Valley sub-model boundary

San Diego

Soledad

MONTEREY COUNTY

Los Angeles

n Sa

10 36° 30'

Monterey

Salinas

CA NE San L VA Francisco IFO DA RN IA Map area

MONTEREY BAY

SAN LUIS OBISPO COUNTY

Pismo Beach 0 0

10 10

20 20

30 MILES

30 KILOMETERS

Figure 1. Salinas Valley study area, with land-surface elevation, major rivers and streams, reservoirs, and U.S. Geological Survey streamgages (Henson and others, 2022; U.S. Geological Survey, 2023; Hevesi and others, 2025).

4 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley The Salinas Valley Watershed Model (SVWM), developed in cooperation with the Monterey County Water Resources Agency, includes an area coincident with the Salinas Valley study area and was developed to simulate the natural hydrologic system of the Salinas Valley study area, with a focus on precipitation-runoff processes and the need to estimate surface-water inflows from mountainous upland areas draining into the area of the SVIHM. Application of the SVWM is intended to improve the understanding of the land-surface and shallow subsurface (soil zone) hydrology of the upland drainages that are tributaries to the main branch of the Salinas River and adjacent developed areas, including groundwater basins and surface-water reservoirs. Results obtained using the SVWM could be used to help develop a broader, more comprehensive analysis of the natural hydrologic system for the entire Salinas Valley study area. The SVWM was applied to simulate the natural hydrology of the Salinas Valley study area for a 70-year historical climate period from water years 1949 to 2018 (October 1, 1948, to September 30, 2018); a water year is the 12-month period from October 1 through September 30 designated by the calendar year in which it ends. The SVWM uses an integrated modeling approach consisting of the Basin Characterization Model (BCM; Flint and others, 2021) and the Hydrologic Simulation Program—Fortran (HSPF; Bicknell and others, 2005) computer codes to simulate the natural hydrologic system in response to daily climate inputs and the physical characteristics of the surface-water drainages within the Salinas Valley study area. The SVWM simulations account for climate, surface, and shallow subsurface components of the natural hydrologic system, including surface water and shallow groundwater, with an emphasis on natural hydrologic processes at the land surface and in the shallow subsurface, including the upper soil layer and the root zone. The BCM component of the SVWM was used to develop the historical daily climate inputs for the HSPF component, consisting of daily values of precipitation, air temperature, and potential evapotranspiration (PET). The climate inputs are required to run HSPF for a continuous simulation of the hydrologic system for the 70-year target period. The BCM used a 270-meter (m) gridded representation of the watershed to account for localized orographic effects on precipitation and air temperature and topographic controls on PET. The HSPF component of the SVWM was discretized as a connected network of 690 hydrologic response units (HRUs) defined by surface hydrography and was run using an hourly time step, with daily and monthly model outputs used for model calibration and for the analysis of simulated water balance components. The HSPF outputs included surface-water outflows from ungaged tributary drainages upstream of the SVIHM and Salinas Valley lowlands that include areas of productive groundwater development. The simulated outflows can be used as inflow boundary conditions for the SVIHM. The results generated by the SVWM are intended to help water managers evaluate and adjust to

projected effects on water supplies and demands in the Salinas Valley watershed caused by changes in land use, population, and climate.

Purpose and Scope The purpose of this study was to develop a precipitation-runoff model, the SVWM, to simulate the surface-water inflows and shallow subsurface components of the natural hydrologic system of the Salinas Valley study area, including all tributary upland areas that provide surface-water inflows to lower elevation areas overlying groundwater basins in the Salinas Valley study area. The SVWM was used to simulate the natural hydrologic system of the Salinas Valley study area from water years 1949 to 2018 with the goal of developing a better understanding of the long-term historical water balance. This report describes the development, calibration, and application of the SVWM (consisting of the BCM and HSPF model components) to quantify precipitation-runoff processes, including climate, surface-water flow, evapotranspiration (ET), and recharge in the Salinas Valley study area. The specific objectives of this study were to (1) apply the SVWM to quantify the historical distribution of precipitation falling on the land surface, infiltrating the root zone, returning to the atmosphere by ET, contributing to streamflow as overland runoff, percolating through the root zone to become recharge or contributing to streamflow as shallow subsurface interflow or deeper base flow, and flowing through the stream channel network; (2) provide a characterization and historical context of the spatial and temporal variability and distribution of the simulated water balance components; and (3) simulate the water inflows from the tributary drainage basins in the upland areas for potential use as boundary conditions for integrated surface water–groundwater modeling in the Salinas Valley study area.

Study Area The Salinas River valley is bounded by the Diablo, Temblor, and Gabilan Ranges to the east and the Santa Lucia and Sierra de Salinas Ranges to the west (fig. 1). The SVWM includes the entire Salinas River watershed and several smaller drainages along the Monterey Bay coast that drain into Monterey Bay. The Salinas River is the third longest river in the State of California and is the largest river in California’s Central Coast region, draining an area of 4,160 mi2. The Salinas River originates in the La Panza Range of central San Luis Obispo County and flows 170 miles (mi) north and northwest through Monterey County before discharging into Monterey Bay, about 80 mi south of San Francisco (fig. 1).

Study Area 5 Major tributaries to the Salinas River include Arroyo Seco, San Lorenzo Creek, and the San Antonio, Nacimiento, and Estrella Rivers (fig. 1). Land-surface elevations in the SVWM average 1,426 ft and range from 0 ft along the coast to a maximum of 5,872 ft at the summit of Junipero Serra Peak in the Santa Lucia Range and the headwaters of the Arroyo Seco drainage (fig. 1). The general area of the groundwater basin in the SVIHM is approximately defined by the extent of the Salinas Valley alluvial basin and the transition from the valley floor to the steeper terrain of the surrounding uplands (fig. 1). Land-surface elevations within the SVIHM range from 0 to about 2,000 ft at various locations along the SVIHM boundary (fig. 1). Land-surface elevations within the area of the upper Salinas River watershed, upstream of the SVIHM boundary, range from about 100 to about 4,000 ft (fig. 1).

Water Use and Management The Salinas River watershed contains three reservoirs: Lake Nacimiento and Lake San Antonio, each with an area of about 22 mi2, and Santa Margarita Lake, with an area of about 1.1 mi2 (fig. 1). The reservoirs are used primarily to provide flood protection and are operated for a variety of uses that include municipal water supplies, agricultural irrigation, recreation, groundwater recharge, and protection of fish habitat. The dams impounding Lakes Nacimiento and San Antonio were constructed to control floodwaters and provide water for summer recharge of the Salinas Valley groundwater basin for urban and agricultural use (Brown and Caldwell, 2014). The dam impounding Lake Nacimiento, located in northern San Luis Obispo County approximately 20 mi from the coast, was completed in 1957 (fig. 1) and provides a maximum storage capacity of 377,900 acre-feet (acre-ft). The dam impounding Lake San Antonio, located in southern Monterey County about 16 mi northwest of Paso Robles, was completed in 1967 and provides a maximum storage capacity of 477,000 acre-ft. The management of Lakes Nacimiento and San Antonio, including operation of the dams, is under the jurisdiction of the California Department of Water Resources, Division of Safety of Dams (Brown and Caldwell, 2014).

Climate The climate in the Salinas Valley study area is characterized by warm, dry summers and cool, moist winters (Baillie and others, 2015). Based on an 80-year average of climate records from 1931 to 2015, from the National Climatic Data Center station at the Salinas airport (station USW00023233 at https://www.ncdc.noaa.gov/cdo-web/ datatools/findstation) located on the valley floor, the average annual temperature is 57 degrees Fahrenheit (°F), and the average annual precipitation is 13 inches (in.) falling as rain primarily during the winter and early spring. The distribution of precipitation across the study area is dependent on the topography, prevailing winds, and proximity to the coastline

(Daly and others, 2004). Precipitation generally increases with increasing altitude due to orographic lifting and adiabatic cooling of moist air, but also decreases with increasing distance from the coastline, such that the higher land elevations for summit locations on the east side of the Salinas Valley receive less precipitation compared to equivalent land elevations on the west side of the valley (Tinsley, 1975; Montgomery–Watson Consulting Engineers, 1994; Daly and others, 2004).

Hydrography For reasons discussed in the “Model Layout and Discretization” section, the SVWM was divided into two sub-model areas south and north of the U.S. Geological Survey (USGS) streamgage 11150500 (gage 19, Salinas River at Bradley; in this report, streamgage is synonymous with gage; the streamgage numbering sequence does not include numbers 9, 12, 26, 33, and 37; fig. 2); the upper Salinas Valley sub-model (USVS), south of the streamgage and the lower Salinas Valley sub-model (LSVS), north of the streamgage (fig. 2; tables 1, 2). In addition to the two sub-model areas, the Salinas Valley study area was further subdivided into 10 surface-water subbasins and 28 surface-water subdrainages (figs. 2, 3; tables 1, 3). The subbasins and subdrainages were used in this study to analyze and compare characteristics and model results for different parts and tributaries of the study area. The subbasin and subdrainage areas were defined based on hydrography, including the location of tributary junctions, drainage areas, streamgages, and reservoirs. The USVS and LSVS each include 5 subbasins and 14 subdrainages. The Estella River (EST) subbasin drains the southeastern part of the SVWM and includes four subdrainages: the Cholame Creek (CHO), the upper and lower San Juan Creek (USJ and LSJ) subdrainages, and the lower Estrella River (LER) subdrainage. The Salinas River headwaters (SRH) subbasin drains the southwestern part of the SVWM and includes the Santa Margarita Lake (SML), Salinas River near Paso Robles (SRP), and Paso Robles Creek (PRC) subdrainages. The upper Salinas River (USR) subbasin drains the south-central part of the SVWM, upstream of gage 19 and downstream of the city of Paso Robles, and includes the Huerhuero Creek (HUE), Big Sandy Creek (BSC), and Salinas River near Bradley (SNB) subdrainages. The Arroyo Seco (ARR), San Antonio River (SAN), and Nacimiento River (NAC) subbasins drain the western part of the SVWM with each subbasin including upper and lower subdrainages; the upper and lower Arroyo Seco (UAS and LAS) in the ARR, the upper and lower San Antonio River (USA and LSA) in the SAN, and upper and lower Nacimiento River (UNR and LNR) in the NAC. The middle Salinas River (MSR) subbasin includes five subdrainages in the west-central part of the SVWM downstream of gage 19 and upstream of USGS streamgage 11151700 (gage 6, Salinas River at Soledad): the Sargent–Pancho Rico Creeks (SPR), Pine Creek drainage (PCD), upper Salinas Valley (USV), Chalone–Stonewall Creek (CSC), and Monroe Creek–Salinas River (MCS) subdrainages.

6 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley 121°30'

122°

River aro aj

120°

101

EXPLANATION

MERCED COUNTY

MCB

Salinas Valley Watershed Model (SVWM) subbasins FRESNO COUNTY

17 SAN BENITO COUNTY

Salinas 21 7

LSR

n Sa

en it

36° 30'

120°30'

MCB 39

MONTEREY BAY

Monterey

121°

MONTEREY COUNTY

Soledad

ARR

Arroyo Seco

EST

Estrella River

LSR

Lower Salinas River

MSR

Middle Salinas River

MCB

Monterey Coastal Basins

NAC

Nacimiento River

SRH

Salinas River headwaters

SAN

San Antonio River

LOR

San Lorenzo Creek

USR

Upper Salinas River SVWM boundary

6 Se

SVWM sub-model boundary

Ar r o y

101

ARR

King City

Sa n

as in

Sa l

Lo re n

re e

r ve Ri

NAC

U.S. Geological Survey streamgage at Bradley (see table 2)

SAN ive r

Na cim 16 ien to

U.S. Geological Survey streamgage and identifier (see table 2)

LOR

MSR

San Ant

io on

36°

Lower Salinas Valley sub-model boundary

Lake San Antonio

19 18

USR

Lake Nacimiento

36 Estrel la R i 29 8

SRH 32

PACIFIC OCEAN

KERN COUNTY

r ve

Paso Robles 20 35° 30'

KINGS COUNTY

EST

Upper Salinas Valley sub-model boundary

San Luis Obispo 101

24 Santa Margarita Lake

SAN LUIS OBISPO COUNTY

Pismo Beach Base from U.S. Geological Survey, The National Map (2023) and other Federal digital data, various scales; California IV State Plane Coordinate System (FIPS 0404); North American Datum of 1983

Figure 2. Surface-water subbasins in the Salinas Valley study area (Hevesi and others, 2025).

0 0

10 10

20 20

30 KILOMETERS

30 MILES

Study Area 7 122°

121°30'

121°

120°30'

120°

101

MERCED COUNTY

ELK

MONTEREY BAY

SAN BENITO COUNTY

Salinas Monterey

ETC

SRO

QCC

n Sa

36° 30'

MSB

Be n

r ive

LCS

MONTEREY COUNTY

ito

CSC

Soledad

FRESNO COUNTY

LAS MCS

Sa l

USV

USA

iv oR

UNR

ve r

101

SPR

Na cim ien t

re e

PCD

i ve r

San Ant

R io on

36°

Sa n SLC Lo re nz o

King City

s ina

UAS Lower Salinas Valley sub-model boundary

Lake San Antonio

CHO

LSA SNB

Lake Nacimiento

LNR

LER

Paso Robles

San Ju an C

PRC HUE

San Luis Obispo

10 10

20 20

LSJ

SRP

Upper Salinas Valley sub-model boundary 0

KERN COUNTY

k ree

PACIFIC OCEAN

35° 30'

KINGS COUNTY

BSC

Santa 101 Margarita Lake

30 MILES

30 KILOMETERS

Pismo Beach

SML

USJ

SAN LUIS OBISPO COUNTY

Base from U.S. Geological Survey, The National Map (2023) and other Federal digital data, various scales; California IV State Plane Coordinate System (FIPS 0404); North American Datum of 1983

EXPLANATION Salinas Valley Watershed Model (SVWM) subdrainages Upper Salinas Valley sub-model SRP Salinas River near Paso Robles LAS

SVWM boundary Lower Arroyo Seco

BSC

Big Sandy Creek

SML

Santa Margarita Lake

MCS

Monroe Creek-Salinas River

CHO

Cholame Creek

UNR

Upper Nacimiento River

MSB

Monterey-Seaside basin

HUE

Huerhuero Creek

USA

Upper San Antonio River

PCD

Pine Creek drainages

LER

Lower Estrella River

USJ

Upper San Juan Creek

QCC

Quail-Chualar Creek

LNR

Lower Nacimiento River

Salinas River outflow

Lower San Antonio River

Lower Salinas Valley sub-model

SRO

LSA

CSC

SLC

San Lorenzo Creek

LSJ

Lower San Juan Creek

Chalone-Stonewall Creek Elkhorn Slough

SPR

Sargent-Pancho Rico Creeks

PRC

Paso Robles Creek

ELK

El Toro Creek

UAS

Upper Arroyo Seco

SNB

Salinas River near Bradley

ETC LCS

Limekiln Creek-Salinas River

USV

Upper Salinas Valley

Salinas Valley Hydrologic Model sub-model boundary SVWM subbasin boundary 19

Figure 3. Surface-water subdrainages in the Salinas Valley study area (Hevesi and others, 2025).

U.S. Geological Survey streamgage at Bradley (see table 2)

[Subbasins and sub-models are listing in order of upstream to downstream tributary connections. Abbreviations: mi2, square mile; Min, minimum; Max, maximum; —, not applicable]

Subbasin name/ sub-model name

Elevation (feet)

Slope (rise/run)

Subbasin abbreviation

Area (mi2)

Downstream subbasin abbreviation

Sub-model abbreviation

Mean

Max

Min

Range

Mean

Max

Min

Salinas River headwaters

SRH

389

USR

USVS

1,518

4,057

667

3,390

0.24

1.61

0.00

Estrella River

EST

924

USR

USVS

1,794

4,327

674

3,653

0.19

1.69

0.00

Nacimiento River

NAC

325

USR

USVS

1,592

3,749

761

2,988

0.30

1.58

0.00

San Antonio River

SAN

323

USR

USVS

1,554

5,870

678

5,192

0.26

1.52

0.00

Upper Salinas River

USR

575

LSV

USVS

1,285

3,930

455

3,475

0.20

1.40

0.00

—

2,536

LSV

USVS

1,580

5,870

455

5,414

0.22

1.69

0.00

Arroyo Seco

ARR

297

MSR

LSVS

2,225

5,872

173

5,699

0.43

2.08

0.00

San Lorenzo Creek

LOR

261

MSR

LSVS

1,936

4,483

283

4,200

0.27

1.33

0.00

Middle Salinas River

MSR

769

LSR

LSVS

1,125

3,938

133

3,805

0.24

2.37

0.00

Lower Salinas River

LSR

374

Ocean

LSVS

800

4,403

0.20

1.17

0.00

Monterey Coastal Basins

MCB

292

Ocean

LSVS

410

3,137

0.14

1.07

0.00

Lower Salinas Valley sub-model

—

1,992

Ocean

LSVS

1,229

5,872

0.25

2.37

0.00

Salinas Valley Watershed Model (SVWM)

—

4,529

Ocean

—

1,425

5,872

0.23

2.37

0.00

Upper Salinas Valley sub-model

Table 2. U.S. Geological Survey (USGS) streamgages in the Salinas Valley study area with daily streamflow records between water years 1948 and 2018. [ID, identification; mi2, square mile; mm/dd/yyyy, month/day/year; SVWM, Salinas Valley Watershed Model]

USGS streamgage ID

SVWM streamgage number

USGS streamgage name

Start of record (mm/dd/yyyy)

End of record at time of study (mm/dd/yyyy)

Status at time of study

Drainage area (mi2)

11152000

ARROYO SECO NR SOLEDAD CA

10/01/1901

09/30/2018

Active

244

11152050

ARROYO SECO BL RELIZ C NR SOLEDAD CA

10/01/1994

09/30/2018

Active

304

11149900

SAN ANTONIO R NR LOCKWOOD CA

10/01/1965

09/30/2018

Active

217

11144200

SALSIPUEDES C NR POZO CA

10/01/1969

09/30/1983

Historic

11150800

COW C NR SAN ARDO CA

10/01/1960

09/30/1964

Historic

11151700

SALINAS R A SOLEDAD CA

10/01/1968

09/30/2018

Active

3,563

11152540

EL TORO C NR SPRECKELS CA

10/01/1961

09/30/2001

Historic

11147600

HUERHUERO C NR CRESTON CA

10/01/1958

09/30/1972

Historic

101

11152300

SALINAS R NR CHUALAR CA

10/01/1976

09/30/2018

Active

4,042

11147040

SANTA RITA C TRIB NR TEMPLETON CA

08/01/1967

09/30/1972

Inactive

8 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Table 1. Subbasin areas and topographic characteristics of subbasins in the Salinas Valley study area.

Table 2. U.S. Geological Survey (USGS) streamgages in the Salinas Valley study area with daily streamflow records between water years 1948 and 2018.—Continued [ID, identification; mi2, square mile; mm/dd/yyyy, month/day/year; SVWM, Salinas Valley Watershed Model]

USGS streamgage ID

SVWM streamgage number

USGS streamgage name

Start of record (mm/dd/yyyy)

End of record at time of study (mm/dd/yyyy)

Status at time of study

Drainage area (mi2)

11151870

ARROYO SECO NR GREENFIELD CA

10/01/1961

09/30/1986

Inactive

113

11144600

SALINAS R BL SALINAS DAM NR POZO CA

10/01/1973

04/09/1986

Inactive

112

11151300

SAN LORENZO C BL BITTERWATER C

10/01/1958

09/30/2018

Active

233

11149650

SULPHUR SPRINGS CYN NR JOLON CA

10/01/1967

09/30/1969

Inactive

11152570

ALISAL C NR SALINAS CA

10/01/1970

09/30/1974

Inactive

11150000

SAN ANTONIO R A PLEYTO CA

10/01/1929

09/30/1965

Inactive

277

11150500

SALINAS R NR BRADLEY CA

10/01/1948

09/30/2018

Active

2,535

11147070

SANTA RITA C NR TEMPLETON CA

10/01/1961

09/30/1994

Inactive

11152500

SALINAS R NR SPRECKELS CA

10/01/1929

09/30/2018

Active

4,156

11148800

NACIMIENTO R NR BRYSON CA

10/01/1955

09/30/1971

Inactive

147

11149700

SAN ANTONIO R A SAM JNS BR NR LOCKW

07/01/1958

09/30/1965

Inactive

147

11145000

SALINAS R AB PILITAS C NR SANTA MAR

07/28/1942

10/03/1975

Inactive

114

11148900

NACIMIENTO R BL SAPAQUE C NR BRYSON

09/16/1971

09/30/2018

Active

162

11144000

TORO C NR POZO CA

10/01/1960

09/30/1983

Inactive

11149500

NACIMIENTO R NR SAN MIGUEL CA

10/01/1939

09/30/1957

Inactive

349

11147500

SALINAS R A PASO ROBLES CA

11/01/1939

09/30/2018

Active

390

11149400

NACIMIENTO R BL NACIMIENTO DAM NR B

10/01/1957

09/30/2018

Active

329

11143500

SALINAS R NR POZO CA

10/01/1942

09/30/1983

Inactive

11147000

JACK C NR TEMPLETON CA

10/01/1949

09/30/1978

Inactive

11147700

CHOLAME C TRIB NR CHOLAME CA

10/01/1958

09/30/1965

Inactive

11147800

CHOLAME C NR SHANDON CA

10/01/1958

09/30/1972

Inactive

227

11148500

ESTRELLA R NR ESTRELLA CA

10/01/1954

09/30/2018

Active

922

11152650

RECLAMATION DITCH NR SALINAS CA

10/01/1970

09/30/2018

Active

11152600

GABILAN C NR SALINAS CA

10/01/1970

09/30/2014

Inactive

11143300

ARROYO DEL REY A DEL REY OAKS CA

10/01/1966

09/30/1978

Inactive

Study Area 9

[Subdrainages are listing in order of upstream to downstream tributary connections. Abbreviations: mi2, square mile; Min, minimum; Max, maximum]

Subdrainage abbreviation

Area (mi2)

SML

112

SRP

SRH

Salinas River near Paso Robles

SRP

170

SNB

Paso Robles Creek

PRC

107

SRP

Upper San Juan Creek

USJ

172

Lower San Juan Creek

LSJ

263

Cholame Creek

CHO

237

EST

Lower Estrella River

LER

251

Upper Nacimiento River

UNR

126

Lower Nacimiento River

LNR

199

Upper San Antonio River

USA

175

Lower San Antonio River

LSA

Huerhuero Creek

HUE

Big Sandy Creek

BSC

Salinas River near Bradley

SNB

Name

Downstream subdrainage abbreviation

Subbasin abbreviation

Elevation (feet) Mean

Slope (rise/run)

Max

Min

Range

Mean

Max

Min

2,039

4,057

1,220

2,837

0.28

1.61

0.00

SRH

1,279

2,776

667

2,109

0.20

1.13

0.00

SRH

1,349

2,469

741

1,727

0.24

0.98

0.00

LSJ

EST

2,215

3,929

1,420

2,509

0.22

1.69

0.00

LER

EST

1,755

3,656

1,013

2,643

0.20

1.39

0.00

EST

1,878

4,327

1,013

3,314

0.17

1.06

0.00

SNB

EST

1,465

2,874

674

2,200

0.18

0.87

0.00

LNR

NAC

1,928

3,743

894

2,848

0.34

1.58

0.00

SNB

NAC

1,377

3,584

761

2,823

0.27

1.27

0.00

LSA

SAN

1,865

5,870

866

5,003

0.32

1.52

0.00

149

SNB

SAN

1,189

2,780

672

2,108

0.20

0.94

0.00

162

SNB

USR

1,350

3,322

676

2,646

0.15

0.75

0.00

172

SNB

USR

1,693

3,930

617

3,313

0.25

1.40

0.00

241

USV

USR

950

2,778

455

2,322

0.19

1.13

0.00

Subdrainages in the upper Salinas Valley sub-model Santa Margarita Lake

Subdrainages in the lower Salinas Valley sub-model Upper Arroyo Seco

UAS

110

LAS

ARR

2,957

5,872

781

5,091

0.52

2.08

0.00

Lower Arroyo Seco

LAS

187

LCS

ARR

1,796

5,254

173

5,082

0.37

1.53

0.00

San Lorenzo Creek

SLC

261

MCS

LOR

1,936

4,483

283

4,200

0.27

1.33

0.00

Sargent–Pancho Rico Creeks

SPR

143

USV

MSR

1,537

3,938

404

3,534

0.31

1.45

0.00

Pine Creek drainages

PCD

119

USV

MSR

1,142

2,196

316

1,881

0.28

0.97

0.00

Upper Salinas Valley

USV

187

MCS

MSR

822

3,043

282

2,761

0.21

1.38

0.00

Chalone–Stonewall Creek

CSC

183

MCS

MSR

1,445

3,291

183

3,108

0.27

2.37

0.00

Monroe Creek–Salinas River

MCS

138

LCS

MSR

669

2,985

133

2,852

0.17

1.14

0.00

Quail–Chualar Creek

QCC

101

LSV

LSR

907

3,423

3,363

0.20

1.16

0.00

Limekiln Creek–Salinas River

LCS

143

SRO

LSR

927

4,403

4,326

0.23

1.17

0.00

El Toro Creek

ETC

LSV

LSR

951

3,551

3,523

0.27

0.96

0.00

Salinas River outflow

SRO

Ocean

LSR

331

3,539

3,540

0.10

0.94

0.00

Elkhorn Slough

ELK

226

Ocean

MCB

434

3,137

0.15

1.07

0.00

Monterey–Seaside basin

MSB

Ocean

MCB

317

1,334

0.11

0.94

0.00

10 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Table 3. Subdrainage areas and topographic characteristics of subdrainages in the Salinas Valley study area.

Study Area 11 The San Lorenzo Creek (SLC) subdrainage is coincident with the San Lorenzo Creek (LOR) subbasin and is a major tributary to the MSR subbasin, draining the eastern side of the Salinas River valley. The lower Salinas River (LSR) subbasin includes four subdrainages in the lower Salinas Valley between gage 6 and the mouth of the Salinas River: the Limekiln Creek–Salinas River (LCS), Quail–Chualar Creek (QCC), El Toro Creek (ETC), and Salinas River outflow (SRO) subdrainages. The Monterey Coastal Basin (MCB) subbasin includes two separate areas consisting of several small drainages emptying into Monterey Bay: the Elkhorn Slough (ELK) subdrainage adjacent to the northeastern side of the SRO subdrainage and the Monterey–Seaside basin (MSB) subdrainage adjacent to the southwestern side of the SRO.

Physiography An important characteristic of the Salinas Valley study area is the substantial amount (more than 3,000 ft) of topographic relief on both sides of the valley, particularly the central valley, and between the headwaters and mouth of the Salinas River (fig. 1; table 1). Based on the National Elevation Dataset (U.S. Geological Survey, 2023), the mean land-surface elevation of the Salinas Valley study area is 1,425 ft above the North American Vertical Datum of 1988 (NAVD 88) and ranges from a minimum of 0 ft at the mouth of the Salinas River to a maximum of 5,872 ft at the summit of Junipero Serra Peak in the headwaters of the Arroyo Seco drainage along the western boundary of the SRW (fig. 1). Relief in the UAS and LAS subdrainages and the USA subdrainage on the west side of Salinas Valley is about 5,000 ft. The relief in the LCS and SLC subdrainages on the east side of the valley is more than 4,000 ft (figs. 1, 3; table 3). The land-surface slope is generally greatest in the headwater drainages along the western and eastern boundaries of the SRW. Calculated as rise over run, land-surface slope is less than 0.l for most of the valley floor and locations along the coastal plain, and it increases to more than 0.3 for most of the more rugged terrain in the upland areas, reaching values as high as 1.0–2.37 for steeper slopes and canyons in the uplands on both sides of the valley (fig. 4; table 1). The high relief and comparatively steep slopes for many of the tributary drainages to the Salinas River result in rapid runoff response times and accumulation of channelized streamflow during storms, often referred to as flashiness in the characteristics of streamflow.

Streamflow The characteristics of high relief and mountainous terrain combined with a focused distribution of annual precipitation from a limited number of winter (December–March) storms result in large variations in streamflow, seasonally and between peak and mean streamflow conditions (fig. 5). Mean

monthly streamflow in February ranges from about 1,200 to 1,450 cubic feet per second (ft3/s) at three streamgages along the main branch of the Salinas River (USGS streamgages 11150500 [gage 19], 11152300 [gage 10], and 11151700 [gage 6]), in the middle and LSR subbasins, compared to mean monthly flows of about 220 ft3/s and less from October to December at these locations (fig. 5A). Controlled reservoir releases from May to September cause increased mean monthly streamflow of as much as about 500 ft3/s at USGS streamgage 11150500 (gage 19); however, mean monthly streamflow is less than 200 ft3/s at USGS streamgages 11152300 (gage 10) and 11151700 (gage 6), downstream from USGS streamgage 11150500 (gage 19). Maximum monthly streamflows are approximately an order of magnitude greater than mean monthly flows along the Salinas River and minor tributaries (figs. 5B, F) and about five times greater for major tributaries (fig. 5D). Mean monthly July–October streamflow of about 15 ft3/s and less for major tributaries and 2 ft3/s and less for minor tributaries are very low compared to January– March flows of more than 200 ft3/s for major tributaries and more than 35 ft3/s for minor tributaries (fig. 5A). An important characteristic of streamflow in the Salinas River is that mean flows are not the highest at the mouth of the river. The long-term average streamflow close to the mouth of the Salinas River to Monterey Bay, as measured at the USGS streamgage 11152500 (gage 21, fig. 1), is approximately 333 ft3/s or about 241,000 acre-feet per year (acre-ft/yr). In comparison, the average flow at USGS streamgage 11152300 (gage 10) is 377 ft3/s, the average flow at USGS streamgage 11151700 (gage 6) is 343 ft3/s, and the average flow at USGS streamgage 11150500 (gage 19) is 487 ft3/s. The differences in streamflow between the streamgages on the main branch of the Salinas River indicate a loss of streamflow likely caused by seepage through the streambed and into the underlying unsaturated zone along a section of the channel where the water table is lower than the streambed elevation. Characteristics of streamflow in the SRW also include managed streamflow conditions downstream from the reservoirs, particularly Lake Nacimiento. In addition to controlled releases from Lake Nacimiento and Lake San Antonio to augment crop irrigation during the dry summer months, a portion of the Salinas River outflow to Monterey Bay occurs in response to controlled releases for environmental purposes to promote the threatened anadromous steelhead run in the Central Coast (Brown and Caldwell, 2014). Controlled reservoir releases are indicated by mean monthly streamflows greater than 400 ft3/s for the months of July and August at USGS streamgage 11150500 (gage 19; Salinas River near Bradley, downstream from the NAC junction; figs. 1, 5A) and to a lesser degree at USGS streamgage 11151700 (gage 6; Salinas River at Soledad; fig. 1), with mean monthly streamflows of more than 150 ft3/s during July and August (fig. 5A).

12 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley 122°

121°30'

121°

120°30'

120°

101

EXPLANATION

MERCED COUNTY

MONTEREY BAY

17 SAN BENITO COUNTY

Salinas Monterey

Land surface slope, in rise/run

0.41 to 0.50

0.11 to 0.20

0.51 to 0.60

0.21 to 0.30

0.61 to 0.70

0.31 to 0.40

0.71 to 2.37

Salinas Valley Watershed Model (SVWM) boundary

10 36° 30'

0.00 to 0.10

SVWM sub-model boundary

MONTEREY COUNTY

U.S. Geological Survey streamgage and identifier (see table 2)

U.S. Geological Survey streamgage at Bradley (see table 2)

Soledad

1 King City

FRESNO COUNTY

36°

Lower Salinas Valley sub-model boundary

Lake San Antonio

KINGS COUNTY

3 22

Lake Nacimiento

36 Paso Robles

35° 30'

KERN COUNTY

29 8

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary

San Luis Obispo 101

Santa Margarita Lake

SAN LUIS OBISPO COUNTY

0 0

10 10

20 20

30 MILES

30 KILOMETERS

Figure 4. Land-surface slope, calculated as rise over run, in the Salinas Valley study area (U.S. Geological Survey, 2023; Hevesi and others, 2025)

Study Area 13

Mean monthly streamflow, in cubic feet per second

1,500

A Salinas River main branch streamgages

1,400

EXPLANATION

1,300

Gage 19 (mean flow=487)

1,200

Gage 10 (mean flow=377)

1,100

Gage 6 (mean flow=343)

1,000 900 800 700 600 500 400 300 200 100 0

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Water year

Maximum monthly streamflow, in cubic feet per second

1,500

1,400

EXPLANATION

1,300

Gage 19 (maximum flow=10,180)

1,200

Salinas River main branch streamgages

Gage 10 (maximum flow=14,349)

1,100

Gage 6 (maximum flow=11,170)

1,000 900 800 700 600 500 400 300 200 100 0

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Water year

Mean monthly streamflow, in cubic feet per second

650

C Streamgages on major tributaries

600

EXPLANATION

550

Gage 25 (mean flow=170)

500

Gage 1 (mean flow=167)

450

Gage 3 (mean flow=100)

400

Gage 29 (mean flow=97)

350 300 250 200 150 100 50 0

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Water year

Figure 5. Monthly streamflow at selected streamgages in the Salinas Valley study area; A, mean monthly streamflow at three streamgages on the Salinas River; B, maximum monthly streamflow at three streamgages on the Salinas River; C, mean monthly streamflow at major tributaries to the Salinas River and the Salinas River headwater subbasin; D, maximum monthly streamflow at major tributaries to the Salinas River and the Salinas River headwater subbasin; E, mean monthly streamflow at two minor tributaries to the Salinas River; and F, maximum monthly streamflow at two minor tributaries to the Salinas River (U.S. Geological Survey, 2016; Hevesi and others, 2022, 2025).

14 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

3,600

Maximum monthly streamflow, in cubic feet per second

Streamgages on major tributaries

EXPLANATION

3,200

Gage 25 (mean flow=170)

2,800

Gage 1 (mean flow=167)

2,400

Gage 3 (mean flow=100) Gage 29 (mean flow=97)

2,000 1,600 1,200 800 400 0

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Water year 120

E Gage 36 (mean flow=24)

100

Mean monthly streamflow, in cubic feet per second

Streamgages on minor tributaries

EXPLANATION

110

Gage 15 (mean flow=14)

90 80 70 60 50 40 30 20 10 0

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Maximum monthly streamflow, in cubic feet per second

Water year 1,700 1,600 1,500 1,400 1,300 1,200 1,100 1,000 900 800 700 600 500 400 300 200 100 0

F Streamgages on minor tributaries

EXPLANATION Gage 36 (mean flow=1,671) Gage 15 (maximum flow=583)

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

Water year

Figure 5.—Continued

May

June

July

Aug.

Sept.

Study Area 15

Groundwater Groundwater flow generally follows the topography of the Salinas Valley study area, going from the mountain ranges, down to the Salinas Valley floor and then flowing toward Monterey Bay (Hamlin, 1904; Jenkins, 1943; Simpson and others, 1946; Kennedy Jenks L.L.C., 2004). Sources of groundwater recharge include percolation of precipitation, streamflow infiltration, and return flows from agricultural irrigation. The primary source of groundwater discharge in the Salinas Valley is through the pumping of wells (Hamlin, 1904; Simpson and others, 1946; Monterey County Water Resources Agency, 2006; Burton and Wright, 2018). A significant portion of runoff that originates in the uplands and contributes to streamflow in the middle and lower reaches of the Salinas River channel does not reach Monterey Bay, but rather infiltrates the riverbed and recharges the valley-fill aquifers that are subsequently pumped for irrigation or municipal use (Montgomery–Watson Consulting Engineers, 1994; Fugro West, Inc., 1995; Harding ESE, 2001; Kennedy Jenks L.L.C., 2004). Some of the infiltrated streamflow also may contribute to riparian ET. Natural groundwater recharge to the Salinas Valley occurs during the wet winter months (December–March) as infiltrated streamflow from the Salinas River; with major inflows from the Arroyo Seco, Nacimiento, and San Antonio Rivers; as infiltrated streamflow from smaller tributaries, and as direct percolation from precipitation (Brown and Caldwell, 2014). Evidence of infiltrated streamflow is indicated by the streamflow records along the main branch of the Salinas River, with decreasing streamflow occurring at downstream streamgages. The natural groundwater recharge in the Salinas Valley occurring in response to winter precipitation is augmented during the late spring and summer months by controlled releases from Lakes Nacimiento and San Antonio.

Land Cover Land cover can have an important effect on surface hydrology in terms of interception and retention storage and surface roughness affecting infiltration and overland flow. In addition, differences in vegetation type can result in substantial differences in ET. As defined by the 30-m resolution National Land Cover Database (NLCD) in 2011 (U.S. Geological Survey, 2014), natural vegetation constituted about 82 percent of the total land cover in the Salinas Valley study area, consisting mostly of grasslands and shrublands for most subdrainages, especially the mid to lower elevation subdrainages in the eastern and southeastern parts of the Salinas Valley watershed (figs. 3, 6; table 4). In contrast,

forestlands have the highest percentage of land cover for several subdrainages along the wetter western side of the Salinas Valley watershed, including the UAS, PRC, and LNR subdrainages. The percentage of shrubland is about the same for the USVS and LSVS sub-model areas, whereas the USVS area includes a higher percentage of grassland and a lower percentage of cultivated crop areas compared to the LSVS. The coastal subdrainages, including the LSV, ELK, and MSB, contain the highest percentages of combined low, medium, and high-density developed lands (about 11–33 percent). The lowest elevation subdrainages generally contain the highest percentage of developed open space (about 17 percent). Open water comprises only about 0.5 percent of the land cover in the Salinas Valley study area. The 2011 NLCD 30-m resolution, percentage of forest canopy cover supplements the NLCD land cover types and provides an additional measure of the density of tree cover. As with land cover type, forest canopy cover is an important characteristic affecting interception storage and ET. The percentage of canopy cover based on NLCD averages about 11 percent in the Salinas Valley study area and ranges from 0 percent throughout the study area to a maximum of 93 percent in the Monterey Coastal subdrainage (fig. 7; table 5). In general, the wetter and cooler western side of the Salinas Valley has a higher percentage of canopy cover, with greater than 20 percent average canopy cover for many subdrainages, compared with the eastern side of the valley, with less than 10 percent average canopy cover for many subdrainages. Several subdrainages in the coastal region also have comparatively higher average canopy cover resulting from higher precipitation and lower PET demand compared to inland areas. The 2011 NLCD 30-m resolution impervious land cover dataset indicates the percentage coverage of impervious developed lands such as parking lots, roads, and rooftops. Impervious land cover can have a significant localized effect on runoff generation and streamflow in terms of increased peak flows, increased flow volumes, and an overall increase in the flashiness of runoff. Impervious land cover, based on the 2011 data, averages only 1.2 percent of the total land area within the Salinas Valley study area (fig. 7; table 5). However, in subdrainages with population centers such as the cities of Salinas, Seaside, and Paso Robles, the percentage of impervious land cover can be as high as 100 percent locally. The average percentage of impervious cover is higher for the subdrainages in the coastal region, ranging from about 6 to 18 percent impervious land, compared to most of the more remote inland subdrainages with an average impervious cover of 0.2 percent and less.

16 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley 121°30'

122°

River aro aj

121°

101

120°30'

EXPLANATION

MERCED COUNTY

National Land Cover Database 2011

Salinas

SAN BENITO COUNTY

21 Monterey

7 n Sa

en it

Salinas Valley Watershed Model (SVWM) boundary

36° 30'

SVWM sub-model boundaries

MONTEREY COUNTY

Soledad

2 6

101

King City

Sa n

Lo re n

as in

Sa l

U.S. Geological Survey streamgage and identifier (see table 2)

U.S. Geological Survey streamgage at Bradley (see table 2)

re e

San Ant

FRESNO COUNTY

io on

36°

Lower Salinas Valley sub-model boundary

Developed, high intensity Deciduous forest Cultivated crops Barren land Shrub/scrub Mixed forest

Woody wetlands Pasture/hay Grassland/herbaceous Evergreen forest Emergent herbaceous wetlands Developed, open space Developed, low intensity Developed, medium intensity

39 MONTEREY BAY

120°

ive r

Na cim 16 ien to

r ve Ri

Lake San Antonio

KINGS COUNTY

3 22

Lake Nacimiento

KERN COUNTY

r ve

Paso Robles 20

Estrel la R i 29 8

35° 30'

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary

San Luis Obispo 101

Santa Margarita Lake

SAN LUIS OBISPO COUNTY

0 0

10 10

20 20

30 MILES

30 KILOMETERS

Figure 6. National Land Cover Data (NLCD) 2011 land cover types in the Salinas Valley study area (U.S. Geological Survey, 2014).

Table 4. National Land Cover Data (NLCD) 2011 land cover types, as a percentage of subdrainage and sub-model areas, for the Salinas Valley study area. [Subdrainages are listed in order of upstream to downstream tributary connections. Abbreviation: Cult., cultivated; —, not applicable]

NLCD 2011 land cover Subdrainage/ sub-model

Subbasin abbreviation

Water

SML SRP PRC USJ LSJ CHO LER UNR LNR USA LSA HUE BSC SNB — UAS LAS SLC SPR PCD USV CSC MCS QCC LCS ETC SRO ELK MSB — —

SRH SRH SRH EST EST EST EST NAC NAC SAN SAN USR USR USR — ARR ARR LOR MSR MSR MSR MSR MSR LSR LSR LSR LSR MCB MCB — —

1.0 0.2 0.2 0.0 0.0 0.0 0.1 0.2 3.4 0.2 5.2 0.3 0.0 0.2 0.7 0.1 0.1 0.1 0.0 0.0 0.1 0.1 0.1 0.1 0.1 0.2 0.6 0.9 0.3 0.2 0.5

Developed Open Low to space high 1.9 0.0 10.5 4.5 7.4 1.3 2.3 0.0 3.5 0.0 3.0 0.1 3.9 0.2 3.3 0.0 3.1 0.2 4.1 0.2 5.6 0.1 4.7 0.4 3.8 0.0 9.2 2.0 4.7 0.6 0.7 0.0 2.9 0.1 4.0 0.3 3.9 0.1 5.0 0.0 6.8 1.1 3.1 0.0 8.1 3.4 4.4 3.4 4.7 3.1 8.6 2.7 6.9 10.7 15.0 12.4 17.5 32.6 6.2 3.6 5.4 2.0

Barren land

Forest land

Shrub/ scrub

Grassland

Pasture/ hay

Cult. crops

Wetlands

0.1 0.4 0.7 2.9 5.6 1.7 4.9 0.7 0.1 0.3 1.0 5.7 1.6 2.1 2.3 0.0 2.8 1.2 2.5 12.2 3.3 2.9 3.2 0.1 1.0 0.1 0.6 0.2 0.7 2.3 2.3

20.8 20.1 39.4 5.4 0.8 3.6 1.4 34.3 40.4 27.0 12.0 3.0 7.1 8.9 13.8 59.3 28.0 6.1 6.6 1.6 9.6 3.6 7.5 14.1 13.1 27.6 11.1 16.3 19.0 14.3 14.0

54.7 27.5 14.7 48.2 21.6 26.3 15.7 41.3 30.4 41.5 29.2 24.1 38.1 17.4 29.2 38.3 40.9 38.5 40.6 20.9 24.3 40.7 16.7 21.0 25.2 29.5 8.8 10.8 14.7 28.1 28.7

17.0 25.2 30.3 40.7 63.8 61.7 64.1 18.6 20.7 24.3 38.5 53.4 46.7 48.6 42.9 1.0 15.0 41.3 45.4 58.4 24.4 47.2 19.4 22.4 12.8 30.5 10.7 19.1 13.0 27.5 36.1

3.6 4.1 1.4 0.2 1.4 1.3 2.2 0.0 0.0 0.0 1.7 2.2 2.3 1.8 1.6 0.0 0.8 4.9 0.1 0.2 2.0 0.7 1.0 0.9 1.0 0.0 0.6 0.8 0.0 1.3 1.4

0.1 5.2 4.2 0.2 3.1 1.8 7.3 0.0 0.1 0.3 5.1 5.8 0.3 7.3 3.2 0.0 8.9 3.1 0.7 1.7 26.2 0.8 36.7 33.4 35.9 0.3 46.7 22.2 1.5 15.2 8.5

0.7 2.3 0.5 0.0 0.1 0.4 0.2 1.5 1.5 2.1 1.6 0.3 0.1 2.4 1.0 0.6 0.3 0.4 0.0 0.0 2.5 0.8 3.9 0.2 3.2 0.5 3.2 2.3 0.7 1.4 1.1

Study Area 17

Santa Margarita Lake Salinas River near Paso Robles Paso Robles Creek Upper San Juan Creek Lower San Juan Creek Cholame Creek Lower Estrella River Upper Nacimiento River Lower Nacimiento River Upper San Antonio River Lower San Antonio River Huerhuero Creek Big Sandy Creek Salinas River near Bradley Upper Salinas Valley sub-model Upper Arroyo Seco Lower Arroyo Seco San Lorenzo Creek Sargent–Pancho Rico Creeks Pine Creek drainages Upper Salinas Valley Chalone–Stonewall Creek Monroe Creek–Salinas River Quail–Chualar Creek Limekiln Creek–Salinas River El Toro Creek Salinas River outflow Elkhorn Slough Monterey–Seaside basin Lower Salinas Valley sub-model Salinas Valley Watershed Model

Subdrainage abbreviation

18 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley 122°

121°30'

River aro aj

MONTEREY BAY

Monterey

121°

EXPLANATION

MERCED COUNTY

National Land Cover Database (2011) percentage forest canopy cover

120°

101

FRESNO COUNTY

17 SAN BENITO COUNTY

Salinas 21 7

en it

Soledad

101

King City

Sa n

Lo re n

as in

Sa l

U.S. Geological Survey streamgage at Bradley (see table 2)

re e

San Ant

60.1 to 93

io on

36°

Lower Salinas Valley sub-model boundary

40.1 to 60

10.1 to 20

U.S. Geological Survey streamgage and identifier (see table 2)

5.1 to 10

MONTEREY COUNTY

20.1 to 40

SVWM sub-model boundary

0 to 5

Salinas Valley Watershed Model (SVWM) boundary

n Sa

10 36° 30'

120°30'

ive r

Na cim ien 16 to

r ve Ri

Lake San Antonio

KINGS COUNTY

3 22 25

Lake Nacimiento

36 Estrel la 29

KERN COUNTY

r ve

Paso Robles 20

35° 30'

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary

San Luis Obispo 101

Santa Margarita Lake

SAN LUIS OBISPO COUNTY

0 0

10 10

20 20

30 KILOMETERS

Figure 7. National Land Cover Database (NLCD) 2011 percentage of A, forest canopy cover and B, impervious land cover in the Salinas Valley study area (U.S. Geological Survey, 2014).

30 MILES

Study Area 19 122°

121°30'

River aro aj

MONTEREY BAY

121°

EXPLANATION

MERCED COUNTY

National Land Cover Database (2011) percentage impervious land

5 FRESNO COUNTY

17 SAN BENITO COUNTY

Salinas 21 40

120°

101

Monterey

n Sa

en it

Soledad

101

King City

60.1 to 93

U.S. Geological Survey streamgage at Bradley (see table 2)

Sa n

Lo re n

as in

Sa l

re e

San Ant

io on

36°

Lower Salinas Valley sub-model boundary

40.1 to 60

10.1 to 20

U.S. Geological Survey streamgage and identifier (see table 2)

5.1 to 10

MONTEREY COUNTY

20.1 to 40

SVWM sub-model boundary

0 to 5

Salinas Valley Watershed Model (SVWM) boundary

7 10

36° 30'

120°30'

ive r

Na cim ien 16 to

r ve Ri

Lake San Antonio 23

3 18

KINGS COUNTY

Lake Nacimiento

36 Estrel la 29

KERN COUNTY

r ve

Paso Robles 20

35° 30'

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary

San Luis Obispo 101

Santa Margarita Lake

Figure 7.—Continued

SAN LUIS OBISPO COUNTY

0 0

10 10

20 20

30 KILOMETERS

30 MILES

[Subdrainages are listed in order of upstream to downstream tributary connections. Abbreviations: ARR, Arroyo Seco; EST, Estrella River; LOR, San Lorenzo Creek; LSR, lower Salinas River; MSR, middle Salinas River; NAC, Nacimiento River; SAN, San Antonio River; SD, standard deviation; SRH, Salinas River headwaters; USR, upper Salinas River; —, not applicable]

Subdrainage/ sub-model Santa Margarita Lake Salinas River near Paso Robles Paso Robles Creek Upper San Juan Creek Lower San Juan Creek Cholame Creek Lower Estrella River Upper Nacimiento River Lower Nacimiento River Upper San Antonio River Lower San Antonio River Huerhuero Creek Big Sandy Creek Salinas River near Bradley Upper Salinas Valley sub-model Upper Arroyo Seco Lower Arroyo Seco San Lorenzo Creek Sargent–Pancho Rico Creeks Pine Creek drainages Upper Salinas Valley Chalone–Stonewall Creek Monroe Creek–Salinas River Quail–Chualar Creek Limekiln Creek–Salinas River El Toro Creek Salinas River outflow Elkhorn Slough Monterey–Seaside basin Lower Salinas Valley sub-model Salinas Valley Watershed Model

Subdrainage abbreviation

Subbasin abbreviation

SML SRP PRC USJ LSJ CHO LER UNR LNR USA LSA HUE BSC SNB — UAS LAS SLC SPR PCD USV CSC MCS QCC LCS ETC SRO ELK MSB — —

SRH SRH SRH EST EST EST EST NAC NAC SAN SAN USR USR USR — ARR ARR LOR MSR MSR MSR MSR MSR LSR LSR LSR LSR MCB MCB — —

NLCD 2011 land cover Percentage forest canopy Average Maximum Minimum 20.2 74.0 0.0 15.6 77.0 0.0 25.8 75.0 0.0 7.3 83.0 0.0 1.6 65.0 0.0 4.6 70.0 0.0 2.6 65.0 0.0 23.5 80.0 0.0 25.6 79.0 0.0 17.0 88.0 0.0 8.6 76.0 0.0 4.6 65.0 0.0 8.3 75.0 0.0 6.2 71.0 0.0 10.7 88.0 0.0 22.0 87.0 0.0 15.4 82.0 0.0 7.0 68.0 0.0 8.1 69.0 0.0 2.3 55.0 0.0 9.1 74.0 0.0 4.8 69.0 0.0 5.9 75.0 0.0 9.5 78.0 0.0 11.1 82.0 0.0 20.1 83.0 0.0 9.4 84.0 0.0 15.4 86.0 0.0 20.6 93.0 0.0 0.8 93.0 0.0 10.6 93.0 0.0

SD 19.3 19.4 23.2 12.9 6.0 10.1 7.4 21.9 21.4 18.6 13.6 9.8 12.9 12.4 — 18.1 17.9 12.2 12.8 7.3 15.7 10.1 13.0 17.9 18.6 22.5 19.3 24.3 25.6 — —

Percentage impervious area Average Maximum Minimum 0.0 48.0 0.0 2.6 100.0 0.0 0.8 92.0 0.0 0.0 26.0 0.0 0.1 76.0 0.0 0.1 62.0 0.0 0.2 99.0 0.0 0.1 42.0 0.0 0.2 75.0 0.0 0.2 99.0 0.0 0.2 82.0 0.0 0.4 100.0 0.0 0.1 56.0 0.0 1.2 94.0 0.0 0.4 100.0 0.0 0.0 24.0 0.0 0.1 61.0 0.0 0.3 100.0 0.0 0.1 87.0 0.0 0.1 52.0 0.0 0.7 93.0 0.0 0.1 45.0 0.0 2.1 98.0 0.0 1.8 99.0 0.0 1.7 98.0 0.0 1.5 97.0 0.0 6.1 100.0 0.0 7.4 100.0 0.0 18.5 100.0 0.0 2.2 100.0 0.0 1.2 100.0 0.0

SD 0.4 9.7 4.8 0.3 0.7 1.1 1.7 0.5 1.7 2.1 1.1 3.5 0.7 6.1 — 0.2 1.2 2.8 1.5 0.8 4.0 0.6 9.5 8.3 8.3 6.0 16.5 17.5 25.5 — —

20 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Table 5. National Land Cover Database (NLCD) 2011, percentage of forest canopy and percentage of impervious area for subdrainage and sub-model areas, Salinas Valley study.

Study Area 21

Soils Soils properties affecting hydrologic processes include the soil storage capacity and soil hydraulic conductivity, both of which can be highly variable because of differences in soil thickness, texture, and structure. Soil texture, including grain-size distribution and structure, are important properties affecting water infiltration and percolation. The available water capacity of soils is an indicator of the potential amount of water available for plant transpiration when precipitation (or irrigation) is not limited and is dependent on soil texture and thickness. Soils with high permeability and storage capacities tend to favor recharge over runoff, depending on climate and the hydraulic conductivity of the underlying bedrock or alluvium. Soil texture classes, defined according to particle size distribution based on the Soil Survey Geographic database (SSURGO; U.S. Department of Agriculture [USDA], 2017), range from clayey to fine soils to coarse-loamy and sandy-skeletal soils in the Salinas Valley study area (fig. 8; table 6). Fine-loamy soils are the most prevalent basinwide soil texture for the USVS and LSVS areas. On the scale of subdrainages, however, soil texture is variable, with clayey-skeletal soils being the most prevalent in 4 of the 28 subdrainages, and loamy soils being most prevalent in 4 different subdrainages. Clayey to fine soils are most prevalent in the LNR and ELK subdrainages, whereas sandy soil is the most prevalent in the MSB subdrainage, and loamy-skeletal soil is the most prevalent in the UAS subdrainage. The available water capacity for soils in the study area, based on SSURGO data for a maximum soil thickness of 150 centimeters (cm; about 5 ft), averages 4.5 in. and varies from 0 to 16.5 in. (fig. 9). High soil storage capacities of 8.5 in. and more are generally located in the larger valley bottoms where medium to fine-grained (clayey and loamy) soils overlay unconsolidated valley-fill sediments and tend to be the thickest compared to locations on hillsides. In comparison, low available water capacities of 2.5 in. and less are dominantly located at the higher elevations of headwater

areas with steep slopes and relatively thinner coarser-grained soils. Locations with very low soil storage capacities of 0.5 in. and less occur throughout the upper parts of the ARR, SAN, and NAC subdrainages, coinciding with locations having the steepest terrain within the study area and locations having clayey-skeletal, loamy-skeletal, and undefined soils. Locations with intermediate available water capacities from 4.5 to 8.5 in. include the uplands on the eastern side of the valley and the central region of the upper Salinas Valley and tend to be coincident with more intermediate slopes and finer, well-structured soils underlain by sedimentary rocks.

Generalized Surface Geology In addition to topography, land cover, and soils, surficial geology consisting of unconsolidated deposits and consolidated rock forming the land surface is an important characteristic affecting the hydrologic system of the Salinas Valley study area. Alluvium and porous sedimentary rocks with high permeability are more conducive to recharge rather than runoff generation, whereas shales, siltstones, and igneous and metamorphic rocks with low permeability are more conducive to runoff generation. The Salinas Valley study area includes a wide variety of different rock types, from unconsolidated alluvium to consolidated sediments and igneous, metamorphic, and volcanic rock types (fig. 10; table 7). The dominant consolidated rock type outcropping in the Salinas Valley study area is conglomerate, mostly throughout the USR watershed, followed by unconsolidated alluvium consisting of valley-fill sediments and river wash deposits. Sandstone and siltstone are present throughout the western side and east–central side of the valley, respectively. Shale and siltstone are present throughout most of the Nacimiento subdrainage, and igneous and metamorphic rocks crop out in the upland areas of the LSVS, the UAS subdrainage, and the upper parts of the SRH subbasin. Volcanic rocks are the least common consolidated rock type outcropping in the SVWM, located mostly in the uplands adjacent to the northeastern boundary of the SVWM.

22 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley 121°30'

122°

River aro aj

MONTEREY BAY

Monterey

121°

EXPLANATION

MERCED COUNTY

Soil texture, Soil Survey Geographic Database (SSURGO)

39 38

SAN BENITO COUNTY

Salinas 21 7

n Sa

Other

Coarse-loamy

Sandy

Fine

Sandy- skeletal

Fine-loamy

Undefined

Fine-silty

Very-fine

SVWM sub-model boundary

6 1

101

King City

Sa n

Sa l

Lo re n

as in

re e

U.S. Geological Survey streamgage and identifier (see table 2)

U.S. Geological Survey streamgage at Bradley (see table 2)

k FRESNO COUNTY

io on

San Ant

Lower Salinas Valley sub-model boundary

Loamy-skeletal

Clayey-skeletal

Salinas Valley Watershed Model (SVWM) boundary

Soledad

Clayey

Loamy

en it

MONTEREY COUNTY

36°

120°

101

10 36° 30'

120°30'

ive r

Na cim 16 ien to

r ve Ri

KINGS COUNTY

3 22 25

34 36

KERN COUNTY

r ve

Paso Robles 20

Estrel la R i 29 8

35° 30'

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary

San Luis Obispo

101

SAN LUIS OBISPO COUNTY

0 0

10 10

20 20

30 MILES

30 KILOMETERS

Figure 8. Soil Survey Geographic database information on soil texture classes in the Salinas Valley study area (U.S. Department of Agriculture, 2017).

Table 6. Soil Survey Geographic database (SSURGO) soil texture classes for subdrainage areas in the Salinas Valley study area. [Subdrainages are listed in order of upstream to downstream tributary connections. Abbreviations: ARR, Arroyo Seco; EST, Estrella River; LOR, San Lorenzo Creek; LSR, lower Salinas River; MCB, Monterey Coastal Basins; MSR, middle Salinas River; NAC, Nacimiento River; SAN, San Antonio River; SRH, Salinas River headwaters; USR, upper Salinas River; —, not applicable]

Subdrainage/ sub-model

Subbasin abbreviation

SML SRP PRC USJ LSJ CHO LER UNR LNR USA LSA HUE BSC SNB — UAS LAS SLC SPR PCD USV CSC MCS QCC LCS ETC SRO ELK MSB — —

SRH SRH SRH EST EST EST EST NAC NAC SAN SAN USR USR USR — ARR ARR LOR MSR MSR MSR MSR MSR LSR LSR LSR LSR MCB MCB — —

Clayey to fine 7.0 11.6 11.4 1.0 3.8 36.0 8.5 7.0 32.0 5.8 14.8 3.5 15.9 8.8 12.5 0.3 8.1 24.9 20.3 5.8 7.0 10.9 9.6 24.0 13.9 19.8 21.8 27.9 12.9 15.3 13.7

Finesilty 0.0 0.0 0.0 0.0 0.0 0.6 5.7 0.4 0.3 0.2 2.0 0.0 9.8 1.2 1.6 0.1 1.8 9.3 29.8 48.7 3.6 7.3 0.7 0.0 0.0 0.0 0.0 0.0 0.0 7.5 4.2

Fineloamy 21.1 35.3 60.7 33.9 57.4 39.8 74.8 4.6 27.2 10.4 27.8 61.0 44.3 58.8 42.5 0.1 11.9 31.9 30.1 39.3 21.2 43.1 30.1 33.7 23.9 1.8 33.2 24.1 0.6 25.5 35.0

SSURGO soil texture classes (percentage coverage) CoarseClayeyLoamyLoamy Sandy loamy skeletal skeletal 47.1 3.4 1.4 0.6 3.6 31.4 9.0 1.3 3.5 2.5 18.3 1.1 0.0 5.2 0.0 47.4 8.8 0.1 0.0 3.5 14.5 16.5 0.5 0.4 2.7 12.0 1.8 0.9 0.7 0.1 2.9 4.1 0.6 1.1 0.0 33.1 6.1 0.1 3.5 21.0 21.1 1.4 0.1 5.6 0.0 18.5 11.4 0.5 29.8 7.5 2.5 1.6 0.6 41.0 0.4 18.8 8.5 2.1 0.2 2.7 1.3 1.0 0.4 10.7 0.3 1.1 5.1 2.2 16.0 0.1 17.2 6.1 0.8 8.0 2.6 32.5 3.0 0.0 0.0 32.7 19.2 9.8 1.3 26.9 8.5 9.7 4.0 0.8 0.3 0.0 3.6 1.8 1.9 0.5 0.0 0.0 3.5 0.0 0.1 0.0 0.0 9.9 6.9 42.9 0.0 0.6 15.6 0.1 0.6 5.3 0.0 16.6 6.2 27.9 0.1 9.4 31.2 0.6 0.0 0.0 13.1 35.5 5.0 0.0 0.0 3.3 29.2 5.2 6.5 0.0 5.4 16.9 8.3 0.1 0.1 4.2 19.7 0.9 0.1 0.0 0.0 2.0 28.9 14.2 0.0 7.4 13.4 3.2 9.2 3.1 12.9 9.3 1.9 8.6 2.8

Sandyskeletal 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.3 0.0 1.4 0.0 0.0 0.0 0.0 0.1 0.1 0.9 0.0 0.0 0.0 0.0 10.0 0.0 0.1 0.2 0.0 0.0 0.1 0.0 1.0 0.5

Other/ undefined 15.8 5.5 3.2 5.2 4.3 8.1 2.2 23.8 12.3 14.6 9.3 3.4 16.3 6.9 8.7 31.1 11.6 19.2 12.0 2.7 8.4 6.4 8.7 1.0 8.5 34.2 14.3 23.1 41.4 14.3 11.1

Study Area 23

Subdrainage abbreviation

24 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley 121°30'

122°

River aro aj

MONTEREY BAY

101

SAN BENITO COUNTY

Salinas 21 7

r ve Ri

10 36° 30'

120°

MERCED COUNTY

120°30'

it en nB Sa

Monterey

121°

FRESNO COUNTY

Lower Salinas Valley sub-model boundary

MONTEREY COUNTY

Soledad

2 5

6 1

101

King City

Sa n

Lo re n

as in

Sa l

re e

io on

San Ant

Upper Salinas Valley sub-model boundary

r ve Ri

PACIFIC OCEAN

ive r

Na cim 16 ien to

36°

KINGS COUNTY

3 22 25

34 36

EXPLANATION 35° 30'

Soil Survey Geographic Database (SSURGO) soil available water storage capacity (AWS150), in inches 0 to 1

>3 to 4

>7 to 8

Greater than (>) 1 to 2

>4 to 5

>8 to 9

>5 to 6

>2 to 3

11 32

>6 to 7

Salinas Valley Watershed Model (SVWM) boundary SVWM sub-model boundary 2

U.S. Geological Survey (USGS) streamgage and identifier (see table 2)

U.S. Geological Survey streamgage at Bradley (see table 2)

Base from U.S. Geological Survey, The National Map (2023) and other Federal digital data, various scales; California IV State Plane Coordinate System (FIPS 0404); North American Datum of 1983

KERN COUNTY

r ve

Paso Robles 20

Estrel la R i 29 8

San Luis Obispo

101

SAN LUIS OBISPO COUNTY

Pismo Beach 0 0

10 10

20 20

30 MILES

30 KILOMETERS

Figure 9. Soil Survey Geographic database (SSURGO) soil available water storage capacity up to 150 centimeters soil depth, Salinas Valley study area (U.S. Department of Agriculture, 2017).

Study Area 25 121°30'

122°

River aro aj

121°

120°30'

101

120°

EXPLANATION

MERCED COUNTY

Generalized surficial geology, dominant rock type Alluvium Claystone

MONTEREY BAY

Conglomerate Salinas

Igneous rocks

SAN BENITO COUNTY

Monterey

Metamorphic rocks Sandstone

n Sa

en it

Shale

36° 30'

Siltstone

MONTEREY COUNTY

Volcanic rocks

Soledad

Salinas Valley Watershed Model (SVWM) boundary SVWM sub-model boundary 101

King City

Sa n as in

Sa l

Lo re n

re e

FRESNO COUNTY

ive r

Na cim ien to

io on

San Ant

U.S. Geological Survey streamgage at Bradley (see table 2)

Lower Salinas Valley sub-model boundary

36°

r ve Ri

Lake San Antonio

KINGS COUNTY

Lake Nacimiento

KERN COUNTY

r ve

Paso Robles

Estre lla Ri

35° 30'

Upper Salinas Valley sub-model boundary

PACIFIC OCEAN

San Luis Obispo 0 0

10 10

20 20

30 MILES

30 KILOMETERS

Base from U.S. Geological Survey, The National Map (2023) and other Federal digital data, various scales; California IV State Plane Coordinate System (FIPS 0404); North American Datum of 1983

101

Pismo Beach

Santa Margarita Lake SAN LUIS OBISPO COUNTY Geology modified from Jennings and others (2010)

Figure 10. Generalized California surficial geology for the Salinas Valley Watershed Model (SVWM) study area (Jennings, 1977).

[Subdrainages are listed in order of upstream to downstream tributary connections. Abbreviations: ARR, Arroyo Seco; EST, Estrella River; LOR, San Lorenzo Creek; LSR, lower Salinas River; MCB, Monterey Coastal Basins; MSR, middle Salinas River; NAC, Nacimiento River; SRH, Salinas River headwaters; USR, upper Salinas River; —, not applicable]

Surficial geology, as percentage of total land area

Subdrainage/ sub-model

Subdrainage abbreviation

Subbasin abbreviation

Alluvium

Conglomerate

Volcanic rocks

Claystone

Sandstone

Shale

Siltstone

Metamorphic rocks

Igneous rocks

Water

Santa Margarita Lake

SML

SRH

0.0

3.8

0.0

2.2

16.9

67.6

0.0

8.0

1.4

Salinas River near Paso Robles

SRP

SRH

11.9

19.1

0.3

12.2

18.6

18.2

0.0

1.3

18.5

0.1

Paso Robles Creek

PRC

SRH

4.4

6.3

0.1

16.4

52.5

20.3

0.0

Upper San Juan Creek

USJ

EST

8.1

34.4

0.3

0.0

40.4

10.0

0.0

0.6

6.1

0.0

Lower San Juan Creek

LSJ

EST

9.7

70.0

0.0

9.3

1.4

0.0

9.6

0.0

Cholame Creek

CHO

EST

34.6

24.2

0.0

3.7

5.6

14.1

11.8

5.8

0.2

0.0

Lower Estrella River

LER

EST

12.6

80.2

0.0

4.9

1.0

0.1

0.0

1.2

0.0

Upper Nacimiento River

UNR

NAC

0.0

30.9

3.9

49.7

0.0

10.1

5.4

0.0

Lower Nacimiento River

LNR

NAC

0.0

18.6

28.0

46.5

0.0

1.4

0.0

5.5

Upper San Antonio River

USA

SAN

8.0

13.5

0.0

37.1

12.3

1.3

15.8

12.0

0.0

Lower San Antonio River

LSA

SAN

18.0

16.1

0.0

57.8

0.3

1.4

0.0

6.4

Huerhuero Creek

HUE

USR

7.4

61.2

0.0

6.3

3.4

0.0

21.7

0.0

Big Sandy Creek

BSC

USR

6.9

54.9

0.0

0.7

18.9

2.6

15.8

0.0

0.1

0.0

Salinas River near Bradley

SNB

USR

22.9

49.6

0.0

19.9

4.6

1.9

0.0

1.2

0.0

11.7

35.7

0.1

5.0

20.9

15.1

2.5

2.4

5.7

0.9

Upper Salinas Valley sub-model

26 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Table 7. Generalized surficial geology for subdrainage areas in the Salinas Valley study area.

Table 7. Generalized surficial geology for subdrainage areas in the Salinas Valley study area.—Continued [Subdrainages are listed in order of upstream to downstream tributary connections. Abbreviations: ARR, Arroyo Seco; EST, Estrella River; LOR, San Lorenzo Creek; LSR, lower Salinas River; MCB, Monterey Coastal Basins; MSR, middle Salinas River; NAC, Nacimiento River; SRH, Salinas River headwaters; USR, upper Salinas River; —, not applicable]

Surficial geology, as percentage of total land area

Subdrainage/ sub-model

Subdrainage abbreviation

Subbasin abbreviation

Alluvium

Upper Arroyo Seco

UAS

ARR

Lower Arroyo Seco

LAS

San Lorenzo Creek

Claystone

Sandstone

Shale

Siltstone

Metamorphic rocks

Igneous rocks

Water

0.0

1.9

24.1

0.0

51.3

22.7

0.0

ARR

13.7

3.4

0.0

42.2

7.5

2.8

19.9

10.5

0.0

SLC

LOR

9.5

11.6

0.0

2.5

4.7

2.7

62.1

2.3

4.5

0.0

Sargent– Pancho Rico Creeks

SPR

MSR

5.5

23.4

0.0

0.2

2.8

1.3

66.7

0.0

0.1

0.0

Pine Creek drainages

PCD

MSR

12.2

10.5

0.0

77.0

0.2

0.0

Upper Salinas Valley

USV

MSR

42.0

6.4

0.0

45.5

0.0

6.2

0.0

Chalone– Stonewall Creek

CSC

MSR

0.5

25.7

8.2

0.0

2.7

14.9

11.8

2.8

33.4

0.0

Monroe Creek– Salinas River

MCS

MSR

53.5

10.2

0.0

26.9

1.6

1.8

2.6

3.5

0.0

Quail–Chualar Creek

QCC

LSR

46.4

0.0

0.7

52.9

0.0

Limekiln Creek– Salinas River

LCS

LSR

53.8

4.5

0.0

20.1

21.5

0.0

El Toro Creek

ETC

LSR

12.1

37.3

0.0

23.9

1.9

0.0

9.8

15.0

0.0

Salinas River outflow

SRO

LSR

81.3

1.4

0.0

0.8

0.0

5.6

9.7

1.1

Elkhorn Slough

ELK

MCB

66.1

0.0

1.0

0.0

0.7

1.9

1.0

2.2

26.5

0.6

Monterey– Seaside basin

MSB

MCB

64.7

10.3

0.0

20.4

0.0

3.9

0.7

Lower Salinas Valley sub-model

30.6

9.5

0.9

0.3

12.6

4.3

19.8

7.7

14.3

0.1

Salinas Valley Watershed Model

20.0

24.2

0.4

3.0

17.2

10.3

10.1

4.7

9.5

0.6

Study Area 27

Conglomerate

Volcanic rocks

28 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Model Development Development of the SVWM required nine generalized steps: (1) defining the model domain; (2) defining a conceptual model and simplifying assumptions appropriate for the intended model application; (3) selecting an appropriate model code to simulate selected processes of the natural hydrologic system; (4) selecting the simulation period; (5) estimating the initial conditions; (6) defining the model boundary, layout, and spatial discretization; (7) developing climate inputs consisting of spatially interpolated daily precipitation, maximum and minimum daily air temperature, and simulated daily PET; (8) estimating initial values for model parameters representing the physical characteristics of the Salinas Valley study area; and (9) performing model calibration to refine and finalize parameter values. These nine steps are described in more detail in the subsections below.

Model Domain The SVWM model domain was defined mostly by surface-water drainage divides based on hydrologic unit code 12 boundaries (HUC-12, https://water.usgs.gov/GIS/huc.html; Seaber and others, 1987), with minor modifications using National Hydrography Dataset (NHD) flowlines and flow directions based on calculated land-surface slope using 10-m (98.4-ft) resolution digital elevation data. The 4,530 mi2 SVWM area includes 35 complete or partial HUC-12 areas. The SVWM domain encompasses the entire SRW and smaller coastal drainages along the Monterey Bay coastline adjacent to the Salinas River outflow (figs. 2, 3). The SVWM consists of two connected HSPF sub-model components: the 2,540 mi2 USVS and the 1,990 mi2 LSVS (fig. 2) that are connected in the upper Salinas Valley at the Salinas River USGS streamgage 11150500 (gage 19, near Bradley; fig. 2; table 2). All surface-water outflows from the USVS enter the LSVS at this streamgage location.

Conceptual Model The components of the natural hydrologic system represented by the SVWM include climate (precipitation and maximum and minimum air temperature), PET, ET, soil moisture, runoff, interflow, baseflow, recharge, and streamflow for the Salinas Valley study area. The SVWM was developed using the BCM (Flint and others, 2021) and the HSPF (version 12.4; Bicknell and others, 1997, 2005). The daily climate inputs required by HSPF are provided by the BCM (precipitation maximum and minimum air temperature and PET). The HSPF application is a widely used and well documented modeling program originally developed and supported by the U.S. Environmental Protection Agency and USGS in 1984 and based on the Stanford Watershed Model

(Bicknell and others, 2001, 2005). The HSPF application has been used extensively for hydrologic studies covering a broad range of areas and for a variety of objectives (Atkins and others, 2005; Amirhossien and others, 2015; Stern and others, 2016). Documentation of HSPF version history and the use and theory that led to the development of HSPF is widely available (Donigian and Imhoff, 2009). The conceptual model of the natural hydrologic system represented by the SVWM is based on the HSPF algorithm. The HSPF computer program is a semi-distributed, process-based, mostly deterministic, and continuous-simulation algorithm for simulating water flow and storage processes with options for simulating water quality and transport processes (Donigian and others, 1984; Bicknell and others, 2001). Applications of HSPF can be used to simulate basin response to normal and extreme precipitation (or lack of precipitation) and to simulate spatial and temporal variability and trends in climate; to evaluate water budgets and water quality; and to evaluate changes in the hydrologic system, such as flow regimes, flood peaks and volumes, soil-water relationships, and groundwater recharge. Through parameter optimization and sensitivity analysis, models developed using HSPF can be calibrated to multiple streamgages to reflect a variety of physiographic characteristics. Hydrologic processes simulated by HSPF include pervious and impervious surface storage (including interception and retention storage), pervious and impervious surface runoff (overland flow), pervious land infiltration, soil water storage, percolation, ET, interflow, recharge, streamflow, stream losses to evaporation and seepage, and shallow (active) groundwater reservoir storage and discharge (baseflow) contributions to streamflow and riparian ET (fig. 11; Bicknell and others, 2005). The HSPF model provides a comprehensive simulation of rainfall-runoff and streamflow processes, allowing for analysis of surface and shallow subsurface-water budget components and processes such as soil moisture, ET, and recharge for inter-channel areas and components of streamflow (overland runoff, interflow, baseflow, and streamflow seepage) for intra-channel areas. The dominant components of the HSPF conceptual model are the surface and shallow-subsurface (primarily the root zone), stream channels, and larger water bodies (lakes and reservoirs). The surface and shallow-subsurface systems include the plant canopy, land surface, soil zone, and groundwater reservoir supplying groundwater discharge to the stream channels. The plant canopy includes natural vegetation, crops, and landscaped urbanized areas. Two types of land surfaces are represented by HSPF: (1) pervious land areas (PERLNDs) and (2) impervious land areas (IMPLNDs), which are further broken down according to soil type, land cover type, vegetation density, geology, and topography. Pervious land areas represent natural and developed land areas with soil cover, and IMPLNDs represent impervious developed land surfaces, such as rooftops, roads, and parking lots.

Model Development 29 A IFWO Interflow outflow IFWS Interflow storage

SURO Surface outflow SUPY Precipitation or rain and snowpack water yield CEPS Interception storage

TAET Total actual evapotranspiration

CEPE Interception evapotranspiration

CEPO Interception outflow

SURI Surface inflow

IFWLI External lateral interflow input

IFWI Interflow input from surface

SURS Surface detention storage

SURLI External lateral surface inflow

1 Modified from Bicknell and others (2001)

B LZS Lower zone storage UZS Upper zone storage

UZI Upper zone inflow INFIL Infiltration

PERC Percolation

AGWS Active groundwater inflow

LZI Lower zone inflow IPERC Infiltration and percolation to lower zones

AGWI Active groundwater inflow

AGWO Groundwater outflow

AGWLI External lateral groundwater inflow

2 UZET Upper zone evapotranspiration

LZET Lower zone evapotranspiration

AGWET Groundwater evapotranspiration

IGWI Deep percolation

1 Modified from Bicknell and others (2001)

Figure 11. Diagram of water flow and storages simulated by the Hydrologic Simulation Program—Fortran (HSPF): A, surface storage and flow processes simulated on pervious land areas; B, subsurface storage and flow processes simulated in the subsurface underlying pervious land areas; C, surface storage and flow processes simulated on impervious surfaces; and D, streamflow simulated in channels (Bicknell and others, 2005) Abbreviations: NEXITS, number of outflow exits from a RCHRES; RCHRES, stream reach or reservoir in HSPF.

30 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley C

D SUPY Precipitation or rain + snowpack water yield

RETI Retention inflow

VOLEV Evaporation RETS Impervious retention storage

Path depends on value of RTLFG1

SURI Surface detention inflow

RTLFG, retention lateral flow

OVOL (NEXITS2) Outflow through exit NEXITS

RETO Retention outflow

PRSUPY Precipitation on RCHRES1 surface VOL Volume of water in RCHRES

EVAP Impervious evaporation

NEXITS

SURLI Lateral surface inflow

IVOL Inflow

SURS Surface detention storage

OVOL(1) Outflow through exit 1

ROVOL Total outflow

SURO Surface outflow

Modified from Bicknell and others (2001)

RCHRES, stream reach or reservoir NEXITS, number of exits (outflows) from RCHRES

1 2

Modified from Bicknell and others (2001)

Figure 11.—Continued

Each PERLND is partitioned into several storage zones, including interception storage, surface detention (upper zone) storage, interflow storage, root-zone (lower-zone) storage representing soils and the upper subsurface of consolidated or unconsolidated rock underlying PERLNDs (for example, fractured or weathered bedrock), and active groundwater storage (Bicknell and others, 2005). The lower zone storage is conceptually defined as extending from the ground surface to the base of the root zone. Pervious land areas are used to simulate the storage and transfer of water to the atmosphere as ET, shallow subsurface flow through the soil to stream channels as interflow, and deeper percolation to groundwater reservoirs as recharge. The HSPF layout for the SVWM was configured such that all runoff generated by the PERLND and IMPLND model components (including overland runoff, interflow runoff, and groundwater discharge) is routed to a single stream reach or reservoir (RCHRES) model component. In addition to receiving inflows from upstream PERLNDs and IMPLNDs, RCHRESs along the downstream sections of flow paths receive inflows from one or more tributary RCHRESs. Streamflow through the connected stream network is simulated using the kinematic wave approximation of

the Saint-Venant equations of one-dimensional channel flow, where water accumulated in each RCHRES from contributing land areas and upstream RCHRESs is routed to the downstream RCHRES. The Hydrologic Simulation Program—Fortran uses a simplified representation of the deeper subsurface below the root zone that includes the groundwater-flow system. Recharge to the groundwater system is partitioned between active and inactive groundwater reservoirs. Water that is stored in the active groundwater reservoir is available for ET and groundwater discharge to streams, whereas recharge to the inactive groundwater reservoir is an outflow from the hydrologic system simulated by HSPF. The simplified representation accounts for the component of total streamflow that originates as groundwater discharge to stream channels (also referred to as the baseflow component of streamflow), which is important for model calibration and the simulation of water budgets. The inactive groundwater reservoir is used to represent outflows from the hydrologic system that are not explicitly defined by the HSPF model, such as pumping, groundwater underflows across basin boundaries, or increases to deep aquifer storage (deep recharge).

Model Development 31 In the conceptual model applied for the SVWM, irrigation and flow diversions were assumed to be minor compared to precipitation, especially in the upland areas of the Salinas Valley study area where the percentage of irrigated agricultural land is small. Furthermore, it was assumed in the conceptual model that most of the irrigation water is returned to the atmosphere by ET. Reservoir operations were not simulated by the SVWM. However, the daily water budgets in the subdrainages contributing water inflows to the reservoir areas in response to precipitation were simulated. Modules in HSPF used to simulate snowfall, snow storage, and snowmelt were not activated in the SVWM; all precipitation was simulated as rain. Although snowfall can occur at higher elevations in the SVWM study area, the accumulation and subsequent melting of snow was not considered to be a significant factor affecting the natural hydrologic system of the Salinas Valley study area.

Simulation Period and Initial Conditions The SVWM was run using an hourly time step to simulate a continuous daily water balance for each HRU and RCHRES. Daily climate inputs developed by the BCM were partitioned into hourly increments for the HSPF simulation. The target simulation period used for the SVWM started with water year 1949 (October 1, 1948) and ended with water year 2018 (September 30, 2018). The target simulation period was intended to provide an estimate of the historical natural water balance and to allow for a sufficiently long period of time to analyze the spatial and temporal variability of water-balance components, including ET, surface- and subsurface-water storage, recharge, and streamflow. Precipitation-runoff models generally require an initialization period to help minimize uncertainties associated with the assumed or estimated initial conditions (Markstrom and others, 2008; Hevesi and others, 2019). The SVWM simulations were run starting on October 1, 1947, to provide for a 1-year initialization period (water year 1948) before the target simulation period (water years 1949–2018) to reduce the effect of transients associated with initial conditions specified at start-up. The initial conditions include water contents for interception storage, retention storage, soil moisture (upper and lower zones), interflow storage, and active groundwater storage that were either set to zero or based on values defined in the HSPF manual (Bicknell and others, 2005).

Model Layout and Discretization The SVWM discretization is used to account for the spatially varying physical characteristics of the Salinas Valley study area, including spatial heterogeneity in topography, vegetation and land cover, soils, geology, water bodies, and climate. The discretization also is used to define the various tributary surface-water drainages connected to the main channel of the Salinas River, the smaller drainages along the Monterey Bay coast, and drainages supplying boundary inflows from upland areas to the lower elevation developed land areas of the Salinas Valley. The HRU is the basic homogeneous model segment used to spatially partition and discretize the SVWM domain. The SVWM was discretized such that each HRU contains a single PERLND connected to a single RCHRES, with the network of connected RCHRESs representing the natural surface-water drainage system of the Salinas Valley study area as a one-dimensional flow routing network. Outflows from each HRU are ET, surface-water discharges to RCHREs, and inflows to the inactive groundwater reservoir. Surface-water discharges from HRUs to RCHRESs are overland runoff, interflow, and groundwater discharge from the active groundwater reservoir. Water storage, inflows, and outflows are simulated as a water-equivalent depth for the area of each HRU and groundwater reservoir associated with each HRU. Inflows to each RCHRES are hourly rainfall, outflows from the connected HRU, and surface-water inflows from upstream RCHRESs. The SVWM discretization includes 690 HRUs, ranging in area from 65 to 25,376 acres (0.1–39.6 mi2), with mean HRU land-surface elevations ranging from 12 to 4,252 ft (fig. 12). The HRU delineation was developed using irregular-polygon areas representing surface-water sub-drainages that were defined using a combination of 10-m (98-ft) digital elevation models (DEMs), the NHD streamlines and sub-drainage boundaries (U.S. Geological Survey, 2019), and Calwater version 2.2.1 watershed areas (California Department of Forestry and Fire Protection, 2004). The percentage of impervious developed land cover averaged over the area of each HRU ranged from 0 to 55 percent (fig. 13). Hydrologic response units with a substantial percentage of impervious cover include one IMPLND element as a fraction of the total HRU area, with a corresponding reduction in the PERLND area such that the combined IMPLND and PERLND area equals the total area for that HRU. A total of 181 HRUs include IMPLND elements, with impervious areas ranging from 11 to 2,216 acres.

32 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley 121°30'

122°

MONTEREY BAY

River aro aj

121°

120°30'

101

EXPLANATION

MERCED COUNTY

Hydrologic response unit (HRU) mean elevation, in feet

MCB

MCB SAN BENITO COUNTY

Salinas Monterey

en it

1,401 to 1,700

301 to 600

1,701 to 2,000

601 to 900

2,001 to 2,300

901 to 1,100

2,301 to 2,600

1,101 to 1,400

2,601 to 4,252

SVWM sub-model boundary

MONTEREY COUNTY

0 to 300

Salinas Valley Watershed Model (SVWM) boundary

n Sa

LSR 36° 30'

120°

Soledad

LOR

SVWM subbasin boundary and identifier1

U.S. Geological Survey streamgage at Bradley (see table 2)

101

ARR

King City

Sa n as in

Sa l

LOR Lo re n

re e

San Ant

FRESNO COUNTY

SAN ive r

Na cim ien to

io on

36°

Lower Salinas Valley sub-model boundary

MSR

ARR, Arroyo Seco; EST, Estrella River; LOR, San Lorenzo Creek; LSR, Lower Salinas River; MCB, Monterey Coastal Basins; MSR, Middle Salinas River; NAC, Nacimiento River; SAN, San Antonio River; SRH, Salinas River headwaters; USR, Upper Salinas River

r ve Ri

NAC

Lake San Antonio

KINGS COUNTY

USR

Lake Nacimiento

Estrel la R i

KERN COUNTY

r ve

Paso Robles

SRH

35° 30'

EST

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary 1

San Luis Obispo 101

Santa Margarita Lake SAN LUIS OBISPO COUNTY

0 0

10 10

20 20

30 MILES

30 KILOMETERS

Figure 12. Mean land-surface elevation for 690 hydrologic response units (HRUs) used in the Salinas Valley Watershed Model (SVWM; U.S. Geological Survey, 2023; Hevesi and others, 2025).

Model Development 33 121°30'

122°

River aro aj

121°

120°30'

101

EXPLANATION

MERCED COUNTY

Hydrologic response unit (HRU) mean impervious area, in percent

MONTEREY BAY SAN BENITO COUNTY

Salinas Monterey

King City

Sa n as in

Sa l

15.1 to 20.0

2.1 to 5.0

20.1 to 55.4

U.S. Geological Survey streamgage at Bradley (see table 2)

re e

Lo re n

San Ant

FRESNO COUNTY

5 ive r

Na cim ien to

io on

36°

10.1 to 15.0

1.1 to 2.0

Soledad

101

Lower Salinas Valley sub-model boundary

MONTEREY COUNTY

5.1 to 10.0

0.2 to 1.0

SVWM sub-model boundary

en it

0.0 to 0.1

Salinas Valley Watershed Model (SVWM) boundary

n Sa

36° 30'

120°

r ve Ri

Lake San Antonio

KINGS COUNTY

Lake Nacimiento

Estrel la R i

KERN COUNTY

r ve

Paso Robles 35° 30'

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary 1

San Luis Obispo 101

Santa Margarita Lake SAN LUIS OBISPO COUNTY

0 0

10 10

20 20

30 MILES

30 KILOMETERS

Figure 13. Mean percentage of impervious land cover for 690 hydrologic response units (HRUs) used in the Salinas Valley Watershed Model, calculated using the 2011 National Land Cover Database 30-meter (98 foot) percentage of impervious developed land cover (U.S. Department of Agriculture, 2017).

34 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley The SVWM discretization was defined such that each HRU is connected to one RCHRES, resulting in a stream channel network of 690 linked RCHRESs, with a given RCHRES having zero to many tributary connections to upstream RCHRESs, depending on the position of the RCHRES in the routing network. All RCHRESs in the stream channel network discharge to a single downstream RCHRES, with exceptions for a small number of RCHRESs discharging to Monterey Bay from the SRO, ELK, and MSB subdrainages. First-order RCHRESs in the headwater drainages do not have a tributary connection to upstream RCHRESs and only receive inflows from the single connected HRU. The headwater RCHRESs represented by the RCHRES network have drainage areas ranging from 0.1 to 50 mi2 with 46 headwater RCHRESs having drainage areas less than 1 mi2 and 179 RCHRESs having drainage areas of 1–5 mi2 (fig. 14). There are 231 RCHRESs that have contributing drainage areas between 5 and 20 mi2, and there are 150 RCHRESs that have drainage areas greater than 50 mi2. The HSPF code version used to develop the SVWM is limited to a maximum number of 1,000 model elements, with each PERLND, IMPLND, and RCHRES defining separate model elements. The SVWM model discretization includes a total of 1,561 elements (the sum of 690 PERLNDs, 181 IMPLNDs, and 690 RCHRESs), exceeding the capacity of the HSPF code version used. To preserve the degree of spatial detail provided by the 1,561 model elements, the SVWM was divided into the 2 HSPF sub-model domains, the USVS and the LSVS (fig. 2). The USVS has an area of 2,536 mi2 containing 387 HRUs with 36 HRUs including IMPLNDs. The LSVS has an area of 1,992 mi2 containing 303 HRUs with 69 HRUs including IMPLNDs. The LSVS is connected to the USVS by the outflow of the Salinas River at USGS streamgage 11150500 (gage 19, Salinas River near Bradley). The more detailed discretization provided by the two connected sub-models allowed for an improved representation of variability in climate and watershed characteristics such as slope, land cover, soil properties, and surficial geology, compared to a single model representing the entire study area. In addition, the higher level of discretization using two sub-models with 1,561 elements compared to a single model with 1,000 elements, resulted in a more precise delineation of all tributary drainages to the lower Salinas Valley study area (defined by the SVIHM boundary as mentioned earlier; fig. 15). The SVWM simulated surface-water tributary inflows at 148 locations along the SVIHM boundary from the tributary drainages. The simulated inflows can be used to define boundary conditions for integrated hydrologic modeling of the lower Salinas Valley study area. The drainage areas for the 148 inflow locations were grouped into 25 tributary drainages with areas ranging from 11 mi2 for the Quail Creek (QUAI) tributary drainage in the northwest part of the SVWM (southeast of the city of Salinas) to 1,574 mi2 for the Salinas River (SALI) tributary drainage supplying surface-water inflow from the upper Salinas Valley watershed (fig. 15; table 8). Average elevations for the tributary drainages range from 997 ft for the Cherry Canyon (CHER) tributary drainage

to 3,393 ft for the Arroyo Seco (SECO) tributary drainage (table 8). The total 3,625 mi2 area comprising all tributary drainages, referred to as the Salinas Valley upland area (SVU), has an average elevation of 1,671 ft and elevations ranging from −1 to 5,872 ft. In contrast, the 904 mi2 land area of the SVIHM has an average elevation of 441 ft with elevations ranging from −1 to 3,003 ft (table 8).

Basin Characterization Model Climate Inputs The BCM was used to develop climate inputs for the SVWM. The BCM consists of a set of computer codes for grid-based water balance simulations and includes preprocessing applications for spatially distributing and downscaling climate variables and for simulating PET for historical climate and future climate scenarios and projections (Flint and Flint, 2007, 2012; Flint and others, 2013, 2021; Stern and others, 2016). The BCM applications use monthly climate data from the Parameter-elevation Regression on Independent Slopes Model (PRISM; Daly and others, 1994, 2004), available daily climate records, and the Gradient-Invers e-Distance-Squared method (Nalder and Wein, 1998) to downscale and spatially interpolate climate data to a 270-m (886-ft) grid covering the SVWM study area. Inputs used by the BCM to develop the HSPF climate inputs consisted of daily precipitation records from 155 climate stations (fig. 16), daily maximum and minimum air temperature records from 113 climate stations (fig. 17), gridded PRISM maps of monthly precipitation, monthly maximum and minimum air temperature, and land-surface elevations for the 270-m grid of the SVWM. The precipitation and air temperature climate grids developed by the BCM were applied to simulate daily PET using the Priestley–Taylor method (Flint and others, 2021). A listing of the 155 climate stations shown on figure 16 with daily precipitation records used to develop climate inputs is provided in appendix 1, table 1.1. A listing of the 113 climate stations shown on figure 17 with daily minimum and maximum air temperature records used to develop climate inputs is provided in appendix 1, tables 1.2 and 1.3, respectively. The number of years of data for daily precipitation ranged from 1.8 years for Salinas 6 SSW (station 77, app. 1, table 1) to 70.8 years for Paso Robles (station 68, app. 1, table 1). The number of years of data for daily minimum and maximum air temperature ranged from 1.9 years for King City Airport (station 16, app. 1, tables 1.2, 1.3) to 70.4 years for Santa Cruz (station 43, app. 1, tables 1.2, 1.3). Mean annual precipitation ranged from 5.7 in. for Orchard Sunflower Valley (station 61, app. 1, table 1.1) to 54.2 in. for Ben Lomond (station 107, app. 1, table 1.1). Mean minimum daily air temperature ranged from 37.7 °F for station Priest Valley (station 32, app. 1, table 1.2) to 57.0 °F for station Kettleman Hills (station 64, app. 1, table 1.2) and mean maximum daily air temperature ranged from 60.4 °F for San Simeon Point Piedras Blancas (station 31, app. 1, table 1.3) to 80.1 °F for Avenal 9 SSE (station 1, app. 1, table 1.3).

Model Development 35 122°

121°30'

121°

120°30'

120°

101

EXPLANATION

MERCED COUNTY MONTEREY BAY

Monterey

Salinas Valley Watershed Model (SVWM) RCHRES stream segment upstream drainage area, in square miles 0 to 5 101 to 500

39 38

SAN BENITO COUNTY

Salinas 21 7 10

36° 30'

501 to 1,000

11 to 20

1,001 to 2,000

21 to 50

2,001 to 3,000

51 to 100

3,001 to 4,227

SVWM boundary SVWM sub-model boundary

MONTEREY COUNTY

Soledad

101

King City

U.S. Geological Survey streamgage and identifier (see table 2)

U.S. Geological Survey streamgage at Bradley (see table 2)

Sa n

Lo re n

as in

Sa l

re e

San Ant

FRESNO COUNTY

io on

Na cim 16 ien to

ive r

Lower Salinas Valley sub-model boundary

2 2 6

36°

6 to 10

r ve Ri

KINGS COUNTY

3 22 25

34 36

KERN COUNTY

r ve

Paso Robles 20

Estrel la R i 29 8

35° 30'

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary

San Luis Obispo

101

SAN LUIS OBISPO COUNTY

0 0

10 10

20 20

30 MILES

30 KILOMETERS

Figure 14. Location and size of total contributing drainage areas for 690 stream reaches or reservoirs (RCHRES) used to discretize the Salinas Valley Watershed Model (Hevesi and others, 2025)

36 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley 121°30'

MONTEREY BAY

122°

River aro aj

121°

120°30'

101

EXPLANATION Lower Salinas Valley tributary drainages (see table 8)

39 38

Monterey

Salinas 21 40

7 n Sa

en it

ALIS

HARE

PINE

STON

BIGS

LSRW

QUAI

TORO

CHAL

MCOY

SALI

VINE

CHER

MONR

SANL

WILD

CHUA

MSRW

SANR

GABI

NACI

SARG

HAME

PANR

SECO

Salinas Valley Watershed Model (SVWM) boundary

SVWM sub-model boundary

36° 30'

120°

Soledad

Lower Salinas Valley study area

U.S. Geological Survey streamgage and identifier (see table 2)

U.S. Geological Survey streamgage at Bradley (see table 2)

6 1

101

King City

Sa n

Lo re n

as in

Sa l

re e

San Ant

16 ive r

Na cim ien to

io on

36°

Lower Salinas Valley sub-model boundary

Location of 148 surface-water inflows from tributary drainages to the lower Salinas Valley study area

r ve Ri

Lake San Antonio

3 18

25 Lake Nacimiento

28 34

35° 30'

PACIFIC OCEAN

Estrel la R i 29

r ve

Paso Robles 20

Upper Salinas Valley sub-model boundary

San Luis Obispo 101

27 31

24 Santa Margarita Lake

0 0

10 10

20 20

30 MILES

30 KILOMETERS

Figure 15. Lower Salinas Valley study area tributary drainages and locations of 148 surface-water inflows (Henson and others, 2022; Hevesi and others, 2025) Abbreviations: ALIS, Alisal Creek; BIGS, Big Sandy Creek; CHAL, Chalone Creek; CHER, Cherry Canyon; CHUA, Chualar Creek; GABI, Gabilan Creek; HAME, Hames Creek; HARE, Hare Canyon; LSRW, lower Salinas River West; MCOY, McCoy Creek; MONR, Monroe Creek; MSRW, middle Salinas River West; NACI, Nacimiento River; PANR, Pancho Rico Creek; PINE, Pine Creek; QUAI, Quail Creek; SALI, Salinas River; SANL, San Lorenzo Creek; SANR, San Antonio River; SARG, Sargent Creek; SECO, Arroyo Seco; STON, Stonewall Creek; TORO, El Toro Creek; VINE, Vineyard Canyon; WILD, Wildhorse Canyon.

Model Development 37 Table 8. Drainage basin areas and topographic characteristics of tributary drainages to the lower Salinas Valley study area. [mi2, square mile; Min, minimum; Max, maximum]

Lower Salinas Valley study area tributary drainage

Tributary drainage abbreviation

Area (mi2)

Elevation (feet) Mean

Max

Slope (rise/run)

Min

Range

Mean

Max

Min

Alisal Creek

ALIS

1,326

2,911

−1

2,912

0.39

1.17

0.14

Arroyo Seco

SECO

271

2,393

5,872

305

5,567

0.46

1.07

0.14

Big Sandy Creek

BIGS

2,006

3,930

357

3,572

0.28

2.08

0.21

Chalone Creek

CHAL

141

1,519

3,291

1,036

2,256

0.28

1.05

0.16

Cherry Canyon

CHER

997

1,682

259

1,423

0.19

2.37

0.17

Chualar Creek

CHUA

1,926

3,423

265

3,158

0.38

0.90

0.12

El Toro Creek

TORO

1,358

3,551

537

3,015

0.30

1.16

0.18

Gabilan Creek

GABI

1,340

3,137

349

2,788

0.34

0.91

0.16

Hames Creek

HAME

1,369

2,778

299

2,479

0.32

1.07

0.17

Hare Canyon

HARE

1,200

2,048

670

1,378

0.25

0.95

0.16

Lower Salinas River West

LSRW

2,105

4,403

671

3,732

0.52

0.81

0.12

McCoy Creek

MCOY

1,621

3,326

415

2,911

0.37

1.14

0.18

Middle Salinas River West

MSRW

1,288

2,571

612

1,959

0.38

0.93

0.16

Monroe Creek

MONR

1,346

3,043

536

2,507

0.38

1.38

0.18

Nacimiento River

NACI

358

1,533

3,749

483

3,266

0.29

1.23

0.18

Pancho Rico Creek

PANR

1,658

3,938

561

3,377

0.32

1.58

0.17

Pine Creek

PINE

1,174

2,196

504

1,693

0.32

1.45

0.18

Quail Creek

QUAI

1,570

3,046

515

2,531

0.44

0.97

0.17

Salinas River

SALI

1,574

1,635

4,327

449

3,878

0.20

1.01

0.17

San Antonio River

SANR

328

1,549

5,870

587

5,282

0.26

1.69

0.14

San Lorenzo Creek

SANL

248

2,009

4,483

678

3,805

0.28

1.52

0.20

Sargent Creek

SARG

1,515

2,452

423

2,028

0.33

1.33

0.18

Stonewall Creek

STON

1,470

2,522

603

1,919

0.27

1.01

0.17

Vineyard Canyon

VINE

1,815

2,751

323

2,427

0.24

0.93

0.15

Wildhorse Canyon

WILD

1,182

2,100

1,051

1,049

0.27

1.40

0.14

Salinas Valley upland area

SVU

3,625

1,671

5,872

−1

5,873

0.26

2.37

0.12

Lower Salinas Valley study area

SVIHM

904

441

3,003

−1

3,004

0.11

0.88

0.00

38 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley 122° 280 84

121 94 112 107

6 24

SVWM sub-model boundary 148 53

140

152 130

74 77

106

California Irrigation Management Information System (CIMIS)

113

Wildland Fire Remote Automatic Weather Stations (RAWS)

Soledad

SAN BENITO COUNTY 101

70 127

119 125

143

U.S. Geological Survey streamgage at Bradley (see table 2)

101

146

14 64

111

National Weather Service Cooperative Observer Program (COOP)

135 118

120

144

7 8 108

149

139

MONTEREY COUNTY 65

146

145 Salinas 75 136

97 117 155 154 50 16 82

36°

Climate station with records of daily precipitation and identifier (see appendix 1, table 1.1)

147 133

132

131

153

MONTEREY BAY 116

MERCED COUNTY

113 54

140 21

Salinas Valley Watershed Model (SVWM) boundary

101

104

EXPLANATION

STANISLAUS COUNTY 5

44 45

37°

SANTA CLARA COUNTY 26 57 San Jose

680

124

120°

121°

King City

115

20 73

134 142

Lower Salinas Valley sub-model boundary

FRESNO COUNTY

129

95 79

42 19

126

102

109

KINGS COUNTY

59 72

32 123 128

Paso Robles

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary

138

151 56 55 150 15

90 87 137 71

San Luis Obispo

122

60 99

100

10 10

SANTA BARBARA COUNTY

101

110

105 1

35°

Base from U.S. Geological Survey, The National Map (2023) and other Federal digital data, various scales; California IV State Plane Coordinate System (FIPS 0404); North American Datum of 1983

KERN COUNTY

SAN LUIS OBISPO COUNTY

40 KILOMETERS

40 MILES

Figure 16. Locations of climate stations having records of the daily precipitation used in the Basin Characterization Model (BCM) to develop daily climate input for the Salinas Valley Watershed Model (Hevesi and others, 2022).

Model Development 39 122° 280 280

680 680

SAN MATEO COUNTY 37

74 76

37°

3 8

SANTA 21 CRUZ COUNTY 93 80 10 56 43 94 47 108 81 105 79 100 92 MONTEREY BAY 9

106

18 109

82 98

Salinas 34

112 6

MONTEREY COUNTY

110

63 26

California Irrigation Management Information System (CIMIS)

Wildland Fire Remote Automatic Weather Stations (RAWS)

U.S. Geological Survey streamgage at Bradley (see table 2)

111 91

97 61

113 84 Soledad

Lower Salinas Valley sub-model boundary

FRESNO COUNTY 14

62 70

16 15

SAN BENITO COUNTY

36°

National Weather Service Cooperative Observer Program (COOP)

4 51

102

Climate stations with records of daily maximum and daily minimum air temperature and identifier (see appendix 1, tables 1.2, 1.3)

MERCED COUNTY

101

SVWM sub-model boundary

101

Salinas Valley Watershed Model (SVWM) boundary

SANTA CLARA COUNTY

EXPLANATION

STANISLAUS COUNTY 23

San Jose

65 45

120°

121°

King City

107 7

95 101

KINGS COUNTY

64 19

52 24 31

11 67 77

Paso Robles

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary

103

KERN COUNTY

SAN LUIS OBISPO COUNTY

104

86 San Luis Obispo 30

Pismo Beach

35° 46 1

Base from U.S. Geological Survey, The National Map (2023) and other Federal digital data, various scales; California IV State Plane Coordinate System (FIPS 0404); North American Datum of 1983

101

0 0

SANTA BARBARA COUNTY 10

40 KILOMETERS

40 MILES

Figure 17. Locations of climate stations having records of daily maximum and minimum air temperature used in the Basin Characterization Model (BCM) to simulate daily potential evapotranspiration (PET) for the Salinas Valley Watershed Model (Hevesi and others, 2022).

40 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley The four BCM-simulated 270-m gridded climate inputs (precipitation, maximum and minimum air temperature, and PET) were averaged over the area of each HRU. Using a uniform hourly distribution, the four daily climate time series developed for each HRU were disaggregated into hourly time series, starting 1 second after midnight on October 1, 1947, and ending at midnight on September 30, 2018. The 690 unique sets of hourly climate time-series inputs were compiled and stored in a single binary Watershed Data Management (WDM) file used by the HSPF code. The annual (water year) basinwide mean precipitation estimated for the Salinas Valley study area using the BCM indicates high interannual variability, with annual precipitation greater than 32 in. for 6 water years and less than 12 in. for 10 water years, compared to a 70-year mean precipitation of 18.5 inches per year (in/yr; fig. 18A). The ARR subbasin had the highest annual precipitation for all water years, with a maximum annual precipitation of about 61 in. for water year 1998 (fig. 18B). In comparison, water year 1983 was the wettest year for the SRH and NAC subbasins, with about 51 in. of annual precipitation for both subbasins (fig. 18B). Water year 2014 was the driest year in the 70-year period with about 7 in. of precipitation basinwide for the SVWM (fig. 18A), varying from about 13 in. for the ARR subbasin to about 6 in. for the LOR and MSR subbasins (fig. 18B). The 70-year (water years 1949–2018) mean precipitation simulated by the BCM and averaged over the 690 HRU areas indicates a high degree of spatial variability, with about 36–60 in/yr for the high-elevation HRUs along the western boundary to less than 12 in/yr for low-lying HRUs in the central part of the valley and the southeastern part of the study area, and values less than 10 in/yr along the southeastern boundary (fig. 19). Precipitation was less variable in the Salinas Valley lowlands and coastal basins, ranging from about 12 to 15 in/yr for most locations, with higher values of 15–21 in/yr for the coastal basins in the northwest part of the lower Salinas Valley. Annual (water year) 70-year mean PET simulated by the BCM and averaged over the 690 HRU areas also indicates substantial spatial variability in climate (fig. 20). The mean

PET for the SVWM was 58.1 in. and varied from a minimum of about 55.3 in. for water year 1998 to high values of more than 59.5 in. for several water years including 1959, 1984, and 2014 (fig. 20A). Water year 2014, the driest year, also had the highest PET of 60 in. The MCB subbasin had the lowest PET for all water years, ranging from about 48 in. for water years 1999 and 2011 to 54 in. for water year 1997 (fig. 20B). The EST and USR subbasins had the highest annual PET values of about 62 in. or higher for water years 1959, 1960, 1984, 1996–97, and 2014–15 (fig. 21). The BCM-simulated 70-year mean PET varied from high values of about 61–63 in/yr for HRUs in the more inland, southeast part of the SVWM to low values of about 42–46 in/yr for HRUs closer to the coastline (fig. 21). Northto northeast-facing slopes on the west side of the Salinas Valley (particularly in northwestern part of the SVWM) also had lower mean annual PET values of about 49–52 in/yr compared to the basinwide mean of 58.1 in/yr. Mean annual PET within the lower elevation of the upper Salinas Valley varied from about 60 to 63 in/yr and overall had higher values compared to most locations in the Salinas Valley study area. Seasonal variability in precipitation and PET is an important characteristic of the Salinas Valley study area. The basinwide mean monthly precipitation is the highest for January for the SVWM and all subbasins, ranging from about 3 in. for the EST and MSR subbasins to 6.4 in. for the ARR subbasin (fig. 22A). February is the second wettest month for the SVWM and all subbasins, varying from about 2.8 in. for the LSR, EST, and MSR subbasins to 5.8 in. for the ARR subbasin. For the dry-season months, June through August, mean monthly precipitation is approximately zero for all subbasins. In contrast to precipitation, mean monthly PET is highest during the dry season months of May through August (fig. 22B). Although the basinwide 70-year mean PET of 58.1 in/yr is more than three times the basinwide 70-year mean precipitation of 18.4 in/yr, the mean monthly precipitation for December through February exceeds mean monthly PET.

Model Development 41 A

–8

–16

–24

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Cumulative departure from mean, in inches

Annual precipitation, in inches

–32

Water Year

EXPLANATION

Annual precipitation

20-year moving mean

Mean annual precipitation

10-year moving mean

Cumulative departure from mean

60 55

Annual precipitation, in inches

50 45 40 35 30 25 20 15 10 5 0

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Water Year

EXPLANATION Salinas Valley Watershed Model subbasin (see table 1) ARR

LOR

MCB

NAC

SRH

EST

LSR

MSR

SAN

USR

Figure 18. Precipitation for water years 1948–2018 estimated using the Basin Characterization Model (BCM): A, annual precipitation averaged for the area of the Salinas Valley Watershed Model (SVWM) and B, annual precipitation averaged over 10 subbasin areas in the Salinas Valley study area. Abbreviations: ARR, Arroyo Seco; EST, Estrella River; LOR, San Lorenzo Creek; LSR, lower Salinas River; MCB, Monterey Coastal Basins; MSR, middle Salinas River; NAC, Nacimiento River; SAN, San Antonio River; SRH, Salinas River headwaters; USR, upper Salinas River (Hevesi and others, 2022).

42 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley 121°30'

122°

River aro aj

121°

120°30'

101

EXPLANATION

MERCED COUNTY

Salinas Valley Watershed Model (SVWM) mean annual precipitation (water years 1949–2018), in inches per year

MONTEREY BAY SAN BENITO COUNTY

Salinas Monterey

n Sa

er Sa n

as in

Sa l

Lo re n

re e

ive r

33.1 to 36.0

21.1 to 24.0

36.1 to 60.0

FRESNO COUNTY

Na cim ien to

30.1 to 33.0

18.1 to 21.0

San Ant

27.1 to 30.0

15.1 to18.0

U.S. Geological Survey streamgage at Bradley (see table 2)

io on

36°

Soledad

King City

Lower Salinas Valley sub-model boundary

24.1 to 27.0

12.1 to 15.0

SVWM sub-model boundary

MONTEREY COUNTY

9.0 to 12.0

SVWM boundary

en it

36° 30'

120°

r ve Ri

Lake San Antonio

KINGS COUNTY

Lake Nacimiento

Estrel la R i

KERN COUNTY

r ve

Paso Robles 35° 30'

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary 1

San Luis Obispo 101

Santa Margarita Lake SAN LUIS OBISPO COUNTY

0 0

10 10

20 20

30 MILES

30 KILOMETERS

Figure 19. Mean annual precipitation estimated using the Basin Characterization Model (BCM) for 690 hydrologic response units (HRUs) used in the Salinas Valley Watershed Model (Hevesi and others, 2025).

Model Development 43 A

3 2

59.5

59.0

0 –1

58.5

–2

58.0

–3

57.5

–4 –5

57.0

–6

56.5

–7 –8

56.0

–9

55.5 55.0

–10 1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Cumulative departure from mean, in inches

Annual potential evapotranspiration, in inches

60.0

–11

Water Year

EXPLANATION

Annual precipitation

20-year moving mean

Mean annual precipitation

10-year moving mean

Cumulative departure from mean

Annual potential evapotranspiration, in inches

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Water Year

EXPLANATION Salinas Valley Watershed Model subbasin (see table 1) ARR

LOR

MCB

NAC

SRH

EST

LSR

MSR

SAN

USR

Figure 20. Potential evapotranspiration (PET) simulated using the Basin Characterization Model (BCM): A, annual PET averaged for the Salinas Valley Watershed Model (SVWM) and B, annual PET averaged for subbasins in the SVWM. Abbreviations: ARR, Arroyo Seco; EST, Estrella River; LOR, San Lorenzo Creek; LSR, lower Salinas River; MCB, Monterey Coastal Basins; MSR, middle Salinas River; NAC, Nacimiento River; SAN, San Antonio River; SRH, Salinas River headwaters; USR, upper Salinas River (Hevesi and others, 2022).

44 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley 121°30'

122°

River aro aj

121°

120°30'

101

EXPLANATION

MERCED COUNTY

Salinas Valley Watershed Model (SVWM) mean annual potential evapotranspiration (water years 1949–2018), in inches

MONTEREY BAY SAN BENITO COUNTY

Salinas Monterey

n Sa

55.1 to 56.5

49.1 to 50.5

56.6 to 58.0

50.6 to 52.0

58.1 to 59.5

52.1 to 53.5

59.6 to 61.0

53.6 to 55.0

61.1 to 62.8

SVWM sub-model boundary

MONTEREY COUNTY

42.4 to 49.0

SVWM boundary

en it

36° 30'

120°

Soledad

U.S. Geological Survey streamgage at Bradley (see table 2)

101

King City

Sa n as in

Sa l

re e

San Ant

FRESNO COUNTY

ive r

Na cim ien to

io on

36°

Lower Salinas Valley sub-model boundary

Lo re n

r ve Ri

Lake San Antonio

KINGS COUNTY

Lake Nacimiento

Estrel la R i

KERN COUNTY

r ve

Paso Robles 35° 30'

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary 1

San Luis Obispo 101

Santa Margarita Lake SAN LUIS OBISPO COUNTY

0 0

10 10

20 20

30 MILES

30 KILOMETERS

Figure 21. Mean annual potential evapotranspiration (PET) for water years 1949–2018 simulated using the Basin Characterization Model (BCM) and averaged for 690 hydrologic response units (HRUs) used in the Salinas Valley Watershed Model (Hevesi and others, 2022).

Model Development 45

6.5

6.0

EXPLANATION

5.5

Salinas Valley Watershed Model Salinas Valley Watershed Model subbasin (see table 1) ARR MSR

Mean monthly precipitation, in inches

5.0 4.5

EST

NAC

4.0

LOR

SAN

3.5

LSR

SRH

MCB

USR

3.0 2.5 2.0 1.5 1.0 0.5 0.0

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

May

June

July

Aug.

Sept.

Water Year

Mean monthly potential evapotranspiration, in inches

9.5

9.0

EXPLANATION

8.5

Salinas Valley Watershed Model

8.0 7.5 7.0

Salinas Valley Watershed Model subbasin (see table 1) ARR MSR

6.5

EST

NAC

6.0

LOR

SAN

5.5

LSR

SRH

5.0

MCB

USR

4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

Water Year

Figure 22. Climate inputs developed using the Basin Characterization Model (BCM) for the Salinas Valley Watershed Model (SVWM) and subbasins; A, mean monthly precipitation and B, mean monthly potential evapotranspiration. Abbreviations: ARR, Arroyo Seco; EST, Estrella River; LSR, lower Salinas River; MSR, middle Salinas River; MCB, Monterey Coastal Basins; NAC, Nacimiento River; SRH, Salinas River headwaters; SAN, San Antonio River; LOR, San Lorenzo Creek; USR, upper Salinas River (Hevesi and others, 2022)

46 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Model Parameters The HSPF code requires a User Control Input (UCI) file to specify model control options, the simulation period, model parameters, and input and output filenames needed to run a simulation. The UCI file is an American Standard Code for Information Interchange text file with a formatted, column-specified input structure. The model parameters are organized according to the modular structure of HSPF, with separate input groups used for simulating water flow and storage within PERLND areas, IMPLND areas, and the connected RCHRES network. In addition to the UCI file, simulations require the WDM file containing the time series inputs and outputs, including hourly climate (precipitation, air temperature, and PET) and hourly and daily streamflow. Parameters used to simulate water flow and storage in PERLNDs define spatially varying catchment properties throughout the SVWM and were the most important for representing the physical characteristics of the Salinas Valley study area (table 9). Initially, model parameters were defined using a combination of geospatial data and representative values from previous studies, including suggested values provided in the HSPF user’s manual (U.S. Environmental Protection Agency, 2000; Bicknell and others, 2001). During the model-calibration procedure, 15 of the 17 parameters listed in table 9 were then adjusted and refined, as discussed in more detail in the “Model Calibration” section. Parameters (defined in table 9) LZSN, INFILT, LSUR, KVARY, AGWRC, INFEXP, DEEPFR, BASETP, and AGWETP were scaled using geospatial data and calibrated as a set of unique values for each HRU. Monthly parameters INTERCEP, UZSN, MANNING, INTERFLW, IRC, and LZETPARM were scaled using geospatial data and calibrated for each HRU as a unique set of 12 monthly values. Initial estimates of LZSN, the lower zone storage capacity, were varied as a function of the mean AWS150 value from SSURGO (fig. 9), the mean 30-m DEM slope (fig. 4), and the weighted average of DEM slope classes for each HRU. The model parameter INFILT affects the infiltration capacity of the soil and was varied for each HRU as the weighted average of the 13 SSURGO soil texture classes within the SVWM, where the weighting factors were defined by the area-fraction of each soil texture class for each HRU (fig. 8). The model parameter LSUR, the length of the overland flow plane used to simulate re-infiltration of surface runoff, was estimated based on the inverse of DEM-derived slope, with steeper slopes resulting in lower LSUR values, and then adjusted during calibration. The model parameters KVARY and AGWRC were varied for each HRU using the surficial geologic map (fig. 10; Jennings, 1977). The KVARY parameter affects the simulation of groundwater recession flow, enabling the recession flow to be non-exponential in its decay with time, and was calculated

as the weighted average of KVARY values defined for each of the 9 surficial geologic rock types (for this study all metamorphic rock types from Jennings [1977] were grouped as metamorphic rocks and all volcanic rock types from Jennings [1977] were grouped as volcanic rocks), with the weighting factors calculated as the area-fraction of the surficial geologic rock type within each HRU (fig. 10). The AGWRC parameter is the basic groundwater recession rate and also was calculated for each HRU as the area-fraction weighted average of values defined for each of 9 surficial geologic rock types. The INFEXP parameter controls the infiltration rate into the root zone and was estimated as a function of the mean DEM slope for each HRU. The DEEPFR parameter controls the rate of inflow to the inactive groundwater reservoir (groundwater that does not contribute to groundwater discharge in the HSPF simulation). As with KVARY and AGWRC, DEEPFR was varied for each HRU based on the surficial geologic map and the area fraction of the different surficial geologic rock types within each HRU (fig. 10). The AGWETP parameter controls groundwater ET losses and was estimated based on the area-fraction of cropland in each HRU. Six parameters, INTERCEP, UZSN, MANNING, INTERFLW, IRC, and LZETPARM (table 9), were defined using the HSPF option of having a set of 12 monthly values for each HRU to represent seasonal variability. The INTERCEP parameter defines the interception storage capacity for vegetation and was varied for each HRU as the area-fraction weighted average of values defined for each NLCD land cover type (fig. 6). In addition to land cover type, the NLCD 30-m resolution, percentage of forest canopy cover data (fig. 7A) was used to increase the interception storage capacity based on increasing canopy cover. The INTERFLW parameter controls the interflow inflow rate and was varied using monthly scaling factors and the calculated mean 30-m DEM slope for each HRU. The IRC parameter is the interflow recession coefficient and was also varied monthly as a function of the 30-m DEM slope for each HRU. The model parameters LZETPARM, MANNING, and UZSN were varied by month and by the area-weighted average of the different NLCD land cover types within each HRU. The LZETPARM parameter is used for simulating plant transpiration as a function of PET, vegetation type, and growing season. The MANNING parameter is the surface roughness coefficient used for simulating overland flow. The UZSN parameter is the upper zone storage capacity used to account for surface retention storage. In addition to NLCD land cover type, the NLCD percentage of canopy cover was used to scale the parameters LZETPARM and UZSN by increasing parameter values with increasing canopy cover percentage. Mean land-surface slope also was used to scale the MANNING and UZSN parameters, with an increase in slope resulting in a decrease in values for both parameters.

Table 9. Hydrologic Simulation Program—Fortran (HSPF) pervious land-area (PERLND) parameters used in the Salinas Valley Watershed Model (SVWM) to represent basin characteristics for the Salinas Valley study area (modified from U.S. Environmental Protection Agency, 2000). [Modified from U.S. Environmental Protection Agency (2000); Abbreviations: ET, evapotranspiration; ft, foot; ft/ft, foot per foot; GW, groundwater; in., inch; in/hr, inch per hour; Max, maximum; Min, minimum]

Baseline models HSPF PERLND parameters

Typical values Name

Possible values

SVWM values defined by

Description

Units

Min

Max

Mean

Min

Max

Mean

LZSN

Lower zone nominal soil moisture storage; representative of soils, climate

in.

3.0

8.0

5.5

2.0

15.0

8.5

Calibration

INFILT

Index to infiltration capacity; divides surface and subsurface flow; representative of soils, land use

in/hr

0.01

0.25

0.13

0.001

0.50

0.25

Calibration

LSUR

Length of overland flow; estimated using topography

SLSUR

Slope of overland flow plane; estimated using topography

ft/ft

0.01

0.15

0.08

0.001

0.30

0.15

Defined using digital elevation model

KVARY

Variable groundwater recession; used when recession rate varies with GW levels; representative of baseflow recession variation

1/in.

0.0

3.0

1.5

0.0

5.0

2.5

Calibration

AGWRC

Baseflow recession coefficient

unitless

0.92

0.99

0.96

0.85

0.999

0.92

Calibration

INFEXP

Exponent in infiltration equation; usually default to 2.0; representative of soils variability

unitless

2.0

1.0

3.0

2.0

Calibration

INFILD

Ratio of max/mean infiltration capacities; representative of soils variability; usually default to 2.0

unitless

2.0

1.0

2.0

1.5

Set to default value of 2.0

DEEPFR

Fraction of GW inflow to deep recharge; accounts for subsurface losses; representative of geology, recharge to groundwater that does not contribute to baseflow

unitless

0.0

0.20

0.10

0.0

0.50

0.25

Estimated using geology, then calibrated

BASETP

Fraction of remaining ET from baseflow; accounts for direct ET from riparian vegetation

unitless

0.0

0.05

0.03

0.0

0.20

0.10

Calibration

AGWETP

Fraction of remaining ET from groundwater available for baseflow; accounts for ET from shallow groundwater (marshes, wetlands)

unitless

0.0

0.05

0.03

0.0

0.20

0.10

Calibration

INTERCEP

Interception storage capacity; representative of vegetation type and density, land use

in.

0.03

0.20

0.12

0.01

0.40

0.21

Estimated from land cover, then calibrated (monthly values)

UZSN

Upper zone nominal soil moisture storage; accounts for near surface retention; representative of soil conditions, land use

in.

0.10

1.0

0.55

0.05

2.0

1.03

Calibration (monthly values)

MANNING

Manning’s n (roughness) for overland flow; representative of surface conditions, residue, etc.

unitless

0.15

0.35

0.25

0.05

0.50

0.28

Calibration (monthly values)

200

500

350

100

700

400

Calibration

Model Development 47

[Modified from U.S. Environmental Protection Agency (2000); Abbreviations: ET, evapotranspiration; ft, foot; ft/ft, foot per foot; GW, groundwater; in., inch; in/hr, inch per hour; Max, maximum; Min, minimum]

Baseline models HSPF PERLND parameters

Typical values Name

Possible values

SVWM values defined by

Description

Units

Min

Max

Mean

Min

Max

Mean

INTERFLW

Interflow inflow parameter; representative of soils, topography, land cover

unitless

1.0

3.0

2.0

1.0

10.0

5.5

Calibration (monthly values)

IRC

Interflow recession parameter. representative of soils, topography, land cover

unitless

0.5

0.7

0.60

0.3

0.85

0.58

Calibration (monthly values)

LZETPARM

Lower zone ET parameter; representative of vegetation type/density, root depth

unitless

0.2

0.7

0.45

0.1

0.9

0.50

Calibration (monthly values)

48 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Model Calibration 49 Several parameters were defined directly using the Geographic Information System (GIS) applications or by the suggested default values and were held constant during model calibration. The SLSUR parameter, the slope of the overland flow plane used by HSPF to simulate overland runoff and infiltration of overland flow, was defined directly as the mean calculated rise-over-run slope of all 30-m DEM grid cells within each HRU. The INFILD parameter was set to a constant value of 2.0, as recommended in the HSPF user’s manual and supporting documentation (U.S. Environmental Protection Agency, 2000; Bicknell and others, 2001), and was not adjusted during model calibration. Impervious land-area parameters also were defined based on suggested values in HSPF documentation and were not adjusted during model calibration. Impervious land-area parameters LSUR, SLSUR, and NSUR (NSUR is equivalent to the MANNING roughness coefficient used for pervious land areas) were set to 200, 0.117, and 0.05 ft, respectively. The impervious land-area parameter RETSC, the retention storage capacity, was set to 0.1 in. Parameters for simulating surface-water flow through the 690 RCHRES elements comprising the SVWM drainage network include stream reach length, the change in elevation over the length of the stream reach, and a table defining the stage-area-volume-discharge relation for each stream reach, referred to as the Flow-table (Ftable). The Ftables used in the SVWM included two outlets, one for streamflow and a second outlet used to simulate seepage losses from the infiltration of surface water through the streambed. Discharge for the second outlet representing seepage loss was set to zero or near-zero outflow for stream reaches assumed to have none to negligible stream seepage. The Ftables were estimated using rating curves, field data, and peak discharge data measured at the USGS streamgages in the Salinas Valley study area (table 2). The data were used to define the Ftables in the RCHRES segments where the streamgages were located. The Ftables for segments without a streamgage were estimated based on the proximity to the nearest streamgage and scaled according to the length and slope of the channel represented by the RCHRES, the channel width, the upstream drainage area, and the estimated permeability of the streambed based on the surficial geology and soil texture. Additionally, Ftables for many stream reaches were adjusted during calibration, particularly with respect to the seepage outflow rate.

Model Calibration Model calibration is done by comparing simulated and measured variables. Measured values, often referred to as observations of the variable of interest, are determined directly from field data or from values calculated from field data. Ideally, the observed variable of interest has continuous records at the desired temporal resolution spanning

multi-year periods that capture a full range of conditions and variability, with at least one drier-than-average period and one wetter-than-average period. The SVWM was calibrated by comparing simulated and observed streamflow, where observed streamflow is calculated using measurements of stream stage, velocity, or both. Streamflow accounts for the integrated effects of the hydrologic processes in the drainage area upstream from the streamgage used for calibration and is generally the variable used for calibrating hydrologic models such as the SVWM. Observed streamflow in the Salinas Valley is characterized by rapid surface runoff response to precipitation in upland drainages, with ephemeral streamflow in most channels and seasonally distributed baseflow after winter storms with very low to no-flow conditions during the dry summer months. In addition to seasonal variability, there is substantial annual variability in streamflow in response to variability in annual precipitation. To achieve a good calibration for the SVWM, simulated streamflow needed to represent a wide range of flow conditions for a variety of catchment areas, from flashy peak flows having short durations of several days or less to long periods of several months with very low flows (less than 10 ft3/s) and extended no-flow conditions at most streamgages.

Calibration Procedure Streamflow records for 29 USGS streamgages within the SVWM (fig. 1; table 10) with observed streamflow between October 1, 1948, and October 1, 2015, (water years 1949–2015) were used for calibration. In addition to the USGS streamflow data, estimates of surface-water inflows to Lake Nacimiento from October 1, 1958, to September 30, 2013, and to Lake San Antonio from October 1, 1966, to September 30, 2013, were provided by Monterey County Water Resources Agency (MCWRA) and included as observations. All calibration runs were started on October 1, 1947, to allow for a minimum of a 1-year model initialization period before the target period used for calibration at each streamgage. Calibration was done using an iterative trial-and-error approach of adjusting PERLND parameters identified in table 9 to improve the match between simulated and observed streamflow. Trial-and-error adjustments for spatially distributed parameters were done primarily by changing scaling factors that were defined using geospatial data, which included topography, soils, land cover, and geology. In the case of discrete geospatial data such as land cover type, soil texture, and geology, the scaling factors were calculated as the weighted mean value based on the area-fraction of discrete data within each HRU. In the case of continuous 30-m (98-ft) gridded data such as land elevation, slope, percentage of forest canopy, and percentage of imperviousness, parameter values were calculated as the average of the gridded data values within each HRU area. This method of parameter estimation and calibration used in the SVWM resulted in a unique set of pervious land parameters for each HRU.

50 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley Table 10. U.S. Geological Survey (USGS) streamgages and reservoirs with estimated inflows along with calibration periods used for calibrating the Salinas Valley Watershed Model (SVWM). [ID, identification; HSPF, Hydrologic Simulation Program–Fortran; mi2, square mile; mm/dd/yyyy, month/day/year; NWIS, National Water Information System; RCHRES, stream reach or reservoir; —, not applicable]

USGS streamgage ID or reservoir number

Drainage area (mi2)

Start date (mm/dd/yyyy)

End date (mm/dd/yyyy)

Years in period

ARROYO SECO NR SOLEDAD

11152000

244

10/01/1948

9/30/2015

67.0

159

ARROYO SECO BL RELIZ C NR SOLEDAD

11152050

304

10/01/1994

9/30/2015

21.0

629

SAN ANTONIO R NR LOCKWOOD

11149900

217

10/01/1965

9/30/2015

50.0

312

SALSIPUEDES C NR POZO

11144200

10/01/1969

9/30/1983

14.0

962

EL TORO C NR SPRECKELS

11152540

10/01/1961

9/30/2001

40.0

710

HUERHUERO C NR CRESTON

11147600

101

10/01/1958

9/30/1972

14.0

SVWM streamgage number

HSPF RCHRES ID

150

NWIS streamgage name or reservoir name

Calibration period

344

SANTA RITA C TRIB

11147040

08/01/1967

9/30/1972

5.2

124

ARROYO SECO NR GREENFIELD

11151870

113

10/01/1966

9/30/1978

12.0

315

SALINAS R BL SALINAS DAM NR POZO

11144600

112

10/01/1973

4/9/1986

12.5

222

SAN LORENZO C BL BITTERWATER C

11151300

233

10/01/1958

9/30/2015

56.7

637

SAN ANTONIO R A PLEYTO

11150000

277

10/01/1948

9/30/1965

17.0

768

SALINAS R NR BRADLEY

11150500

2,535

10/01/1948

9/30/1957

8.3

348

SANTA RITA C NR TEMPLETON

11147070

10/01/1961

9/30/1994

33.0

954

SALINAS R NR SPRECKELS

11152500

4,156

10/01/1948

1/31/1957

8.3

519

NACIMIENTO R NR BRYSON

11148800

147

10/01/1955

9/30/1971

16.0

627

SAN ANTONIO R A SAM JNS BR

11149700

147

07/01/1958

9/30/1965

5.9

317

SALINAS R AB PILITAS C NR SANTA MAR

11145000

114

10/01/1948

10/3/1975

27.0

521

NACIMIENTO R BL SAPAQUE C

11148900

162

09/16/1971

9/30/2015

44.0

311

TORO C NR POZO

11144000

10/01/1960

9/30/1983

20.7

748

NACIMIENTO R NR SAN MIGUEL

11149500

349

10/01/1948

9/30/1957

9.0

354

SALINAS R A PASO ROBLES

11147500

390

10/01/1948

9/30/2015

63.0

307

SALINAS R NR POZO

11143500

10/01/1948

9/30/1983

35.0

339

JACK C NR TEMPLETON

11147000

10/01/1949

9/30/1978

29.0

443

CHOLAME C TRIB

11147700

10/01/1958

9/30/1965

7.0

Model Calibration 51 Table 10. U.S. Geological Survey (USGS) streamgages and reservoirs with estimated inflows along with calibration periods used for calibrating the Salinas Valley Watershed Model (SVWM).—Continued [ID, identification; HSPF, Hydrologic Simulation Program–Fortran; mi2, square mile; mm/dd/yyyy, month/day/year; NWIS, National Water Information System; RCHRES, stream reach or reservoir; —, not applicable]

USGS streamgage ID or reservoir number

Drainage area (mi2)

Start date (mm/dd/yyyy)

End date (mm/dd/yyyy)

Years in period

CHOLAME C NR SHANDON

11147800

227

10/01/1958

9/30/1972

14.0

473

ESTRELLA R NR ESTRELLA

11148500

922

10/01/1954

9/30/1996

42.0

RECLAMATION DITCH NR SALINAS

11152650

10/01/1970

9/30/2015

28.7

GABILAN C NR SALINAS

11152600

10/01/1970

9/30/2014

44.0

ARROYO DEL REY A DEL REY OAKS

11143300

10/02/1970

10/1/2015

14.3

—

539

Lake Nacimiento inflow

325

10/01/1958

9/30/2013

55.0

—

640

Lake San Antonio inflow

323

10/01/1966

9/30/2013

47.0

SVWM streamgage number

HSPF RCHRES ID

498

NWIS streamgage name or reservoir name

Calibration period

Summary for all gages Mean

—

377

—

Maximum

—

4,156

—

Minimum

—

5.2

Weighted mean

—

Calibration Statistics The calibration procedure used a quantitative analysis of the goodness-of-fit between simulated and observed time series based on the available period of record at each streamgage from October 1, 1948, to September 30, 2015, (table 10) and was done using the percent-average estimation error (PAE) and the Nash–Sutcliffe model efficiency (NSME) statistics (Nash and Sutcliffe, 1970; Donigian and Imhoff, 2009; Duda and others, 2012; Hevesi and Johnson, 2016; Hevesi and others, 2019). Calibration periods varied from a minimum of 5.2 years for USGS streamgage 11147040 (gage 11) to a maximum of 67 years at USGS streamgage 11152000 (gage 1), with a mean calibration period of 28 years for all streamgages (table 10). The PAE statistic provides a measure of model bias, where a value of 0.0 percent indicates an unbiased model fit. Values of PAE in the range of plus or minus 10 percent indicate an acceptable calibration (Donigian and Imhoff, 2009). The NSME statistic is a standardized mean squared-error statistic, similar to the coefficient of determination (R2) used in regression analysis, that is often used to compare goodness-of-fit results between different models (Nash and Sutcliffe, 1970; Markstrom and others, 2008; Hevesi and others, 2019). An NSME value greater than 0.0 indicates the model provides a better match to the observed values than the mean of the observed values (the mean has an

NSME equal to 0.0). The closer the NSME is to 1.0, the better the match is between simulated and observed values, with a value of 1.0 indicating a perfect match to observed values. The PAE and NSME goodness-of-fit statistics between simulated and observed streamflow were evaluated using daily mean, monthly mean, annual (water year) mean, and mean monthly streamflow at each streamgage for varying values of parameters selected for calibration (table 9). To summarize the calibration results for the 31 streamflow comparisons, weighted mean PAE, percent estimation error (PE), and NSME statistics were calculated for each parameter set tested. The weighting factors were defined for each streamgage using the average of two products consisting of (1) the length of the calibration period at each streamgage divided by the total length of record for all streamgages and (2) the square root of the contributing drainage area for each streamgage divided by the sum of the square root of the total drainage area for all streamgages. The weighted mean PAE was calculated using the absolute value of PAE calculated at each individual streamgage. Calibration criteria used for the SVWM are provided in table 11. To achieve a minimum level of calibration for the SVWM, the weighted mean goodness-of-fit statistics needed to meet the satisfactory criteria or better for daily, monthly, and annual streamflow, with the goal of having results meet the good to very good criteria.

52 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley Table 11. Calibration criteria applied to goodness-of-fit results for daily, monthly, and annual streamflow for calibrating the Salinas Valley Watershed Model (SVWM). [NSME, Nash-Sutcliffe model efficiency; PAE, percent-average estimation error; >, greater than; <, less than]

Streamflow Daily Monthly Annual

Statistic

Goodness-of-fit results Poor

Satisfactory

Good

Very Good

>10

5–10

2–5

NSME

<0.4

0.4–0.6

0.6–0.7

>0.7

PAE

>10

5–10

2–5

NSME

<0.6

0.6–0.7

0.7–0.8

>0.8

PAE

>10

5–10

2–5

NSME

<0.7

0.7–0.8

0.8–0.9

>0.9

PAE

Parameter Sensitivity Before the process of trial-and-error model calibration, an initial set of parameter values was defined using the recommended range of values provided by the HSPF user’s manual and supporting documentation (U.S. Environmental Protection Agency, 2000; Bicknell and others, 2001, 2005). Six baseline models were defined using the range of typical and possible parameter values listed in BASINS Technical Note 6, Estimating Hydrology and Hydraulic Parameters for HSPF (U.S. Environmental Protection Agency, 2000). Typical and possible mean parameter values were calculated using the minimum and maximum values for the typical and possible ranges. The six baseline parameter sets consisted of (1) typical minimum values, (2) typical maximum values, (3) typical mean values, (4) possible minimum values, (5) possible maximum values, and (6) possible mean values, with all HRUs having uniform parameter values as listed in table 9. Goodness-of-fit statistics were calculated for the 31 simulated daily streamflow comparisons using the six baseline models as a means of evaluating initial estimates for pervious land parameters. The weighted mean PAE, PE, and NSME results indicated that the possible mean and typical maximum baseline models provided better initial parameter values compared to the typical minimum, typical mean, possible minimum, and possible maximum baseline values (table 12). The possible mean baseline model with PAE of 39.2, PE of −15.2, and NSME of 0.355 provided the best overall goodness-of-fit results compared to the four other baseline models; however, all six baseline models failed to provide a satisfactory calibration result. The typical and possible minimum baseline models greatly overestimated the amount of streamflow, and the typical minimum, possible minimum, and mean baseline models provided a very poor fit to observed streamflow based on the negative NSME results. The possible mean, typical maximum, and possible maximum baseline models provided an improved model fit compared to the other three baseline models but underestimated

the weighted average mean, maximum, and minimum streamflows. All six baseline models underestimated the weighted average median streamflow. As part of parameter estimation, the sensitivity of the SVWM to pervious land parameters was evaluated using a simple approach of varying selected parameters across a range of possible values and comparing the weighted mean goodness-of-fit statistics for simulated daily streamflow. The analysis was used to help indicate which parameters would likely have the greatest effect on streamflow and therefore have the greatest effect on calibration results. To perform the sensitivity analysis, pervious land parameters were set to uniform values across all HRUs for each simulation. Minimum and maximum values in table 9 provided an approximate guide for defining the range of parameter values tested. Each test simulation was done using parameter values defined by the typical mean baseline model (table 9), whereas a single parameter was varied across a range of minimum to maximum values. Parameters having the option for monthly values (table 9) were set to a constant value for all months. For each parameter tested, all other parameters were set to constant uniform values for all HRUs as defined by the typical mean baseline model (table 9). The procedure was repeated for each parameter in table 9. Results indicated a high degree of model sensitivity in terms of simulated daily streamflow to parameters LZSN, INFILT, and LZETPARM (table 13). The best goodness-of-fit results using the typical mean baseline model were obtained with values from 8.0 to 15.0 in. for LZSN, 0.05 in/hr for INFILT, and 0.99 for LZETPARM. The LZSN values less than 5.5 in. resulted in a substantial overestimation of streamflow because of insufficient water storage capacity in the root zone causing a reduction in ET. The LZETPARM values of 0.3 and less also resulted in a reduction in ET and a corresponding overestimation of streamflow. The LZSN values greater than 8 in. resulted in an underestimation of streamflow because of the high root zone storage capacity causing a high percentage of precipitation being lost to ET.

Model Calibration 53 Table 12. Goodness-of-fit results comparing observed streamflow with six baseline models developed using documented ranges in the Hydrologic Simulation Program—Fortran (HSPF) parameter values. [ft3/s, cubic foot per second; Max, maximum; Min, minimum; NSME, Nash-Sutcliffe model efficiency; PAE, percent-average estimation error; PE, percent estimation error; %, percent]

Weighted average observed or simulated streamflow (ft3/s)

Observed or baseline scenario

Weighted average goodness-of-fit statistics

Mean

Median

Max

Min

PAE (%)

PE (%)

NSME

Observed

110.1

5.33

15,269

0.023

0.0

1.000

Typical minimum baseline

242.2

2.15

39,744

0.075

248.3

248.2

−11.292

Typical maximum baseline

56.7

0.13

5,241

0.002

41.2

−22.1

0.329

Typical mean baseline

111.9

0.52

10,500

0.008

68.4

57.0

−0.224

Possible minimum baseline

368.5

1.49

45,918

0.113

422.4

424.0

−28.003

Possible maximum baseline

4.9

0.00

474

0.001

93.6

−93.6

0.016

Possible mean baseline

62.1

0.14

8,812

0.002

39.2

−15.2

0.355

Table 13. Goodness-of-fit results used to evaluate model sensitivity to adjustments in values of pervious land parameters used in the Salinas Valley Watershed Model (SVWM). [ft, foot; ft/ft, foot per foot; ft3/s, cubic foot per second; in., inch; in/hr, inch per hour; Max, maximum; Min, minimum; NSME, Nash-Sutcliffe model efficiency; PAE, percent-average estimation error; PE, percent estimation error; *, asterisks designate parameter value for typical mean baseline model; %, percent]

Parameter values

Mean

0.1 1.0 3.0 5.5* 8.0 15.0 30.0 60.0

259.6 208.4 154.4 111.9 84.0 45.2 22.7 14.5

0.001 0.010 0.050 0.130* 0.200 0.500 2.000 5.000

206.5 137.0 114.0 111.9 112.6 115.4 119.4 120.7

10 50 100 350* 500 1,000 5,000

112.1 112.0 111.9 111.9 111.9 111.9 111.9

Simulated streamflow (ft3/s) Median Max Min Parameter LZSN (in.) 1.38 20,415 0.115 0.96 16,776 0.073 0.70 12,522 0.031 0.52 10,500 0.008 0.28 8,927 0.003 0.11 5,838 0.002 0.05 3,109 0.002 0.02 1,628 0.002 Parameter INFILT (in/hr) 0.57 38,657 0.036 0.49 23,837 0.008 0.50 13,210 0.007 0.52 10,500 0.008 0.53 10,560 0.009 0.54 11,032 0.010 0.54 11,927 0.010 0.55 12,268 0.010 Parameter LSUR (ft) 0.52 10,620 0.008 0.52 10,566 0.008 0.52 10,536 0.008 0.52 10,500 0.008 0.52 10,508 0.008 0.52 10,547 0.008 0.52 10,636 0.008

Goodness of fit statistics PAE (%) PE (%) NSME 280 199 125 68 46 47 73 85

280 198 119 57 16 −40 −73 −85

−5.005 −3.033 −1.264 −0.224 0.206 0.410 0.256 0.133

191 98 69 68 70 74 78 80

188 89 58 57 59 63 69 70

−10.671 −1.928 −0.199 −0.224 −0.269 −0.363 −0.478 −0.516

69 69 69 68 68 68 68

57 57 57 57 57 57 57

−0.188 −0.195 −0.206 −0.224 −0.230 −0.240 −0.254

54 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley Table 13. Goodness-of-fit results used to evaluate model sensitivity to adjustments in values of pervious land parameters used in the Salinas Valley Watershed Model (SVWM).—Continued [ft, foot; ft/ft, foot per foot; ft3/s, cubic foot per second; in., inch; in/hr, inch per hour; Max, maximum; Min, minimum; NSME, Nash-Sutcliffe model efficiency; PAE, percent-average estimation error; PE, percent estimation error; *, asterisks designate parameter value for typical mean baseline model; %, percent]

Simulated streamflow (ft3/s) Median Max Min

Parameter values

Mean

0.001 0.010 0.040 0.080* 0.200 0.500 1.000

111.9 111.9 111.9 111.9 111.9 111.9 111.9

0.52 0.52 0.52 0.52 0.52 0.52 0.52

0.0 0.5 1.0 1.5* 2.0 3.0 6.0

92.9 104.0 108.8 111.9 114.1 117.3 122.6

0.73 0.52 0.52 0.52 0.52 0.51 0.51

0.800 0.850 0.900 0.955* 0.980 0.990 0.999

126.6 124.0 120.1 111.9 102.7 92.8 58.4

0.51 0.51 0.51 0.52 0.51 0.50 7.13

0.01 0.10 0.50 1.00 2.00* 3.00 5.00 10.00

110.2 110.3 110.6 111.0 111.9 112.9 115.4 121.4

0.51 0.51 0.51 0.52 0.52 0.52 0.52 0.52

0.00 0.02 0.04 0.06 0.10* 0.15 0.30 0.50

125.6 122.8 120.1 117.3 111.9 105.2 85.6 61.2

0.57 0.56 0.55 0.54 0.52 0.49 0.32 0.15

Parameter SLSUR (ft/ft) 10,616 0.008 10,546 0.008 10,507 0.008 10,500 0.008 10,508 0.008 10,522 0.008 10,536 0.008 Parameter KVARY (1/in.) 6,265 0.004 8,484 0.006 9,724 0.007 10,500 0.008 11,054 0.009 11,854 0.010 13,184 0.012 Parameter AGWRC (unitless) 13,928 0.017 13,161 0.015 12,275 0.012 10,500 0.008 8,722 0.005 7,326 0.003 5,443 0.021 Parameter INFEXP (unitless) 10,739 0.007 10,716 0.007 10,622 0.007 10,542 0.007 10,500 0.008 10,995 0.008 13,728 0.008 18,672 0.010 Parameter DEEPFR (unitless) 11,478 0.011 11,280 0.011 11,083 0.010 10,888 0.010 10,500 0.008 10,023 0.006 8,644 0.003 7,011 0.002

Goodness of fit statistics PAE (%) PE (%) NSME 68.4 68.4 68.4 68.4 68.5 68.6 68.6

57.0 57.0 57.0 57.0 57.1 57.1 57.2

−0.251 −0.240 −0.230 −0.224 −0.218 −0.212 −0.206

53.0 60.1 64.9 68.4 71.0 74.6 80.8

32.0 46.5 52.9 57.0 60.0 64.1 70.8

0.328 0.088 −0.089 −0.224 −0.332 −0.496 −0.788

85.5 82.4 77.7 68.4 59.0 52.0 41.7

75.7 72.5 67.6 57.0 44.7 30.7 −22.4

−0.938 −0.792 −0.591 −0.224 0.069 0.248 0.444

67.0 67.1 67.5 67.8 68.4 69.2 71.5 78.9

55.0 55.1 55.7 56.1 57.0 58.1 60.9 69.6

−0.231 −0.232 −0.234 −0.232 −0.224 −0.214 −0.233 −0.749

84.9 81.4 77.9 74.7 68.4 61.4 48.4 41.1

76.3 72.4 68.5 64.7 57.0 47.5 19.8 −15.0

−0.544 −0.474 −0.408 −0.344 −0.224 −0.090 0.215 0.421

Model Calibration 55 Table 13. Goodness-of-fit results used to evaluate model sensitivity to adjustments in values of pervious land parameters used in the Salinas Valley Watershed Model (SVWM).—Continued [ft, foot; ft/ft, foot per foot; ft3/s, cubic foot per second; in., inch; in/hr, inch per hour; Max, maximum; Min, minimum; NSME, Nash-Sutcliffe model efficiency; PAE, percent-average estimation error; PE, percent estimation error; *, asterisks designate parameter value for typical mean baseline model; %, percent]

Parameter values

Mean

Simulated streamflow (ft3/s) Median Max Min

0.000 0.010 0.025* 0.050 0.100 0.150 0.200 0.300

113.7 112.9 111.9 110.5 108.3 106.5 105.0 102.4

0.56 0.54 0.52 0.50 0.47 0.46 0.44 0.38

0.000 0.010 0.025* 0.050 0.100 0.150 0.200 0.300

112.6 112.3 111.9 111.4 110.6 110.0 109.5 108.7

0.53 0.52 0.52 0.51 0.50 0.49 0.48 0.48

0.000 0.020 0.050 0.115* 0.200 0.300 0.400 0.800 1.500

132.7 116.8 115.0 111.9 108.8 105.8 103.1 95.4 86.1

0.65 0.55 0.54 0.52 0.50 0.48 0.46 0.35 0.25

0.01 0.10 0.25 0.55* 0.75 1.25 2.00 3.00 5.00

128.7 123.0 117.9 111.9 109.0 103.7 98.3 93.7 88.7

0.56 0.54 0.53 0.52 0.51 0.50 0.50 0.49 0.48

0.001 0.100 0.300 0.500*

112.4 112.4 112.3 112.1

0.52 0.52 0.52 0.52

Parameter BASETP (unitless) 10,467 0.012 10,481 0.009 10,500 0.008 10,530 0.006 10,581 0.005 10,626 0.004 10,666 0.004 10,732 0.004 Parameter AGWETP (unitless) 10,498 0.009 10,499 0.008 10,500 0.008 10,503 0.007 10,508 0.006 10,512 0.006 10,517 0.005 10,524 0.005 Parameter INTERCEP (in.) 10,747 0.024 10,569 0.011 10,546 0.010 10,500 0.008 10,440 0.006 10,376 0.005 10,313 0.005 10,106 0.004 9,753 0.003 Parameter UZSN (in.) 11,688 0.012 11,463 0.010 11,075 0.009 10,500 0.008 10,154 0.007 9,415 0.007 8,612 0.007 7,899 0.007 7,002 0.006 Parameter IRC (unitless) 15,248 0.008 13,721 0.008 12,301 0.008 11,115 0.008

Goodness of fit statistics PAE (%) PE (%) NSME 71.2 69.9 68.4 66.5 63.5 61.2 59.3 56.4

60.4 58.9 57.0 54.5 50.5 47.4 44.7 40.3

−0.224 −0.224 −0.224 −0.223 −0.219 −0.215 −0.209 −0.199

69.5 69.0 68.4 67.6 66.5 65.6 64.9 63.8

58.3 57.7 57.0 56.0 54.5 53.4 52.5 51.0

−0.226 −0.225 −0.224 −0.223 −0.221 −0.220 −0.219 −0.219

97.9 74.9 72.5 68.4 64.5 61.0 57.9 50.3 43.5

90.4 64.9 62.0 57.0 52.0 47.2 43.0 30.3 15.9

−0.561 −0.288 −0.263 −0.224 −0.185 −0.146 −0.111 0.000 0.115

88.2 80.6 74.7 68.4 65.7 61.2 58.2 56.8 56.3

79.7 71.5 64.8 57.0 53.4 46.9 40.4 35.1 29.6

−0.585 −0.483 −0.362 −0.224 −0.166 −0.073 −0.004 0.033 0.048

69.2 69.1 69.0 68.7

57.8 57.8 57.6 57.3

−0.177 −0.205 −0.229 −0.232

56 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley Table 13. Goodness-of-fit results used to evaluate model sensitivity to adjustments in values of pervious land parameters used in the Salinas Valley Watershed Model (SVWM).—Continued [ft, foot; ft/ft, foot per foot; ft3/s, cubic foot per second; in., inch; in/hr, inch per hour; Max, maximum; Min, minimum; NSME, Nash-Sutcliffe model efficiency; PAE, percent-average estimation error; PE, percent estimation error; *, asterisks designate parameter value for typical mean baseline model; %, percent]

Parameter values

Mean

0.600 0.700 0.800 0.990

111.9 111.5 110.7 102.1

0.01 0.10 0.20 0.25* 0.30 0.40 0.50 0.60

112.0 111.9 111.9 111.9 111.9 111.9 111.9 111.9

0.01 0.50 1.00 2.00* 3.00 4.00 6.00 10.00

111.7 111.6 111.7 111.9 112.0 112.0 112.0 112.1

0.000 0.100 0.200 0.300 0.450* 0.600 0.800 0.990

268.2 166.4 131.2 119.8 111.9 108.9 107.3 105.9

Simulated streamflow (ft3/s) Median Max Min

Goodness of fit statistics PAE (%) PE (%) NSME

Parameter IRC (unitless)—Continued 0.52 10,500 0.008 68.4 0.52 9,877 0.008 68.0 0.52 9,104 0.008 67.2 4.69 6,712 0.010 60.6 Parameter MANNING (unitless) 0.52 10,613 0.008 68.6 0.52 10,522 0.008 68.6 0.52 10,502 0.008 68.5 0.52 10,500 0.008 68.4 0.52 10,502 0.008 68.4 0.52 10,513 0.008 68.4 0.52 10,526 0.008 68.4 0.52 10,537 0.008 68.4 Parameter INTERFLW (unitless) 0.52 15,732 0.008 69.3 0.52 14,372 0.008 68.8 0.52 11,437 0.008 68.4 0.52 10,500 0.008 68.4 0.52 10,549 0.008 68.5 0.52 10,593 0.008 68.5 0.52 10,634 0.008 68.5 0.52 10,665 0.008 68.6 Parameter LZETPARM (unitless) 1.51 21,219 0.100 296.4 0.90 12,273 0.056 144.4 0.65 11,028 0.030 95.1 0.57 10,757 0.017 79.3 0.52 10,500 0.008 68.4 0.50 10,382 0.006 64.3 0.49 10,301 0.004 62.1 0.48 10,247 0.004 60.2

Model sensitivity was shown as being intermediate to variations in parameters affecting baseflow recession (KVARY and AGWRC), the parameter controlling deep recharge to groundwater not contributing to baseflow (DEEPFR), parameters defining the interception storage capacity of vegetation and the retention storage capacity of the land surface (INTERCEP and UZSN), and the parameter

57.0 56.5 55.7 46.1

−0.224 −0.205 −0.169 0.004

57.2 57.1 57.0 57.0 57.0 57.0 57.0 57.0

−0.188 −0.212 −0.221 −0.224 −0.227 −0.232 −0.235 −0.238

57.7 57.3 56.9 57.0 57.1 57.1 57.2 57.2

−0.251 −0.187 −0.182 −0.224 −0.241 −0.248 −0.255 −0.259

296.4 140.8 87.3 70.0 57.0 51.9 49.0 46.5

−5.625 −1.318 −0.570 −0.370 −0.224 −0.170 −0.138 −0.111

controlling interflow recession (IRC; table 13). Model sensitivity was shown to be low for parameters controlling the re-infiltration of overland flow (LSUR and SLSUR), the exponent in the infiltration equation (INFEXP), the parameters controlling groundwater ET (BASETP and AGWETP), the roughness coefficient for overland flow (MANNING), and the coefficient controlling interflow (INTERFLW).

Model Calibration 57

Calibrated Parameters Summarized parameter values calculated as the mean, median, maximum, minimum, and variance for the 690 calibrated HRU values for the SVWM are listed in tables 14 and 15. Summarized values for parameters with constant monthly values (values are the same for each month) are in table 14. Summarized values for parameters with monthly varying values are in table 15. The goodness-of-fit statistics for the set of parameters in tables 14 and 15 are provided in the “Calibration Results” section. Calibrated values for the length of the overland flow plane, LSUR, were defined by using the equation 0.7 minus HRU slope times 600 ft, resulting in a range of values from 49.2 to 418.5, with a mean value of 275.5 for the SVWM. The soil zone water storage capacity, LZSN, was the most critical parameter adjusted during calibration, with values for the calibrated model ranging from 0.10 to 29.74 in. and a mean value of 8.69 in. for the SVWM. The mean calibrated value for INFILT is 0.24, ranging from 0.0005 to 13.3, with high values of 0.81 and greater mostly in the lowlands of Salinas Valley and locations with loamy soils. The KVARY parameter, adjusted for each geologic rock type, includes a minimum value of 0.2 for metamorphic rock types and a maximum value of 2.0 for alluvium and conglomerate surficial geology. Calibrated values for AGWETP, used to empirically represent groundwater pumped for crop irrigation, include high values of 0.2 and greater in areas with substantial

cropland in the middle and lower Salinas Valley, and intermediate values between 0.06 and 0.2 are in the upper Salinas Valley for HRUs containing vineyards. Calibrated values for AGWRC, scaled according to surficial geologic rock type, range from low values of 0.91 for alluvium and 0.937 for conglomerate to high values of 0.97 for rock types in upland areas, including a maximum of 0.999 for igneous and metamorphic rock types. Calibrated values for INTERCEP are highest in May and lowest in October (table 15). Higher values for INTERCEP are assigned to HRUs having mostly forested land cover and a high percentage of canopy cover, compared to HRUs with mostly grassland. The INTERFLW parameter, controlling the rate of interflow to streams, is highest for February and March and lowest for July through October (table 15). The highest INTERFLW values for March and October are for HRUs in the UAS and along the western boundary of the LSR subbasins. Calibrated values for the IRC parameter have a similar spatial pattern as INTERFLW, with high values in the Arroyo Seco subdrainage (table 15). Calibrated values for LZETPARM are highest in April and lowest in October (table 15). Calibrated values for MANNING are highest in March and April and lowest in September and October (table 15). Calibrated values for UZSN include maximum values for March through May and a minimum mean for the SVWM for October (table 15). Locations with the highest values in April include the lower Salinas Valley floor and the SRH.

Table 14. Summary statistics for parameters with constant monthly values calibrated to 690 hydrologic response units (HRUs) used in the Salinas Valley Watershed Model (SVWM). [See table 9 for all Hydrologic Simulation Program—Fortran (HSPF) parameter descriptions. Abbreviations: ft, foot; ft/ft, foot per foot; in., inch; in/hr, inch per hour]

Parameter group

Parameter name

Units

Mean

Median

Maximum

Minimum

Variance

PWAT-PARM2

LZSN

in,

8.69

7.57

29.74

0.10

38.85

PWAT-PARM2

INFILT

in/hr

PWAT-PARM2

LSUR

0.240 265.8

0.071 275.5

13.292

0.0005

418.5

49.2

0.562 6,089.0

PWAT-PARM2

SLSUR

ft/ft

0.257

0.241

0.618

0.003

0.017

PWAT-PARM2

KVARY

1/in.

1.266

1.235

2.175

0.200

0.270

PWAT-PARM2

AGWRC

1/day

0.959

0.962

0.999

0.910

0.001

PWAT-PARM3

INFEXP

unitless

2.58

2.41

6.18

0.03

1.70

PWAT-PARM3

INFILD

unitless

2.0

0.0

PWAT-PARM3

DEEPFR

unitless

0.072

0.040

0.300

0.000

0.007

PWAT-PARM3

BASETP

unitless

0.05

0.0

PWAT-PARM3

AGWETP

unitless

0.070

0.008

0.842

0.000

0.022

58 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley Table 15. Summary statistics for monthly varying parameters calibrated to 690 hydrologic response units (HRUs) used in the Salinas Valley Watershed Model (SVWM). [See table 9 for all Hydrologic Simulation Program—Fortran (HSPF) parameter descriptions. Abbreviation: in., inch]

Month

Mean

Median

January

0.043

0.037

February

0.050

March

0.056

April May

Maximum

Minimum

Variance

0.127

0.005

0.00079

0.042

0.140

0.006

0.00096

0.048

0.153

0.007

0.00115

0.062

0.054

0.166

0.008

0.00135

0.069

0.059

0.179

0.009

0.00158

June

0.056

0.048

0.153

0.007

0.00115

July

0.050

0.042

0.140

0.006

0.00096

August

0.037

0.031

0.113

0.003

0.00064

September

0.031

0.025

0.100

0.002

0.00051

October

0.028

0.022

0.094

0.002

0.00045

November

0.031

0.025

0.100

0.002

0.00051

December

0.037

0.031

0.113

0.003

0.00064

Mean

0.046

0.039

0.131

0.005

0.001

Maximum

0.069

0.059

0.179

0.009

0.002

Minimum

0.028

0.022

0.094

0.002

0.000

January

1.285

1.205

3.090

0.013

0.4228

February

1.542

1.446

3.708

0.015

0.6089

March

1.542

1.446

3.708

0.015

0.6089

April

1.285

1.205

3.090

0.013

0.4228

May

1.028

0.964

2.472

0.010

0.2706

June

0.771

0.723

1.854

0.008

0.1522

July

0.514

0.482

1.236

0.005

0.0677

August

0.514

0.482

1.236

0.005

0.0677

September

0.514

0.482

1.236

0.005

0.0677

October

0.514

0.482

1.236

0.005

0.0677

November

0.771

0.723

1.854

0.008

0.1522

December

1.028

0.964

2.472

0.010

0.2706

Mean

0.942

0.883

2.266

0.009

0.265

Maximum

1.542

1.446

3.708

0.015

0.609

Minimum

0.514

0.482

1.236

0.005

0.068

January

0.360

0.337

0.865

0.0035

0.0332

February

0.385

0.361

0.927

0.0038

0.0381

March

0.411

0.385

0.989

0.0040

0.0433

April

0.411

0.385

0.989

0.0040

0.0433

May

0.385

0.361

0.927

0.0038

0.0381

June

0.308

0.289

0.742

0.0030

0.0244

July

0.257

0.241

0.618

0.0025

0.0169

August

0.206

0.193

0.494

0.0020

0.0108

September

0.206

0.193

0.494

0.0020

0.0108

Monthly-INTERCEP (in.)

Monthly-INTERFLW (unitless)

Monthly-IRC (1/day)

Model Calibration 59 Table 15. Summary statistics for monthly varying parameters calibrated to 690 hydrologic response units (HRUs) used in the Salinas Valley Watershed Model (SVWM).—Continued [See table 9 for all Hydrologic Simulation Program—Fortran (HSPF) parameter descriptions. Abbreviation: in., inch]

Month

Mean

Median

Maximum

Minimum

Variance

Monthly-IRC (1/day)—Continued October

0.206

0.193

0.494

0.0020

0.0108

November

0.257

0.241

0.618

0.0025

0.0169

December

0.308

0.289

0.742

0.0030

0.0244

Mean

0.308

0.289

0.742

0.003

0.026

Maximum

0.411

0.385

0.989

0.004

0.043

Minimum

0.206

0.193

0.494

0.002

0.011

Monthly-LZETPARM (unitless) January

0.578

0.563

0.808

0.3172

0.0087

February

0.651

0.648

0.869

0.3175

0.0099

March

0.712

0.706

0.967

0.4026

0.0144

April

0.719

0.706

0.997

0.4008

0.0155

May

0.701

0.702

0.994

0.3622

0.0181

June

0.692

0.693

0.964

0.3622

0.0166

July

0.575

0.569

0.845

0.3001

0.0132

August

0.449

0.441

0.669

0.2209

0.0097

September

0.437

0.425

0.668

0.2182

0.0105

October

0.432

0.421

0.668

0.2182

0.0116

November

0.469

0.464

0.671

0.2323

0.0084

December

0.554

0.557

0.770

0.2330

0.0110

Mean

0.581

0.575

0.824

0.299

0.012

Maximum

0.719

0.706

0.997

0.403

0.018

Minimum

0.432

0.421

0.668

0.218

0.008

January

0.302

0.318

0.470

0.0769

0.0044

February

0.302

0.318

0.470

0.0769

0.0044

March

0.366

0.385

0.575

0.0837

0.0068

April

0.366

0.385

0.575

0.0837

0.0068

May

0.302

0.318

0.470

0.0769

0.0044

June

0.239

0.251

0.365

0.0702

0.0024

July

0.208

0.217

0.313

0.0668

0.0017

August

0.208

0.217

0.313

0.0668

0.0017

September

0.176

0.184

0.260

0.0635

0.0011

October

0.176

0.184

0.260

0.0635

0.0011

November

0.239

0.251

0.365

0.0702

0.0024

December

0.271

0.284

0.418

0.0736

0.0033

Mean

0.263

0.276

0.404

0.073

0.003

Maximum

0.366

0.385

0.575

0.084

0.007

Minimum

0.176

0.184

0.260

0.063

0.001

January

0.771

0.777

1.553

0.1619

0.0440

February

0.981

1.013

1.956

0.1811

0.0748

Monthly-MANNING (unitless)

Monthly-UZSN (in.)

60 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley Table 15. Summary statistics for monthly varying parameters calibrated to 690 hydrologic response units (HRUs) used in the Salinas Valley Watershed Model (SVWM).—Continued [See table 9 for all Hydrologic Simulation Program—Fortran (HSPF) parameter descriptions. Abbreviation: in., inch]

Month

Mean

Median

Maximum

Minimum

Variance

Monthly-UZSN (in.)—Continued March

1.081

1.120

2.025

0.1936

0.0918

April

1.081

1.120

2.025

0.1936

0.0918

May

1.081

1.120

2.025

0.1936

0.0918

June

0.834

1.627

0.1918

0.0600

July

0.621

0.573

1.569

0.1901

0.0551

August

0.595

0.562

1.237

0.1776

0.0379

September

0.563

0.533

1.194

0.1299

0.0312

October

0.406

0.375

1.146

0.1014

0.0362

November

0.548

0.517

1.186

0.1224

0.0328

December

0.701

0.703

1.481

0.1316

0.0363

Mean

0.772

0.771

1.585

0.164

0.057

Maximum

1.081

1.120

2.025

0.194

0.092

Minimum

0.406

0.375

1.146

0.101

0.031

Calibration Results The overall goodness-of-fit statistics for the calibrated model included weighted mean PAE and NSME values of 2.0 and 0.64 percent for daily mean streamflow, 2.0 and 0.84 percent for monthly streamflow, and 2.1 and 0.88 percent for annual streamflow, respectively (table 16). The results were considered satisfactory for daily streamflow, very good for monthly streamflow, and good for annual streamflow. The NSME statistic for daily streamflow ranged from 0.09 to 0.85 (table 16). Sixteen of the USGS streamgages and both reservoir inflows had very good NSME results of 0.7 or higher for daily streamflow, with 10 of the 29 locations having NSME values of 0.75 or higher, including the two reservoir inflows. Comparisons of simulated and observed daily streamflow for 15 selected streamgages using the calibrated model are provided on figure 23. The model performed well in terms of matching the timing and magnitude of most storms. The simulated daily streamflow at USGS streamgages 11150500 (gage 19) and 11152500 (gage 21) were well matched to the large runoff event during water year 1951, resulting in more than 20,000 ft3/s daily mean streamflow for the Salinas River (figs. 23N, 24O). The 8-year calibration period used for USGS streamgages 11150500 (gage 19) and 11152500 (gage 21), water years 1949–1957, was before the completion of Lake Nacimiento and was considered representative of natural streamflow conditions in the main branch of the Salinas River. As expected, calibration results for monthly streamflow showed improved NSME results compared to daily streamflow for all locations, with values ranging from 0.47 to 0.97 (table 16). There were 26 locations that had very good

NSME values of 0.80 or higher for monthly streamflow, and 6 locations that had excellent values of 0.9 or higher. The monthly hydrographs indicate a good visual fit to wetter-than-average periods at most streamgages during water years 1969, 1978, and 1998 (fig. 24). Considering the bias of the calibration statistics to high flows, visual comparison of log-transformed streamflow was used qualitatively as part of calibration (figs. 24A, C, E, J). The calibration results for annual streamflow were similar to results for monthly streamflow, with NSME ranging from 0.47 to 0.99 (table 16). Only 4 locations had NSME results less than 0.8 for annual streamflow, and 13 locations had results of 0.9 or higher. The annual (water year) hydrographs at 10 selected streamgages indicate a good to very good fit between simulated and observed flows at most streamgages for the wetter-than-average years of 1969 and 1998, and a fair to good fit for water year 1983 (fig. 25). Results also indicated a good general match to drier-than-average periods, such as water years 1988–1990, at most streamgages. The combined calibration result, based on the comparison of simulated versus observed long-term (period of record) mean streamflow and maximum daily, monthly, and annual streamflow for all 31 streamflow records (29 USGS streamgages and 2 reservoir inflows), indicated a good calibration in terms of low overall estimation bias and R2 values of 0.798–0.999 (figs. 26A, B, C, D). Mean monthly streamflow as a percentage of total annual streamflow, calculated as the average result for all 31 records, indicated a satisfactory comparison of the seasonal distribution of simulated versus observed streamflow (figs. 26E, F), with February having the highest mean monthly percentage of annual streamflow and August having the lowest mean monthly percentage of annual streamflow.

Table 16. Salinas Valley Watershed Model calibration results showing goodness-of-fit statistics using daily, monthly, and annual streamflow. [ft3/s, cubic foot per second; ID, identification; Max, maximum; Min, minimum; NSME, Nash-Sutcliffe model efficiency; USGS, U.S. Geological Survey; —, not applicable; %, percent]

USGS streamgage or reservoir name

USGS streamgage ID or reservoir number

Observed daily streamflow Mean (ft3/s)

Median (ft3/s)

Max (ft3/s)

Simulated daily streamflow

Min (ft3/s)

Mean (ft3/s)

Median (ft3/s)

Max (ft3/s)

Calibration statistics

Min (ft3/s)

% error

NSME

Daily streamflow GABILAN C NR SALINAS

11152600

4.4

0.00

646

0.00

4.3

1.11

727

0.00

−3.0

0.71

RECLAMATION DITCH NR SALINAS

11152650

18.4

6.70

524

0.00

18.7

2.77

1,013

0.07

1.8

0.51

ARROYO DEL REY A DEL REY OAKS

11143300

0.7

0.09

0.00

0.7

0.00

–2.8

0.28

ARROYO SECO NR GREENFIELD

11151870

172.0

31.00

9,520

0.00

169.0

29.43

10,698

0.08

–1.7

0.75

ARROYO SECO NR SOLEDAD

11152000

161.3

27.00

16,500

0.00

157.9

25.14

11,810

0.00

–2.1

0.73

ARROYO SECO BL RELIZ C NR SOLEDAD

11152050

111.8

0.00

17,000

0.00

112.9

0.00

11,517

0.00

1.0

0.73

SAN LORENZO C BL BITTERWATER C

11151300

14.1

1.23

5,860

0.00

13.8

0.72

4,365

0.00

–1.9

0.63

SALINAS R NR POZO

11143500

20.3

1.40

7,150

0.00

20.7

0.00

4,567

0.00

2.0

0.70

TORO C NR POZO

11144000

1.0

0.31

321

0.00

1.1

0.24

309

0.00

3.7

0.18

SALSIPUEDES C NR POZO

11144200

2.6

0.01

296

0.00

2.6

0.00

386

0.00

–0.8

0.38

SALINAS R BL SALINAS DAM

11144600

30.6

1.80

5,310

0.00

30.0

0.34

3,960

0.00

–1.9

0.77

SALINAS R AB PILITAS Ck

11145000

17.8

0.10

10,200

0.00

17.5

0.00

6,840

0.00

–1.6

0.79

JACK C NR TEMPLETON

11147000

14.2

0.40

2,780

0.00

14.7

0.00

2,247

0.00

3.4

0.75

SANTA RITA C TRIB

11147040

3.4

0.00

496

0.00

3.5

0.00

411

0.00

1.9

0.85

SANTA RITA C NR TEMPLETON

11147070

13.2

0.22

2,190

0.00

13.2

0.00

2,047

0.00

–0.2

0.73

SALINAS R A PASO ROBLES

11147500

92.3

0.00

19,600

0.00

92.6

0.02

19,528

0.00

0.3

0.83

CHOLAME C TRIB

11147700

0.1

0.00

0.1

0.00

0.4

0.09

CHOLAME C NR SHANDON

11147800

5.8

0.00

3,320

0.00

5.6

0.00

2,222

0.00

–3.2

0.27

ESTRELLA R NR ESTRELLA

11148500

25.1

0.00

18,500

0.00

24.9

0.00

9,989

0.00

–0.9

0.44

NACIMIENTO R NR BRYSON

11148800

169.4

10.00

24,600

0.00

162.7

2.46

20,918

0.00

–3.9

0.84

11148900

168.5

6.20

24,400

0.00

173.2

2.92

15,601

0.00

2.8

0.68

281.6

13.55

49,053

0.00

284.7

2.71

37,741

0.00

1.1

0.75

SAN ANTONIO R A SAM JNS BR

11149700

58.1

5.90

7,480

0.00

59.0

0.26

8,710

0.00

1.6

0.71

SAN ANTONIO R NR LOCKWOOD

11149900

99.5

3.60

14,000

0.00

98.2

0.37

13,752

0.00

–1.3

0.73

SAN ANTONIO R A PLEYTO

11150000

65.6

0.80

10,900

0.00

64.7

0.05

7,941

0.00

–1.3

0.70

San Antonio Reservoir inflow

107.2

5.41

15,040

0.00

96.4

0.07

16,757

0.00

–10.1

0.75

HUERHUERO C NR CRESTON

11147600

5.9

0.00

5,000

0.00

6.0

0.00

2,745

0.00

2.6

0.54

NACIMIENTO R NR SAN MIGUEL

11149500

218.0

2.20

35,000

0.00

213.8

1.31

17,160

0.00

–1.9

0.68

Model Calibration 61

NACIMIENTO R BL SAPAQUE C Nacimiento Reservoir inflow

[ft3/s, cubic foot per second; ID, identification; Max, maximum; Min, minimum; NSME, Nash-Sutcliffe model efficiency; USGS, U.S. Geological Survey; —, not applicable; %, percent]

USGS streamgage or reservoir name

USGS streamgage ID or reservoir number

Observed daily streamflow Mean (ft3/s)

Median (ft3/s)

Max (ft3/s)

Simulated daily streamflow

Min (ft3/s)

Mean (ft3/s)

Median (ft3/s)

Max (ft3/s)

Calibration statistics

Min (ft3/s)

% error

NSME

Daily streamflow—Continued SALINAS R NR BRADLEY

11150500

363.1

15.00

22,000

0.00

357.9

1.05

27,782

0.00

–1.4

0.76

SALINAS R NR SPRECKELS

11152500

235.6

2.00

25,500

0.30

235.9

5.06

39,373

0.06

0.1

0.32

EL TORO C NR SPRECKELS

11152540

2.2

0.10

390

0.00

2.3

0.00

349

0.00

2.9

0.50

Summary of daily streamflow for all gages Mean

80.1

4.4

11,408

0.0

79.3

2.5

9,726

0.0

–0.5

0.62

Maximum

363.1

31.0

49,053

0.3

357.9

29.4

39,373

0.1

3.7

0.85

Minimum

0.1

0.0

0.1

0.0

–10.1

0.09

Weighted mean

—

2.1

0.64

—

Monthly streamflow GABILAN C NR SALINAS

11152600

4.5

0.0

239

0.0

4.3

1.1

218

0.0

–3.1

0.88

RECLAMATION DITCH NR SALINAS

11152650

18.5

7.9

337

1.7

18.8

4.9

307

0.1

2.0

0.81

ARROYO DEL REY A DEL REY OAKS

11143300

0.7

0.1

0.0

0.7

0.1

0.0

–3.4

0.64

ARROYO SECO NR GREENFIELD

11151870

174.2

42.4

2,044

0.0

171.1

34.2

2,384

0.4

–1.8

0.86

ARROYO SECO NR SOLEDAD

11152000

161.1

36.2

2,697

0.0

157.6

27.0

3,535

0.0

–2.2

0.87

ARROYO SECO BL RELIZ C NR SOLEDAD

11152050

113.1

0.0

2,806

0.0

114.1

0.0

3,486

0.0

0.9

0.87

SAN LORENZO C BL BITTERWATER C

11151300

14.3

1.6

583

0.0

14.0

0.8

502

0.0

–1.9

0.86

SALINAS R NR POZO

11143500

20.6

1.6

710

0.0

21.1

0.0

648

0.0

2.2

0.91

TORO C NR POZO

11144000

1.1

0.4

0.0

1.1

0.3

0.0

3.9

0.47

SALSIPUEDES C NR POZO

11144200

2.7

0.0

2.6

0.0

–1.0

0.89

SALINAS R BL SALINAS DAM

11144600

30.9

2.6

642

0.0

30.2

0.4

548

0.0

–2.2

0.83

SALINAS R AB PILITAS Ck

11145000

18.0

0.3

961

0.0

17.8

0.0

851

0.0

–0.9

0.84

JACK C NR TEMPLETON

11147000

14.4

0.7

303

0.0

14.9

0.0

334

0.0

3.7

0.90

SANTA RITA C TRIB

11147040

3.4

0.0

3.5

0.0

2.1

0.97

SANTA RITA C NR TEMPLETON

11147070

13.4

0.7

227

0.0

13.4

0.0

301

0.0

–0.1

0.84

SALINAS R A PASO ROBLES

11147500

93.6

0.0

2,884

0.0

93.8

1.5

3,627

0.0

0.3

0.87

CHOLAME C TRIB

11147700

0.1

0.0

0.1

0.0

3.0

0.62

CHOLAME C NR SHANDON

11147800

6.0

0.0

379

0.0

5.8

0.0

425

0.0

–2.8

0.95

ESTRELLA R NR ESTRELLA

11148500

25.7

0.0

1,672

0.0

25.4

0.0

1,544

0.0

–1.3

0.83

62 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Table 16. Salinas Valley Watershed Model calibration results showing goodness-of-fit statistics using daily, monthly, and annual streamflow.—Continued

Table 16. Salinas Valley Watershed Model calibration results showing goodness-of-fit statistics using daily, monthly, and annual streamflow.—Continued [ft3/s, cubic foot per second; ID, identification; Max, maximum; Min, minimum; NSME, Nash-Sutcliffe model efficiency; USGS, U.S. Geological Survey; —, not applicable; %, percent]

USGS streamgage or reservoir name

USGS streamgage ID or reservoir number

Observed daily streamflow Mean (ft3/s)

Median (ft3/s)

Max (ft3/s)

Simulated daily streamflow

Min (ft3/s)

Mean (ft3/s)

Median (ft3/s)

Max (ft3/s)

Calibration statistics

Min (ft3/s)

% error

NSME

Monthly streamflow—Continued NACIMIENTO R NR BRYSON

11148800

164.8

18.8

2,858

0.0

164.8

7.9

2,418

0.0

–4.0

0.90

NACIMIENTO R BL SAPAQUE C

11148900

174.9

11.1

3,545

0.0

174.9

12.3

2,796

0.0

2.4

0.84

Nacimiento Reservoir inflow

—

287.8

24.8

5,831

0.0

287.8

13.3

5,063

0.0

0.8

0.87

SAN ANTONIO R A SAM JNS BR

11149700

60.8

9.9

748

0.0

60.8

0.4

1,069

0.0

1.9

0.76

SAN ANTONIO R NR LOCKWOOD

11149900

99.4

7.0

2,351

1.7

99.4

0.9

2,675

0.0

–1.4

0.84

SAN ANTONIO R A PLEYTO

11150000

65.6

3.3

1,212

0.0

65.6

0.2

1,289

0.0

–1.4

0.83

San Antonio Reservoir inflow

—

97.7

10.7

2,494

0.0

97.7

0.2

2,980

0.0

–10.0

0.85

HUERHUERO C NR CRESTON

11147600

6.2

0.0

581

0.0

6.2

0.0

471

0.0

1.7

0.84

NACIMIENTO R NR SAN MIGUEL

11149500

213.8

16.2

3,248

0.0

213.8

17.5

2,663

0.0

–1.6

0.87

SALINAS R NR BRADLEY

11150500

356.7

39.8

5,372

0.0

356.7

21.5

6,101

0.0

–1.6

0.91

SALINAS R NR SPRECKELS

11152500

234.2

2.5

5,610

0.7

234.2

8.7

5,506

0.2

0.0

0.80

EL TORO C NR SPRECKELS

11152540

2.3

0.1

0.0

2.3

0.1

151

0.0

3.4

0.73

Summary of monthly streamflow for all gages Mean

80.0

7.7

1,632

0.13

79.8

4.9

1,679

0.02

–0.4

0.83

Maximum

356.7

42.4

5,831

1.70

356.7

34.2

6,101

0.39

3.9

0.97

Minimum

0.1

0.0

0.00

0.1

0.0

0.00

–10.0

0.47

Weighted mean

—

2.1

0.84

—

Annual streamflow GABILAN C NR SALINAS

11152600

4.4

0.8

0.0

4.3

2.2

0.2

–3.1

0.87

RECLAMATION DITCH NR SALINAS

11152650

17.4

11.7

2.7

17.8

11.7

3.1

2.4

0.92

11143300

0.7

0.3

0.1

0.7

0.5

0.2

–2.8

0.75

11151870

172.0

130.9

564

7.6

169.1

134.9

479

17.0

–1.7

0.83

ARROYO SECO NR SOLEDAD

11152000

161.3

107.9

709

7.0

157.9

118.3

517

12.4

–2.1

0.84

ARROYO SECO BL RELIZ C NR SOLEDAD

11152050

111.8

52.2

354

1.3

112.9

81.5

440

0.0

1.0

0.85

SAN LORENZO C BL BITTERWATER C

11151300

14.0

6.6

0.0

13.8

4.1

0.0

–1.9

0.87

SALINAS R NR POZO

11143500

18.9

7.2

122

0.7

18.4

5.5

116

0.1

–2.4

0.94

TORO C NR POZO

11144000

1.0

0.5

0.1

1.1

0.6

0.0

3.9

0.47

SALSIPUEDES C NR POZO

11144200

2.6

1.2

0.0

2.6

1.1

0.0

–0.8

0.95

Model Calibration 63

ARROYO DEL REY A DEL REY OAKS ARROYO SECO NR GREENFIELD

[ft3/s, cubic foot per second; ID, identification; Max, maximum; Min, minimum; NSME, Nash-Sutcliffe model efficiency; USGS, U.S. Geological Survey; —, not applicable; %, percent]

USGS streamgage or reservoir name

USGS streamgage ID or reservoir number

Observed daily streamflow Mean (ft3/s)

Median (ft3/s)

Max (ft3/s)

Simulated daily streamflow

Min (ft3/s)

Mean (ft3/s)

Median (ft3/s)

Max (ft3/s)

Calibration statistics

Min (ft3/s)

% error

NSME

Annual streamflow—Continued SALINAS R BL SALINAS DAM

11144600

29.5

9.6

127

1.0

28.9

8.9

125

0.1

–1.9

0.94

SALINAS R AB PILITAS Ck

11145000

17.8

2.9

173

0.0

17.5

1.9

153

0.0

–1.6

0.89

JACK C NR TEMPLETON

11147000

14.2

8.0

0.1

14.7

8.9

0.6

3.4

0.86

SANTA RITA C TRIB

11147040

2.9

2.0

0.0

3.0

1.6

0.0

1.9

0.97

SANTA RITA C NR TEMPLETON

11147070

13.2

8.8

0.2

13.2

8.1

0.1

–0.2

0.86

SALINAS R A PASO ROBLES

11147500

92.3

40.3

526

0.0

92.6

38.1

454

1.2

0.3

0.91

CHOLAME C TRIB

11147700

0.1

0.0

0.1

0.0

0.5

0.77

CHOLAME C NR SHANDON

11147800

5.8

0.4

0.0

5.6

0.1

0.0

–3.2

0.98

ESTRELLA R NR ESTRELLA

11148500

25.1

3.5

256

0.0

24.9

1.9

235

0.1

–0.9

0.84

NACIMIENTO R NR BRYSON

11148800

169.4

128.5

479

26.2

162.7

142.6

415

41.7

–3.9

0.90

NACIMIENTO R BL SAPAQUE C

11148900

168.7

110.6

623

5.7

173.4

135.2

443

32.7

2.8

0.87

Nacimiento Reservoir inflow

—

281.7

206.5

987

12.1

284.8

226.9

776

51.6

1.1

0.92

SAN ANTONIO R A SAM JNS BR

11149700

49.1

41.2

121

1.4

49.9

55.9

129

0.0

1.6

0.66

SAN ANTONIO R NR LOCKWOOD

11149900

99.6

60.6

455

0.0

98.2

64.4

335

2.3

–1.3

0.90

SAN ANTONIO R A PLEYTO

11150000

65.6

42.9

239

5.2

64.7

41.3

216

5.5

–1.3

0.90

San Antonio Reservoir inflow

—

107.2

68.8

444

0.0

96.4

51.3

347

1.0

–10.1

0.89

HUERHUERO C NR CRESTON

11147600

5.9

0.1

0.0

6.0

0.0

2.6

0.97

NACIMIENTO R NR SAN MIGUEL

11149500

217.8

149.6

553

66.3

213.7

180.6

453

108.3

–1.9

0.85

SALINAS R NR BRADLEY

11150500

362.8

232.9

945

151.9

357.7

243.0

919

160.3

–1.4

0.99

SALINAS R NR SPRECKELS

11152500

235.3

84.1

921

2.7

235.6

103.8

747

56.7

0.1

0.93

EL TORO C NR SPRECKELS

11152540

2.2

1.0

0.0

2.3

0.5

0.1

2.9

0.89

–0.6

0.87

Summary of annual streamflow for all gages Mean

79.7

49.1

292

78.8

54.0

251

Maximum

362.8

232.9

987

152

357.7

243.0

919

160

3.9

0.99

Minimum

0.1

0.0

0.1

0.0

–10.1

0.47

Weighted mean

—

2.1

0.88

—

64 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Table 16. Salinas Valley Watershed Model calibration results showing goodness-of-fit statistics using daily, monthly, and annual streamflow.—Continued

Model Calibration 65

Daily streamflow, in cubic feet per second

400 350

EXPLANATION

300

Observed streamflow

200 150 100 50

Daily streamflow, in cubic feet per second

500

1950

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

B. Gage 11 (Santa Rita Creek tributary) EXPLANATION

400

Observed streamflow Simulated streamflow

300 200 100 0

800

Daily streamflow, in cubic feet per second

Simulated streamflow

250

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Water year

C. Gage 39 (Galiban Creek near Salinas)

700

EXPLANATION

600

Observed streamflow Simulated streamflow

500 400 300 200 100 0

3,000

Daily streamflow, in cubic feet per second

A. Gage 4 (Salsipuedes Creek near Pozo)

1950

1955

1960

1965

Water year

D. Gage 32 (Jack Creek near Templeton)

2,500

EXPLANATION

2,000

Simulated streamflow

Observed streamflow

1,500 1,000 500 0

1950

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

Figure 23. Comparison of observed (light blue) versus simulated (dashed dark blue) daily streamflow at selected U.S. Geological Survey streamgages: A, gage 4; B, gage 11; C, gage 39; D, gage 32; E, gage 20; F, gage 14; G, gage 15; H, gage 31; I, gage 24; J, gage 13; K, gage 18; L, gage 1; M, gage 22; N, gage 19; and O, gage 21 (U.S. Geological Survey, 2016; Hevesi and others, 2025).

66 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Daily streamflow, in cubic feet per second

2,500

E. Gage 20 (Santa Rita Creek near Templeton) EXPLANATION

2,000

Observed streamflow Simulated streamflow

1,500 1,000 500 0

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Water year

Daily streamflow, in cubic feet per second

6,000

F. Gage 14 (Salinas River below Salinas Dam)

5,000

EXPLANATION Observed streamflow

4,000

Simulated streamflow

3,000 2,000 1,000 0

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

1990

1995

2000

2005

2010

2015

Water year

Daily streamflow, in cubic feet per second

6,000

G. Gage 15 (San Lorenzo Creek) EXPLANATION

5,000

Observed streamflow 4,000

Simulated streamflow

3,000 2,000 1,000 0

1950

1955

1960

1965

1970

1975

1980

1985

Water year

Daily streamflow, in cubic feet per second

8,000

H. Gage 31 (Salinas River near Pozo)

7,000

EXPLANATION

6,000

Observed streamflow

5,000

Simulated streamflow

4,000 3,000 2,000 1,000 0

1950

Figure 23.—Continued

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

Model Calibration 67

Daily streamflow, in cubic feet per second

12,000

Observed streamflow 8,000

Daily streamflow, in cubic feet per second

Simulated streamflow

6,000 4,000 2,000

12,000

1950

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

J. Gage 13 (Arroyo Seco near Greenfield)

10,000

EXPLANATION Observed streamflow

8,000

Simulated streamflow

6,000 4,000 2,000 0

12,000

Daily streamflow, in cubic feet per second

EXPLANATION

10,000

1950

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

K. Gage 18 (San Antonio River at Pleyto) EXPLANATION

10,000

Observed streamflow 8,000

Simulated streamflow

6,000 4,000 2,000 0

18,000

Daily streamflow, in cubic feet per second

I. Gage 24 (Salinas River above Pilitas Creek)

1950

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

L. Gage 1 (Arroyo Seco near Soledad)

16,000

EXPLANATION

14,000

Observed streamflow

12,000

Simulated streamflow

10,000 8,000 6,000 4,000 2,000 0

1950

Figure 23.—Continued

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

68 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Daily streamflow, in cubic feet per second

25,000

Observed streamflow Simulated streamflow

15,000 10,000 5,000

30,000

Daily streamflow, in cubic feet per second

EXPLANATION

20,000

1950

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

N. Gage 19 (Salinas River near Bradley)

25,000

EXPLANATION Observed streamflow

20,000

Simulated streamflow

15,000 10,000 5,000 0

40,000

Daily streamflow, in cubic feet per second

M. Gage 22 (Nacimiento River near Bryson)

1950

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

O. Gage 21 (Salinas River near Spreckels)

35,000

EXPLANATION

30,000

Observed streamflow Simulated streamflow

25,000 20,000 15,000 10,000 5,000 0

1950

Figure 23.—Continued

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

Model Calibration 69

Monthly streamflow, in cubic feet per second

100

Observed streamflow Simulated streamflow

1 0.1 0.01

350

Monthly streamflow, in cubic feet per second

EXPLANATION

0.001

1950

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2010

2015

EXPLANATION Observed streamflow

250

Simulated streamflow

200 150 100 50

1,000

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Water year

C. Gage 15 (San Lorenzo Creek) EXPLANATION

100

Observed streamflow Simulated streamflow

10 1 0.1 0.01

1,800

1950

1955

1960

Water year

D. Gage 36 (Estrella River)

1,600

Monthly streamflow, in cubic feet per second

2005

B. Gage 32 (Jack Creek near Templeton)

300

Monthly streamflow, in cubic feet per second

A. Gage 4 (Salsipuedes Creek near Pozo)

EXPLANATION

1,400

Observed streamflow

1,200

Simulated streamflow

1,000 800 600 400 200 0

1950

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

Figure 24. Comparison of observed (light blue) versus simulated (dashed dark blue) monthly streamflow at selected U.S. Geological Survey streamgages: A, gage 4; B, gage 32; C, gage 15; D, gage 36; E, gage 13; F, gage 3; G, gage 25; H, gage 1; I, gage 19; and J, gage 21.

70 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Monthly streamflow, in cubic feet per second

10,000

Monthly streamflow, in cubic feet per second

Observed streamflow Simulated streamflow

100 10 1

3,000

1950

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

F. Gage 3 (San Antonio River near Lockwood) EXPLANATION

2,500

Observed streamflow 2,000

Simulated streamflow

1,500 1,000 500 0

4,000

Monthly streamflow, in cubic feet per second

EXPLANATION

1,000

0.1

1950

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

G. Gage 25 (Nacimiento River below Sapaque Creek)

3,500

EXPLANATION

3,000

Observed streamflow Simulated streamflow

2,500 2,000 1,500 1,000 500 0

4,000

Monthly streamflow, in cubic feet per second

E. Gage 13 (Arroyo Seco near Greenfield)

1950

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

H. Gage 1 (Arroyo Seco near Soledad)

3,500

EXPLANATION

3,000

Observed streamflow Simulated streamflow

2,500 2,000 1,500 1,000 500 0

1950

Figure 24.—Continued

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

Monthly streamflow, in cubic feet per second

Model Calibration 71

6,500 6,000 5,500 5,000 4,500 4,000 3,500 3,000 2,500 2,000 1,500 1,000 500 0

Monthly streamflow, in cubic feet per second

10,000

I. Gage 19 (Salinas River near Bradley) EXPLANATION Observed streamflow Simulated streamflow

1950

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

J. Gage 21 (Salinas River near Spreckles) EXPLANATION

1,000

Observed streamflow Simulated streamflow

100 10 1 0.1

1950

Figure 24.—Continued

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

72 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Annual streamflow, in cubic feet per second

60 50

EXPLANATION

Simulated streamflow

Observed streamflow

30 20 10 0

800

Monthly streamflow, in cubic feet per second

A. Gage 20 (Santa Rita Creek near Templeton)

1950

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2010

2015

B. Gage 1 (Arroyo Seco near Soledad) EXPLANATION

600

Observed streamflow

500

Simulated streamflow

400 300 200 100

450

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Water year

C. Gage 2 (Arroyo Seco below Reliz Creek)

400

EXPLANATION

350

Observed streamflow

300

Simulated streamflow

250 200 150 100 50 0

450

1950

1955

1960

1965

Water year

D. Lake San Antonio inflows

400

Monthly streamflow, in cubic feet per second

2005

700

Monthly streamflow, in cubic feet per second

1955

EXPLANATION

350

Observed streamflow

300

Simulated streamflow

250 200 150 100 50 0

1950

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

Figure 25. Comparison of observed (light blue) versus simulated (dashed dark blue) annual and monthly streamflow at selected U.S. Geological Survey streamgages: A, gage 20; B, gage 1; C, gage 2; D, Lake San Antonio inflows; E, Lake Nacimiento inflows; F, gage 36; G, gage 15; H, gage 24; I, gage 29; and J, gage 31 (U.S. Geological Survey, 2016; Hevesi and others, 2025).

2015

Model Calibration 73

Annual streamflow, in cubic feet per second

1,000

EXPLANATION

800

Observed streamflow Simulated streamflow

600 400 200 0

1,000

Annual streamflow, in cubic feet per second

E. Lake Nacimiento inflows

1950

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

F. Gage 36 (Estrella River near Estrella) EXPLANATION Observed streamflow Simulated streamflow

10 1 0.1

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Water year

G. Gage 15 (San Lorenzo Creek)

Annual streamflow, in cubic feet per second

1960

100

0.01

EXPLANATION

Observed streamflow

Simulated streamflow

50 40 30 20 10 0

1,000

Annual streamflow, in cubic feet per second

1955

1950

1955

1960

Water year

H. Gage 24 (Salinas River above Pilitas Creek)

100

EXPLANATION

Simulated streamflow

Observed streamflow

1 0.1 0.01 0.001

1950

Figure 25.—Continued

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

74 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

550

I. Gage 29 (Salinas River at Paso Robles)

Annual streamflow, in cubic feet per second

500

EXPLANATION

450

Observed streamflow

400 350

Simulated streamflow

300 250 200 150 100 50 0

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Water year

Annual streamflow, in cubic feet per second

140

J. Gage 31 (Salinas River near Pozo)

120

EXPLANATION Observed streamflow

100

Simulated streamflow

80 60 40 20 0

1950

1955

1960

1965

1970

1975

1980

1985

Water year

Figure 25.—Continued

1990

1995

2000

2005

2010

2015

Model Calibration 75

1,000

100,000

EXPLANATION

Simulated maximum daily mean streamflow, in cubic feet per second

Linear (mean streamflow, all gages)

Simulated mean streamflow, in cubic feet per second

1 to 1 line Mean streamflow, all gages

100

y=0.9915x–0.1336 Coefficient of determination statistic used in regression (R²) = 0.9992

10,000

y=0.8052x–50.29 R² = 0.798

1,000

EXPLANATION Linear (maximum streamflow, all gages)

100

1 to 1 line Mean streamflow, all gages

100

1,000

Observed mean streamflow, in cubic feet per second

10,000

100

1,000

10,000

100,000

Observed maximum daily mean streamflow, in cubic feet per second

10,000

Simulated maximum annual streamflow, in cubic feet per second

Simulated maximum monthly mean streamflow, in cubic feet per second

EXPLANATION Linear (maximum streamflow, all gages) 1 to 1 line

1,000

Maximum streamflow, all gages

y=0.9942x–56.347 R² = 0.9519

100

1,000

100

y=0.8189x+11.254 R² = 0.9714 10

EXPLANATION Linear (maximum streamflow, all gages)

1 to 1 line Mean streamflow, all gages 1

100

1,000

Observed maximum monthly mean streamflow, in cubic feet per second

10,000

0.1

100

1,000

10,000

Observed maximum annual streamflow, in cubic feet per second

Figure 26. Summarized comparison of observed versus simulated streamflow for 29 U.S. Geological Survey streamgages and 2 reservoir inflows: A, mean streamflow; B, maximum daily mean streamflow; C, maximum monthly mean streamflow; D, maximum annual streamflow; E, mean monthly streamflow; and F, mean monthly streamflow, shown on a log scale (U.S. Geological Survey, 2016; Hevesi and others, 2025).

76 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Mean monthly streamflow, in percent of total annual streamflow

E EXPLANATION

Observed streamflow

Simulated streamflow

20 15 10 5 0

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Month

Mean monthly streamflow, in percent of total annual streamflow

100

F EXPLANATION Observed streamflow Simulated streamflow

0.1

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

Month

Figure 26.—Continued

May

June

July

Aug.

Sept.

Model Calibration 77 Comparisons between simulated and observed mean monthly streamflow were used as an indication of the goodness-of-fit to the seasonal distribution of streamflow for all streamgages. Monthly streamflow was calculated as a mean monthly percentage of total annual flow to standardized results for all streamgages and provided an equal weight to each streamgage (table 17). The R2 statistic was used as a measure of the goodness-of-fit and ranged from a minimum of 0.67 for USGS streamgage 11143300 (gage 40), to a maximum of 0.99 for several streamgages, including USGS streamgage 11152000 (gage 1), 11151300 (gage 15), and 11143500 (gage 31). An R2 of 0.97 was calculated for the average of the mean monthly streamflow, as a percentage of total annual streamflow, for the 29 USGS streamgages and the 2 reservoir inflows.

The percentage departure of annual streamflow from the long-term (period of record) mean streamflow was compared for all simulated and observed annual flows (fig. 27A). The comparison was used to help evaluate the combined goodness-of-fit for all annual flows based on a standardized measure of flow. The R2 value of 0.88 indicated a good fit between simulated and observed annual flows for the combined set of annual streamflows. In addition, the residuals of the difference between simulated and observed percentage departure of annual flow from mean streamflow did not indicate estimation bias relative to the annual time series (fig. 27B).

78 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley Table 17. Salinas Valley Watershed Model calibration results showing goodness-of-fit statistics using mean monthly streamflow. [ID, identification; R2, coefficient of determination]

Streamgage name

Streamgage ID

Observed mean monthly streamflow as percentage of total annual streamflow Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

Jun.

Jul.

Aug.

Sep.

GABILAN C NR SALINAS

11152600

RECLAMATION DITCH NR SALINAS

11152650

ARROYO DEL REY A DEL REY OAKS

11143300

ARROYO SECO NR GREENFIELD

11151870

ARROYO SECO NR SOLEDAD

11152000

ARROYO SECO BL RELIZ C NR SOLEDAD

11152050

SAN LORENZO C BL BITTERWATER C

11151300

SALINAS R NR POZO

11143500

TORO C NR POZO

11144000

SALSIPUEDES C NR POZO

11144200

SALINAS R BL SALINAS DAM

11144600

SALINAS R AB PILITAS Ck

11145000

JACK C NR TEMPLETON

11147000

SANTA RITA C TRIB

11147040

SANTA RITA C NR TEMPLETON

11147070

SALINAS R A PASO ROBLES

11147500

CHOLAME C TRIB 11147700

CHOLAME C NR SHANDON

11147800

ESTRELLA R NR ESTRELLA

11148500

NACIMIENTO R NR BRYSON

11148800

Model Calibration 79 Table 17. Salinas Valley Watershed Model calibration results showing goodness-of-fit statistics using mean monthly streamflow.— Continued [ID, identification; R2, coefficient of determination]

Streamgage name

Streamgage ID

Simulated mean monthly streamflow as percentage of total annual streamflow

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

Jun.

Jul.

Aug.

Sep.

GABILAN C NR SALINAS

11152600

0.97

RECLAMATION DITCH NR SALINAS

11152650

0.93

ARROYO DEL REY A DEL REY OAKS

11143300

0.67

ARROYO SECO NR GREENFIELD

11151870

0.98

ARROYO SECO NR SOLEDAD

11152000

0.99

ARROYO SECO BL RELIZ C NR SOLEDAD

11152050

0.97

SAN LORENZO C BL BITTERWATER C

11151300

0.99

SALINAS R NR POZO

11143500

0.99

TORO C NR POZO

11144000

0.90

SALSIPUEDES C NR POZO

11144200

0.98

SALINAS R BL SALINAS DAM

11144600

0.94

SALINAS R AB PILITAS Ck

11145000

0.91

JACK C NR TEMPLETON

11147000

0.99

SANTA RITA C TRIB

11147040

0.99

SANTA RITA C NR TEMPLETON

11147070

0.97

SALINAS R A PASO ROBLES

11147500

0.92

CHOLAME C TRIB 11147700

0.95

CHOLAME C NR SHANDON

11147800

0.96

ESTRELLA R NR ESTRELLA

11148500

0.96

NACIMIENTO R NR BRYSON

11148800

0.92

80 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley Table 17. Salinas Valley Watershed Model calibration results showing goodness-of-fit statistics using mean monthly streamflow.—Continued [ID, identification; R2, coefficient of determination]

Streamgage name

Streamgage ID

Observed mean monthly streamflow as percentage of total annual streamflow Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

Jun.

Jul.

Aug.

Sep.

NACIMIENTO R BL SAPAQUE C

11148900

Nacimiento Reservoir inflow

SAN ANTONIO R A SAM JNS BR

11149700

SAN ANTONIO R 11149900 NR LOCKWOOD

SAN ANTONIO R A PLEYTO

11150000

San Antonio Reservoir inflow

HUERHUERO C NR CRESTON

11147600

NACIMIENTO R NR SAN MIGUEL

11149500

SALINAS R NR BRADLEY

11150500

SALINAS R NR SPRECKELS

11152500

EL TORO C NR SPRECKELS

11152540

All gages, averaged mean monthly streamflow as percentage of total annual streamflow

Model Calibration 81 Table 17. Salinas Valley Watershed Model calibration results showing goodness-of-fit statistics using mean monthly streamflow.— Continued [ID, identification; R2, coefficient of determination]

Streamgage name

Streamgage ID

Simulated mean monthly streamflow as percentage of total annual streamflow

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

NACIMIENTO R BL SAPAQUE C

11148900

0.88

Nacimiento Reservoir inflow

0.90

SAN ANTONIO R A SAM JNS BR

11149700

0.96

SAN ANTONIO R 11149900 NR LOCKWOOD

0.90

SAN ANTONIO R A PLEYTO

11150000

0.86

San Antonio Reservoir inflow

0.88

HUERHUERO C NR CRESTON

11147600

0.87

NACIMIENTO R NR SAN MIGUEL

11149500

0.91

SALINAS R NR BRADLEY

11150500

0.88

SALINAS R NR SPRECKELS

11152500

0.81

EL TORO C NR SPRECKELS

11152540

0.97

All gages, averaged mean monthly streamflow as percentage of total annual streamflow

82 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Simulated annual streamflow, in percentage difference from long-term mean

1,200

1,100 1,000 900 800 700

y=0.9195x–0.8196 Coefficient of determination statistic used in regression (R² )=0.8844

600 500 400 300 200

EXPLANATION

100

Difference from long-term mean

Linear regression line (difference from long-term mean)

–100 –100

100

200

300

400

500

600

700

800

900

1,000

1,100

1,200

Observed annual streamflow, in percentage difference from long-term mean

Residual of annual streamflow, simulated minus observed, as percentage difference from long-term mean

400

300 200

y=0.0037x–8.18 R ²=2E-06

100 0 –100 –200

EXPLANATION Difference from long-term mean

–300 –400 1948

Linear regression line (difference from long-term mean) 1952

1956

1960

1964

1968

1972

1976

1980

1984

1988

1992

1996

2000

2004

2008

2012

2016

Water year

Figure 27. Summarized comparison of simulated versus observed streamflow for all U.S. Geological Survey streamgages and reservoir inflows: A, annual streamflow as a percentage of long-term mean streamflow and B, residual of annual streamflow, simulated minus observed, as percentage difference from mean streamflow (U.S. Geological Survey, 2016; Henson and others, 2022; Hevesi and others, 2025).

Model Validation 83

Model Validation Model validation provides an indication of model uncertainty and forecasting accuracy by comparing simulated and observed streamflow using records that were not included as part of the calibration procedure. The SVWM was validated using streamflow records for five streamgages having streamflow records that were not included in the calibration procedure: USGS streamgages 11152650 (gage 38), 11150500 (gage 19), 11151700 (gage 6), 11152300 (gage 10), and 11152500 (gage 21). The validation period for USGS streamgage 11152650 (gage 38) included the later part of the record, from 2002 to 2016, whereas the calibration period included the earlier record, from 1971 to 1986. Records from USGS streamgages 11152300 (gage 10) and 11151700 (gage 6) were not used in model calibration because of the effect of reservoir releases on daily and monthly streamflow. U.S. Geological Survey streamgages 11152500 (gage 21) and 11150500 (gage 19) were included in model calibration using records before the completion of Lake Nacimiento (water years 1949–56), whereas the validation period for these streamgages included records from 1957 to 2016. The validation periods using the four streamgages on the Salinas River post-dated the completion of the dam impounding Lake Nacimiento, and therefore, the observed streamflow included managed flow conditions. The comparison between simulated and observed streamflow at the four streamgages on the Salinas River was done to evaluate changes in flow conditions relative to unimpaired streamflow conditions represented by the SVWM, in addition to model validation. The validations results were considered good for USGS streamgages 11152500 (gage 21), 11150500 (gage 19), and 11152650 (gage 38), in terms of low estimation bias, as

indicated by PAE values from −0.1 to 2.1 percent (table 18). The monthly and annual NSME results for USGS streamgages 11152500 (gage 21) and 11152650 (gage 38) were good (0.70 and 0.87, respectively). In contrast, the monthly NSME result for USGS streamgage 11150500 (gage 19) was poor and the annual result was fair, most likely because of the direct effect of flow releases from Lakes Nacimiento and San Antonio. The annual hydrograph comparing simulated and observed streamflow at USGS streamgage 11150500 (gage 19) indicates an overestimation of high annual flows and an underestimation of low annual flows, which is an expected result in terms of comparing simulated unimpaired streamflow to the managed observed flows (fig. 28D). The PAE results of 1.6–2.1 for USGS streamgage 11150500 (gage 19), however, indicate a good result in terms of the USVS accurately representing the long-term mean streamflow conditions at this location. In general, the model validation result was considered satisfactory to good based on the visual comparison of the annual hydrographs at all five streamgages (fig. 28). Results for USGS streamgages 11152300 (gage 10) and 11151700 (gage 6) indicated overestimation of the long-term mean streamflow by 22 and 32 percent, respectively. These two streamgages were not included in model calibration and the overestimation could be indicative of the effect of groundwater pumping along this section of the Salinas Valley, between gages 6 and 10 (with pumping decreasing baseflow), and the high degree of uncertainty associated with simulating streamflow losses caused by seepage through the streambed. In addition, local flow diversions and return flows from irrigation may have an effect on the Salinas River streamflow in this part of the valley, and these processes are not simulated by the SVWM.

84 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

18,000

A. Gage 21 (Salinas River near Spreckles)

Monthly streamflow, in cubic feet per second

16,000

10,000 8,000 6,000 4,000 2,000

Annual streamflow, in cubic feet per second

3,000

1950

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

B. Gage 10 (Salinas River near Chualar) EXPLANATION

2,500

Observed streamflow 2,000

Simulated streamflow

1,500 1,000 500 0

2,500

Annual streamflow, in cubic feet per second

Simulated streamflow

12,000

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Water year

C. Gage 6 (Salinas River at Soledad) EXPLANATION

2,000

Observed streamflow Simulated streamflow

1,500 1,000 500 0

2,500

Annual streamflow, in cubic feet per second

EXPLANATION Observed Observed streamflow Simulated

14,000

1950

1955

1960

1965

Water year

D. Gage 19 (Salinas River near Bradley) EXPLANATION

2,000

Observed streamflow Simulated streamflow

1,500 1,000 500 0

1950

1955

1960

1965

1970

1975

1980

1985

Water year

1990

1995

2000

2005

2010

2015

Figure 28. Model validation using comparisons of observed and simulated streamflow at A, gage 21; B, gage 10; C, gage 6; D, gage 19; and E, gage 38 (U.S. Geological Survey, 2016; Hevesi and others, 2025).

Model Validation 85

Monthly streamflow, in cubic feet per second

E. Gage 38 (Reclamation Ditch near Salinas)

EXPLANATION

Simulated streamflow

Observed streamflow

50 40 30 0

1950

1955

1960

1965

1970

1975

1980

1985

Water year

Figure 28.—Continued

1990

1995

2000

2005

2010

2015

86 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley Table 18. Model validation results for five selected U.S. Geological Survey streamgages. [ft3/s, cubic foot per second; ID, identification; NSME, Nash-Sutcliffe model efficiency; Obs., observed; Sim., simulated; SVWM, Salinas Valley Watershed Model; USGS, U.S. Geological Survey; %, percent; —, not applicable]

SVWM streamgage number 21

11152500 Salinas River near Spreckels

11152300 Salinas River near Chualar

11150500 Salinas River near Bradley

11152650 Reclamation Ditch near Salinas

1958–2015

2003–16

USGS gage ID and name

Results

11151700 Salinas River at Soledad Validation period (water years)

1958–2018

1977–2016

1969–2016 Number of years

Obs. (ft3/s)

Sim. (ft3/s)

Obs. (ft3/s)

43 Sim. (ft3/s)

Obs. (ft3/s)

Sim. (ft3/s)

Obs. (ft3/s)

Sim. (ft3/s)

Obs. (ft3/s)

Sim. (ft3/s)

Daily streamflow Mean

354

390

516

351

429

492

500

10.5

10.7

Median

132

301

2.4

1.4

64,800

89,408

68,000

92,027

68,300

82,648

63,900

72,597

Maximum Minimum

0.00

0.02

0.00

0.07

477

770

0.00

0.10

0.07

% error

—

0.1

—

32.1

—

22.4

—

1.6

—

1.4

NSME

—

0.11

—

–0.14

—

–0.09

—

–0.22

—

0.40

Monthly streamflow Mean

361

360

396

522

356

436

497

507

10.5

10.7

Median

142

334

3.7

2.3

16,262

16,393

14,346

19,124

11,166

15,439

10,182

14,869

Maximum Minimum

0.00

0.03

0.00

0.37

0.00

0.82

0.08

% error

—

–0.1

—

31.9

—

22.4

—

2.1

—

1.6

NSME

—

0.70

—

0.56

—

0.36

—

0.10

—

0.86

Annual streamflow Mean

354

390

516

351

430

492

500

10.5

10.7

Median

145

133

253

170

215

354

299

8.4

9.5

Maximum

2,997

2,165

2,796

2,077

1,981

2,174

1,997

2,184

Minimum

0.0

7.8

—

16.7

—

19.8

9.4

36.2

2.7

3.1

% error

—

0.1

—

32.1

—

22.4

—

1.6

—

1.5

NSME

—

0.81

—

0.71

—

0.75

—

0.62

—

0.87

Model Limitations 87

Model Limitations The SVWM is considered successful for its intended purpose; however, understanding the model limitations is important for determining appropriate applications, evaluating results, and qualitatively assessing model uncertainty. The timing, magnitude, frequency, and spatial distribution of daily precipitation are the most important factors affecting simulated streamflow by the SVWM. Although a total of 194 climate stations with records of daily precipitation and air temperature were used to develop the climate inputs for the SVWM, many of these stations had gaps in the record or short records (10 years or less) compared to the entire simulation period of 70 years. For many periods in the 71-year simulation period that included water year 1948 as the model initialization period, only a fraction of the 194 stations were active, and a greater degree of uncertainty was associated with the spatially interpolated climate inputs for HRUs that were far from the nearest climate station. Additionally, some areas of the SVWM have a lower density of stations throughout the simulation period, and therefore, the simulation results for these areas may be associated with a higher degree of uncertainty relative to results in areas with a higher density of climate stations. The daily climate inputs developed by the BCM are adequate for estimating daily, monthly, and annual water budgets. However, a daily time step with uniformly distributed hourly precipitation might not adequately represent high hourly precipitation rates that may occur during severe storms. High hourly precipitation rates can result in a greater proportion of precipitation contributing to Hortonian surface-water runoff as compared to the proportion of precipitation contributing to pervious land infiltration. Hortonian surface-water runoff can be a substantial component of streamflow during storms. Inadequate representation of the high precipitation rates might cause the SVWM to potentially underestimate surface runoff and the magnitude of peak flows during some storm periods. Many of the critical HSPF parameters used in the SVWM, such as the parameters controlling ET and recharge from the soil zone; the partitioning of overland runoff, interflow, and recharge; and the rates of interflow and groundwater discharge to streams, were not measured, but rather were estimated and then adjusted during the model calibration process. The final calibrated values are considered reasonable estimates of HSPF parameters based on values used in previous studies; however, the values might not be representative of actual field conditions and might not agree with the measured values if such values become available.

In addition, many of the parameters were estimated using available geospatial datasets representing average historical field conditions at specific times, such as the NLCD 2011 land-cover type, percentage of impervious land cover, and percentage of forest canopy cover. At some locations, these estimated values might not be representative of conditions in the early or final parts of the simulation period. All parameters representing the physical characteristics of the Salinas Valley study area, such as land cover, topography, hydrography, soil properties, and surficial geology, were set to constant, time-invariant values for the simulation period. Anthropogenic effects on the hydrologic system, such as the construction of reservoirs and the implementation of managed river flows at different times in the simulation period, are not represented by the SVWM. Other changes through time not represented by the model include increased diversions of surface water and changes in pumping and irrigation practices. The inclusion of crop and landscape irrigation in the SVWM would likely cause an increase in mean soil moisture simulated for the central and lower valley areas of the SVWM where agricultural land cover is prevalent and developed lands are associated with urbanized areas. The increased soil moisture would result in increased ET and might also result in increased recharge and runoff. The SVWM uses a simplified, empirical representation of the groundwater system that does not account for the three-dimensional heterogeneity of aquifer systems and the physics of groundwater storage and flow. The groundwater reservoirs simulated by the SVWM represented only the active part of the groundwater system contributing to streamflow and ET and were assumed to be coincident with the boundaries of the associated HRUs that were defined mostly by surface-water drainage divides rather than groundwater-flow divides. The AGWRC coefficient controlling the time-varying rate of groundwater discharge to streams was estimated and calibrated based on surficial characteristics of drainage basins such as topography, land cover, soils, and surficial geology; the underlying geologic structure is not represented by the SVWM. The SVWM-simulated groundwater reservoirs do not represent the total groundwater-flow system in underlying aquifers, and therefore, the SVWM cannot be used to quantify the groundwater system such as changes in aquifer storage. Groundwater travel times and flow paths through the unsaturated zone are not explicitly simulated by the SVWM, and therefore, rejected recharge for locations with a high water table is not simulated; this could cause an overestimation of recharge for these locations.

88 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Simulation Results, Water Years 1949–2018 Simulation results for the 70-year target period, water years 1949–2018, were used to evaluate and quantify water balance components of the natural hydrologic system for the Salinas Valley study area, including the Salinas River watershed and several smaller, adjacent drainages along the Monterey Bay coast. Simulated components include ET, recharge, surface runoff (overland flow), interflow, and groundwater discharge (baseflow) for the 690 HRUs, and streamflow and stream seepage losses for the 690 stream reaches connecting the HRUs. The results were used to analyze the spatial and temporal variability of the natural hydrologic system for the 70-year historical simulation period. Results were compared between the 10 subbasin areas, 28 subdrainage areas, and 25 upland tributary drainages having outflows to the lower Salinas Valley study area, an area generally coincident with developed lands and agricultural land irrigated with groundwater. Precipitation is the only inflow to the natural hydrologic system of the Salinas Valley study area simulated by the SVWM. The three outflows simulated by the SVWM are ET, surface-water outflow (streamflow) to Monterey Bay from the Salinas River and smaller adjacent basins along the Monterey Bay coastal region, and recharge to the inactive groundwater reservoir. Recharge to the inactive groundwater reservoir accounts for water that does not contribute to groundwater discharge either as baseflow to streams or transpiration from the saturated zone and is an outflow of water from the hydrologic system represented by the SVWM. As simulated by the SVWM, recharge to the inactive groundwater reservoir consists of two components: (1) streamflow seepage losses in stream reaches and (2) inter-channel recharge from PERLNDs, also referred to in this study as net land-area recharge. In addition to precipitation and the three outflows, components of the water balance that were analyzed included total recharge, surface runoff (overland flow), interflow runoff, and groundwater discharge to stream reaches (baseflow) from the active groundwater reservoir. Total recharge was calculated as the sum of recharge to the active and inactive groundwater reservoirs. Results for selected inflows and outflows are provided in tables 19 and 20 for subbasin areas, subdrainage

areas, and upland drainage areas tributary to the central and lower Salinas Valley lowlands (fig. 15). Table 19 shows the 1948–2018 70-year mean annual inflows and outflows as an average water equivalent depth, in units of in/yr, for the area of each subbasin, subdrainage, and upland tributary drainage. Table 20 shows the 70-year mean inflows and outflows in terms of water volumes, in units of thousands of acre-ft/yr, for subbasins, subdrainages, and upland tributary drainages. Results in terms of area-averaged water equivalent depths provided in table 19 provide a more direct comparison of the differences in the hydrologic character between the various subbasins and subdrainages because the effect of drainage area size is excluded, whereas the water volumes shown in table 20 provide a more direct comparison of the absolute magnitude of water balance components between subbasins and subdrainages.

Evapotranspiration The simulated 70-year mean ET of 14.9 in/yr for the Salinas Valley study area was the largest outflow of water from the SVWM, about 3.6 million acre-ft/yr, accounting for about 80 percent of the 70-year mean precipitation inflow of 18.5 in/yr, about 4.5 million acre-ft/yr (tables 19, 20). Results for HRUs ranged from mean values of 17–26 in/yr for higher elevations with substantial forest cover and thick soils along the western and eastern sides of the SVWM, to less than 11 in/yr throughout the valley floor and in some parts of the western and eastern sides, and less than 9.5 in. in the southeastern part of the Salinas River watershed (fig. 29). The spatial distribution of ET is partly consistent with the spatial distribution of precipitation (fig. 19) because the magnitude of ET is dependent on water availability; however, the spatial distribution also is affected by spatial variations in PET (fig. 21), soil water storage capacity (fig. 9), land-surface slope (fig. 4) and other factors controlling runoff, and vegetation. The highest mean ET of more than 19 in/yr was simulated for the ARR subbasin and the UAS, LAS, and SRH subdrainages, which also has the highest mean precipitation of 30.1 in/yr (table 19). The lowest ET of 12.9 in/yr was simulated for the NAC and MSR subbasins in part because of coarse soils on steep slopes limiting the storage capacity of the root zone.

Table 19. Simulation results for selected water balance components calculated as mean annual inches of water inflows and outflows, water years 1949–2018, for subbasins, subdrainages, and tributary drainages in the Salinas Valley study area. [AET, actual evapotranspiration; in/yr, inch per year]

Inflows (in/yr) Name

Abbreviation

Precipitation

Surface water

Outflow total AET (in/yr)

Land area runoff (in/yr) Overland runoff

Interflow

Baseflow

Recharge (in/yr)

Outflows (in/yr)

Total runoff

Net land area

Stream seepage

Net recharge

Surface water

5.10

0.06

1.69

1.76

3.43

Salinas Valley subbasin and sub-model areas Salinas River headwaters

SRH

23.99

0.00

18.83

0.92

1.27

2.92

Estrella River

EST

14.82

0.00

13.40

0.03

0.10

1.18

1.31

0.11

1.01

1.12

0.30

Nacimiento River

NAC

24.68

0.00

12.86

5.46

3.36

2.99

11.82

0.00

0.23

11.59

San Antonio River

SAN

21.92

0.00

16.38

1.34

1.45

2.66

5.45

0.10

1.78

1.88

3.68

Upper Salinas River

USR

16.95

11.43

14.76

0.34

0.32

1.42

2.08

0.13

2.24

2.38

11.28

Upper Salinas Valley

USV

18.88

0.00

14.85

1.10

0.92

1.92

3.94

0.09

1.39

1.49

2.56

Arroyo Seco

ARR

30.08

0.00

19.91

0.98

3.08

6.07

10.13

0.02

5.27

5.29

4.87

San Lorenzo Creek

LOR

16.92

0.00

15.70

0.39

0.42

1.20

0.02

0.57

0.59

0.68

Middle Salinas River

MSR

14.69

8.67

12.88

0.75

0.38

0.65

1.78

0.04

3.17

3.21

7.29

Lower Salinas River

LSR

15.91

18.87

14.13

0.85

0.44

0.45

1.74

0.06

10.74

10.80

10.11

Monterey Coastal Basins

MCB

18.12

0.00

15.75

1.08

0.09

0.92

2.08

0.28

0.18

0.46

1.97

Lower Salinas Valley

LSV

18.01

3.26

14.95

0.80

0.75

1.43

2.99

0.07

4.12

4.20

2.18

Salinas River watershed

SRW

18.52

0.00

14.84

0.96

0.90

1.76

3.62

0.07

2.76

2.83

0.89

Salinas Valley Watershed Model

SVWM

18.50

0.00

14.89

0.97

0.84

1.71

3.52

0.08

2.59

2.68

0.96

Santa Margarita Lake

SML

25.90

0.00

20.95

0.51

1.17

3.26

4.93

0.02

2.29

2.31

2.68

Salinas River near Paso Robles

SRP

23.40

4.67

18.19

1.07

1.22

2.80

5.10

0.11

1.91

2.03

7.87

Paso Robles Creek

PRC

22.94

0.00

17.62

1.11

1.44

2.75

5.29

0.04

0.72

0.76

4.58

Upper San Juan Creek

USJ

15.23

0.00

13.05

0.02

0.11

1.97

2.10

0.08

1.71

1.80

0.39

Lower San Juan Creek

LSJ

13.13

0.25

11.84

0.02

0.11

1.08

1.21

0.07

0.98

1.05

0.49

Cholame Creek

CHO

15.35

0.00

14.42

0.05

0.09

0.67

0.81

0.12

0.56

0.68

0.25

Lower Estrella River

LER

15.80

0.75

14.29

0.04

0.09

1.22

1.36

0.16

0.99

1.15

1.12

Upper Nacimiento River

UNR

30.09

0.00

13.06

8.37

4.95

3.72

17.03

0.00

0.06

16.98

Lower Nacimiento River

LNR

21.26

10.71

12.74

3.63

2.36

2.54

8.52

0.00

0.34

18.91

Upper San Antonio River

USA

27.29

0.00

18.01

2.46

2.61

4.10

9.18

0.10

1.57

1.67

7.61

Lower San Antonio River

LSA

16.22

8.09

14.65

0.15

0.21

1.12

1.48

0.09

2.00

2.09

7.58

Simulation Results, Water Years 1949–2018 89

Salinas Valley subdrainage areas Subdrainages in the upper Salinas Valley sub-model

[AET, actual evapotranspiration; in/yr, inch per year]

Inflows (in/yr) Name

Abbreviation

Precipitation

Surface water

Outflow total AET (in/yr)

Land area runoff (in/yr) Overland runoff

Interflow

Baseflow

Recharge (in/yr)

Outflows (in/yr)

Total runoff

Net land area

Stream seepage

Net recharge

Surface water 0.82

Subdrainages in the upper Salinas Valley sub-model—Continued Huerhuero Creek

HUE

21.10

0.00

18.55

0.06

0.13

2.20

2.39

0.16

1.58

1.74

Big Sandy Creek

BSC

Salinas River near Bradley

SNB

17.16

0.00

14.43

0.45

0.55

1.62

2.61

0.13

1.32

1.45

1.18

14.03

28.61

12.46

0.45

0.28

0.77

1.50

0.11

3.35

3.46

26.87

Subdrainages in the lower Salinas Valley sub-model Upper Arroyo Seco

UAS

42.10

0.00

21.10

2.39

7.19

11.34

20.92

0.00

1.70

19.24

Lower Arroyo Seco

LAS

23.04

11.26

19.21

0.15

0.67

2.99

3.81

0.03

7.36

7.39

7.72

San Lorenzo Creek

SLC

16.92

0.00

15.70

0.39

0.42

1.20

0.02

0.57

0.59

0.68

Sargent–Pancho Rico Creeks

SPR

16.06

0.00

13.90

0.47

0.69

0.94

2.11

0.05

1.17

1.22

0.95

Pine Creek drainages

PCD

13.80

0.00

12.91

0.17

0.29

0.38

0.85

0.04

0.46

0.50

0.39

Upper Salinas Valley

USV

14.44

35.75

12.30

1.45

0.50

0.17

2.12

0.04

6.37

6.41

31.54

Chalone–Stonewall Creek

CSC

14.83

0.00

13.27

0.01

0.05

1.46

1.52

0.03

1.03

1.06

0.50

Monroe Creek–Salinas River

MCS

14.20

44.60

12.06

1.56

0.42

0.13

2.12

0.04

6.10

6.14

40.66

Quail–Chualar Creek

QCC

14.20

0.00

13.42

0.17

0.19

0.36

0.72

0.04

0.58

0.62

0.14

Limekiln Creek–Salinas River

LCS

15.39

49.36

12.73

1.54

0.75

0.35

2.64

0.05

5.35

5.40

46.69

El Toro Creek

ETC

20.41

0.00

18.89

0.18

0.05

1.21

1.44

0.08

0.88

0.96

0.57

Salinas River outflow

SRO

16.14

85.35

14.47

0.92

0.43

0.27

1.62

0.10

39.96

40.07

48.00

Elkhorn Slough

ELK

18.38

0.00

16.13

0.82

0.11

1.01

1.94

0.31

0.22

0.53

1.81

Monterey–Seaside basin

MSB

17.21

0.00

14.48

1.94

0.00

0.62

2.57

0.16

0.06

0.22

2.50

Alisal Creek

ALIS

18.98

0.00

16.81

0.11

0.69

1.30

2.11

0.00

2.11

Arroyo Seco

SECO

31.68

0.00

20.53

1.07

3.38

6.65

11.11

0.01

2.93

8.19

Big Sandy Creek

BIGS

19.00

0.00

14.80

0.93

0.96

2.21

4.10

0.11

1.75

1.86

2.38

Chalone Creek

CHAL

15.12

0.00

13.25

0.02

0.06

1.75

1.83

0.03

1.23

1.26

0.61

Cherry Canyon

CHER

12.78

0.00

12.37

0.00

0.36

0.05

0.25

0.31

0.11

Tributary drainages to the lower Salinas Valley study area

90 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Table 19. Simulation results for selected water balance components calculated as mean annual inches of water inflows and outflows, water years 1949–2018, for subbasins, subdrainages, and tributary drainages in the Salinas Valley study area.—Continued [AET, actual evapotranspiration; in/yr, inch per year]

Inflows (in/yr) Name

Abbreviation

Precipitation

Surface water

Outflow total AET (in/yr)

Land area runoff (in/yr) Overland runoff

Interflow

Baseflow

Recharge (in/yr)

Outflows (in/yr)

Total runoff

Net land area

Stream seepage

Net recharge

Surface water

Tributary drainages to the lower Salinas Valley study area—Continued CHUA

16.45

0.00

15.05

0.06

0.31

0.98

1.34

0.01

0.80

0.81

0.54

El Toro Creek

TORO

22.98

0.00

21.36

0.05

0.11

1.46

1.61

0.01

1.04

1.05

0.58

Gabilan Creek

GABI

23.09

0.00

21.47

0.07

0.41

1.13

1.60

0.00

1.60

Hames Creek

HAME

16.92

0.00

12.65

2.46

1.15

0.65

4.27

0.01

1.00

1.01

3.27

Hare Canyon

HARE

13.68

0.00

12.54

0.03

0.10

0.89

1.02

0.13

0.64

0.76

0.38

Lower Salinas River West

LSRW

24.52

0.00

13.36

6.30

3.72

1.08

11.10

0.03

3.70

3.74

7.44

McCoy Creek

MCOY

15.50

0.00

14.18

0.08

0.36

0.84

1.27

0.00

0.89

0.90

0.38

Middle Salinas River West

MSRW

16.99

0.00

12.24

3.31

1.20

0.23

4.74

0.00

1.51

3.25

Monroe Creek

MONR

20.52

0.00

13.30

5.26

1.60

0.36

7.22

0.00

1.76

5.48

Nacimiento River

NACI

23.74

0.00

12.87

4.98

3.08

2.81

10.86

0.01

1.04

9.84

Pancho Rico Creek

PANR

16.78

0.00

14.39

0.59

0.83

0.93

2.35

0.05

1.22

1.26

1.14

Pine Creek

PINE

14.17

0.00

13.03

0.29

0.47

0.35

1.11

0.03

0.39

0.42

0.72

Quail Creek

QUAI

16.69

0.00

14.64

0.21

0.91

0.86

1.98

0.00

0.94

1.04

Salinas River

SALI

17.74

0.00

15.25

0.27

0.40

1.71

2.38

0.11

1.20

1.31

1.19

San Antonio River

SANR

21.83

0.00

16.32

1.34

1.44

2.63

5.41

0.10

1.76

1.86

3.65

San Lorenzo Creek

SANL

17.22

0.00

15.96

0.40

0.41

0.45

1.25

0.02

0.56

0.58

0.74

Sargent Creek

SARG

15.73

0.00

13.57

0.34

0.60

1.16

2.11

0.06

0.99

1.05

1.19

Stonewall Creek

STON

15.04

0.00

14.26

0.02

0.09

0.64

0.74

0.00

0.47

0.27

Vineyard Canyon

VINE

17.89

0.00

15.34

0.23

0.48

1.67

2.39

0.16

1.09

1.25

1.31

Wildhorse Canyon

WILD

13.75

0.00

12.99

0.10

0.18

0.44

0.72

0.04

0.40

0.43

0.32

Salinas Valley upland area

SVU

19.55

0.00

15.30

1.09

1.05

2.04

4.18

0.07

1.31

1.37

2.88

0.88

0.15

7.76

7.91

4.82

Lower Salinas Valley study area receiving inflows from tributary drainages Lower Salinas Valley study area

SVIHM

14.28

11.56

13.28

0.48

0.01

0.38

Simulation Results, Water Years 1949–2018 91

Chualar Creek

[acre-ft/yr, acre-foot per year; AET, actual evapotranspiration; ft3/s, cubic feet per second]

Inflows,in thousands of acre-ft/ yr Name

Abbreviation Precipitation

Surface water

Outflows, in thousands of acre-ft/yr Outflow total AET

Recharge Net land area

Stream seepage

Outflow recharge

Outflow surface water

Surface water outflow (ft3/s)

Salinas Valley subbasins and sub-model areas Salinas River headwaters

SRH

497.9

0.0

390.7

1.3

35.1

36.5

71.2

98.3

Estrella River

EST

729.9

0.0

660.0

5.5

49.8

55.3

15.0

20.7

Nacimiento River

NAC

428.0

0.0

223.1

0.0

4.0

201.0

277.5

San Antonio River

SAN

377.9

0.0

282.4

1.7

30.7

32.3

63.4

87.5

Upper Salinas River

USR

520.1

350.6

452.7

4.0

68.9

72.9

346.0

477.6

Upper Salinas Valley

USV

2,553.8

0.0

2,008.8

12.5

188.5

201.0

346.0

477.6

Arroyo Seco

ARR

476.5

0.0

315.4

0.3

83.4

83.7

77.1

106.4

San Lorenzo Creek

LOR

235.2

0.0

218.3

0.3

7.9

8.1

9.5

13.1

Middle Salinas River

MSR

602.3

355.5

528.1

1.6

130.1

131.7

299.1

412.9

Lower Salinas River

LSR

317.2

376.2

281.6

1.2

214.0

215.2

201.4

278.0

Monterey Coastal Basins

MCB

282.2

0.0

245.4

4.3

2.8

7.2

30.7

42.3

Lower Salinas Valley

LSV

1,913.4

346.0

1,588.7

7.8

438.2

446.0

232.1

320.4

Salinas River watershed

SRW

4,185.0

0.0

3,352.2

15.9

623.9

639.8

201.4

278.0

Salinas Valley Watershed Model

SVWM

4,467.1

0.0

3,597.5

20.2

626.7

646.9

232.1

320.4

Salinas Valley subdrainage areas Subdrainages in the upper Salinas Valley sub-model Santa Margarita Lake

SML

154.8

0.0

125.2

0.1

13.7

13.8

16.1

22.2

Salinas River near Paso Robles

SRP

211.7

42.3

164.6

1.0

17.3

18.3

71.2

98.3

Paso Robles Creek

PRC

131.4

0.0

100.9

0.2

4.1

4.4

26.2

36.2

Upper San Juan Creek

USJ

139.9

0.0

119.9

0.8

15.7

16.5

3.6

4.9

Lower San Juan Creek

LSJ

184.2

3.6

166.2

1.0

13.8

14.8

6.8

9.4

Cholame Creek

CHO

194.1

0.0

182.4

1.6

7.1

8.6

3.2

4.4

Lower Estrella River

LER

211.8

10.0

191.5

2.1

13.2

15.4

15.0

20.7

Upper Nacimiento River

UNR

201.9

0.0

87.6

0.0

0.4

113.9

157.2

Lower Nacimiento River

LNR

226.1

113.9

135.5

0.0

3.6

201.0

277.5

92 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Table 20. Simulation results for selected water balance components calculated as mean annual inflow and outflow volumes, in thousands of acre-feet per year (acre-ft/yr), water years 1949–2018, for subbasins, subdrainages, and tributary drainages in the Salinas Valley study area.

Inflows,in thousands of acre-ft/ yr Name

Abbreviation Precipitation

Surface water

Outflows, in thousands of acre-ft/yr Outflow total AET

Recharge Net land area

Stream seepage

Outflow recharge

Outflow surface water

Surface water outflow (ft3/s)

Subdrainages in the upper Salinas Valley sub-model—Continued Upper San Antonio River

USA

242.3

0.0

159.9

0.9

13.9

14.8

67.6

93.3

Lower San Antonio River

LSA

135.6

67.6

122.5

0.8

16.8

17.5

63.4

87.5

Huerhuero Creek

HUE

181.9

0.0

159.9

1.4

13.6

15.0

7.1

9.7

Big Sandy Creek

BSC

157.5

0.0

132.4

1.2

12.1

13.3

10.8

14.9

Salinas River near Bradley

SNB

180.7

368.5

160.5

1.4

43.2

44.6

346.0

477.6

Upper Arroyo Seco

UAS

246.3

0.0

123.4

0.0

9.9

112.5

155.3

Lower Arroyo Seco

LAS

230.2

112.5

192.0

0.3

73.5

73.8

77.1

106.4

San Lorenzo Creek

SLC

235.2

0.0

218.3

0.3

7.9

8.1

9.5

13.1

Sargent–Pancho Rico Creeks

SPR

122.2

0.0

105.8

0.4

8.9

9.3

7.2

9.9

Pine Creek drainages

PCD

87.6

0.0

82.0

0.2

2.9

3.1

2.5

3.4

Upper Salinas Valley

USV

143.6

355.7

122.3

0.4

63.4

63.8

313.8

433.1

Chalone–Stonewall Creek

CSC

144.4

0.0

129.2

0.3

10.0

10.4

4.9

6.7

Monroe Creek–Salinas River

MCS

104.5

328.1

88.7

0.3

44.9

45.2

299.1

412.9

Quail–Chualar Creek

QCC

76.4

0.0

72.3

0.2

3.1

3.3

0.7

1.0

Limekiln Creek–Salinas River

LCS

117.3

376.2

97.0

0.4

40.8

41.1

355.9

491.3

El Toro Creek

ETC

55.7

0.0

51.6

0.2

2.4

2.6

1.6

2.1

Salinas River outflow

SRO

67.7

358.2

60.7

0.4

167.7

168.1

201.4

278.0

Elkhorn Slough

ELK

221.5

0.0

194.3

3.8

2.6

6.4

21.9

30.2

Monterey–Seaside basin

MSB

60.7

0.0

51.1

0.6

0.2

0.8

8.8

12.2

Alisal Creek

ALIS

14.5

0.0

12.8

0.0

1.6

2.2

Arroyo Seco

SECO

457.2

0.0

296.3

0.1

42.2

42.4

118.2

163.2

Subdrainages in the lower Salinas Valley sub-model

Simulation Results, Water Years 1949–2018 93

Tributary drainages to the lower Salinas Valley study area

[acre-ft/yr, acre-foot per year; AET, actual evapotranspiration; ft3/s, cubic feet per second]

Inflows,in thousands of acre-ft/ yr Name

Abbreviation Precipitation

Surface water

Outflows, in thousands of acre-ft/yr Outflow total AET

Recharge Net land area

Stream seepage

Outflow recharge

Outflow surface water

Surface water outflow (ft3/s)

Tributary drainages to the lower Salinas Valley study area—Continued Big Sandy Creek

BIGS

69.7

0.0

54.3

0.4

6.4

6.8

8.7

12.1

Chalone Creek

CHAL

113.5

0.0

99.5

0.3

9.2

9.5

4.6

6.3

Cherry Canyon

CHER

14.6

0.0

14.1

0.1

0.3

0.4

0.1

0.2

Chualar Creek

CHUA

19.9

0.0

18.2

0.0

1.0

0.7

0.9

El Toro Creek

TORO

27.1

0.0

25.2

0.0

1.2

0.7

0.9

Gabilan Creek

GABI

37.5

0.0

34.8

0.0

2.6

3.6

Hames Creek

HAME

24.4

0.0

18.2

0.0

1.5

4.7

6.5

Hare Canyon

HARE

17.0

0.0

15.5

0.2

0.8

1.0

0.5

0.7

Lower Salinas River West

LSRW

47.3

0.0

25.8

0.1

7.1

7.2

14.3

19.8

McCoy Creek

MCOY

17.7

0.0

16.2

0.0

1.0

0.4

0.6

Middle Salinas River West

MSRW

42.6

0.0

30.7

0.0

3.8

8.2

11.3

Monroe Creek

MONR

62.2

0.0

40.3

0.0

5.3

16.6

22.9

Nacimiento River

NACI

453.1

0.0

245.7

0.1

19.8

19.9

187.7

259.1

Pancho Rico Creek

PANR

72.7

0.0

62.4

0.2

5.3

5.5

4.9

6.8

Pine Creek

PINE

41.1

0.0

37.8

0.1

1.1

1.2

2.1

2.9

Quail Creek

QUAI

9.9

0.0

8.7

0.0

0.6

0.9

Salinas River

SALI

1,489.3

0.0

1,280.7

9.0

101.0

110.0

99.6

137.5

San Antonio River

SANR

381.8

0.0

285.6

1.7

30.8

32.5

63.8

88.1

San Lorenzo Creek

SANL

227.8

0.0

211.0

0.3

7.4

7.7

9.7

13.4

Sargent Creek

SARG

41.3

0.0

35.6

0.2

2.6

2.8

3.1

4.3

Stonewall Creek

STON

12.0

0.0

11.4

0.0

0.4

0.2

0.3

Vineyard Canyon

VINE

40.1

0.0

34.4

0.4

2.4

2.8

2.9

4.1

Wildhorse Canyon

WILD

44.8

0.0

42.3

0.1

1.3

1.4

1.1

1.5

Salinas Valley upland area

SVU

3,779.0

0.0

2,957.5

13.0

252.5

265.6

557.7

769.9

SVIHM

688.1

381.3

232.1

320.4

Lower Salinas Valley study area receiving inflows from tributary drainages Lower Salinas Valley study area

557.7

640.0

7.2

374.2

94 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Simulation Results, Water Years 1949–2018 95 122°

121°30'

121°

120°30'

120°

101

EXPLANATION

MERCED COUNTY

Simulated mean evapotranspiration, water years 1949–2018, in inches per year

MONTEREY BAY SAN BENITO COUNTY

Salinas Monterey

36° 30'

8.8 to 11.0

15.1 to 17.0

11.1 to 12.0

17.1 to 19.0

12.1 to 13.0

19.1 to 21.0

13.1 to 14.0

21.1 to 23.0

14.1 to 15.0

23.1 to 26.1

Salinas Valley Watershed Model (SVWM) boundary SVWM sub-model boundary

MONTEREY COUNTY

U.S. Geological Survey streamgage at Bradley (see table 2)

Soledad

101

King City

36°

FRESNO COUNTY

Lower Salinas Valley sub-model boundary Lake San Antonio

KINGS COUNTY

Lake Nacimiento

KERN COUNTY

Paso Robles 35° 30'

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary 1

San Luis Obispo 101

Santa Margarita Lake SAN LUIS OBISPO COUNTY

0 0

10 10

20 20

30 MILES

30 KILOMETERS

Figure 29. Salinas Valley Watershed Model simulation results for mean evapotranspiration (total actual evapotranspiration [AET]), water years 1949–2018 (Hevesi and others, 2025).

96 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Runoff Total runoff simulated by the SVWM from each HRU to the connected RCHRES stream segment includes three separate flow components: overland runoff, interflow runoff, and groundwater discharge to streams (baseflow). Total HRU runoff is the unrouted surface-water outflow from HRUs to the stream network. The accumulated total runoff is routed through the connected RCHRES network using the kinematic wave algorithm in HSPF to simulate streamflow. The 70-year mean total runoff simulated by the SVWM for the Salinas Valley study area was of 3.52 in/yr for the SVWM, or 19 percent of precipitation (table 19). The NAC subbasin had the highest mean total runoff of 11.82 in/yr, about 48 percent of precipitation. In comparison, the LOR had the lowest mean total runoff of only 1.2 in/yr, about 7 percent of precipitation. Mean total runoff of more than 10 in/yr and higher was simulated for HRUs along the western boundary of the SVWM in response to the high precipitation, low PET, steep terrain, and thin soil cover (fig. 30). Low runoff of 1.0 in/yr and less was simulated for the hotter and drier areas of the SVWM, such as the central valley of the Salinas River and along the southeast part of the SVWM domain. The low runoff simulated for the Salinas River valley does not include irrigation return flows because irrigation is not accounted for by the SVWM. Overland runoff is simulated by the SVWM when precipitation exceeds the storage capacity of the land surface and soil zone or when the precipitation rate exceeds the infiltration capacity of PERLNDs. Overland runoff can reinfiltrate the ground surface depending on the slope, length, and roughness of the overland flow plane (defined by parameters SSLUR, LSUR, and MANNING). Simulation results for overland runoff included a 70-year basinwide mean of 0.97 in/yr for the Salinas Valley study area, or about 5.2 percent of precipitation and 28 percent of total runoff (table 19). A maximum subbasin mean overland runoff of 5.5 in/yr was simulated for the NAC subbasin, which was about 22 percent of precipitation and 46 percent of total runoff (table 19). In contrast, a mean overland runoff of only 0.03 in/ yr was simulated for the EST subbasin. High overland runoff was generally limited to the high-elevation HRUs along the

western boundary of the Salinas Valley study area, coinciding with the locations of the highest precipitation and the steepest terrain (fig. 31). The simulated 70-year mean basinwide interflow runoff to stream channels of 0.84 in/yr was similar in magnitude to overland runoff (table 19). The highest interflow runoff of 3.36 in/yr, about 13.6 percent of precipitation, was simulated for the NAC subbasin. The ARR subbasin also had a high mean interflow runoff of 3.08 in/yr compared to the other 8 subbasins having mean values of 1.45 in/yr and less. As with overland runoff, the highest mean interflow runoff of 4 in/yr and more was simulated along the western boundary of the Salinas Valley study area in response to the high precipitation and steep terrain (fig. 32). Low mean interflow values of 0.5 in/yr and less were simulated for the areas that are drier, more inland, less rugged, and more flat-lying, which comprise much of the Salinas Valley study area. Groundwater discharge to streams from the active groundwater reservoir, referred to as baseflow, is simulated by the SVWM as a function of the groundwater recession parameters AGWRC and KVARY and the amount of water recharging and stored in the active groundwater reservoir, minus groundwater losses to transpiration. The 70-year mean groundwater discharge to streams simulated for the Salinas Valley study area was 1.71 in/yr, which was 48 percent of the total outflow to stream channels and about 9.2 percent of precipitation (table 19). The ARR subbasin had the highest mean groundwater discharge to streams of 6.1 in/yr, about 20 percent of precipitation, and 60 percent of the total outflow from HRUs. In contrast, the mean groundwater discharge to streams for the NAC subbasin was only 3.0 in/yr, about 12 percent of precipitation and 25 percent of total outflow. Mean values of groundwater discharge to streams of 3.0 in/yr and higher were simulated for HRUs in the SRH, NAC, SAN, and ARR subbasins, with very high values of 8–28 in/ yr simulated for HRUs in the headwaters of the ARR and SAN subbasins (fig. 33). Mean groundwater discharge to streams of at least 0.1 in/yr was simulated for most HRUs in the SVWM. The lowest mean values of less than 0.1 in/yr were simulated for low-elevation, flat-lying HRUs in the valley bottom of the middle and lower Salinas Valley, generally coincident with the location of unconsolidated alluvium.

Simulation Results, Water Years 1949–2018 97 122°

121°30'

121°

120°30'

120°

101

EXPLANATION

MERCED COUNTY

Simulated mean total runoff, water years 1949–2018, in inches per year

MONTEREY BAY SAN BENITO COUNTY

Salinas Monterey

36° 30'

8.8 to 11.0

15.1 to 17.0

11.1 to 12.0

17.1 to 19.0

12.1 to 13.0

19.1 to 21.0

13.1 to 14.0

21.1 to 23.0

14.1 to 15.0

23.1 to 26.1

Salinas Valley Watershed Model (SVWM) boundary 1

SVWM sub-model boundary MONTEREY COUNTY

Soledad

U.S. Geological Survey streamgage at Bradley (see table 2)

101

King City

36°

FRESNO COUNTY

Lower Salinas Valley sub-model boundary Lake San Antonio

KINGS COUNTY

Lake Nacimiento

KERN COUNTY

Paso Robles 35° 30'

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary 1

San Luis Obispo 101

Santa Margarita Lake SAN LUIS OBISPO COUNTY

0 0

10 10

20 20

30 MILES

30 KILOMETERS

Figure 30. Simulated mean total runoff to stream channels using the Salinas Valley Watershed Model, water years 1949–2018 (Hevesi and others, 2025).

98 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley 121°30'

122°

River aro aj

121°

120°30'

120°

101

EXPLANATION

MERCED COUNTY

Simulated mean overland runoff, water years 1949–2018, in inches per year

MONTEREY BAY

FRESNO COUNTY SAN BENITO COUNTY

Salinas Monterey

0.00 to 0.01

1.01 to 1.50

0.02 to 0.20

1.51 to 2.00

0.21 to 0.40

2.01 to 4.00

0.41 to 0.60

4.01 to 8.00

0.61 to 1.00

8.01 to 24.39

n Sa

Salinas Valley Watershed Model (SVWM) boundary

en it

36° 30'

SVWM sub-model boundary

MONTEREY COUNTY

Soledad

U.S. Geological Survey streamgage at Bradley (see table 2)

101

King City

Sa n as in

Sa l

re e

Lower Salinas Valley sub-model boundary

San Ant

ive r

Na cim ien to

io on

36°

Lo re n

r ve Ri

Lake San Antonio

KINGS COUNTY

Lake Nacimiento

Estrel la R i

KERN COUNTY

r ve

Paso Robles 35° 30'

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary 1

San Luis Obispo 101

Santa Margarita Lake SAN LUIS OBISPO COUNTY

0 0

10 10

20 20

30 MILES

30 KILOMETERS

Figure 31. Salinas Valley Watershed Model simulation results for mean overland runoff to stream channels, water years 1949–2018 (Hevesi and others, 2025).

Simulation Results, Water Years 1949–2018 99 121°30'

122°

River aro aj

121°

120°30'

101

EXPLANATION

MERCED COUNTY

Simulated mean interflow runoff, water years 1949–2018, in inches per year

MONTEREY BAY

FRESNO COUNTY SAN BENITO COUNTY

Salinas Monterey

n Sa

1.51 to 2.00

0.02 to 0.10

2.01 to 4.00

0.11 to 0.50

4.01 to 6.00

0.51 to 1.00

6.01 to 8.00

1.01 to 1.50

8.01 to 14.46

SVWM sub-model boundary

MONTEREY COUNTY

0.00 to 0.01

Salinas Valley Watershed Model (SVWM) boundary

en it

36° 30'

120°

Soledad

U.S. Geological Survey streamgage at Bradley (see table 2)

101

King City

Sa n as in

Sa l

re e

Lower Salinas Valley sub-model boundary

San Ant

ive r

Na cim ien to

io on

36°

Lo re n

r ve Ri

Lake San Antonio

KINGS COUNTY

Lake Nacimiento

Estrel la R i

KERN COUNTY

r ve

Paso Robles

Sa nJ ua nC

ek re

35° 30'

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary 1

San Luis Obispo 101

Santa Margarita Lake SAN LUIS OBISPO COUNTY

0 0

10 10

20 20

30 MILES

30 KILOMETERS

Figure 32. Salinas Valley Watershed Model simulation results for mean interflow runoff to stream channels, water years 1949–2018 (Hevesi and others, 2025).

100 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley 122°

121°30'

121°

120°30'

120°

101

EXPLANATION

MERCED COUNTY

Simulated mean groundwater discharge to streams (baseflow), water years 1949–2018, in inches per year

MONTEREY BAY SAN BENITO COUNTY

Salinas Monterey

36° 30'

0.00 to 0.10

2.01 to 3.00

0.11 to 0.50

3.01 to 4.00

0.51 to 1.00

4.01 to 8.00

1.01 to 1.50

8.01 to 16.00

1.51 to 2.00

16.01 to 28.26

Salinas Valley Watershed Model (SVWM) boundary

SVWM sub-model boundary

MONTEREY COUNTY

Soledad

U.S. Geological Survey streamgage at Bradley (see table 2)

101

King City

36°

FRESNO COUNTY

Lower Salinas Valley sub-model boundary Lake San Antonio

KINGS COUNTY

Lake Nacimiento

KERN COUNTY

Paso Robles 35° 30'

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary 1

San Luis Obispo 101

Santa Margarita Lake SAN LUIS OBISPO COUNTY

0 0

10 10

20 20

30 MILES

30 KILOMETERS

Figure 33. Salinas Valley Watershed Model simulation results for mean groundwater discharge to streams, water years 1949–2018 (Hevesi and others, 2025).

Simulation Results, Water Years 1949–2018 101

Recharge Groundwater recharge simulated by the SVWM includes inflows to the active and inactive groundwater reservoirs. Unlike recharge supplying the active groundwater reservoir used to simulate groundwater discharge to streams and transpiration from the saturated zone, recharge to the inactive groundwater reservoir is an outflow from the hydrologic system represented by the SVWM. Recharge to the inactive groundwater reservoir is referred to in this study as net recharge. Net recharge does not contribute groundwater discharge to streams or transpiration from the saturated zone. As simulated by the SVWM, net recharge consists of two components: (1) stream seepage recharge, or the infiltration of streamflow through the streambed and (2) deep recharge from PERLNDs. Conceptually, net recharge simulated by the SVWM may represent increases in groundwater storage, recharge to groundwater reservoirs having flow paths with long travel times or flow paths contributing to basin underflows, riparian ET, or outflows from wells. The 70-year mean inter-channel recharge (net land-area recharge) was the smallest component of the water balance simulated by the SVWM, with a 70-year mean value of 0.08 in/yr, or about 20,000 acre-ft/yr and 0.5 percent of precipitation for the Salinas Valley study area (tables 19, 20). The MCB subbasin had the highest net land-area recharge of 0.28 in/yr, or about 1.5 percent of precipitation, with HRUs along the northern boundary of the SVWM having the highest rates of more than 0.5 in/yr (fig. 34). All other subbasins had net land-area recharge rates of 0.13 in/yr and less, with the NAC, ARR, and LOR subbasins having rates of approximately zero. The lowest net land-area recharge rates were mostly coincident with consolidated bedrock types having low permeability, such as igneous and metamorphic rocks, shales, and claystones. Overall, the upper Salinas Valley had a greater mean annual net land-area recharge volume compared to the lower Salinas Valley (table 20), mostly because of a high percentage of permeable conglomerate bedrock underlying soils (fig. 10). Stream losses resulting from the downward seepage of streamflow through the streambed are an important component of the water balance simulated by the SVWM. Simulating the streamflow losses, or stream seepage recharge, was needed to achieve a satisfactory calibration of many of the streamgages in the Salinas Valley study area. As indicated by the streamflow records in the Salinas Valley study area,

a substantial portion of the runoff generated in the upland areas of the Salinas River watershed is not discharged to Monterey Bay as streamflow, but rather is reinfiltrated into the main channel of the Salinas River and becomes recharge to aquifers that are potential sources of groundwater pumped for crop irrigation and other uses. For example, the observed mean streamflow at USGS streamgage 11151700 (gage 6; table 2) on the Salinas River is less than the sum of observed streamflow at upstream USGS streamgages 11150500 (gage 19) and 11151300 (gage 15) for water years 1984–2014, with a cumulative decrease in annual streamflow of about 3.2 million acre-ft, or a mean annual decrease in streamflow of 128 ft3/s (fig. 35A). Differences in observed streamflow at other locations also indicate stream losses. There was a cumulative decrease of 3.6 million acre-ft from water years 1984 to 2014 at USGS streamgage 11152300 (gage 10; table 2) on the Salinas River compared to the sum of observed streamflow at upstream USGS streamgages 11152000 (gage 1), 11151700 (gage 6), and 11151300 (gage 15; fig. 35C); a cumulative decrease of 0.8 million acre-ft from water years 1995 to 2016 between downstream USGS streamgage 11152050 (gage 2) and upstream USGS streamgage 11152000 (gage 1) on the Arroyo Seco (fig. 35E); and a cumulative decrease of 13 million acre-ft (a long-term mean streamflow loss of 310 ft3/s) from water years 1959 to 2014 at USGS streamgage 11152500 (gage 21) on the Salinas River compared to the sum of streamflow at upstream USGS streamgages 11152000 (gage 1), 11151300 (gage 15), and 11150500 (gage 19; fig. 35F). Stream seepage recharge is simulated by the SVWM using a second outflow node for each stream reach (the first outflow node is used for simulating streamflow) and was conceptualized as a component of deep recharge to the inactive groundwater reservoir as opposed to recharge to the active groundwater reservoir used for simulating baseflow. Total deep recharge to the inactive groundwater reservoir, calculated as the sum of net land-area recharge and stream seepage recharge, is an outflow from the SVWM because this water does not contribute to simulated baseflow or simulated ET from shallow groundwater. Rather, deep recharge is conceptualized as contributing to groundwater reservoirs that may supply water for crop irrigation or consumptive use, contributing to deep groundwater storage, or contributing to groundwater underflow.

102 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley 122°

121°30'

121°

120°30'

120°

101

EXPLANATION

MERCED COUNTY

Simulated mean groundwater losses (deep recharge), water years 1949–2018, in inches per year

MONTEREY BAY SAN BENITO COUNTY

Salinas Monterey

36° 30'

0.000 to 0.010

0.101 to 0.150

0.011 to 0.025

0.151 to 0.200

0.026 to 0.050

0.201 to 0.400

0.051 to 0.075

0.401 to 0.600

0.076 to 0.100

0.601 to 1.086

Salinas Valley Watershed Model (SVWM) boundary

SVWM sub-model boundary

MONTEREY COUNTY

Soledad

U.S. Geological Survey streamgage at Bradley (see table 2)

101

King City

36°

FRESNO COUNTY

Lower Salinas Valley sub-model boundary Lake San Antonio

KINGS COUNTY

Lake Nacimiento

KERN COUNTY

Paso Robles 35° 30'

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary 1

San Luis Obispo 101

Santa Margarita Lake SAN LUIS OBISPO COUNTY

0 0

10 10

20 20

30 MILES

30 KILOMETERS

Figure 34. Salinas Valley Watershed Model simulation results for mean groundwater losses (deep recharge), water years 1949–2018 (Hevesi and others, 2025).

Simulation Results, Water Years 1949–2018 103

Cumulative difference in annual streamflow, in thousands of acre-feet

A Streamflow at gage 6 minus streamflow at gages 19 and 15

–500 –1,000 –1,500 –2,000 –2,500

EXPLANATION Observed streamflow

–3,000

Simulated streamflow

–3,500

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Water year

Cumulative difference in annual streamflow, in thousands of acre-feet

600

B Streamflow at gage 19 minus reservoir outflows and streamflow at gages 29 and 36

500

EXPLANATION Observed streamflow Simulated streamflow

400 300 200 100 0 –100

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

1990

1995

2000

2005

2010

2015

Water year

Cumulative difference in annual streamflow, in thousands of acre-feet

0 –500

C Streamflow at gage 10 minus streamflow at gages 1, 6 and 15

–1,000 –1,500 –2,000 –2,500 –3,000

EXPLANATION

–3,500

Simulated streamflow

–4,000

Observed streamflow

1950

1955

1960

1965

1970

1975

1980

1985

Water year

Figure 35. Comparison of observed and simulated differences in annual streamflow at selected U.S. Geological Survey streamgages: A, streamflow at gage 6 minus streamflow at gages 19 and 15; B, streamflow at gage 19 minus reservoir outflows and streamflow at gages 29 and 36; C, streamflow at gage 10 minus streamflow at gages 1, 6, and 15; D, streamflow at gage 21 minus streamflow at gage 10; E, streamflow at gage 2 minus streamflow at gage 1; and F, streamflow at gage 21 minus streamflow at gages 1, 15, and 19 (U.S. Geological Survey, 2016; Hevesi and others, 2025).

104 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Cumulative difference in annual streamflow, in thousands of acre-feet

500

Streamflow at gage 21 minus streamflow at gage 10

–500 –1,000 –1,500 –2,000 –2,500 –3,000 –3,500

EXPLANATION

–4,000

Observed streamflow Simulated streamflow

–4,500 –5,000

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Water year

Cumulative difference in annual streamflow, in thousands of acre-feet

E Streamflow at gage 2 minus streamflow at gage 1

–100

EXPLANATION Observed streamflow

–200

Simulated streamflow

–300 –400 –500 –600 –700 –800 –900

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Water year

Cumulative difference in annual streamflow, in thousands of acre-feet

F Streamflow at gage 21 minus streamflow at gages 1, 15 and 19

–2,000 –4,000 –6,000 –8,000 –10,000

EXPLANATION Observed streamflow

–12,000 –14,000

Simulated streamflow

1950

1955

1960

1965

1970

1975

1980

1985

Water year

Figure 35.—Continued

1990

1995

2000

2005

2010

2015

Simulation Results, Water Years 1949–2018 105 The simulated decreases between streamgages indicated a good match to the observed differences in streamflow between USGS streamgages 11152050 (gage 2) and 11152000 (gage 1) on the Arroyo Seco and between USGS streamgage 11152500 (gage 21) on the Salinas River and the sum of streamflow at upstream USGS streamgages 11152000 (gage 1), 11151300 (gage 15), and 11150500 (gage 19; figs. 35E, F). Simulated decreases in streamflow were about half the observed decreases between USGS streamgages 11151700 (gage 6) and the sum of streamflow at upstream USGS streamgages 11151300 (gage 15) and 11150500 (gage 19; fig. 35A). The simulated decreases in streamflow also were about half the observed decreases between USGS streamgage 11152300 (gage 10) and upstream USGS streamgages 11152000 (gage 1), 11151700 (gage 6), and 11151300 (gage 15; fig. 35C). Although the results indicated discrepancy between simulated and observed decreases in streamflow at several locations that were analyzed, the simulation results provided a good overall match to the combined decrease in streamflow in the MSR and LSR subbasins, as indicated by the observed differences in streamflow between USGS streamgage 11152500 (gage 21) and upstream USGS streamgages 11152000 (gage 1), 11151300 (gage 15), and 11150500 (gage 19; fig. 35F). The LSR subbasin had the highest mean simulated stream seepage recharge, as a water equivalent depth for the subbasin area, of 10.74 in/yr (table 19). The ARR subbasin had the second highest mean stream seepage recharge, expressed as a water equivalent depth, of 5.27 in/yr. The MCB subbasin had the smallest stream seepage of 0.18 in/yr, followed by the NAC subbasin with the next smallest stream seepage of 0.23 in/yr (table 19). Simulated streamflow losses were highest along the middle and lower reaches of the Salinas River, and along the lowermost reaches of the ARR, varied from 16 to 32 ft3/s in the middle Salinas Valley and LAS, and varied from 16 to 70 ft3/s in the lower Salinas Valley (figs. 1, 36). Mean stream losses in the main channels of tributary drainages ranged from 1 to 16 ft3/s, and stream losses in the headwater drainages ranged from 0 to 0.5 ft3/s for most RCHRESs in the drainage network. Overall, deep recharge to the inactive groundwater reservoir from stream seepage was substantially greater than net land-area recharge to the inactive groundwater reservoir for all subbasins and subdrainages except for the MCB subbasin and the ELK and MSB subdrainages (tables 19, 20). Total stream seepage for the Salinas Valley study area was 627,000 acre-ft/yr for the 70-year simulation period and was substantially greater than the total net land-area recharge of only 20,000 acre-ft/yr (table 20). The total recharge volume to the inactive groundwater reservoir of 446,000 acre-ft/yr for the LSVS was more than double the total recharge volume for the USVS and also was greater than the surface-water outflow of 232,000 acre-ft/yr for the Salinas Valley study area. In contrast, total recharge to the inactive groundwater reservoir was only 201,000 acre-ft/yr for the USVS, substantially less than the surface-water outflow of 346,000 acre-ft/yr.

Streamflow Simulated 70-year mean streamflow ranged from a maximum of 505 ft3/s for the Salinas River directly downstream from the juncture of the Arroyo Seco tributary to less than 1 ft3/s for most of the small tributary drainages in drier sections of the SVWM (figs. 1, 37). The simulated 70-year mean streamflow at the mouth of the Salinas River was only 278 ft3/s, substantially less than the mean streamflow of 478 ft3/s simulated at USGS streamgage 11150500 (gage 19), because of the large number of streamflow losses caused by streambed seepage (table 20). The simulated 70-year mean streamflow for most sections of the Salinas River in the LSVS, downstream of gage 19, was between 200 and 500 ft3/s (fig. 37). The highest mean tributary inflow of 278 ft3/s was simulated for the NAC subbasin (figs. 1, 37; table 20). High tributary inflows to the Salinas River also were simulated for the ARR (106 ft3/s) and SAN (87 ft3/s) subbasins (table 20). The simulated total mean surface-water inflow from all tributary drainages to the lower Salinas Valley study area was 770 ft3/s (table 20). Expressed as an equivalent runoff depth of the total upstream contributing area for each RCHRES stream segment, the highest 70-year mean runoff depth of 20 to more than 30 in/yr were simulated for the rugged uplands along the western side of the valley (fig. 38). In contrast, simulated runoff depths were only 2 in/yr and less for most stream segments on the east side of the Salinas Valley. Along the section of the Salinas River within the LSVS, downstream of gage 19, runoff depths varied from 1 to 4 in/yr above USGS streamgage 11152500 (gage 21) but decreased to less than 1 in/yr downstream from USGS streamgage 11152500 (gage 21). Simulated runoff depths for many of the lower elevation drainages on the drier, eastern side of the Salinas Valley were less than 0.5 in/yr. Simulated maximum daily streamflow for water years 1949–2018 varied from less than 200 ft3/s for headwater drainages to 80,000–92,000 ft3/s along the main channel of the Salinas River in the MSR and LSR subbasins (figs. 1, 2, 39). Total daily inflow from all tributary drainages to the lower Salinas Valley study area reached a maximum of 124,000 ft3/s during water year 1995 and exceeded 80,000 ft3/s during water years 1969 and 1966 (fig. 40A). Maximum daily streamflow was 10,000–20,000 ft3/s in the ARR, SAN, EST, and SRH subbasins, and 20,000–40,000 ft3/s in the NAC subbasin (figs. 2, 39). Maximum daily inflows from the SECO tributary drainage into the lower Salinas Valley study area were 10,000 ft3/s during water years 1969 and 1998 and 12,000 ft3/s during water year 1995 (fig. 40B). In contrast, maximum daily inflows were higher from the Nacimiento River (NACI) and SALI tributary drainages, with maximum daily inflows of about 37,000 ft3/s during water year 1967 for NACI and during water year 1995 for SALI (figs. 40C, D).

106 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley 122°

121°30'

121°

120°30'

120°

101

EXPLANATION

MERCED COUNTY MONTEREY BAY

Monterey

Simulated mean streamflow seepage losses, water years 1949–2018, in cubic feet per second

39 38

17 SAN BENITO COUNTY

Salinas 21 7 10

36° 30'

0.00 to 0.01

1.51 to 2.00

0.02 to 0.20

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0.21 to 0.50

4.01 to 16.00

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1.01 to 1.50

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Salinas Valley Watershed Model (SVWM) boundary

SVWM sub-model boundary

MONTEREY COUNTY

Soledad

2 6 101

1 13

U.S. Geological Survey streamgage and identifier (see table 2)

U.S. Geological Survey streamgage at Bradley (see table 2)

King City

36°

FRESNO COUNTY

Lower Salinas Valley sub-model boundary

16 19

KINGS COUNTY

3 22

34 36 Paso Robles 20

KERN COUNTY

35° 30'

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary

San Luis Obispo

101

SAN LUIS OBISPO COUNTY

0 0

10 10

20 20

30 MILES

30 KILOMETERS

Figure 36. Salinas Valley Watershed Model simulation results for mean streamflow seepage losses, in cubic feet per second, water years 1949–2018 (Hevesi and others, 2025).

Simulation Results, Water Years 1949–2018 107 122°

121°30'

121°

120°30'

120°

101

EXPLANATION

MERCED COUNTY MONTEREY BAY

Monterey

Simulated mean streamflow, water years 1949–2018, in cubic feet per second

39 38

17 SAN BENITO COUNTY

Salinas 21 7 10

36° 30'

0.0 to 0.5

10.1 to 20.0

0.6 to 1.0

20.1 to 50.0

1.1 to 2.0

50.1 to 100.0

2.1 to 5.0

100.1 to 300.0

5.1 to 10.0

300.1 to 504.9

Salinas Valley Watershed Model (SVWM) boundary

SVWM sub-model boundary

MONTEREY COUNTY

Soledad

U.S. Geological Survey streamgage and identifier (see table 2)

U.S. Geological Survey streamgage at Bradley (see table 2)

6 101

1 13

King City

36°

FRESNO COUNTY

Lower Salinas Valley sub-model boundary

16 19

KINGS COUNTY

3 22

34 36 Paso Robles 20

KERN COUNTY

35° 30'

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary

San Luis Obispo

101

SAN LUIS OBISPO COUNTY

0 0

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20 20

30 KILOMETERS

Figure 37. Salinas Valley Watershed Model simulation results for mean streamflow, in cubic feet per second (ft3/s), water years 1949–2018 (Hevesi and others, 2025).

30 MILES

108 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley 122°

121°30'

121°

120°30'

120°

101

EXPLANATION

MERCED COUNTY MONTEREY BAY

Monterey

Simulated mean streamflow runoff equivalent, water years 1949–2018, in inches per year

39 38

FRESNO COUNTY

17 SAN BENITO COUNTY

Salinas 21 7

6.1 to 8.0

0.6 to 1.0

8.1 to 10.0

1.1 to 2.0

10.1 to 20.0

2.1 to 4.0

20.1 to 30.0

4.1 to 6.0

30.1 to 357.7

Salinas Valley Watershed Model (SVWM) boundary

10 36° 30'

0.0 to 0.5

SVWM sub-model boundary

MONTEREY COUNTY

Soledad

U.S. Geological Survey streamgage and identifier (see table 2)

U.S. Geological Survey streamgage at Bradley (see table 2)

2 6 101

1 13

King City

36°

Lower Salinas Valley sub-model boundary

16 19

KINGS COUNTY

3 22

34 36 Paso Robles 20

KERN COUNTY

35° 30'

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary

San Luis Obispo

101

SAN LUIS OBISPO COUNTY

0 0

10 10

20 20

30 MILES

30 KILOMETERS

Figure 38. Salinas Valley Watershed Model simulation results for mean streamflow runoff equivalent in inches per year, water years 1949–2018 (Hevesi and others, 2025).

Simulation Results, Water Years 1949–2018 109 122°

121°30'

121°

120°30'

120°

101

EXPLANATION

MERCED COUNTY MONTEREY BAY

Monterey

Simulated maximum daily streamflow, 1949–2018, in cubic feet per second

39 38

FRESNO COUNTY

17 SAN BENITO COUNTY

Salinas 21 7

10,001 to 20,000

201 to 500

20,001 to 40,000

501 to 1,000

40,001 to 60,000

1,001 to 5,000

60,001 to 80,000

5,001 to 10,000

80,001 to 92,027

Salinas Valley Watershed Model (SVWM) boundary

10 36° 30'

0 to 200

SVWM sub-model boundary MONTEREY COUNTY

Soledad

U.S. Geological Survey streamgage and identifier (see table 2)

U.S. Geological Survey streamgage at Bradley (see table 2)

2 6 101

1 13

King City

36°

Lower Salinas Valley sub-model boundary

16 19

KINGS COUNTY

3 22

34 36 Paso Robles 20

KERN COUNTY

35° 30'

PACIFIC OCEAN

Upper Salinas Valley sub-model boundary

San Luis Obispo

101

SAN LUIS OBISPO COUNTY

0 0

10 10

20 20

30 MILES

30 KILOMETERS

Figure 39. Salinas Valley Watershed Model simulation results for maximum daily streamflow seepage losses, in cubic feet per second, water years 1949–2018 (Hevesi and others, 2025).

110 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Daily mean streamflow, in cubic feet per second

140,000

A Total surface water inflow to lower Salinas Valley study area from all tributary drainage

120,000 100,000 80,000 60,000 40,000 20,000 0

1950

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Water year 14,000

B Surface water inflow to lower Salinas Valley study area from the Arroyo Seco tributary drainage

Daily mean streamflow, in cubic feet per second

12,000 10,000 8,000 6,000 4,000 2,000 0

1950

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Water year 40,000

Daily mean streamflow, in cubic feet per second

35,000

Surface water inflow to lower Salinas Valley study area from the Nacimiento River tributary drainage

30,000 25,000 20,000 15,000 10,000 5,000 0

1950

1955

1960

1965

1970

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1980

1985

Water year 40,000

Daily mean streamflow, in cubic feet per second

35,000 30,000

D Surface water inflow to lower Salinas Valley study area from the Salinas River tributary drainage

25,000 20,000 15,000 10,000 5,000 0

1950

1955

1960

1965

1970

1975

1980

1985

Water year

Figure 40. Simulated total daily surface-water inflows to the lower Salinas Valley study area from A, all tributary drainages; B, the Arroyo Seco tributary drainage (SECO); C, the Nacimiento River tributary drainage (NACI); and D, from the Salinas River tributary drainage (SALI; Hevesi and others, 2025).

Simulation Results, Water Years 1949–2018 111 Differences in simulated water balance components between the 10 subbasins were strongly dependent on differences in precipitation, with the ARR subbasin having the highest precipitation of about 30.1 in/yr, and the middle Salinas Valley subbasin having the lowest precipitation of about 14.7 in/yr. The NAC subbasin had the highest total flow (total runoff) to streams of about 11.8 in/yr, the highest percentage of total runoff relative to precipitation, and the highest percentage of overland runoff relative to total runoff to streams. In contrast, the ARR subbasin had the highest percentage of groundwater discharge to streams relative to the total runoff to streams. A large percentage of the 10.1 in/yr total outflow to streams in the ARR subbasin, about 5.3 in/yr, was lost to stream seepage. Only 4.9 in/yr (about 77,000 acre-ft/yr) of the total runoff simulated in the ARR subbasin reached the Salinas River channel. Total inflow from upland tributaries to the Salinas Valley lowlands was about 2.9 in/yr (558,000 acre-ft/yr) or about 15 percent of the 19.6 in/yr precipitation falling over the tributary subbasins.

Annual Results The time series of annual ET for the Salinas Valley study area indicated a high degree of year-to-year variability, coinciding with the annual variability in precipitation, with high annual totals of more than 22 in. for water years 1958, 1983, 1995, and 1998 (fig. 41A). Comparison of annual results for the subbasins (fig. 42) indicated that ET was the largest water outflow for all years and all subbasins except NAC, where streamflow exceeded ET for water years with more than about 40 in. total precipitation, including 1958, 1969, 1978, 1983, 1996, 1998, and 2017 (the wettest water years for all subbasins). Water year 2014, the driest year, with less than 8 in. of precipitation for the Salinas Valley study area, had the lowest simulated annual ET of about 7 in. (fig. 41A). Comparison of annual ET between the 10 subbasin areas indicated differences of more than 6 in. between subbasins for some years, such as between the EST and ARR subbasins (fig. 43A). Relative differences between the subbasins were not consistent year to year, with the highest annual ET alternating between the SRH and ARR subbasins, mostly in response to spatial variations in annual precipitation. Water year 2014, the driest water year, had the lowest ET for all subbasins, and water years 1983 and 1998, the two wettest water years, had higher than average ET for all subbasins. Water year 1969 had the highest annual runoff (total outflow to streams) of about 14 in. for the Salinas Valley study area (fig. 41B). In contrast, many dry years (such as 2014) had low runoff values of less than 1.0 in. The cumulative departure from mean runoff for the Salinas Valley study area indicated a drier-than-normal period, relative to the 70-year mean, starting after water year 1998. The ARR and NAC

subbasins had the highest total runoff outflow to streams for all water years, with both subbasins exceeding 25 in. during the three wet years 1969, 1983, and 1998 (fig. 43B). As with total runoff to streams, annual variability in overland runoff, interflow runoff, and groundwater outflow to streams was high for most subbasins (figs. 43C, D, E). Water year 1969 had the highest overland and interflow runoff for most subbasins, whereas water year 1998 had the highest groundwater outflow to streams for most subbasins. Basinwide annual streamflow seepage (streamflow losses) simulated for the Salinas Valley study area was highest for water year 1983 (about 7.9 in. equivalent basinwide water depth) and water year 1998 (about 7.8 in. basinwide water depth; fig. 41C). A minimum mean basinwide streamflow loss of about 0.2 in. was simulated for water year 2014. The cumulative departure from the 70-year mean-annual stream seepage recharge indicates a period of below-average stream seepage after water year 2006 (fig. 41C). Annual results for subbasins indicated that the simulated streamflow losses were much higher for the LSR subbasin compared to all other subbasins, with 12 water years exceeding 20 in. (fig. 43F).

Mean Monthly Results Comparison of the mean monthly simulation results for the SVWM and the 10 subbasins indicated large variations in components of the water balance across different areas of the Salinas Valley study area (fig. 44). Mean monthly actual evapotranspiration (AET), the largest outflow of water from the SVWM, reached a maximum of about 4.2 in. during April for the ARR subbasin, whereas the NAC subbasin reached a maximum of 2.7 in. during March (fig. 44A). The NAC subbasin has a maximum mean monthly total runoff outflow (land-area runoff) of 3.5 in. to streams during January, compared to the ARR subbasin where the maximum mean monthly total runoff outflow of 2.4 in. to streams occurs during February (fig. 44B). Mean monthly overland runoff to streams is much greater for the NAC subbasin compared to all other subbasins, with a maximum mean monthly overland runoff of about 1.65 in. occurring during January (fig. 44C). The NAC and ARR subbasins have similar results for mean monthly interflow runoff to streams (fig. 44D), whereas the ARR subbasin has a much higher mean monthly groundwater outflow (baseflow) to streams compared to other subbasins, with the highest values of about 1.3 in. occurring during February and March (fig. 44E). The MCB subbasin had a much higher mean monthly inflow to inactive groundwater (deep recharge) compared to other subbasins, with a maximum of 0.09 in. occurring during February (fig. 44F). The LSR subbasin had a much higher mean monthly streamflow loss compared to other subbasins, with the highest values of about 2.8 in. occurring during January and February (fig. 44G).

112 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley A

–5

–10

–15

8 6

Cumulative departure from mean, in inches

Annual evapotranspiration, in inches

1950

1955

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1980

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1990

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2000

2005

2010

2015

–20

Water Year EXPLANATION Annual Mean annual

Cumulative departure from mean

–5

–10

–15

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2015

Cumulative departure from mean, in inches

Total outflow to streams, in inches

20-year moving mean 10-year moving mean

–20

Water Year EXPLANATION Annual Mean annual

Cumulative departure from mean

10 8

Annual streamflow losses, in inches

0 –2

–4

–6

1 0

Cumulative departure from mean, in inches

20-year moving mean 10-year moving mean

–8 1950

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2015

–10

Water Year EXPLANATION Annual 70-year mean annual

20-year moving mean 10-year moving mean

Cumulative departure from 70-year mean

Figure 41. Annual simulation results for the Salinas Valley study area using the Salinas Valley Watershed Model (SVWM): A, actual evapotranspiration; B, total outflow to streams; C, streamflow losses; D, inflow to inactive groundwater reservoirs; E, groundwater outflow to streams; F, interflow runoff to streams; and G, overland runoff to streams (Hevesi and others, 2025).

0.50

0.5

0.45

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0.35

0.2 0.1

0.30

0.25

–0.1

0.20

–0.2

0.15

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0.10

–0.4

0.05

–0.5

1950

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2015

Cumulative departure from mean, in inches

Annual inflow to inactive groundwater reservoir, in inches

Simulation Results, Water Years 1949–2018 113

–0.6

Water Year EXPLANATION 20-year moving mean 10-year moving mean

Annual Mean annual

–2 –4

–6

1 0

–8 1950

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Water Year EXPLANATION 20-year moving mean 10-year moving mean

Annual Mean annual 4.0

1995

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2010

2015

Annual interflow runoff to streams, in inches

–10

Cumulative departure from mean

5 4

3.5

3.0

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Cumulative departure from mean, in inches

1950

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Water Year Annual Mean annual

Figure 41.—Continued

EXPLANATION 20-year moving mean 10-year moving mean

Cumulative departure from mean

2015

–4

Cumulative departure from mean, in inches

Annual groundwater outflow to streams, in inches

Cumulative departure from mean

114 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley G

7 6

Annual overalnd runoff to streams, in inches

4.0

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Water Year

Annual Mean annual

Figure 41.—Continued

EXPLANATION 20-year moving mean 10-year moving mean

Cumulative departure from mean

2015

–4

Cumulative departure from mean, in inches

4.5

Simulation Results, Water Years 1949–2018 115

A SRH

Inflows 40

Annual totals, in inches

30 20 10 0 –10 –20 –30 –40

Outflows –50 –60

1950

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Water Year EXPLANATION Precipitation

35 30 25

Evapotranspiration

Streamflow

Groundwater loss

Storage inflow/outflow

B EST Inflows

Annual totals, in inches

15 10 5 0 –5 –10 –15 –20 –25 –30

Outflows

–35 1950

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Water Year EXPLANATION Precipitation

Evapotranspiration

Streamflow

Groundwater loss

Storage inflow/outflow

Figure 42. Simulated annual (water year) inflows and outflows for subbasins A, SRH, Salinas River headwaters; B, EST, Estrella River; C, USR, upper Salinas River; D, NAC, Nacimiento River; E, SAN, San Antonio River; F, MSR, middle Salinas River; G, LOR, San Lorenzo Creek; H, ARR, Arroyo Seco; I, LSR, lower Salinas River; and J, MCB, Monterey Coastal Basins (Hevesi and others, 2025).

116 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

C USR

Inflows

Annual totals, in inches

–10

–20

Outflows

–30

–40

1950

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Water Year EXPLANATION Precipitation

Evapotranspiration

Streamflow

Groundwater loss

Storage inflow/outflow

D NAC

50 40

Inflows

Annual totals, in inches

30 20 10 0 –10 –20 –30 –40

Outflows

–50 –60 1950

1955

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Water Year EXPLANATION Precipitation

Figure 42.—Continued

Evapotranspiration

Streamflow

Storage inflow/outflow

2010

2015

Simulation Results, Water Years 1949–2018 117

E SAN

Inflows

40 30

Annual totals, in inches

20 10 0 –10 –20 –30 –40

Outflows

–50

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Water Year EXPLANATION Precipitation

35 30 25

Evapotranspiration

Streamflow

Groundwater loss

Storage inflow/outflow

F MSR Inflows

Annual totals, in inches

15 10 5 0 –5 –10 –15 –20 –25 –30

Outflows

–35 1950

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Water Year EXPLANATION Precipitation

Figure 42.—Continued

Evapotranspiration

Streamflow

Groundwater loss

Storage inflow/outflow

118 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

G LOR

Inflows 30

Annual totals, in inches

–10

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–30

Outflows –40

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Water Year EXPLANATION Precipitation

70 60 50

Evapotranspiration

Streamflow

Groundwater loss

Storage inflow/outflow

H ARR Inflows

Annual totals, in inches

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Outflows

–70 1950

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Water Year EXPLANATION Precipitation

Figure 42.—Continued

Evapotranspiration

Streamflow

Groundwater loss

Storage inflow/outflow

Simulation Results, Water Years 1949–2018 119

I LSR

Inflows

30 25 20

Annual totals, in inches

15 10 5 0 –5 –10 –15 –20 –25 –30

Outflows

–35 –40

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Water Year EXPLANATION Precipitation

45 40 35

Evapotranspiration

Streamflow

Groundwater loss

Storage inflow/outflow

J MCB Inflows

30 25

Annual totals, in inches

20 15 10 5 0 –5 –10 –15 –20 –25 –30 –35 –40

Outflows

–45 1950

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Water Year EXPLANATION Precipitation

Figure 42.—Continued

Evapotranspiration

Streamflow

Groundwater loss

Storage inflow/outflow

120 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Annual actual evapotranspiration, in inches

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Water year

Annual total outflow to streams, in inches

30 25 20 15 10 5 0

1950

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Water year

Annual overland runoff to streams, in inches

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Water year

EXPLANATION Salinas Valley Watershed Model subbasin (see table 1) ARR

LOR

MCB

NAC

SRH

EST

LSR

MSR

SAN

USR

Figure 43. Comparison of annual simulation results for 10 subbasins in the Salinas Valley study area using the Salinas Valley Watershed Model, water years 1949–2018: A, actual evapotranspiration; B, land-area total outflow to streams; C, overland runoff to streams; D, interflow runoff to streams; E, groundwater outflow to streams; F, streamflow losses; and G, inflow to inactive groundwater reservoir. Abbreviations: ARR, Arroyo Seco; EST, Estrella River; LOR, San Lorenzo Creek; LSR, lower Salinas River; MCB, Monterey Coastal Basins; MSR, middle Salinas River; NAC, Nacimiento River; SAN, San Antonio River; SRH, Salinas River headwaters; and USR, upper Salinas River (Hevesi and others, 2025).

Simulation Results, Water Years 1949–2018 121

Annual interflow runoff to streams, in inches

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

1990

1995

2000

2005

2010

2015

1990

1995

2000

2005

2010

2015

Water year

Annual groundwater outflow to streams, in inches

1950

1955

1960

1965

1970

1975

1980

1985

Water year

Annual streamflow losses, in inches

30 25 20 15 10 5 0

1950

1955

1960

1965

1970

1975

1980

1985

Water year

EXPLANATION Salinas Valley Watershed Model subbasin (see table 1)

Figure 43.—Continued

ARR

LOR

MCB

NAC

SRH

EST

LSR

MSR

SAN

USR

122 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Annual inflow to inactive groundwater reservoir, in inches

2.0

1.5

1.0

0.5

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

Water year

EXPLANATION Salinas Valley Watershed Model subbasin (see table 1)

Figure 43.—Continued

ARR

LOR

MCB

NAC

SRH

EST

LSR

MSR

SAN

USR

2010

2015

Simulation Results, Water Years 1949–2018 123

4.5

A EXPLANATION

Mean monthly actual evapotranspiration, in inches

4.0

Salinas Valley Watershed Model Salinas Valley Watershed Model subbasin (see table 1) ARR MSR

3.5

3.0

2.5

EST

NAC

LOR

SAN

LSR

SRH

MCB

USR

2.0

1.5

1.0

0.5

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Water Year

3.6

B EXPLANATION

Mean monthly total outflow to streams, in inches

3.2

Salinas Valley Watershed Model Salinas Valley Watershed Model subbasin (see table 1) ARR MSR

2.8

2.4

2.0

EST

NAC

LOR

SAN

LSR

SRH

MCB

USR

1.6

1.2

0.8

0.4

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Water Year

Figure 44. Salinas Valley Watershed Model (SVWM) mean monthly simulation results for subbasins during water years 1949–2018: A, total actual evapotranspiration; B, total outflow to streams; C, overland runoff to streams; D, interflow runoff to streams; E, groundwater outflow to streams; F, inflow to inactive groundwater reservoir; and G, streamflow losses. Abbreviations: ARR, Arroyo Seco; EST, Estrella River; LOR, San Lorenzo Creek; LSR, lower Salinas River; MCB, Monterey Coastal Basins; MSR, middle Salinas River; NAC, Nacimiento River; SAN, San Antonio River; SRH, Salinas River headwaters; USR, upper Salinas River (Hevesi and others, 2025).

124 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

1.8

C EXPLANATION

Mean monthly overland runoff to streams, in inches

1.6

Salinas Valley Watershed Model Salinas Valley Watershed Model subbasin (see table 1) ARR MSR

1.4

1.2

1.0

EST

NAC

LOR

SAN

LSR

SRH

MCB

USR

0.8

0.6

0.4

0.2

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Water Year

1.1

D EXPLANATION

Mean monthly interflow runoff to streams, in inches

1.0

Salinas Valley Watershed Model

0.9

Salinas Valley Watershed Model subbasin (see table 1) ARR MSR

0.8 0.7 0.6

EST

NAC

LOR

SAN

LSR

SRH

MCB

USR

0.5 0.4 0.3 0.2 0.1 0

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

Water Year

Figure 44.—Continued

May

June

July

Aug.

Sept.

Simulation Results, Water Years 1949–2018 125

Mean monthly groundwater outflow to streams, in inches

1.3

1.2

EXPLANATION

1.1

Salinas Valley Watershed Model Salinas Valley Watershed Model subbasin (see table 1) ARR MSR

1.0 0.9 0.8 0.7

EST

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0.6 0.5 0.4 0.3 0.2 0.1 0

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Jan.

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Mar.

Apr.

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June

July

Aug.

Sept.

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Mean monthly inflow to inactive groundwater reservoir, in inches

0.10

F EXPLANATION

0.09

Salinas Valley Watershed Model

0.08

Salinas Valley Watershed Model subbasin (see table 1) ARR MSR

0.07

0.06

0.05

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SRH

MCB

USR

0.04

0.03

0.02

0.01

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

Water Year

Figure 44.—Continued

May

June

July

Aug.

Sept.

126 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley G

3.0

EXPLANATION Salinas Valley Watershed Model

Mean monthly streamflow losses, in inches

2.5

Salinas Valley Watershed Model subbasin (see table 1) ARR MSR 2.0

1.5

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LOR

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0.5

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Nov.

Dec.

Jan.

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Mar.

Apr.

May

June

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Aug.

Sept.

Water Year

Figure 44.—Continued

Multi-Year Variability Analysis of annual, basinwide results for the Salinas Valley study area was used to identify and compare multi-year wet and dry periods within the 70-year simulation period (water years 1949–2018). Wet and dry periods were identified using the moving 20-year mean and the cumulative departure from the 70-year annual mean, both calculated as a percentage departure from the 70-year mean. The analysis was done using annual (water year) results for precipitation, ET, recharge, and surface-water outflow (fig. 45), with recharge defined as the sum of stream seepage recharge and inter-channel net land-area recharge to the inactive groundwater reservoir (recharge that does not contribute to groundwater discharge) and surface-water outflow defined as streamflow to Monterey Bay from the Salinas Valley study area.

The percentage of cumulative departure from the 70-year mean was used for a direct comparison of annual variability for the four water balance components (fig. 45B). The cumulative departure for surface-water outflow indicated the highest degree of annual variability in terms of percentage of departures from the mean, with recharge indicating the next highest degree of annual variability, and ET indicating the lowest degree of annual variability relative to the 70-year mean. Three drier-than-average and two wetter-than-average periods were identified based on consistent trends of decreasing values (negative slope) for drier-than-average periods and consistent trends of increasing values (positive slope) for wetter-than-average periods.

Simulation Results, Water Years 1949–2018 127 The three drier-than-average periods were water years 1949–68, 1984–92, and 1999–2018, and the two wetter-than-average periods were water years 1969–83 and 1993–98 (fig. 46). The 9-year period of 1984–92 was the driest with 3.7 million acre-ft/yr precipitation, 0.38 million acre-ft/yr recharge, and 0.17 million acre-ft/yr surface-water outflow (fig. 46). The 6-year period of 1993–98 was the wettest with 6.5 million acre-ft/yr precipitation resulting in about 310 percent greater recharge (1.18 million acre-ft/yr) and about 430 percent greater surface-water outflow (0.74 million acre-ft/yr) compared to the 1984–92 dry period. The last 20 years of the simulation period (water years 1999–2018) was the driest 20-year period in the 70-year simulation (fig. 45A), and the second driest of the three drier-than-average periods identified using the cumulative

departure from mean curves, with 0.49 million acre-ft/yr recharge about 20 percent less than the 70-year mean and 0.21 million acre-ft/yr surface-water outflow about 32 percent less than the 70-year mean. In addition to the lowest mean recharge and surface-water outflow for any 20-year period, the last 20 years (water years 1999–2018) also included the driest 10-year period, ending with water year 2016, with a mean recharge of 1.5 in/yr (0.36 million acre-ft/yr) and a mean surface-water outflow of 0.59 in/yr (0.14 million acre-ft/yr). The last 20-year and 10-year periods included water year 2014, the driest year in the simulation, with 7.5 in. precipitation, 0.24 in. recharge (0.06 million acre-ft/yr), and 0.08 in. surface-water outflow (0.02 million acre-ft/yr).

20-year moving mean percentage departure from 70-year mean

30 20 10 0 –10

EXPLANATION

–20

Precipitation

Recharge

Zero line

Evapotranspiration

Surface water outflow

–30 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 2018

Water Year 700

B EXPLANATION

Cumulative departure from 70-year mean, in percent

600 500

Dry period

Recharge

400

Wet period

300

Precipitation

Surface water outflow

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Zero line

Evapotranspiration

100 0 –100 –200 –300 –400 –500

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1955

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2006

2010

2015

Water Year

Figure 45. Salinas Valley Watershed Model (SVWM) multi-year variability in basinwide annual precipitation, evapotranspiration, recharge, and surface-water outflow: A, 20-year moving mean percentage departure from the 70-year mean and B, cumulative percentage departure from the 70-year mean (Hevesi and others, 2025).

128 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Annual averages for wet and dry periods, in millions of acre-feet per year

6.50

6 5.30 5 4

EXPLANATION

4.50

4.30

4.00

4.10

3.70

3.50

Precipitation 3.50

3.20

Evapotranspiration

Recharge Surface water outflow

2 1.18 1 0

0.51

0.82 0.24

1949 to 1968

0.47

1969 to 1983

0.74 0.38

0.17

1984 to 1992

1993 to 1998

0.49

0.21

1999 to 2018

Water years

Figure 46. Salinas Valley Watershed Model (SVWM) variability in basinwide average annual precipitation, evapotranspiration, recharge, and surface-water outflow for multi-year wet and dry periods, in millions of acre-feet per year (Hevesi and others, 2025).

Summary and Conclusions The U.S. Geological Survey (USGS), in cooperation with the Monterey County Water Resources Agency (MCWRA), has been conducting studies to help evaluate the surface-water and groundwater resources of the 4,529 square mile Salinas Valley study area. The study area comprises the entire Salinas River watershed and several smaller, adjacent coastal basins draining into Monterey Bay. The study area includes a highly productive agricultural region that depends on the coordinated use of surface water and groundwater to meet the demands for irrigation and public water supply. To help the MCWRA meet the demands for irrigation and public water supply, the Salinas Valley Watershed Model (SVWM) was developed for the purpose of improving the understanding of the natural hydrologic system and quantifying components of the water balance, including precipitation, evapotranspiration (ET), recharge, and streamflow. The SVWM combines two hydrologic modeling applications: the Basin Characterization Model (BCM) and the Hydrologic Simulation Program–Fortran (HSPF). The SVWM was calibrated and applied to simulate the historical natural water balance for the Salinas Valley study area for water years 1949–2018. Simulation results for the 70-year period were used to provide estimates of daily, monthly, and annual surface-water inflows from mountainous, upland tributary drainages surrounding the more flat-lying lowlands in the central, lower Salinas Valley that include developed lands and irrigated croplands dependent on groundwater.

The BCM was applied using a 270-meter (886-foot) grid covering the Salinas Valley study area to develop spatially distributed daily climate inputs for the SVWM consisting of precipitation, maximum and minimum air temperature, and potential evapotranspiration (PET). The daily climate inputs were disaggregated to hourly time steps used by HSPF to provide a continuous simulation of the natural hydrologic system, with an emphasis on hydrologic processes of the land cover, pervious and impervious land surfaces, and the shallow subsurface including the root zone. The SVWM was discretized into 690 irregular polygon areas referred to as hydrologic response units (HRUs). The HRUs are used to account for spatially varying climate and watershed characteristics and were defined hydrographically and topographically by the stream drainage network, subdrainage areas, and boundaries separating valley lowlands from the surrounding, more rugged upland areas. Geospatial data defining topography, land cover, soil properties, and surficial geology were used to estimate hydrologic parameters for pervious and impervious land areas comprising the HRUs. The 690 HRUs were linked to 690 stream reaches used to simulate the natural surface-water drainage network of the Salinas Valley study area. The SVWM model domain was divided into the upper and lower Salinas Valley sub-model domains (USVS; LSVS), with 387 HRUs in the USVS and 303 HRUs in the LSVS. The use of two separate model domains allowed for a more detailed model discretization representing spatial heterogeneity in climate and watershed characteristics and to better represent the many small tributary subdrainages providing inflows to the Salinas Valley lowlands.

Summary and Conclusions 129 Initial estimates of model parameters were refined by trial-and-error calibration based on the fitting of simulated daily, monthly, and annual (water year) streamflow to available streamflow records at 29 USGS streamgages. Records of estimated daily, monthly, and annual inflows for two reservoirs in the Salinas Valley study area, Lakes Nacimiento and San Antonio, also were used in model calibration. The trial-and-error calibration resulted in a good overall fit between simulated and observed streamflow, as defined by the calibration criteria of plus or minus 10 percent average estimation error and a Nash–Sutcliffe Model Efficiency (NSME) statistic of 0.5 or higher for daily streamflow, 0.6 or higher for monthly streamflow, and 0.7 or higher for annual streamflow. Calibration results indicated the greatest sensitivity to parameters controlling root zone water content, ET, groundwater discharge to streams (baseflow), interflow runoff, and overland runoff. The model archive for the calibrated model is available in Hevesi and others (2025) for citing the data release with the model archive. The SVWM was used to simulate the daily hydrologic water budget for a 70-year period beginning October 1, 1948, and ending September 30, 2018 (water years 1949–2018). The simulations were started on October 1, 1947, to allow a 1-year model initialization period. The simulation results were used to develop and analyze the long-term, 70-year mean water budget and transient annual, monthly, and daily water budgets for 10 subbasins, 28 subdrainages, 25 upland tributary drainages to the Salinas Valley lowlands, and the 690 HRUs and connected stream reaches. During the 70-year mean precipitation for the Salinas Valley study area, the only inflow to the natural hydrologic system represented by the SVWM, was 18.5 inches per year (in/yr) or about 4.47 million acre-feet per year (acre-ft/yr), varying from 9 in/yr for low elevations along the valley floor to as much as 60 in/yr for the highest elevation drainages in the Arroyo Seco subbasin. The simulated 70-year mean ET was 14.9 in/yr, equal to about 3.6 million acre-ft/yr and about 81 percent of the total water outflow from the Salinas Valley study area. Evapotranspiration varied from 8.8 to 26 in/yr in response to spatial variability in precipitation, PET, land cover, and the water-holding capacity of the root zone. Recharge simulated by the SVWM included recharge to the active and inactive reservoirs. The SVWM uses the active groundwater reservoir as the water source, contributing groundwater discharge to streams and ET from groundwater, and is replenished by recharge from water percolating beneath the root zone of pervious land areas. Recharge from the root zone, also referred to as net land-area recharge, is partitioned into recharge to the active and inactive reservoirs, with all or most of the land-area recharge contributing to the active

groundwater reservoir, then subsequently to groundwater discharge. The 70-year mean recharge for the Salinas Valley study area, calculated as the sum of recharge contributing to baseflow, net land area recharge (also referred to as deep recharge in this report), and stream seepage was about 4.4 in/yr (about 1.06 million acre-ft/yr) and 23.7 percent of precipitation, with 1.71 in/yr (about 412,000 acre-ft/yr) contributing to the baseflow component of streamflow. Net land-area recharge to the inactive groundwater reservoir (deep recharge) was only 0.08 in/yr or about 0.5 percent of the total outflow. Recharge to the inactive groundwater reservoir, simulated by the SVWM as recharge that does not contribute groundwater discharge to streams or ET, included intra-channel stream seepage recharge resulting from the infiltration of streamflow through the streambed. The 70-year mean recharge to the inactive groundwater reservoir, including stream seepage recharge and net land-area recharge, was 2.7 in/yr or about 647,000 acre-ft/yr and about 14.5 percent of the total water outflow, which is the second largest outflow of water from the SVWM. The 70-year mean stream seepage recharge for the Salinas Valley study area was about 2.6 in/yr (equal to about 627,000 acre-ft/yr) and accounted for 14 percent of the water outflow from the SVWM hydrologic system. The simulated 70-year mean land-area runoff from HRUs consisted of overland runoff, interflow runoff, and groundwater discharge to streams, and was 3.5 in/yr or about 850,000 acre-ft/yr and 19 percent of precipitation. Most of the runoff was simulated as groundwater discharge; the 70-year mean groundwater discharge to streams accounted for about 48 percent of the total runoff for the Salinas Valley study area. In comparison, the 70-year mean overland runoff was 0.97 in/yr (about 28 percent of total runoff), and the mean interflow runoff was about 0.84 in/yr (about 24 percent of total runoff). Most of the land-area runoff subsequently became stream seepage recharge. The simulated surface-water outflow to the ocean was only 0.96 in/yr (232,000 acre-ft/yr) about 5.2 percent of precipitation and 27 percent of the runoff generated from land areas. This result was found to be generally consistent with streamflow records in the Salinas Valley study area. The simulated 70-year mean surface-water inflow into the lower Salinas Valley from all surrounding upland tributary areas was about 770 cubic feet per second (ft3/s), equal to about 558,000 acre-ft/yr and about 15 percent of precipitation over the upland areas. In contrast, the 70-year mean streamflow at the mouth of the Salinas River was only about 278 ft3/s (about 201,000 acre-ft/yr) and 4.8 percent of precipitation over the entire Salinas River watershed.

130 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley The Nacimiento River (NAC) subbasin was the primary source of streamflow to the main channel of the Salinas River, with a simulated 70-year mean streamflow of about 277 ft3/s. In comparison, the Estrella River (EST) subbasin, with an area of 924 square miles (mi2), almost 3 times the 325 mi2 area of the NAC subbasin, had a 70-year mean streamflow of only 21 ft3/s. The large difference in runoff between the two subbasins is caused primarily by differences in climate between the west and east sides of the Salinas Valley study area: the proximity of the NAC subbasin drainage area close to the Pacific Ocean moisture source results in higher precipitation and lower PET compared to the more inland EST subbasin. In addition to climate, differences in topography, soils, and surface geology cause differences in runoff and streamflow as a percentage of precipitation and the portion of streamflow lost to stream seepage recharge. Simulated groundwater discharge to streams (baseflow) was the main source of streamflow for many locations in the Salinas Valley study area, varying from less than 0.1 in/yr for drier, lower elevation HRUs to about 28 in/yr for high-elevation HRUs along the southwestern boundary of the Arroyo Seco subbasin. Simulated baseflow was seasonal for most locations because winter season recharge was not sufficient to maintain baseflow throughout the dry summer months for most stream reaches. Most of the groundwater discharge occurred over a time span of days to weeks rather than months after winter storms. The lack of sustained baseflow caused streamflow to be mostly ephemeral and highly sensitive to the temporal variability in precipitation, with extended dry periods resulting in no-flow conditions along the main channel of the Salinas River. Simulated streamflow was consistent with streamflow records throughout the Salinas Valley study area in terms of flashy peak flows in response to storms followed by low-flow and no-flow conditions during the dry summer months and extended dry periods. The simulated overland and interflow components of runoff from inter-channel land areas also had a high degree of variability between HRUs and subdrainages. The 70-year mean overland runoff was less than 0.4 in/yr for most locations in the Salinas Valley study area; however, mean values of 4 in/yr and higher were simulated for HRUs with high precipitation, steep slopes, low soil storage capacity, and low-permeability bedrock underlying the soil zone. The simulated 70-year mean interflow runoff also varied spatially

with low values of 0.5 in/yr and less for most locations and high values of 4 in/yr and greater for HRUs with high precipitation and steep slopes. The relative magnitude of interflow runoff compared to overland runoff varied considerably between subbasins and subdrainages. Interflow runoff tended to be higher than overland runoff for HRUs with thicker soils and forested land cover, whereas overland runoff was higher than interflow runoff for HRUs with a high percentage of grasslands and impervious land cover. Analysis of annual, basinwide results for the Salinas Valley study area was used to identify and compare multi-year wet and dry periods within the 70-year simulation period (water years 1949–2018). The percentage of cumulative departures from the 70-year mean for precipitation, ET, net recharge to the inactive groundwater reservoir, and surface-water outflow to Monterey Bay were used to identify three drier-than-average periods (water years 1949–68, 1984–92, and 1999–2018) and two wetter-than-average periods (water years 1969–83 and 1993–98). Surface-water outflow indicated the highest degree of variability between wet and dry periods in terms of percentage of departures from the 70-year mean, with relative differences between multi-year means as high as 430 percent. Relative differences between mean values for wet and dry periods were about 310 percent for recharge, 176 percent for precipitation, and 136 percent for ET. The final 20 years of the simulation period (water years 1999–2018) were the driest 20-year period within the 70-year simulation period. Mean precipitation for water years 1999–2018 was about 10 percent less than the 70-year mean. The 20-year mean PET was the highest for water years 1999–2018 (58.6 in/yr) compared to all other 20-year periods, further decreasing the amount of water available for recharge and streamflow. Mean recharge for water years 1999–2018 was about 2.1 in/yr or 20 percent less than the 70-year mean. The mean surface-water outflow to the ocean for water years 1999–2018 was about 0.65 in/yr or 32 percent less than the 70-year mean. Water years 1999–2018 also included the driest 10-year period, water years 2007–16, with a mean recharge of 1.5 in/yr (44 percent less than the 70-year mean) and a mean surface-water outflow of 0.59 in/yr (39 percent less than the 70-year mean). The last 20-year and 10-year periods included water year 2014, the driest year in the simulation, with 7.5 inches of precipitation, 0.24 inches of recharge, and 0.08 inches of surface-water outflow.

References Cited 131

References Cited Amirhossien, F., Alireza, F., Kazem, J., and Mohammadbagher, S., 2015, A comparison of ANN and HSPF models for runoff simulation in Balkhichai River Watershed, Iran: American Journal of Climate Change, v. 4, no. 3, p. 203–216. [Available at https://doi.org/10.4236/ajcc.2015.43016.] Atkins, J.T., Jr., Wiley, J.B., and Paybins, K.S., 2005, Calibration parameters used to simulate streamflow from application of the Hydrologic Simulation Program-FORTRAN Model (HSPF) to mountainous basins containing coal mines in West Virginia: U.S. Geological Survey Scientific Investigations Report 2005–5099, 79 p. [Available at https://doi.org/10.3133/sir20055099.] Baillie, M., Chau, L., and Turner, J., 2015, State of the Salinas River groundwater basin: Salinas, Calif., Monterey County Resource Management Agency, prepared by Brown and Caldwell, Walnut Creek, Calif., 241 p. [Available at https://digitalcommons.csumb.edu/cgi/viewcontent.cgi? article=1020&context=hornbeck_cgb_6_a.] Bicknell, B.R., Imhoff, J.C., Kittle, J.L., Jr., Donigian, A.S., Jr., and Johanson, R.C., 1997, Hydrological simulation program—FORTRAN user’s manual for version 11: Athens, Ga., U.S. Environmental Protection Agency report. Bicknell, B.R., Imhoff, J.C., Kittle, J.L., Jr., Jobes, T.H., and Donigian, A.S., Jr., 2001, Hydrologic Simulation Program—Fortran user’s manual version 12 : Athens, Ga., U.S. Environmental Protection Agency, prepared by AQUA TERRA Consultants, Mountain View, Calif., 873 p. Bicknell, B.R., Imhoff, J.C., Kittle, J.L., Jr., Jobes, T.H., and Donigian, A.S., Jr., 2005, Hydrological Simulation Program FORTRAN—HSPF version 12.2 user’s manual: Athens, Ga., U.S. Environmental Protection Agency, prepared by AQUA TERRA Consultants, Mountain View, Calif. Brown and Caldwell, 2014, State of the Salinas River Groundwater Basin: Salinas, Calif., Brown and Caldwell, prepared for Monterey County Water Resources Agency, 240 p. [Available at https://digitalcommons.csumb.edu/hornbeck_cgb_5/47/.] Burton, C.A., and Wright, M.T., 2018, Status and understanding of groundwater quality in the Monterey-Salinas Shallow Aquifer study unit, 2012–13— California GAMA Priority Basin Project: U.S. Geological Survey Scientific Investigations Report 2018–5057, 116 p. [Available at https://doi.org/10.3133/sir20185057.]

California Department of Food and Agriculture, 2022, California Agricultural Statistics Review, 2021–2022: California Department of Food and Agriculture report, 159 p. [Available at https://www.cdfa.ca.gov/Statistics/ PDFs/2022_Ag_Stats_Review.pdf.] California Department of Forestry and Fire Protection, 2004, California interagency watershed map of 1999 (Calwater 2.2.1): U.S. Department of Agriculture, Soil Conservation Service, California Interagency Watershed Mapping Committee, 435 p., accessed September 4, 2017, at https://map.dfg.ca.gov/metadata/calw221_polygon_ 20131205_wm.html. California Department of Public Works, 1946, Salinas Basin investigation: California Department of Water Resources Bulletin 52, 252 p. Cook, T.D., 1978, Soil survey of Monterey County, California (vol. 22): U.S. Department of Agriculture, Soil Conservation Service. California Department of Water Resources, 1971, Nitrates in ground waters of the central coast area: San Joaquin District Office Memorandum Report, 16 p. Daly, C., Gibson, W.P., Doggett, M., Smith, J., and Taylor, G., 2004, Up-to-date monthly climate maps for the conterminous United States—Proceedings of the 14th AMS Conference on Applied Climatology, 84th AMS Annual Meeting Combined Preprints, Seattle, Wash., January 13–16, 2004: American Meteorological Society, 8 p. Daly, C., Neilson, R.P., and Phillips, D.L., 1994, A statistical-topographic model for mapping climatological precipitation over mountainous terrain: Journal of Applied Meteorology, v. 33, no. 2, p. 140–158. [Available at https://doi.org/10.1175/1520-0450(1994)033%3C0 140:ASTMFM%3E2.0.CO;2.] Donigian, A.S., Jr., and Imhoff, J.C., 2009, Evaluation and performance assessment of watershed models— Proceedings of the Water Environment Federation TMDLS Conference, Minneapolis, Minn., August 9–12, 2009: WEF Specialty Conference, p. 12–45. [Available at https://www.researchgate.net/publication/233669770_ Evaluation_and_Performance_Assessment_of_ Watershed_Models.] Donigian, A.S., Jr., Imhoff, J.C., Bicknell, B.R., and Kittle, J.L., Jr., 1984, Application guide for Hydrological Simulation Program—FORTRAN (HSPF): Athens, Ga., U.S. Environmental Protection Agency, Office of Research and Development, Environmental Research Laboratory report EPA-600/3-84-065, 177 p. [Available at https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey= 2000HMKI.txt.]

132 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley Duda, P.B., Hummel, P.R., Donigian, A.S., Jr., and Imhoff, J.C., 2012, BASINS/HSPF—Model use, calibration, and validation: American Society of Agricultural and Biological Engineers, v. 55, no. 4, p. 1523–1547. Flint, A.L., and Flint, L.E., 2007, Application of the basin characterization model to estimate in-place recharge and runoff potential in the Basin and Range carbonate-rock aquifer system, White Pine County, Nevada, and adjacent areas in Nevada and Utah: U.S. Geological Survey Scientific Investigations Report 2007–5099, 20 p. [Available at https://pubs.usgs.gov/sir/2007/5099/pdf/ sir20075099.pdf.] Flint, L.E., and Flint, A.L., 2012, Downscaling future climate scenarios to fine scales for hydrologic and ecological modeling and analysis: Ecological Processes, v. 1, no. 2, 15 p. [Available at https://doi.org/10.1186/2192-1709-1-2.] Flint, A.L., Flint, L.E., and Stern, M.A., 2013, Spatial downscaling and mapping of daily precipitation and air temperature using daily station data and monthly mean maps: American Geophysical Union Fall Meeting Abstracts, H43I–1589. Flint, L.E., Flint, A.L., and Stern, M.A., 2021, The basin characterization model—A regional water balance software package.: U.S. Geological Survey Techniques and Methods, book 6, chap. H1, 85 p. [Available at https://doi.org/10.3133/tm6H1.] Fugro West, Inc., 1995, North Monterey County hydrogeologic study: Salinas, Calif., Monterey County Water Resources Agency, prepared by Fugro West, Inc., Monterey Calif., v. 1, 423 p. [Available at http://www.elkhornsloughctp.org/uploads/files/14 21885098Fugro%20Water%20Study%20October% 201995%20Vol%201.pdf.] Hamlin, H., 1904, Water resources of the Salinas Valley, California: Government Printing Office, U.S. Geological Survey Water Supply Paper 89, 91 p. Harding ESE, 2001, Final report—Hydrogeologic investigation of the Salinas Valley basin in the vicinity of Fort Ord and Marina Salinas Valley, California: Salinas, Calif., Monterey County Water Resources Agency, prepared by Harding ESE, Novato, Calif., 165 p. [Available at https://svbgsa.org/wp-content/uploads/2020/08/2001-Final- Report-Hydrog.pdf.]

Harter, T., Lund, J.R., Darby, J.L., Fogg, G.E., Howitt, R., Jessoe, K.K., Pettygrove, G.S., Quinn, J.F., Viers, J.H., Boyle, D.B., Canada, H.E., DeLaMora, N., Dzurella, K.N., Fryjoff-Hung, A., Hollander, A.D., Honeycutt, K.L., Jenkins, M.W., Jensen, V.B., King, A.M., Kourakos, G., Liptzin, D., Lopez, E.M., Mayzelle, M.M., McNally, A., Medellin-Azuara, J., and Rosenstock, T.S., 2012, Addressing nitrate in California’s drinking water—With a focus on Tulare Lake Basin and Salinas Valley Groundwater: Davis, Calif., University of California, Davis, Center for Watershed Sciences, Report for the State Water Resources Control Board Report to the Legislature, 78 p. [Available at https://ucanr.edu/sites/groundwaternitrate/files/138956.pdf.] Henson, W.R., and Jachens, E.R., 2022, Lower Salinas Valley hydrologic models—Discretization data (ver. 1.1, July 2023): U.S. Geological Survey data release, https://doi.org/10.5066/P9850MAK.] Henson, W.R., Hevesi, J.A., Hanson, R.T., Bittner, D., Herbert, D.M., and Jachens, E.R., 2022, Salinas Valley hydrologic models—Surface water data: U.S. Geological Survey data release. [Available at https://doi.org/10.5066/P93COXL6.] Hevesi, J.A., and Johnson, T.D., 2016, Estimating spatially and temporally varying recharge and runoff from precipitation and urban irrigation in the Los Angeles Basin, California: U.S. Geological Survey Scientific Investigations Report 2016–5068, 192 p. [Available at https://doi.org/10.3133/sir20165068.] Hevesi, J.A., Hanson, R.T., and Masoner, J.R., 2019, Precipitation runoff modeling system (PRMS) as part of an integrated hydrologic model for the Osage Nation, northeastern Oklahoma, 1915–2014: U.S. Geological Survey Scientific Investigations Report 2019–5030, 142 p. [Available at https://pubs.er.usgs.gov/publication/sir20195030.] Hevesi, J., Henson, W.R., Hanson, R.T., Earll, M.M., Herbert, D.M., and Jachens, E.R., 2025, Salinas Valley watershed model—Application of Hydrologic Simulation Program—FORTRAN (HSPF): U.S. Geological Survey data release, https://doi.org/10.5066/P9FJAWC4.] Hevesi, J.A., Henson, W.R., Hanson, R.T., Stern, M.A., and Jachens, E.R., 2022, Salinas valley hydrologic system— Regional climate data: U.S. Geological Survey data release, https://doi.org/10.5066/P942J2BC. Jenkins, O.P., 1943, Geomorphic provinces of California— Geologic formations and economic development of the oil and gas fields of California: California Division of Mines Bulletin 118, p. 83–88.

References Cited 133 Jennings, C.W., 1977, Geologic map of California: California Division of Mines and Geology, Geologic Data Map no. 2, scale 1:750,000. Kennedy Jenks L.L.C., 2004, Final report— Hydrostratigraphic analysis of the northern Salinas Valley: Salinas, Calif., Monterey County Water Resources Agency, prepared by Kennedy Jenks, L.L.C., San Francisco, Calif., project no. 035901.00, 113 p. [Available at https://www.countyofmonterey.gov/home/ showpublisheddocument/19582/636232667568400000.] Kulongoski, J.T., and Belitz, K., 2007, Ground-water quality data in the Monterey Bay and Salinas Valley Basins, California, 2005—Results from the California GAMA Program: U.S. Geological Survey Data Series 258, 84 p. [Available at https://doi.org/10.3133/ds258.] Lapham, M.H., and Heileman, W.H., 1901, Soil survey of the lower Salinas Valley, California: U.S. Department of Agriculture, Bureau of Soils report, p. 481–519. [Available at https://digitalcommons.csumb.edu/cgi/viewcontent.cgi? article=1091&context=hornbeck_usa_3_d.] Leedshill-Herkenhoff, Inc., 1985, Salinas Valley seawater intrusion study: Prepared for the Monterey County Flood Control and Water Conservation District. Markstrom, S.L., Niswonger, R.G., Regan, R.S., Prudic, D.E., and Barlow, P.M., 2008, GSFLOW—Coupled ground-water and surface-water flow model based on the integration of the Precipitation-Runoff Modeling System (PRMS) and the Modular Ground-Water Flow Model (MODFLOW-2005): U.S Geological Survey Techniques and Methods, book 6, chap. D1, 240 p. [Available at https://pubs.er.usgs.gov/publication/tm6D1.]

Monterey County Water Resources Agency, 2018, Nacimiento Dam Operation Policy: Monterey County Water Resources Agency, 186 p. [Available at https://www.countyofmonterey.gov/home/showpublishe ddocument/63151/636628427976500000.] Montgomery–Watson Consulting Engineers, 1994, Salinas River basin water resources management plan task 1.09—Salinas Valley groundwater flow and quality model report: Salinas, Calif., Monterey County Water Resources Agency, prepared by Montgomery–Watson Consulting Engineers, 33 p. Moran, J.E., Esser, B.K., Hillegonds, D., Holtz, M., Roberts, S.K., Singleton, M.J., and Visser, A., 2011, California GAMA special study—Nitrate fate and transport in the Salinas Valley: Lawrence Livermore National Laboratory, Report No. LLNL-TR-484186, 52 p. [Available at https://doi.org/10.2172/1122241.] Nalder, I.A., and Wein, R.W., 1998, Spatial interpolation of climatic normals—Test of a new method in the Canadian boreal forest: Agricultural and Forest Meteorology, v. 92, no. 4, p. 211–225. [Available at https://doi.org/10.1016/S0168-1923(98)00102-6.] Nash, J.E., and Sutcliffe, J.V., 1970, River flow forecasting through conceptual models part I—A discussion of principles: Journal of Hydrology, v. 10, no. 3, p. 282–290. [Available at https://doi.org/10.1016/0022-1694(70)90255-6.] Seaber, P.R., Kapinos, F.P., and Knapp, G.L., 1987, Hydrologic unit maps: U.S. Geological Survey Water Supply Paper 2294, 63 p. [Available at https://doi.org/10.3133/wsp2294.]

Monterey County Water Resources Agency, 1995, 1995—Hydrogeology and water supply of Salinas Valley, White paper by Salinas Valley Ground Water Basin Hydrology Conference: Monterey County Water Resources Agency Water Reports. 21 p. [Available at https://digitalcommons.csumb.edu/hornbeck_cgb_6_a/25.]

Simpson, T.R., McPartland, J.W., Nilsson, W.I., Vickroy, G.M., and Farrington, T.K., 1946, Salinas Basin investigation—Summary report: State of California, Department of Public Works, Division of Water Resources Bulletin no. 52-B. [Available at https://digitalcommons.csumb.edu/hornbeck_cgb_1/6/.]

Monterey County Water Resources Agency, 1996, Summary report—1995 groundwater extraction data and agricultural water conservation practices: Monterey County Water Resources Agency, 8 p.

Stern, M.A., Flint, L.E., Minear, J.T., Flint, A.L., and Wright, S.A., 2016, Characterizing changes in streamflow and sediment supply in the Sacramento River Basin, California, using hydrological simulation program— FORTRAN (HSPF): Water, v. 8, no. 10, 21 p. [Available at https://doi.org/10.3390/w8100432.]

Monterey County Water Resources Agency, 2006, Monterey County groundwater management plan: Monterey County Water Resources Agency, 77 p. [Available at https://digitalcommons.csumb.edu/hornbeck_cgb_5/27.]

Tinsley, J.C., III, 1975, Quaternary geology of northern Salinas Valley, Monterey County, California: Palo Alto, Calif., Stanford University, Ph.D. dissertation. U.S. Department of Agriculture [USDA], 2017, Soil data access: U.S. Department of Agriculture web page, accessed January 4, 2017, at https://sdmdataaccess.sc.egov.usda.gov.

134 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley U.S. Environmental Protection Agency, 2000, BASINS technical note 6—Estimating hydrology and hydraulic parameters for HSPF: U.S. Environmental Protection Agency, Office of Water, EPA-823-R00-012, 34 p. [Available at https://www.epa.gov/sites/default/files/2015- 08/documents/2000_08_14_basins_tecnote6.pdf.] U.S. Geological Survey, 2014, National Land Cover Database (NLCD) 2011 land cover conterminous United States: U.S. Geological Survey data release, https://doi.org/10.5066/P97S2IID.]

U.S. Geological Survey, 2016, USGS water data for the Nation: U.S. Geological Survey, National Water Information System Database, accessed December 27, 2018, at http://waterdata.usgs.gov/nwis/. U.S. Geological Survey, 2019, National hydrography dataset: U.S. Geological Survey web page, accessed January 3, 2018, at https://www.usgs.gov/national-hydrography/ national-hydrography-dataset. U.S. Geological Survey, 2023, 1/3rd arc-second digital elevation models (DEMs)—USGS national map 3DEP downloadable data collection: U.S. Geological Survey data. [Available at https://www.sciencebase.gov/catalog/item/ 4f70aa9fe4b058caae3f8de5.]

Appendix 1. Climate Stations with Records of Daily Climate Used to Develop Climate Inputs for the Salinas Valley Watershed Model Table 1.1. Climate stations with records of daily precipitation used to develop daily precipitation input for the Salinas Valley Watershed Model. [COOP, Cooperative Weather Station; CIMIS, California Irrigation Management and Information System; ID, identification; RAWS; Remote Automated Weather Station]

Station ID

Station name

Station network

Station code

Latitude, decimal degrees

Longitude, decimal degrees

Elevation (feet)

Number of years of data

Mean annual precipitation (inches)

Maximum daily precipitation (inches)

Arroyo Grande

COOP

40320

35.1167

−120.5667

112.0

17.2

11.5

2.4

Arroyo Seco

COOP

40322

36.2355

−121.4800

940.0

2.8

23.8

3.9

Arroyo Seco Millers Lg

COOP

40325

36.2500

−121.4167

702.0

3.1

18.1

3.1

Avenal T.E.S.

COOP

40397

35.9983

−120.1181

787.0

2.8

6.1

0.8

Avenal 9 SSE

COOP

40398

35.9000

−120.0500

522.0

6.4

6.3

1.7

Ben Lomond No. 4

COOP

40674

37.0855

−122.0797

420.0

69.3

50.0

11.5

Big Sur Station

COOP

40790

36.2472

−121.7802

200.0

68.0

42.3

9.2

Big Sur 3 SE

COOP

40793

36.2458

−121.7722

240.0

2.6

22.3

6.1

Black Mountain 2 WSW

COOP

40855

37.3167

−122.1667

2,120.0

45.8

35.8

8.0

Boulder Creek Location Reach

COOP

41005

37.1422

−122.1963

2,175.0

3.2

51.3

6.6

Bradley

COOP

41034

35.8667

−120.8000

541.0

21.1

11.3

2.6

Bradley Telemetering

COOP

41037

35.9303

−120.8678

443.0

17.8

8.7

3.6

Bryson

COOP

41142

35.7989

−121.0939

925.0

3.2

16.3

2.7

Buena Vista

COOP

41170

36.7667

−121.1833

1,640.0

3.1

10.9

2.5

Camp San Luis Obispo

COOP

41444

35.3333

−120.6833

620.0

19.7

20.6

4.2

Carmel Valley

COOP

41534

36.4805

−121.7244

480.0

35.1

17.4

3.4

Chittenden Pass

COOP

41739

36.9003

−121.5967

82.0

37.4

18.5

4.0

Cholame Alley Ranch

COOP

41743

35.7167

−120.2500

1,752.0

3.2

7.0

1.3

Coalinga

COOP

41864

36.1356

−120.3606

670.0

68.8

7.7

3.7

Coalinga 14 WNW

COOP

41869

36.2333

−120.5667

1,640.0

22.9

15.9

4.4

Corralitos

COOP

42051

36.9897

−121.8050

270.0

3.2

22.8

5.0

Davenport

COOP

42290

37.0167

−122.2000

279.0

16.7

23.6

3.0

Del Monte

COOP

42362

36.6000

−121.8667

45.0

3.7

13.0

1.6

Felton

COOP

43004

37.0506

−122.0750

400.0

7.7

46.8

7.8

Fritzsche Aaf

COOP

43186

36.6833

−121.7667

128.0

17.1

13.8

2.7

Gerber Ranch

COOP

43387

37.3667

−121.4833

2,142.0

16.5

18.0

3.8

Appendix 1. 135

[COOP, Cooperative Weather Station; CIMIS, California Irrigation Management and Information System; ID, identification; RAWS; Remote Automated Weather Station]

Station ID

Station name

Station network

Station code

Latitude, decimal degrees

Longitude, decimal degrees

Elevation (feet)

Number of years of data

Mean annual precipitation (inches)

Maximum daily precipitation (inches)

Gilroy

COOP

43417

37.0030

−121.5608

194.0

58.3

20.2

6.0

Gilroy 8 NE

COOP

43419

37.0327

−121.4316

1,050.0

3.2

16.6

4.7

Gilroy 14 NE

COOP

43421

37.1167

−121.3667

1,903.0

4.3

17.2

2.7

Gilroy 14 ENE

COOP

43422

37.1000

−121.3333

1,352.0

20.9

17.2

4.7

Gonzales 9 ENE

COOP

43502

36.5333

−121.2833

2,352.0

3.2

10.7

1.5

Hearst Castle

COOP

43882

35.6841

−121.1683

1,526.0

18.5

28.0

4.4

Hernandez 2 NW

COOP

43925

36.4167

−120.9167

2,162.0

32.5

16.3

3.6

Hernandez 7 SE

COOP

43928

36.3052

−120.7047

2,765.0

2.9

12.5

2.3

Freedom 8 NNW

COOP

43953

37.0500

−121.8167

1,503.0

3.1

39.7

11.9

Hollister 1 SW

COOP

44022

36.8333

−121.4167

279.0

26.6

13.1

3.8

Hollister 2

COOP

44025

36.8483

−121.4213

275.0

40.7

13.1

2.8

Huasna

COOP

44144

35.0833

−120.3833

730.0

3.2

12.8

2.1

Idria

COOP

44204

36.4167

−120.6667

2,651.0

28.8

14.7

2.9

King City

COOP

44555

36.2069

−121.1377

320.0

67.6

11.2

3.3

La Honda

COOP

44660

37.3167

−122.2667

751.0

27.3

29.4

6.7

Lockwood 1 N

COOP

45017

35.9667

−121.0833

1,060.0

29.9

13.6

2.8

Los Banos Arburua Ranch

COOP

45119

36.8749

−120.9386

843.0

67.5

8.5

2.4

Los Gatos

COOP

45123

37.2319

−121.9592

365.0

67.1

23.9

8.5

Los Gatos 4 SW

COOP

45125

37.1833

−122.0333

2,415.0

28.4

47.9

10.3

Holy City 3 WNW

COOP

45126

37.1667

−122.0333

2,200.0

8.7

29.3

4.2

Lucia Willow Springs

COOP

45184

35.8780

−121.4497

355.0

3.2

21.1

3.4

Mercey Hot Springs

COOP

45550

36.7000

−120.8667

1,171.0

16.6

5.9

2.3

Monterey

COOP

45795

36.5903

−121.9055

260.0

66.5

19.6

3.9

Monterey Peninsul Ap

COOP

45799

36.5881

−121.8453

165.0

28.6

14.5

3.6

Monterey Wfo

COOP

45802

36.5927

−121.8555

122.0

23.1

16.8

3.6

Morgan Hill 2 E

COOP

45844

37.1333

−121.6167

230.0

28.6

19.1

5.5

Morgan Hill 6 WSW

COOP

45847

37.1000

−121.7500

640.0

3.0

20.9

3.4

Morgan Hill

COOP

45853

37.1363

−121.6025

375.0

16.5

18.9

5.6

Morro Bay Fire Dept

COOP

45866

35.3670

−120.8447

118.0

56.9

16.8

8.8

Morrow Bay 3 N

COOP

45869

35.4167

−120.8500

620.0

17.7

16.7

4.2

136 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Table 1.1. Climate stations with records of daily precipitation used to develop daily precipitation input for the Salinas Valley Watershed Model.—Continued

Table 1.1. Climate stations with records of daily precipitation used to develop daily precipitation input for the Salinas Valley Watershed Model.—Continued [COOP, Cooperative Weather Station; CIMIS, California Irrigation Management and Information System; ID, identification; RAWS; Remote Automated Weather Station]

Station ID

Station name

Station network

Station code

Latitude, decimal degrees

Longitude, decimal degrees

Elevation (feet)

Number of years of data

Mean annual precipitation (inches)

Maximum daily precipitation (inches)

68.6

23.6

6.9

Mount Hamilton

COOP

45933

37.3436

−121.6425

4,206.0

Mount Madonna

COOP

45973

37.0167

−121.7167

1,801.0

3.2

34.2

10.9

Nacimiento Dam

COOP

46056

35.7667

−120.8833

770.0

21.1

14.8

5.0

Nipomo 2 NW

COOP

46207

35.0667

−120.5000

361.0

17.7

16.3

3.7

Orchard Sunflower Valley

COOP

46480

35.8027

−120.1030

753.0

2.8

5.7

0.9

Pacheco Pass

COOP

46583

37.0667

−121.1833

879.0

12.8

12.6

5.8

Pacheco Pass Wind Farm

COOP

46585

37.0333

−121.1833

1,448.0

2.8

6.4

0.9

Paicines 4 W

COOP

46610

36.7150

−121.3492

905.0

59.6

15.7

5.6

Paloma

COOP

46650

36.3503

−121.5400

1,775.0

51.0

24.9

5.3

Panoche 2 W

COOP

46675

36.6066

−120.8841

1,400.0

67.2

9.4

3.0

Parkfield

COOP

46703

35.8833

−120.4333

1,480.0

26.9

14.9

4.1

Paso Robles

COOP

46730

35.6277

−120.6855

730.0

70.8

14.3

5.3

Paso Robles Municipal Ap

COOP

46742

35.6697

−120.6283

810.0

66.5

12.2

5.5

Pinnacles Nm

COOP

46926

36.4819

−121.1822

1,307.0

68.1

16.3

4.7

Pismo Beach

COOP

46943

35.1597

−120.6830

39.0

65.3

16.1

5.2

San Simeon Point Piedras Blancas

COOP

47024

35.6656

−121.2847

59.0

34.3

19.8

5.3

Priest Valley

COOP

47150

36.1883

−120.6953

2,300.0

61.9

21.0

5.1

Salinas No. 2

COOP

47668

36.6594

−121.6663

45.0

57.9

13.9

3.0

Salinas Ap

COOP

47669

36.6636

−121.6081

74.0

70.1

12.6

3.2

Salinas Dam

COOP

47672

35.3372

−120.5038

1,392.0

69.1

21.3

6.8

Salinas 6 SSW

COOP

47675

36.6333

−121.6833

60.0

1.8

11.7

1.1

San Antonio Mission

COOP

47714

36.0167

−121.2500

1,060.0

12.3

18.7

6.4

San Ardo

COOP

47716

36.0333

−120.9000

449.0

17.4

11.5

2.2

San Benito

COOP

47719

36.5092

−121.0869

1,355.0

3.2

8.6

1.7

San Benito Willow Creek

COOP

47721

36.5833

−121.1833

981.0

6.2

12.2

2.3

San Clemente Dam

COOP

47731

36.4375

−121.7092

600.0

68.7

21.2

4.4

San Felipe Hwy Stn

COOP

47755

37.0167

−121.3333

371.0

3.1

14.7

5.9

San Gregorio 2 SE

COOP

47807

37.3117

−122.3617

275.0

52.6

29.6

6.4

San Juan Bautista 3 SS

COOP

47834

36.8000

−121.5167

550.0

3.2

13.6

4.0

San Luis Dam

COOP

47846

37.0533

−121.0578

277.0

44.6

10.4

3.7

Appendix 1. 137

[COOP, Cooperative Weather Station; CIMIS, California Irrigation Management and Information System; ID, identification; RAWS; Remote Automated Weather Station]

Station ID

Station name

Station network

Station code

Latitude, decimal degrees

Longitude, decimal degrees

Elevation (feet)

Number of years of data

Mean annual precipitation (inches)

Maximum daily precipitation (inches)

San Luis Obispo Poly

COOP

47851

35.3056

−120.6619

308.0

69.3

22.2

5.9

Santa Clara University

COOP

47912

37.3500

−121.9333

89.0

28.2

13.9

3.8

Santa Cruz

COOP

47916

36.9879

−121.9995

70.0

69.5

30.4

6.9

Santa Margarita 2 SW

COOP

47930

35.3667

−120.6333

1,201.0

27.1

31.0

7.2

Santa Margarita Boost

COOP

47933

35.3741

−120.6375

1,148.0

69.8

31.2

8.8

Santa Maria 14 ENE

COOP

47942

35.0167

−120.2000

820.0

3.1

21.8

3.1

Santa Rita Peak

COOP

47962

36.3167

−120.5833

4,304.0

4.6

15.6

2.5

Skyline Ridge Preserve

COOP

48273

37.3133

−122.1850

2,270.0

22.4

42.0

5.7

Slack Canyon

COOP

48277

36.0833

−120.6667

1,732.0

3.2

8.6

2.3

Soledad

COOP

48338

36.4333

−121.3167

210.0

27.6

11.1

2.3

Spreckels Hwy Bridge

COOP

48446

36.6311

−121.6714

21.0

39.4

13.9

2.6

Stayton Mine

COOP

48517

36.9167

−121.2167

2,982.0

3.2

17.5

5.9

Suey Ranch

COOP

48627

35.0000

−120.3833

390.0

17.7

14.1

3.6

100

Twitchell Dam

COOP

49111

34.9880

−120.3211

582.0

46.6

17.1

4.8

101

Upper Tres Pinos

COOP

49189

36.6333

−121.0333

2,021.0

3.2

9.1

1.9

102

Valleton

COOP

49221

35.8833

−120.7000

961.0

3.2

6.2

1.2

103

Watsonville Waterworks

COOP

49473

36.9308

−121.7691

95.0

69.7

22.5

5.9

104

Wrights

COOP

49814

37.1333

−121.9500

1,600.0

38.3

46.4

13.8

105

Arroyo Grande

RAWS

50001

35.1792

−120.3919

1,048.0

17.7

14.8

4.0

106

Arroyo Seco

RAWS

50002

36.2300

−121.4917

980.0

22.2

23.6

16.6

107

Ben Lomond

RAWS

50003

37.1317

−122.1700

2,630.0

20.0

54.2

10.6

108

Big Sur

RAWS

50004

36.2356

−121.7850

450.0

17.0

38.3

12.0

109

Bradley

RAWS

50005

35.8644

−120.8031

537.0

15.8

10.8

2.3

110

Branch Mountain

RAWS

50006

35.1889

−120.0833

3,770.0

21.8

21.7

14.8

111

Cahoon

RAWS

50007

36.3469

−121.5108

2,240.0

6.0

24.7

8.9

112

Chalks

RAWS

50008

37.1608

−122.3000

1,585.0

6.3

42.3

7.4

113

Cordoza Ridge

RAWS

50009

37.1683

−121.5283

2,331.0

6.6

23.1

3.2

114

Corralitos

RAWS

50010

36.9911

−121.7978

450.0

27.0

29.4

19.2

115

Fort Hunter Liggett

RAWS

50012

36.0117

−121.2417

1,100.0

17.1

18.1

7.5

116

Fort Ord #1

RAWS

50013

36.6269

−121.7981

460.0

5.2

14.9

3.6

138 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Table 1.1. Climate stations with records of daily precipitation used to develop daily precipitation input for the Salinas Valley Watershed Model.—Continued

Station ID

Station name

Station network

Station code

Latitude, decimal degrees

Longitude, decimal degrees

Elevation (feet)

Number of years of data

Mean annual precipitation (inches)

Maximum daily precipitation (inches)

Fort Ord (Portable)

RAWS

50014

36.5994

−121.7525

768.0

3.8

12.7

3.5

118

Hastings

RAWS

50015

36.3886

−121.5517

1,885.0

21.1

18.8

9.0

119

Hernandez

RAWS

50016

36.3825

−120.8558

3,733.0

26.2

15.3

6.9

120

Hollister

RAWS

50017

36.8422

−121.3622

404.0

15.7

10.7

3.0

121

La Honda

RAWS

50019

37.3053

−122.2550

872.0

27.7

27.4

12.6

122

La Panza

RAWS

50020

35.3811

−120.1875

1,630.0

28.2

10.5

3.7

123

Las Tablas

RAWS

50021

35.6564

−120.9242

994.0

28.1

20.3

14.0

124

Los Altos

RAWS

50022

37.3581

−122.1472

645.0

17.0

21.3

6.1

125

Marquez

RAWS

50024

36.3597

−120.7731

2,571.0

6.8

13.6

2.4

126

Parkfield

RAWS

50026

35.8989

−120.4319

1,535.0

27.6

12.0

10.4

127

Pinnacles

RAWS

50027

36.4708

−121.1472

1,322.0

17.5

10.0

1.7

128

San Luis Obispo Coast

RAWS

50031

35.6050

−121.1142

228.0

3.2

21.1

2.8

129

Santa Rita

RAWS

50032

36.3478

−120.5978

5,000.0

26.5

17.1

4.7

130

Beach /Santa Cruz Co

CIMIS

60003

36.8810

−121.7930

10.0

3.9

25.7

2.8

131

Webb /Santa Cruz Co

CIMIS

60004

36.9720

−121.7260

230.0

5.6

27.6

3.4

132

San Juan

CIMIS

60016

36.9050

−121.7030

44.0

12.6

22.4

3.2

133

Castroville

CIMIS

60019

36.7683

−121.7738

9.0

34.3

15.7

3.9

134

King City

CIMIS

60023

36.1740

−121.1170

300.0

3.0

16.0

1.7

135

Soledad

CIMIS

60028

36.4470

−121.3640

170.0

4.1

20.4

2.7

136

USDA Salinas

CIMIS

60037

36.6200

−121.5450

120.0

9.2

11.8

2.0

137

San Luis Obispo

CIMIS

60052

35.3054

−120.6618

330.0

32.2

20.6

8.1

138

Blackwells Corner

CIMIS

60054

35.6499

−119.9593

705.0

31.7

8.8

2.4

139

Salinas South

CIMIS

60089

36.6094

−121.5293

160.0

19.3

13.9

2.6

140

De Laveaga

CIMIS

60104

36.9978

−121.9969

314.0

28.0

30.0

4.8

141

Green Valley Road

CIMIS

60111

36.9440

−121.7639

110.0

23.6

25.7

13.4

142

King City-Oasis Rd.

CIMIS

60113

36.1213

−121.0845

552.0

25.3

10.7

3.4

143

Arroyo Seco

CIMIS

60114

36.3474

−121.2912

235.0

25.2

9.4

3.1

144

Gonzales

CIMIS

60115

36.5150

−121.5100

146.0

5.4

17.8

4.0

145

Salinas North

CIMIS

60116

36.7168

−121.6919

61.0

25.2

14.5

3.6

146

San Benito

CIMIS

60126

36.8549

−121.3627

340.0

24.1

14.0

3.5

Appendix 1. 139

117

[COOP, Cooperative Weather Station; CIMIS, California Irrigation Management and Information System; ID, identification; RAWS; Remote Automated Weather Station]

Station ID

Station name

Station network

Station code

Latitude, decimal degrees

Longitude, decimal degrees

Elevation (feet)

Number of years of data

Mean annual precipitation (inches)

Maximum daily precipitation (inches)

147

Pajaro

CIMIS

60129

36.9028

−121.7419

65.0

23.0

17.9

6.0

148

Morgan Hill

CIMIS

60132

37.1515

−121.6363

400.0

10.9

16.6

3.8

149

San Juan Valley

CIMIS

60143

36.8229

−121.4679

268.0

20.6

13.0

2.8

150

San Luis Obispo West

CIMIS

60160

35.3353

−120.7357

285.0

17.5

13.7

3.5

151

Atascadero

CIMIS

60163

35.4726

−120.6481

885.0

17.3

12.4

2.7

152

Watsonville West

CIMIS

60177

36.9000

−121.8130

212.0

5.3

21.1

3.4

153

Watsonville West II

CIMIS

60209

36.9132

−121.8235

305.0

10.8

18.2

4.2

154

Carmel

CIMIS

60210

36.5409

−121.8821

75.0

9.9

15.0

3.5

155

Laguna Seca

CIMIS

60229

36.5701

−121.7865

320.0

6.9

17.0

3.2

140 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Table 1.1. Climate stations with records of daily precipitation used to develop daily precipitation input for the Salinas Valley Watershed Model.—Continued

Table 1.2. Climate stations with records of daily minimum air temperature used to develop daily minimum air temperature input for the Salinas Valley Watershed Model. [COOP, Cooperative Weather Station; CIMIS, California Irrigation Management and Information System; ID, identification; RAWS; Remote Automated Weather Station; °F, degrees Fahrenheit]

Station ID

Station name

Station network

Station code

Latitude, decimal degrees

Longitude, decimal degrees

Elevation (feet)

Number of years of data

Mean minimum daily air temperature, (°F)

Maximum minimum daily air temperature, (°F)

Minimum minimum daily air temperature, (°F)

522.0

5.3

48.2

81.0

12.9

Avenal 9 SSE

COOP

40398

35.9000

−120.0500

Avila Beach

COOP

40403

35.1667

−120.7333

30.0

7.8

48.7

69.1

28.9

Ben Lomond No. 4

COOP

40674

37.0855

−122.0797

420.0

70.1

43.0

66.9

15.1

Big Sur Station

COOP

40790

36.2472

−121.7802

200.0

22.9

46.5

75.9

26.1

Blackwells Corner

COOP

40875

35.6167

−119.9000

712.0

7.7

49.3

81.0

21.0

Carmel Valley

COOP

41534

36.4805

−121.7244

480.0

35.8

44.4

75.0

19.9

Coalinga

COOP

41864

36.1356

−120.3606

670.0

67.5

49.4

89.1

10.9

Felton

COOP

43004

37.0506

−122.0750

400.0

7.5

40.8

60.1

14.0

Fritzsche Aaf

COOP

43186

36.6833

−121.7667

128.0

8.5

46.0

63.0

23.0

Gilroy

COOP

43417

37.0030

−121.5608

194.0

57.6

46.2

72.0

17.1

Hearst Castle

COOP

43882

35.6841

−121.1683

1,526.0

17.9

51.9

86.0

25.0

Hollister 1 SW

COOP

44022

36.8333

−121.4167

279.0

26.6

43.6

69.1

14.0

Hollister 2

COOP

44025

36.8483

−121.4213

275.0

39.1

46.0

69.1

14.0

Idria

COOP

44204

36.4167

−120.6667

2,651.0

27.9

48.5

86.0

14.0

King City

COOP

44555

36.2069

−121.1377

320.0

65.3

44.0

68.0

14.0

King City Airport

COOP

44558

36.2333

−121.1167

361.0

1.9

41.8

59.0

16.0

Los Gatos

COOP

45123

37.2319

−121.9592

365.0

67.4

46.3

75.9

16.0

Monterey

COOP

45795

36.5903

−121.9055

260.0

67.7

48.2

73.0

19.9

Monterey Peninsul Ap

COOP

45799

36.5881

−121.8453

165.0

28.1

48.7

72.0

25.0

Monterey Wfo

COOP

45802

36.5927

−121.8555

122.0

23.0

48.9

70.0

26.1

Morgan Hill

COOP

45853

37.1363

−121.6025

375.0

10.5

50.1

87.1

21.0

Morro Bay Fire Dept

COOP

45866

35.3670

−120.8447

118.0

55.3

48.1

75.0

21.9

Mount Hamilton

COOP

45933

37.3436

−121.6425

4,206.0

66.5

47.5

84.0

7.0

Nacimiento Dam

COOP

46056

35.7667

−120.8833

770.0

20.9

43.3

66.9

18.0

New Cuyama Fire Station

COOP

46154

34.9455

−119.6827

2,160.0

42.7

42.5

78.1

7.0

Paicines 4 W

COOP

46610

36.7150

−121.3492

905.0

14.2

40.5

70.0

14.0

Paso Robles

COOP

46730

35.6277

−120.6855

730.0

70.4

41.9

73.9

7.0

Paso Robles Municipal Ap

COOP

46742

35.6697

−120.6283

810.0

66.5

43.6

73.9

8.1

Pinnacles NM

COOP

46926

36.4819

−121.1822

1,307.0

66.9

41.0

73.0

10.0

Pismo Beach

COOP

46943

35.1597

−120.6830

39.0

63.2

48.2

69.1

21.0

Appendix 1. 141

11 12

[COOP, Cooperative Weather Station; CIMIS, California Irrigation Management and Information System; ID, identification; RAWS; Remote Automated Weather Station; °F, degrees Fahrenheit]

Station ID

Station name

Station network

Station code

Latitude, decimal degrees

Longitude, decimal degrees

Elevation (feet)

Number of years of data

Mean minimum daily air temperature, (°F)

Maximum minimum daily air temperature, (°F)

Minimum minimum daily air temperature, (°F) 28.9

San Simeon Point Piedras Blancas

COOP

47024

35.6656

−121.2847

59.0

34.5

48.7

63.0

Priest Valley

COOP

47150

36.1883

−120.6953

2,300.0

61.2

37.7

75.0

5.0

Salinas No. 2

COOP

47668

36.6594

−121.6663

45.0

58.5

47.4

66.9

21.9

Salinas Ap

COOP

47669

36.6636

−121.6081

74.0

68.8

47.6

66.9

19.9

Salinas Dam

COOP

47672

35.3372

−120.5038

1,392.0

4.8

44.6

69.1

18.0

San Antonio Mission

COOP

47714

36.0167

−121.2500

1,060.0

12.2

39.4

69.1

12.0

San Gregorio 2 SE

COOP

47807

37.3117

−122.3617

275.0

52.4

44.5

71.1

19.9

San Jose

COOP

47821

37.3591

−121.9240

51.0

69.4

49.8

78.1

19.0

San Jose Intl Ap

COOP

47824

37.3592

−121.9242

51.0

9.2

50.2

73.9

24.1

San Luis Dam

COOP

47846

37.0533

−121.0578

277.0

43.8

51.1

84.9

14.0

San Luis Obispo Poly

COOP

47851

35.3056

−120.6619

308.0

68.7

47.7

73.9

17.1

Santa Clara University

COOP

47912

37.3500

−121.9333

89.0

27.4

47.1

71.1

19.9

Santa Cruz

COOP

47916

36.9879

−121.9995

70.0

70.4

46.0

70.0

19.0

Santa Margarita Boost

COOP

47933

35.3741

−120.6375

1,148.0

6.0

43.5

69.1

17.1

Skyline Ridge Preserve

COOP

48273

37.3133

−122.1850

2,270.0

19.2

47.6

82.0

18.0

Twitchell Dam

COOP

49111

34.9880

−120.3211

582.0

45.7

46.0

75.9

15.1

Watsonville Waterworks

COOP

49473

36.9308

−121.7691

95.0

69.0

46.3

66.9

12.0

Arroyo Grande

RAWS

50001

35.1792

−120.3919

1,048.0

19.4

46.7

80.1

19.9

Arroyo Seco

RAWS

50002

36.2300

−121.4917

980.0

22.1

45.2

75.9

6.1

Ben Lomond

RAWS

50003

37.1317

−122.1700

2,630.0

19.9

50.3

84.9

25.0

Big Sur

RAWS

50004

36.2356

−121.7850

450.0

17.1

47.4

75.9

28.9

Bradley

RAWS

50005

35.8644

−120.8031

537.0

15.8

44.1

72.0

9.0

Branch Mountain

RAWS

50006

35.1889

−120.0833

3,770.0

21.7

50.0

82.0

18.0

Cahoon

RAWS

50007

36.3469

−121.5108

2,240.0

5.8

46.1

99.0

25.0

Cordoza Ridge

RAWS

50009

37.1683

−121.5283

2,331.0

6.6

49.1

82.0

21.0

Corralitos

RAWS

50010

36.9911

−121.7978

450.0

26.6

46.0

82.9

19.0

Diablo Grande

RAWS

50011

37.3292

−121.2939

1,850.0

20.1

52.5

87.1

23.0

Fort Hunter Liggett

RAWS

50012

36.0117

−121.2417

1,100.0

17.1

43.5

75.9

16.0

142 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Table 1.2. Climate stations with records of daily minimum air temperature used to develop daily minimum air temperature input for the Salinas Valley Watershed Model.— Continued

Table 1.2. Climate stations with records of daily minimum air temperature used to develop daily minimum air temperature input for the Salinas Valley Watershed Model.— Continued [COOP, Cooperative Weather Station; CIMIS, California Irrigation Management and Information System; ID, identification; RAWS; Remote Automated Weather Station; °F, degrees Fahrenheit]

Station ID

Station name

Station network

Station code

Latitude, decimal degrees

Longitude, decimal degrees

Elevation (feet)

Number of years of data

Mean minimum daily air temperature, (°F)

Maximum minimum daily air temperature, (°F)

Minimum minimum daily air temperature, (°F)

460.0

4.9

46.7

64.0

15.1

Fort Ord #1

RAWS

50013

36.6269

−121.7981

Fort Ord (Portable)

RAWS

50014

36.5994

−121.7525

768.0

3.8

49.3

79.0

32.0

Hastings

RAWS

50015

36.3886

−121.5517

1,885.0

21.1

46.8

78.1

19.0

Hernandez

RAWS

50016

36.3825

−120.8558

3,733.0

28.0

50.5

82.9

12.9

Hollister

RAWS

50017

36.8422

−121.3622

404.0

15.7

46.4

84.0

21.9

Kettleman Hills

RAWS

50018

36.0333

−120.0569

810.0

30.5

57.0

93.9

19.0

La Honda

RAWS

50019

37.3053

−122.2550

872.0

28.1

45.5

84.9

18.0

La Panza

RAWS

50020

35.3811

−120.1875

1,630.0

28.2

40.6

98.1

5.0

Las Tablas

RAWS

50021

35.6564

−120.9242

994.0

28.0

40.5

82.9

7.0

Los Altos

RAWS

50022

37.3581

−122.1472

645.0

18.8

48.4

75.9

24.1

Los Banos

RAWS

50023

37.0547

−121.0531

350.0

27.5

53.9

86.0

19.0

Marquez

RAWS

50024

36.3597

−120.7731

2,571.0

6.8

39.4

87.1

7.0

Panoche Road

RAWS

50025

36.7269

−120.7658

2,032.0

24.5

54.8

90.0

14.0

Parkfield

RAWS

50026

35.8989

−120.4319

1,535.0

27.7

42.7

97.0

10.0

Pinnacles

RAWS

50027

36.4708

−121.1472

1,322.0

17.5

45.8

77.0

18.0

Poverty

RAWS

50028

37.4431

−121.7706

2,066.0

14.6

50.8

87.1

23.0

Rose Peak

RAWS

50029

37.5019

−121.7356

3,060.0

23.1

50.0

82.9

17.1

San Jose

RAWS

50030

37.3983

−121.8069

675.0

10.6

48.0

79.0

26.1

San Luis Obispo Coast

RAWS

50031

35.6050

−121.1142

228.0

3.2

50.4

71.1

33.1

Santa Rita

RAWS

50032

36.3478

−120.5978

5,000.0

26.5

47.7

82.9

10.9

Beach /Santa Cruz Co

CIMIS

60003

36.8810

−121.7930

10.0

3.9

46.7

66.6

27.0

Webb /Santa Cruz Co

CIMIS

60004

36.9720

−121.7260

230.0

5.5

46.7

64.9

27.1

San Juan

CIMIS

60016

36.9050

−121.7030

44.0

12.5

46.4

67.3

17.6

Castroville

CIMIS

60019

36.7683

−121.7738

9.0

33.7

45.8

66.6

13.6

King City

CIMIS

60023

36.1740

−121.1170

300.0

3.0

43.4

67.3

20.7

Soledad

CIMIS

60028

36.4470

−121.3640

170.0

4.0

46.7

66.0

17.2

USDA Salinas

CIMIS

60037

36.6200

−121.5450

120.0

9.1

47.4

66.4

13.3

San Luis Obispo

CIMIS

60052

35.3054

−120.6618

330.0

31.7

48.9

72.7

17.6

Greenfield

CIMIS

60053

36.3410

−121.2570

270.0

4.9

42.6

59.9

17.8

Appendix 1. 143

[COOP, Cooperative Weather Station; CIMIS, California Irrigation Management and Information System; ID, identification; RAWS; Remote Automated Weather Station; °F, degrees Fahrenheit]

Station ID

Station name

Station network

Station code

Latitude, decimal degrees

Longitude, decimal degrees

Elevation (feet)

Number of years of data

Mean minimum daily air temperature, (°F)

Maximum minimum daily air temperature, (°F)

Minimum minimum daily air temperature, (°F)

Blackwells Corner

CIMIS

60054

35.6499

−119.9593

705.0

31.4

48.8

83.5

10.6

San Jose

CIMIS

60069

37.3260

−121.9500

125.0

13.7

49.4

73.9

18.9

Cuyama

CIMIS

60088

34.9425

−119.6738

2,192.0

27.6

42.0

73.0

7.7

Salinas South

CIMIS

60089

36.6094

−121.5293

160.0

19.4

46.5

62.8

23.4

Watsonville

CIMIS

60095

36.8860

−121.8090

100.0

4.8

46.6

60.6

22.1

De Laveaga

CIMIS

60104

36.9978

−121.9969

314.0

27.8

46.4

70.0

19.6

Green Valley Road

CIMIS

60111

36.9440

−121.7639

110.0

23.8

46.0

64.9

21.7

King City-Oasis Rd.

CIMIS

60113

36.1213

−121.0845

552.0

25.2

43.6

64.0

14.9

Arroyo Seco

CIMIS

60114

36.3474

−121.2913

235.0

25.1

45.3

66.2

17.2

Gonzales

CIMIS

60115

36.5150

−121.5100

146.0

5.4

46.5

64.0

25.2

Salinas North

CIMIS

60116

36.7168

−121.6919

61.0

24.8

46.5

64.0

22.8

San Benito

CIMIS

60126

36.8549

−121.3627

340.0

24.0

45.9

68.7

19.4

100

Pajaro

CIMIS

60129

36.9028

−121.7419

65.0

22.9

47.1

64.9

22.3

101

Morgan Hill

CIMIS

60132

37.1515

−121.6363

400.0

10.7

46.2

71.4

15.6

102

San Juan Valley

CIMIS

60143

36.8229

−121.4679

268.0

20.5

44.5

68.7

17.1

103

San Luis Obispo West

CIMIS

60160

35.3353

−120.7357

285.0

17.4

45.9

68.7

23.9

104

Atascadero

CIMIS

60163

35.4726

−120.6481

885.0

17.1

41.8

69.6

18.0

105

Watsonville West

CIMIS

60177

36.9000

−121.8130

212.0

5.2

46.6

60.4

28.8

106

Pacific Grove

CIMIS

60193

36.6332

−121.9349

38.0

6.7

49.3

64.2

30.0

107

Coalinga

CIMIS

60205

36.1758

−120.3603

730.0

8.5

51.3

82.4

20.5

108

Watsonville West II

CIMIS

60209

36.9132

−121.8235

305.0

11.1

47.7

64.0

26.8

109

Carmel

CIMIS

60210

36.5409

−121.8821

75.0

9.8

46.0

63.5

26.8

110

Gilroy

CIMIS

60211

37.0150

−121.5370

185.0

9.1

45.1

66.4

18.0

111

Salinas South II

CIMIS

60214

36.6256

−121.5379

153.0

5.1

49.3

65.3

25.7

112

Laguna Seca

CIMIS

60229

36.5701

−121.7865

320.0

6.7

44.5

62.4

21.4

113

Soledad II

CIMIS

60252

36.4567

−121.3444

223.1

2.1

47.0

65.1

25.5

144 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Table 1.2. Climate stations with records of daily minimum air temperature used to develop daily minimum air temperature input for the Salinas Valley Watershed Model.— Continued

Table 1.3. Climate stations with records of daily maximum air temperature used to develop daily maximum air temperature input for the Salinas Valley Watershed Model. [COOP, Cooperative Weather Station; CIMIS, California Irrigation Management and Information System; ID, identification; RAWS; Remote Automated Weather Station; °F, degrees Fahrenheit]

Station ID

Station name

Station network

Station code

Latitude, decimal degrees

Longitude, decimal degrees

Elevation (feet)

Number of years of data

Mean maximum daily air temperature, (°F)

Maximum maximum daily air temperature, (°F)

Minimum maximum daily air temperature, (°F)

522.0

5.5

80.1

117.0

39.9

Avenal 9 SSE

COOP

40398

35.9000

−120.0500

Avila Beach

COOP

40403

35.1667

−120.7333

30.0

7.8

71.8

105.1

43.0

Ben Lomond No. 4

COOP

40674

37.0855

−122.0797

420.0

69.8

73.2

114.1

35.1

Big Sur Station

COOP

40790

36.2472

−121.7802

200.0

23.2

69.1

106.0

46.0

Blackwells Corner

COOP

40875

35.6167

−119.9000

712.0

7.8

76.5

115.0

35.1

Carmel Valley

COOP

41534

36.4805

−121.7244

480.0

35.6

71.2

111.0

42.1

Coalinga

COOP

41864

36.1356

−120.3606

670.0

66.9

78.9

115.0

34.0

Felton

COOP

43004

37.0506

−122.0750

400.0

7.6

73.0

114.1

44.1

Fritzsche Aaf

COOP

43186

36.6833

−121.7667

128.0

8.5

62.6

102.9

39.0

Gilroy

COOP

43417

37.0030

−121.5608

194.0

58.0

74.6

115.0

39.9

Hearst Castle

COOP

43882

35.6841

−121.1683

1,526.0

17.3

69.0

107.1

39.0

Hollister 1 SW

COOP

44022

36.8333

−121.4167

279.0

26.5

72.4

111.0

39.0

Hollister 2

COOP

44025

36.8483

−121.4213

275.0

37.8

71.5

111.9

41.0

Idria

COOP

44204

36.4167

−120.6667

2,651.0

27.8

71.7

109.0

28.0

King City

COOP

44555

36.2069

−121.1377

320.0

63.0

75.1

115.0

39.9

King City Airport

COOP

44558

36.2333

−121.1167

361.0

1.9

72.0

102.9

43.0

Los Gatos

COOP

45123

37.2319

−121.9592

365.0

67.1

72.2

114.1

37.0

Monterey

COOP

45795

36.5903

−121.9055

260.0

67.5

65.1

104.0

39.9

Monterey Peninsul Ap

COOP

45799

36.5881

−121.8453

165.0

28.0

64.3

102.9

46.0

Monterey Wfo

COOP

45802

36.5927

−121.8555

122.0

23.1

65.1

104.0

46.0

Morgan Hill

COOP

45853

37.1363

−121.6025

375.0

16.6

73.0

114.1

43.0

Morro Bay Fire Dept

COOP

45866

35.3670

−120.8447

118.0

55.8

65.4

106.0

43.0

Mount Hamilton

COOP

45933

37.3436

−121.6425

4,206.0

67.0

61.7

102.9

18.0

Nacimiento Dam

COOP

46056

35.7667

−120.8833

770.0

20.9

77.7

117.0

37.0

New Cuyama Fire Station

COOP

46154

34.9455

−119.6827

2,160.0

40.1

76.0

109.9

33.1

Paicines 4 W

COOP

46610

36.7150

−121.3492

905.0

15.2

71.6

109.9

39.0

Paso Robles

COOP

46730

35.6277

−120.6855

730.0

69.7

76.8

114.1

39.0

Paso Robles Municipal Ap

COOP

46742

35.6697

−120.6283

810.0

66.5

76.6

115.0

35.1

Pinnacles NM

COOP

46926

36.4819

−121.1822

1,307.0

67.3

77.3

116.1

37.9

Pismo Beach

COOP

46943

35.1597

−120.6830

39.0

63.8

67.7

102.9

36.0

Appendix 1. 145

11 12

[COOP, Cooperative Weather Station; CIMIS, California Irrigation Management and Information System; ID, identification; RAWS; Remote Automated Weather Station; °F, degrees Fahrenheit]

Station ID

Station name

Station network

Station code

Latitude, decimal degrees

Longitude, decimal degrees

Elevation (feet)

Number of years of data

Mean maximum daily air temperature, (°F)

Maximum maximum daily air temperature, (°F)

Minimum maximum daily air temperature, (°F)

90.0

43.0

San Simeon Point Piedras Blancas

COOP

47024

35.6656

−121.2847

59.0

34.5

60.4

Priest Valley

COOP

47150

36.1883

−120.6953

2,300.0

61.2

74.0

113.0

28.9

Salinas No. 2

COOP

47668

36.6594

−121.6663

45.0

56.6

68.6

107.1

42.1

Salinas Ap

COOP

47669

36.6636

−121.6081

74.0

68.8

67.7

109.0

39.0

Salinas Dam

COOP

47672

35.3372

−120.5038

1,392.0

4.8

78.7

116.1

45.0

San Antonio Mission

COOP

47714

36.0167

−121.2500

1,060.0

12.1

78.4

115.0

37.9

San Gregorio 2 SE

COOP

47807

37.3117

−122.3617

275.0

52.5

64.6

99.0

37.0

San Jose

COOP

47821

37.3591

−121.9240

51.0

68.8

71.0

109.0

37.9

San Jose Intl Ap

COOP

47824

37.3592

−121.9242

51.0

9.2

70.6

106.0

45.0

San Luis Dam

COOP

47846

37.0533

−121.0578

277.0

44.5

73.7

109.9

32.0

San Luis Obispo Poly

COOP

47851

35.3056

−120.6619

308.0

69.4

71.5

111.9

41.0

Santa Clara University

COOP

47912

37.3500

−121.9333

89.0

27.4

70.8

109.0

39.9

Santa Cruz

COOP

47916

36.9879

−121.9995

70.0

70.4

68.6

109.9

41.0

Santa Margarita Boost

COOP

47933

35.3741

−120.6375

1,148.0

6.0

76.2

114.1

44.1

Skyline Ridge Preserve

COOP

48273

37.3133

−122.1850

2,270.0

19.4

64.7

104.0

30.9

Twitchell Dam

COOP

49111

34.9880

−120.3211

582.0

44.5

73.3

111.9

42.1

Watsonville Waterworks

COOP

49473

36.9308

−121.7691

95.0

69.2

67.3

109.9

41.0

Arroyo Grande

RAWS

50001

35.1792

−120.3919

1,048.0

19.4

77.1

116.1

43.0

Arroyo Seco

RAWS

50002

36.2300

−121.4917

980.0

22.1

78.6

115.0

37.9

Ben Lomond

RAWS

50003

37.1317

−122.1700

2,630.0

19.9

66.4

107.1

34.0

Big Sur

RAWS

50004

36.2356

−121.7850

450.0

17.1

71.0

108.0

46.0

Bradley

RAWS

50005

35.8644

−120.8031

537.0

15.8

80.0

120.0

44.1

Branch Mountain

RAWS

50006

35.1889

−120.0833

3,770.0

21.7

69.1

105.1

28.9

Cahoon

RAWS

50007

36.3469

−121.5108

2,240.0

5.8

70.0

104.0

39.0

Cordoza Ridge

RAWS

50009

37.1683

−121.5283

2,331.0

6.6

68.1

104.0

36.0

Corralitos

RAWS

50010

36.9911

−121.7978

450.0

26.6

72.9

116.1

39.9

Diablo Grande

RAWS

50011

37.3292

1,850.0

20.1

71.3

108.0

35.1

Fort Hunter Liggett

RAWS

50012

36.0117

−121.2417

1,100.0

17.1

79.2

116.1

39.9

146 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Table 1.3. Climate stations with records of daily maximum air temperature used to develop daily maximum air temperature input for the Salinas Valley Watershed Model.— Continued

Table 1.3. Climate stations with records of daily maximum air temperature used to develop daily maximum air temperature input for the Salinas Valley Watershed Model.— Continued [COOP, Cooperative Weather Station; CIMIS, California Irrigation Management and Information System; ID, identification; RAWS; Remote Automated Weather Station; °F, degrees Fahrenheit]

Station ID

Station name

Station network

Station code

Latitude, decimal degrees

Longitude, decimal degrees

Elevation (feet)

Number of years of data

Mean maximum daily air temperature, (°F)

Maximum maximum daily air temperature, (°F)

Minimum maximum daily air temperature, (°F)

460.0

4.9

64.4

95.0

36.0

Fort Ord #1

RAWS

50013

36.6269

−121.7981

Fort Ord (Portable)

RAWS

50014

36.5994

−121.7525

768.0

3.8

65.1

99.0

44.1

Hastings

RAWS

50015

36.3886

−121.5517

1,885.0

21.1

70.4

108.0

36.0

Hernandez

RAWS

50016

36.3825

−120.8558

3,733.0

28.0

67.9

120.9

21.0

Hollister

RAWS

50017

36.8422

−121.3622

404.0

15.7

73.0

113.0

46.0

Kettleman Hills

RAWS

50018

36.0333

−120.0569

810.0

30.5

75.8

113.0

33.1

La Honda

RAWS

50019

37.3053

−122.2550

872.0

28.1

67.9

109.9

36.0

La Panza

RAWS

50020

35.3811

−120.1875

1,630.0

28.2

76.6

111.0

30.0

Las Tablas

RAWS

50021

35.6564

−120.9242

994.0

28.0

77.9

114.1

23.0

Los Altos

RAWS

50022

37.3581

−122.1472

645.0

18.8

73.8

111.9

44.1

Los Banos

RAWS

50023

37.0547

−121.0531

350.0

27.5

74.9

111.0

27.0

Marquez

RAWS

50024

36.3597

−120.7731

2,571.0

6.8

76.3

109.0

39.9

Panoche Road

RAWS

50025

36.7269

−120.7658

2,032.0

24.5

72.4

114.1

28.0

Parkfield

RAWS

50026

35.8989

−120.4319

1,535.0

27.7

77.7

114.1

28.9

Pinnacles

RAWS

50027

36.4708

−121.1472

1,322.0

17.5

79.3

115.0

42.1

Poverty

RAWS

50028

37.4431

−121.7706

2,066.0

14.6

67.7

107.1

35.1

Rose Peak

RAWS

50029

37.5019

−121.7356

3,060.0

23.1

64.1

104.0

28.9

San Jose

RAWS

50030

37.3983

−121.8069

675.0

10.6

73.3

111.0

44.1

San Luis Obispo Coast

RAWS

50031

35.6050

−121.1142

228.0

3.2

65.9

98.1

52.0

Santa Rita

RAWS

50032

36.3478

−120.5978

5,000.0

26.5

63.7

97.0

24.1

Beach /Santa Cruz Co

CIMIS

60003

36.8810

−121.7930

10.0

3.9

67.1

97.3

46.8

Webb /Santa Cruz Co

CIMIS

60004

36.9720

−121.7260

230.0

5.5

70.7

111.0

48.0

San Juan

CIMIS

60016

36.9050

−121.7030

44.0

12.5

68.3

107.2

42.1

Castroville

CIMIS

60019

36.7683

−121.7738

9.0

33.7

62.9

104.0

29.8

King City

CIMIS

60023

36.1740

−121.1170

300.0

3.0

74.9

108.5

47.8

Soledad

CIMIS

60028

36.4470

−121.3640

170.0

4.0

72.1

107.6

48.0

USDA Salinas

CIMIS

60037

36.6200

−121.5450

120.0

9.1

69.7

108.3

40.1

San Luis Obispo

CIMIS

60052

35.3054

−120.6618

330.0

31.7

71.6

108.1

41.5

Greenfield

CIMIS

60053

36.3410

−121.2570

270.0

4.9

68.8

108.1

37.6

Appendix 1. 147

[COOP, Cooperative Weather Station; CIMIS, California Irrigation Management and Information System; ID, identification; RAWS; Remote Automated Weather Station; °F, degrees Fahrenheit]

Station ID

Station name

Station network

Station code

Latitude, decimal degrees

Longitude, decimal degrees

Elevation (feet)

Number of years of data

Mean maximum daily air temperature, (°F)

Maximum maximum daily air temperature, (°F)

Minimum maximum daily air temperature, (°F)

Blackwells Corner

CIMIS

60054

35.6499

−119.9593

705.0

31.4

76.5

114.6

30.6

San Jose

CIMIS

60069

37.3260

−121.9500

125.0

13.7

70.9

97.0

37.2

Cuyama

CIMIS

60088

34.9425

−119.6738

2,192.0

27.6

75.4

108.3

33.8

Salinas South

CIMIS

60089

36.6094

−121.5293

160.0

19.4

68.0

99.5

43.0

Watsonville

CIMIS

60095

36.8860

−121.8090

100.0

4.8

63.4

93.2

41.0

De Laveaga

CIMIS

60104

36.9978

−121.9969

314.0

27.8

67.9

109.0

38.8

Green Valley Road

CIMIS

60111

36.9440

−121.7639

110.0

23.8

68.2

103.1

45.0

King City-Oasis Rd.

CIMIS

60113

36.1213

−121.0845

552.0

25.2

74.7

114.6

42.4

Arroyo Seco

CIMIS

60114

36.3474

−121.2913

235.0

25.1

71.7

109.4

43.7

Gonzales

CIMIS

60115

36.5150

−121.5100

146.0

5.4

70.8

104.0

46.9

Salinas North

CIMIS

60116

36.7168

−121.6919

61.0

24.8

64.6

99.7

44.2

San Benito

CIMIS

60126

36.8549

−121.3627

340.0

24.0

72.0

112.8

43.0

100

Pajaro

CIMIS

60129

36.9028

−121.7419

65.0

22.9

66.5

102.2

46.0

101

Morgan Hill

CIMIS

60132

37.1515

−121.6363

400.0

10.7

71.6

111.0

32.4

102

San Juan Valley

CIMIS

60143

36.8229

−121.4679

268.0

20.5

71.3

113.7

45.1

103

San Luis Obispo West

CIMIS

60160

35.3353

−120.7357

285.0

17.4

67.8

106.3

40.1

104

Atascadero

CIMIS

60163

35.4726

−120.6481

885.0

17.1

74.6

109.4

42.4

105

Watsonville West

CIMIS

60177

36.9000

−121.8130

212.0

5.2

63.1

98.6

44.6

106

Pacific Grove

CIMIS

60193

36.6332

−121.9349

38.0

6.7

61.6

92.7

48.9

107

Coalinga

CIMIS

60205

36.1758

−120.3603

730.0

8.5

78.7

109.9

39.0

108

Watsonville West II

CIMIS

60209

36.9132

−121.8235

305.0

11.1

63.7

99.5

41.7

109

Carmel

CIMIS

60210

36.5409

−121.8821

75.0

9.8

66.1

99.1

48.2

110

Gilroy

CIMIS

60211

37.0150

−121.5370

185.0

9.1

74.0

112.8

46.2

111

Salinas South II

CIMIS

60214

36.6256

−121.5379

153.0

5.1

70.7

102.0

48.0

112

Laguna Seca

CIMIS

60229

36.5701

−121.7865

320.0

6.7

67.3

95.2

47.1

113

Soledad II

CIMIS

60252

36.4567

−121.3444

223.1

2.1

74.5

108.7

51.1

148 HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley

Table 1.3. Climate stations with records of daily maximum air temperature used to develop daily maximum air temperature input for the Salinas Valley Watershed Model.— Continued

For more information concerning the research in this report, contact the Director, California Water Science Center U.S. Geological Survey 6000 J Street, Placer Hall Sacramento, California 95819 https://www.usgs.gov/centers/california-water-science-center Publishing support provided by the U.S. Geological Survey Science Publishing Network, Sacramento Publishing Service Center

Hevesi and others—HSPF as Part of an Integrated Hydrologic Model for the Salinas Valley—SIR 2025–5009

ISSN 2328-0328 (online) https://doi.org/10.3133/sir20255009

Application of Hydrologic Simulation Program—FORTRAN (HSPF) as Part of an Integrated Hydrologic Model for the Salinas Valley, California

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