Simulation of Ground-Water/Surface-Water

Flow in the Santa Clara–Calleguas

Ground-Water Basin, Ventura County, California

By R.T. HANSON, PETER MARTIN, and K.M. KOCZOT

A contribution of the

Southern California Regional Aquifer-System 

Analysis Program

Sacramento, California

2003

Water-Resources Investigations Report 02-4136 

U.S. GEOLOGICAL SURVEY 5030-38
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U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director
Contents iii

CONTENTS

Abstract ................................................................................................................................................................ 1

Introduction .......................................................................................................................................................... 3

Purpose and Scope ...................................................................................................................................... 5

Approach ..................................................................................................................................................... 6

Description of Study Area........................................................................................................................... 6

Climate ............................................................................................................................................... 7

Population .......................................................................................................................................... 7

Land and Water Use........................................................................................................................... 7

Acknowledgments....................................................................................................................................... 10

Surface Water....................................................................................................................................................... 10

Precipitation Estimates................................................................................................................................ 10

Streamflow .................................................................................................................................................. 11

Irrigation Diversions ................................................................................................................................... 27

Imported Water ........................................................................................................................................... 27

Sewage Effluent .......................................................................................................................................... 27

Ground Water....................................................................................................................................................... 28

Geologic Framework................................................................................................................................... 28

Consolidated Rocks............................................................................................................................ 28

Unconsolidated Deposits.................................................................................................................... 29

Aquifer Systems .......................................................................................................................................... 39

Upper-Aquifer System .............................................................................................................. 39

Lower-Aquifer System.............................................................................................................. 41

Ground-Water Recharge ............................................................................................................................. 44

Streamflow Infiltration....................................................................................................................... 44

Gaged Streamflow .................................................................................................................... 45

Ungaged Streamflow ................................................................................................................ 48

Direct Infiltration ............................................................................................................................... 49

Artificial Recharge............................................................................................................................. 49

Irrigation Return Flow ....................................................................................................................... 56

Ground-Water Discharge ............................................................................................................................ 56

Pumpage............................................................................................................................................. 56

Evapotranspiration ............................................................................................................................. 58

Coastal Flow along Submarine Outcrops........................................................................................... 58

Ground-Water Levels, Movement, and Occurrence ................................................................................... 59

Upper- and Lower-Aquifer-System Water Levels............................................................................. 59

Water-Level Differences Between Aquifers...................................................................................... 69

Inter-Aquifer Flow ............................................................................................................................. 70

Source of Water to Wells ................................................................................................................... 71

Source, Movement, and Age of Ground Water ................................................................................. 71

Land-Subsidence Effects.................................................................................................................... 74

Simulation of Ground-Water Flow ...................................................................................................................... 75

Model Framework....................................................................................................................................... 76

Previous Models................................................................................................................................. 76

Model Grid......................................................................................................................................... 76
iv Contents

Temporal Discretization..................................................................................................................... 77

Model Boundaries .............................................................................................................................. 77

Streamflow Routing and Ground-Water/Surface-Water Interactions................................................ 83

Mountain-Front Recharge .................................................................................................................. 88

Valley-Floor Recharge....................................................................................................................... 89

Artificial Recharge............................................................................................................................. 89

Other Sources of Recharge ................................................................................................................ 90

Natural Discharge .............................................................................................................................. 90

Pumpage............................................................................................................................................. 91

Hydraulic Properties .......................................................................................................................... 92

Transmissivity........................................................................................................................... 92

Storage Properties ..................................................................................................................... 98

Vertical Leakance ..................................................................................................................... 99

Model Calibration ....................................................................................................................................... 99

Calibration Summary ......................................................................................................................... 99

Predevelopment Initial Conditions ........................................................................................... 101

Transient-State Calibration Parameters .................................................................................... 104

Transient-State Model Comparisons......................................................................................... 106

Model Uncertainty, Sensitivity, and Limitations ............................................................................... 113

Analysis of Ground-Water Flow.......................................................................................................................... 115

Predevelopment Ground-Water Flow ......................................................................................................... 119

Historical Ground-Water Flow, 1984–93 ................................................................................................... 119

Summary of Ground-Water Conditions............................................................................................. 119

Recharge............................................................................................................................................. 120

Coastal Flow ...................................................................................................................................... 121

Flow Between Subareas and Aquifer Systems .................................................................................. 121

Land Subsidence ................................................................................................................................ 124

Projected Future Ground-Water Flow for Existing Management Plan....................................................... 125

Summary of Projected Ground-Water Conditions............................................................................. 127

Recharge............................................................................................................................................. 127

Coastal Flow ...................................................................................................................................... 134

Flow Between Subareas and Aquifer Systems .................................................................................. 135

Land-Subsidence................................................................................................................................ 135

Projected Future Ground-Water Flow for Alternative Water-Supply Projects.................................. 136

Potential Case 1—Seawater Barrier and Increased Pumpage in the Oxnard Plain Forebay ............. 137

Potential Case 2—Artificial Recharge in Happy Camp Canyon ....................................................... 137

Potential Case 3—Eliminate Agricultural Pumpage in the South Oxnard Plain Subarea ................. 147

Potential Case 4—Shift Pumpage to Upper-Aquifer System in PTP Wells ...................................... 147

Potential Case 5—Shift Pumpage to Upper-Aquifer System in the Northeast Oxnard Plain............ 147

Potential Case 6—Shift Pumpage to the Upper-Aquifer System in the South Oxnard Plain 

Subarea .................................................................................................................................. 148

Potential Case 7—Shift Pumpage to Upper-Aquifer System, Pleasant Valley ................................. 148

Summary and Conclusions................................................................................................................................... 148

Selected References.............................................................................................................................................. 152

Appendix 1 ........................................................................................................................................................... 160

Appendix 2 ........................................................................................................................................................... 187

Appendix 3 ........................................................................................................................................................... 194

Appendix 4 ........................................................................................................................................................... 201

Appendix 5 ........................................................................................................................................................... 206

Appendix 6 ........................................................................................................................................................... 211
Figures v

FIGURES

Figure 1. Map showing Santa Clara–Calleguas Hydrologic Unit and ground-water basin......................... 4

Figure 2. Graphs showing cumulative departure of tree-ring indices for southern California 

(1458–1966), cumulative departure of precipitation at Port Hueneme (1891–1993), and wet

and dry climatic periods for the historical simulation period (1891–1993), and cumulative 

departure of tree-ring indices for southern California, cumulative departure of precipitation 

at Port Hueneme and Santa Paula, wet and dry climatic periods, and major climatic

events (1770–1965) ...................................................................................................................... 8

Figure 3. Map showing kriged average total seasonal precipitation 1891–1991 for wet and dry climatic 

periods by season.......................................................................................................................... 12

Figure 4. Map showing surface-water drainage features, selected gaging stations, and selected diversions 

in the Santa Clara–Calleguas ground-water basin........................................................................ 21

Figure 5. Graphs showing daily mean streamflow for wet and dry periods at the major rivers and 

tributaries in the Santa Clara–Calleguas ground-water basin....................................................... 23

Figure 6. Graphs showing streamflow duration during wet and dry periods at the major rivers and 

tributaries in the Santa Clara–Calleguas ground-water basin....................................................... 26

Figure 7. Map showing generalized surficial geology of the Santa Clara–Calleguas ground-water basin 

and extents of layers in the numerical ground-water flow model, and stratigraphic column and 

related aquifer designations of geologic units by source and aquifer-system model layers......... 30

Figure 8. Diagram showing hydrogeology of the Santa Clara–Calleguas ground-water basin................... 34

Figure 9. Map and graphs showing subsidence in Oxnard Plain and Pleasant Valley, Santa 

Clara–Calleguas ground-water basin............................................................................................ 43

Figure 10. Graphs showing estimated seasonal streamflow losses for gaged inflows in the Santa Clara 

River and Calleguas Creek and tributaries ................................................................................... 47

Figure 11. Graphs showing estimates of seasonal ground-water inflows to the subbasins and to the 

Oxnard Plain subareas of the Santa Clara–Calleguas ground-water basin, 1891–1993, and 

annual estimated and reported ground-water pumpage in the Santa Clara–Calleguas 

ground-water basin ....................................................................................................................... 50

Figure 12. Maps showing ground-water level altitudes in the Santa Clara–Calleguas ground-water basin.. 60

Figure 13. Map showing measured and simulated water-level altitudes in wells completed in the 

lower-aquifer system of the Santa Clara–Calleguas ground-water basin..................................... 63

Figure 14. Map showing measured and simulated water-level altitudes in wells completed in the 

upper-aquifer system of the Santa Clara–Calleguas ground-water basin..................................... 65

Figure 15. Map showing measured and simulated water-level altitudes at sites with multiple wells of 

different depths completed in the Santa Clara–Calleguas ground-water basin ............................ 67

Figure 16. Map showing location of model grid, location of general-head boundary cells with associated 

boundary head altitude and conductance, location of horizontal flow barriers and associated 

hydraulic characteristic, and location of evapotranspiration model cells and percent of riparian

vegetation cover per cell in the Santa Clara–Calleugas ground -water basin............................... 78
vi Figures

Figure 17. Maps showing areal extent of model grid, location of model cells representing mountain-front, 

artificial, and treated wastewater recharge, and simulated rates of mountain-front recharge in the 

ground-water flow model of the Santa Clara–Calleguas ground-water basin and modeled subareas 

for the upper- and lower-aquifer systems, percentage of infiltration for seasonal precipitation

during wet and dry climatic periods, location of wells with flowmeter logs, and the related 

percentage of pumpage assigned to wells spanning the upper and lower model layers............... 80

Figure 18 Map showing streamflow network as simulated in the numerical model of the 

Santa Clara–Calleguas ground-water basin, Ventura County, California .................................... 84

Figure 19. Maps showing distribution of hydraulic properties in layers 1 and 2 of the model of the 

Santa Clara–Calleguas ground-water basin.................................................................................. 94

Figure 20. Maps showing distribution of estimated total thickness of coarse-grained and fine-grained

interbeds used to estimate hydraulic properties and storage properties for the model of the 

Santa Clara–Calleguas ground-water basin.................................................................................. 96

Figure 21. Graphs showing relation between measured and simulated water-level altitudes for 

selected years for the transient simulation of developed conditions (1927, 1932, 1950, 1991, 

and 1993) in the Santa Clara–Calleguas ground-water basin....................................................... 100

Figure 22. Map showing measured and simulated seasonal streamflows or diversion rates for the 

Santa Clara–Calleguas ground-water basin.................................................................................. 102

Figure 23. Map showing model cells with simulated evapotranspiration in 1891 (predevelopment), 

1932, 1950, and 1993 in the upper layer of the model of the Santa Clara–Calleugas 

ground-water basin, Ventura County, California ......................................................................... 103

Figure 24. Map showing simulated compaction owing to withdrawal of ground water, 1891–1993, in the 

Santa Clara–Calleguas ground-water basin, and locations of selected bench marks and related 

measured and simulated bench-mark trajectories......................................................................... 107

Figure 25. Map showing simulated water-altitudes (December 1992), decline in ground-water levels from 

1984 to 1994, and mean ground-water flow in the Santa Clara–Calleguas ground-water basin.. 108

Figure 26. Maps showing location of model cells used to simulate proposed water-supply projects for 

the existing management plan and alternative water-supply projects in the model of the 

Santa–Calleguas ground-water basin............................................................................................ 117

Figure 27. Maps showing simulated differences in ground-water levels from 1993 to 2017 for proposed

water-supply projects in the existing management plan for the Santa Clara–Calleguas 

ground-water basin ....................................................................................................................... 128

Figure 28. Maps showing simulated differences in ground-water levels from 1993 to 2017 for 

alternative water-supply projects using the base-case 1 set of projects and assumptions in 

the Santa Clara–Calleguas ground-water basin ............................................................................ 138

Figure A1.1. Map showing location of USGS_BASIN_GW coverage............................................................. 161

Figure A1.2. Map showing location of USGS_FAULTS coverage .................................................................. 162

Figure A1.3. Map showing location of PRECIP_KRIG coverage .................................................................... 163

Figure A1.4. Map showing location of USGS_GWMODEL coverage............................................................. 164

Figure A1.5. Map showing location of WL1931 coverage for both aquifer systems........................................ 165

Figure A1.6. Map showing location of WL1950 coverages for both aquifer systems ...................................... 166

Figure A1.7. Map showing location of WL1991 coverages for both aquifer systems ...................................... 168

Figure A1.8. Map showing location of WL1993 coverages for both aquifer systems ...................................... 170

Figure A1.9. Map showing locastion of OXN_OILFIELD coverage................................................................ 172

Figure A2-1. Diagrams showing examples of revised diversion tyhpes and related decision made 

by the MODFLOW streamflowrouting package regarding simulated distribution of diversion 

and streamflow.............................................................................................................................. 188

Figure A3.1. Graph showing singular-spectrumanalysis (SSA) oscillatory components of 

Santa Clara–Calleguas precipitation, and their long-term projections ......................................... 195
Figures vii

Figure A3.2. Graph showing Santa Clara–Calleguas historical and selected synthetic preciipitation annual

relizations...................................................................................................................................... 198

Figure A3.3. Graph showing Santa Clara–Calleguas cumulative departures of annual precipitation from the

mean.............................................................................................................................................. 199

Figure A5.1. Graph showing resistivity logs (red) and flowmeter logs (blue) for selected wells in the 

Santa Clara–Calleguas basin......................................................................................................... 207

Figure A5.2. Graph showing selected electromagnetic conductivity logs and natural gamma logs 

for selected wells in the Santa Clara–Calleugas ground-water basin............................... 208

Figure A6.1. Diagram showing flow of information in the preparation of the Recharge Package 

for the Santa Clara–Calleugas ground-water model ........................................................ 212

Figure A6.2. Diagram showing flow of information in the preparation of the streamflow package 

for the Santa Clara–Calleugas ground-water model ........................................................ 213

Figure A6.3. Diagram showing flow of information in the preparation of the well package for 

the Santa Clara–Calleugas ground-water model .............................................................. 214
viii Tables

TABLES

Table 1. Summary of coastal precipitation statistics for the Santa Clara–Calleguas Basin ....................... 20

Table 2. Summary of gaged streamflow data for selected streams in the Santa Clara–Calleguas Basin ... 25

Table 3. Summary of estimated ranges and averages of gaged and ungaged streamflow infiltration in 

the Santa Clara–Calleguas Basin.................................................................................................. 46

Table 4. Summary of selected estimates of ranges and averages of direct infiltration of precipitation 

in subbasins in the Santa Clara–Calleguas Basin ......................................................................... 54

Table 5. Summary of well-construction data and discharge rates and inflows from flowmeter logs of 

wells in selected subbasins of the Santa Clara–Calleguas Basin.................................................. 72

Table 6. Summary of simulated ground-water flow components for the Santa Clara–Calleguas Basin.... 122

Table A1–1. Description of variables in PRECIP_KRIG polygon attribute table ............................................ 176

Table A1–2. Summary of boundary array index values used for ground-water flow model of Santa 

Clara–Calleguas Basin.................................................................................................................. 180

Table A4.1. Summary of streamflow regressions used to extrapolate historical winter streamflow in the 

Santa Clara–Calleguas Basin........................................................................................................ 202

Table A4.2. Summary of streamflow regressions used to extrapolate historical spring streamflow in the 

Santa Clara–Calleguas Basin........................................................................................................ 203

Table A4.3. Summary of streamflow regressions used to extrapolate historical summer streamflow in the 

Santa Clara–Calleguas Basin........................................................................................................ 204

Table A4.4. Summary of streamflow regressions used to extrapolate historical fall streamflow in the 

Santa Clara–Calleguas Basin........................................................................................................ 205

Table A5.1. Summary of USGS multiple-well monitoring sites...................................................................... 209
Conversion Factors, Vertical Datum, Abbreviations, and Well-Numbering System ix

CONVERSION FACTORS, VERTICAL DATUM, ABBREVIATIONS AND WELLNUMBERING SYSTEM

CONVERSION FACTORS

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = (1.8 × °C) + 32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C = (°F - 32) / 1.8

VERTICAL DATUM

Sea level: In this report, "mean sea level" refers to the National Geodetic Vertical Datum of 1929

(NGVD of 1929)—a geodetic datum derived from a general adjustment of the first-order level nets of

both the United States and Canada, formerly called Sea Level Datum of 1929.

Multiply By To obtain

Length

inch (in.) 2.54 centimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Area

acre 0.4047 hectare

square mile (mi2) 259.0 hectare

Volume

acre-foot (acre-ft) 1,233 cubic meter 

Flow rate

acre-foot per day (acre-ft/d) 0.01427 meter per day

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year

foot per year (ft/yr) 0.3048 meter per year

cubic foot per second (ft3/s) 0.3048 cubic meter per second

foot per day per foot [(ft/d)/ft] 1.0000 meter per day per meter

gallon per minute (gal/min) 0.06309 liter per minute

inch per year (in/yr) 25.4 millimeter per year

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day

Transmissivity

 foot squared per day (ft2/d) 0.09290 meter squared per day 
Conversion Factors, Vertical Datum, Abbreviations, and Well-Numbering System x

Abbreviations

AR autoregressive

ASR artificial storage and recovery system

BM bench mark

CMWD Calleguas Municipal Water District

DWR [California] Department of Water Resources 

EM electromagnetic induction

ET evapotranspiration

FGMA Fox Canyon Groundwater Management District

GIS Geographic Information System

InSAR Interferometric Synthetic Aperture Radar

LSA land surface altitude

ME mean error

MODFLOW U.S. Geological Survey’s modular flow model

PTP pumping-trough pipeline

PVCWD Pleasant Valley County Water District

RASA Southern California Regional Aquifer-System Analysis

RMSE root mean square error

SSA singular-spectrum analysis

STR1 streamflow routing package 1

USGS U.S. Geological Survey

UWCD United Water Conservation District

VCFCD Ventura County Flood Control District

VCPWD Ventura County Public Works Department
Conversion Factors, Vertical Datum, Abbreviations, and Well-Numbering System xi

Well-Numbering System

Wells are identified and numbered according to their location in the rectangular system for the 

subdivision of public lands. Identification consists of the township number, north or south; the range 

number, east or west; and the section number. Each section is divided into sixteen 40-acre tracts 

lettered consecutively (except I and O), beginning with "A" in the northeast corner of the section and 

progressing in a sinusoidal manner to "R" in the southeast corner. Within the 40-acre tract, wells are 

sequentially numbered in the order they are inventoried. The final letter refers to the base line and 

meridian. In California, there are three base lines and meridians; Humboldt (H), Mount Diablo (M), 

and San Bernardino (S). Well numbers consist of 15 characters and follow the format 

002N002W12F001S. In this report, well numbers may be abbreviated and written 2N/2W-12F1. The 

following diagram shows how the number for well 2N/2W-12F1 is derived.

Well

2N/21W-12F1

Well-numbering diagram

D C B A

E F G H

M L K J

N P Q R

6 5 4 3 2 1

7 8 9 10 11 12

18 17 16 15 14 13

19 20 21 22 23 24

30 29 28 26 25

31 32 33 34 35 36

27

R20W R19W

T1N

T2N

T3N

R21W

RANGE

TOWNSHIP

R23W R22W

T1S

T2S

R21W

T2N BASE LINE

12
Abstract 1

Simulation of Ground-Water/Surface-Water Flow in the 

Santa Clara–Calleguas Ground-Water Basin, Ventura 

County, California

By R.T. Hanson, Peter Martin, and K.M. Koczot

ABSTRACT

Ground water is the main source of water in 

the Santa Clara–Calleguas ground-water basin that 

covers about 310 square miles in Ventura County, 

California. A steady increase in the demand for 

surface- and ground-water resources since the late 

1800s has resulted in streamflow depletion and 

ground-water overdraft. This steady increase in 

water use has resulted in seawater intrusion, 

inter-aquifer flow, land subsidence, and 

ground-water contamination. 

The Santa Clara–Calleguas Basin consists 

of multiple aquifers that are grouped into upperand lower-aquifer systems. The upper-aquifer 

system includes the Shallow, Oxnard, and Mugu 

aquifers. The lower-aquifer system includes the 

upper and lower Hueneme, Fox Canyon, and 

Grimes Canyon aquifers. The layered aquifer 

systems are each bounded below by regional 

unconformities that are overlain by extensive basal 

coarse-grained layers that are the major pathways 

for ground-water production from wells and 

related seawater intrusion. The aquifer systems are 

bounded below and along mountain fronts by 

consolidated bedrock that forms a relatively 

impermeable boundary to ground-water flow. 

Numerous faults act as additional exterior and 

interior boundaries to ground-water flow. The 

aquifer systems extend offshore where they crop 

out along the edge of the submarine shelf and 

within the coastal submarine canyons. Submarine 

canyons have dissected these regional aquifers, 

providing a hydraulic connection to the ocean 

through the submarine outcrops of the aquifer 

systems. Coastal landward flow (seawater 

intrusion) occurs within both the upper- and 

lower-aquifer systems.

A numerical ground-water flow model of 

the Santa Clara–Calleguas Basin was developed 

by the U.S. Geological Survey to better define the 

geohydrologic framework of the regional groundwater flow system and to help analyze the major 

problems affecting water-resources management 

of a typical coastal aquifer system. Construction of 

the Santa Clara–Calleguas Basin model required 

the compilation of geographic, geologic, and 

hydrologic data and estimation of hydraulic 

properties and flows. The model was calibrated to 

historical surface-water and ground-water flow for 

the period 1891–1993.

Sources of water to the regional groundwater flow system are natural and artificial 

recharge, coastal landward flow from the ocean 

(seawater intrusion), storage in the coarse-grained 

beds, and water from compaction of fine-grained 

beds (aquitards). Inflows used in the regional flow 

model simulation include streamflows routed 

through the major rivers and tributaries; infiltration 

of mountain-front runoff and infiltration of 

precipitation on bedrock outcrops and on valley 

floors; and artificial ground-water recharge of 

diverted streamflow, irrigation return flow, and 

treated sewage effluent. 
2 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Most natural recharge occurs through 

infiltration (losses) of streamflow within the major 

rivers and tributaries and the numerous arroyos 

that drain the mountain fronts of the basin. Total 

simulated natural recharge was about 114,100 

acre-feet per year (acre-ft/yr) for 1984–93: 

27,800 acre-ft/yr of mountain-front and bedrock 

recharge, 24,100 acre-ft/yr of valley-floor 

recharge, and 62,200 acre-ft/yr of net streamflow 

recharge.

Artificial recharge (spreading of diverted 

streamflow, irrigation return, and sewage effluent) 

is a major source of ground-water replenishment. 

During the 1984–93 simulation period, the average 

rate of artificial recharge at the spreading grounds 

was about 54,400 acre-ft/yr, 13 percent less than 

the simulated natural recharge rate for streamflow 

infiltration within the major rivers and tributaries. 

Estimated recharge from infiltration of irrigation 

return flow on the valley floors averaged about 

51,000 acre-ft/yr, and treated sewage effluent 

averaged about 9,000 acre-ft/yr. Artificial recharge 

as streamflow diversion to the spreading grounds 

has occurred since 1929, and treated-sewage 

effluent has been discharged to stream channels 

since 1930. 

Under predevelopment conditions, the 

largest discharge from the ground-water system 

was outflow as coastal seaward flow and 

evapotranspiration. Pumpage of ground water 

from thousands of water-supply wells has 

diminished these outflows and is now the largest 

outflow from the ground-water flow system. The 

distribution of pumpage for 1984–93 indicates that 

most of the pumpage occurs in the Oxnard Plain 

subareas (37 percent) and in the upper Santa Clara 

River Valley subareas (37 percent). The total 

average simulated pumpage was about 247,000 

acre-ft/yr (59 percent); of which about 146,000 

acre-ft/yr was from the Fox Canyon Groundwater 

Management Agency (FGMA) subareas and 

101,000 acre-ft/yr (41 percent) from the nonFGMA subareas. Of the total 1984–93 pumpage, 

46 percent was contributed by natural recharge, 

22 percent was contributed by artificial recharge 

from diverted streamflow, 20 percent was 

contributed by irrigation return flow, 4 percent was 

contributed from sewage-effluent infiltration, 6 

percent was contributed from storage depletion, 

and 2 percent was contributed from coastal 

landward flow (seawater intrusion).

Seawater intrusion was first suspected in 

1931 when water levels were below sea level in a 

large part of the Oxnard Plain. The simulation of 

regional ground-water flow indicated that coastal 

landward flow (seawater intrusion) began in 1927 

and continued to the end of the period of 

simulation (1993). During wet periods or periods 

of reduced demand for ground water, the direction 

of coastal flow in the upper-aquifer system 

reverses from landward to seaward. During the 

1984–93 period, the simulated total net seaward 

flow was 9,500 acre-feet in the upper-aquifer 

system, which is considerably less than that 

simulated for predevelopment conditions. During 

the same period, total simulated landward flow in 

the lower-aquifer system was 64,200 acre-feet.

Water-level declines in the basin have 

induced land subsidence that was first measured in 

1939 and have resulted in as much as 2.7 feet land 

subsidence in the southern part of the Oxnard 

Plain. The model simulated a total of 3 feet of land 

subsidence in the southern part of the Oxnard 

Plain and as much as 5 feet in the Las Posas Valley 

subbasins. Model simulations indicate that most of 

the land subsidence occurred after the drought of 

the late 1920s and during the agricultural 

expansion of the 1950s and 1960s. The results also 

indicate that subsidence occurred primarily in the 

upper-aquifer system prior to 1959, but in the 

lower-aquifer system between 1959–93 owing to 

an increase in pumpage from the lower-aquifer 

system.
Introduction 3

The calibrated ground-water flow model 

was used to assess future ground-water conditions 

based on proposed water-supply projects in the 

existing management plan for the Santa Clara–

Calleguas ground-water basin. All the projections 

of the proposed water-supply projects in the 

existing management plan have reduced pumpage 

in the FGMA areas which resulted in a reduction 

but not an elimination of storage depletion and 

related coastal landward flow (seawater intrusion) 

and subsidence, a reduction in streamflow 

recharge, and an increase in coastal seaward flow 

and underflow to adjacent subareas from the 

Oxnard Plain. A comparison of management 

simulations based on historical inflows and a 

spectral estimate of inflows shows increased 

coastal landward flow (seawater intrusion), storage 

depletion, and increased land subsidence due to a 

drought projected earlier in the spectral estimate of 

inflows than in the historical inflows. The spectral 

estimate probably provides a smoother and more 

realistic transition between historical and future 

climatic conditions.

The model also was used to simulate 

potential alternative water-supply projects in the 

Santa Clara–Calleguas ground-water basin. These 

seven alternative water-supply projects were 

proposed to help manage the effects of increasing 

demand and variable supply on seawater intrusion, 

subsidence, increased withdrawal from storage, 

and vertical and lateral flow between subareas and 

aquifers systems. Stopping pumpage primarily in 

the lower-aquifer system in the South Oxnard 

Plain subarea had the largest effect on reducing 

coastal landward flow (seawater intrusion) of all 

the potential cases evaluated. Shifting pumpage 

from the lower- to the upper-aquifer system in the 

South Oxnard Plain subarea yielded the largest 

combined effect on coastal flow with a reduction 

of coastal landward flow in the lower-aquifer 

system and coastal seaward flow from the upperaquifer system. A seawater-barrier injection 

project stopped coastal landward flow (seawater 

intrusion) in the upper-aquifer system but also 

resulted in large quantities of coastal seaward flow. 

The recharge of water in Happy Camp Canyon 

resulted in water-level rises that were above land 

surface (not feasible) in the East Las Posas Valley 

subarea but in no significant changes in hydrologic 

conditions in other parts of the basin.

INTRODUCTION

Ground water from the regional alluvial-aquifer 

systems is the main source of water in the Santa Clara 

and Calleguas watersheds in southern California. In 

Ventura County, for the purposes of this study, the 

alluvial ground-water basins of these watersheds are 

referred to as the Santa Clara–Calleguas ground-water 

basin. Development of the water resources of the Santa 

Clara–Calleguas ground-water basin has steadily 

increased since the late 1800s, resulting in streamflow 

depletion, ground-water overdraft, seawater intrusion, 

inter-aquifer flow, land subsidence, and ground-water 

contamination. The extent of ground-water overdraft, 

which is the withdrawal of potable water from an 

aquifer system in excess of replenishment from natural 

and artificial recharge, varies throughout the basin. 

Overdraft is also dependent on climatic variability and 

associated increases in water use. Overdraft has been 

larger within selected subareas of the ground-water 

basin and in the deeper aquifers. However, there has 

been an increased amount of conjunctive use to 

compensate for the effects of the variability of surfacewater supplies and to mitigate the effects of groundwater overdraft.
4 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Santa

Castaic

Lake

Calleguas

Clara

River

Creek

Lake

Piru

Santa Barbara Channel

Pacific Ocean

Santa

Felicia

Dam

L1 L2

L3

Ventura

Santa

Paula

Camarillo

Moorpark

Oxnard

PortHueneme

Fillmore

Newhall

VENTURA CO.

VENTURA CO.

SANTA BARBARA CO.

LOS ANGELES CO.

KERN CO.

LOS ANGELES CO.

Thousand Oaks Hueneme

Submarine

Canyon

Mugu

Submarine

Canyon

Pt. Mugu

Sulfur Mountain

Topatopa Mountains

Big

Mountain

Camaril ol Hill sCamaril ol Hill s

Las Posas Hill

Simi Hills San at

Monica Mountains

0 20 MILES 10

20 KILOMETERS 10 0

119° 118°30'

34°30'

34°

34

15'

°

45'

B

C

D

E F

G

H I

J

A

0 200 MILES

200 KILOMETERS 0

38°34°

124° 120° 116°

Los Angeles

San

Francisco

San Diego

Hydrologic Units

RegionalAquiferSystemAnalysisStudyArea

CALIFORNIA Calleguas Hydrologic Unit

Santa ClaraHydrologic Unit

L

K

South Mountain

Oak Ridge

Santa

Susana

Mountains

Santa Susana Mountains

Figure 1. Santa Clara–Calleguas Hydrologic Unit and ground-water basin. (Modified from California State Water Resources Board, 1956)
Introduction 5

A U.S. Geological Survey (USGS) study of the 

hydrogeology of the Santa Clara–Calleguas Basin was 

completed as part of the Southern California Regional 

Aquifer-System Analysis (RASA) Program (Martin, 

1986). The purpose of the Southern California RASA 

Program was to analyze the major problems and issues 

affecting ground-water use in southern California, 

including ground-water overdraft, streamflow depletion, 

subsidence, seawater intrusion, and ground-water 

contamination. Because of the large size of the study 

area and the large number of basins involved, only two, 

the Santa Clara–Calleguas Basin (coastal) and the 

Mojave River ground-water basin (desert), of the 89 

hydrologic subunits identified by the California 

Department of Water Resources (1964) were studied for 

the Southern California RASA Program (Martin, 1993). 

The basic assumption of the program was that certain 

characteristics of the geohydrologic processes and 

human activities that control or influence water 

resources are common to many of the basins or groups 

of basins. The development of the Santa Clara–

Calleguas Basin study is an extension of previous 

investigations in the nearby coastal aquifer systems in 

Santa Barbara, California (Martin and Berenbrock, 

1986; Freckleton and others, 1998).

Purpose and Scope

The purpose of this study is to acquire a better 

understanding of the hydrogeologic system in the Santa 

Clara–Calleguas Basin (fig. 1) and to develop a tool to 

help analyze the major problems affecting waterresources management of a typical coastal aquifer 

system. The study included a reevaluation of the basin 

structure and stratigraphy of the water-bearing rocks 

and an evaluation of the hydrologic system under 

predevelopment, historical development, and future 

development conditions. The purposes of this report are 

to describe the regional ground-water flow model that 

was constructed for the RASA Program, to summarize 

the results of simulations of historical and future periods 

using the RASA model, and to describe the model 

limitations and the data needed for future model 

refinements. Also described in this report are groundwater recharge, movement, and discharge.

L1

L2

L3

Santa Clara-Calleguas

ground-water basin

Santa Clara River Valley

Las Posas Valley

Santa Rosa Valley

Pleasant Valley

Coastal Area

Encompassing study area

EXPLANATION

Piru

Fillmore

Santa Paula

South

East

West

North

South

Ground-Water Subbasin

Map area

of this

report

Hydrologic Unit boundary

Ground-water basin boundary

Ground-water subbasin boundary

Bathymetry contour

Northwest Oxnard Plain

Northeast Oxnard Plain

South Oxnard Plain

E

F

G

A

B

C

H

D

K

L

I

J

Mound

Oxnard Plain Forebay

Oxnard Plain
6 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Approach

A regional model of ground-water flow that 

simulates the hydrologic system under pre- and 

post-development conditions was developed to evaluate 

the natural and human-induced controls on the regional 

water resources. Because water-resources development 

began relatively early in the coastal basins of 

California, there is very little quantitative information 

on predevelopment ground-water and surface-water 

conditions. This lack of data required coupling the 

calibration of the steady- and transient-state 

simulations to arrive at a combined fit for pre- and 

post-development conditions.

Previous studies of the aquifer systems (Mann 

and Associates, 1959; Turner, 1975) and numerical 

models of the hydrologic system (California 

Department of Water Resources, 1974a,b; Reichard, 

1995) were used as the starting point for the 

reevaluation of the stratigraphy and structure of the 

water-bearing units and to provide estimates of 

hydraulic properties of each unit. Reevaluation was 

based on additional geophysical data, geochemical 

data, and hydraulic data from selected existing 

production wells and from 23 new monitoring wells 

drilled throughout the basin by the USGS (Izbicki and 

others, 1995; Densmore, 1996). Estimates for many of 

the hydraulic properties and for the quantities and 

locations of recharge and discharge needed to simulate 

ground-water flow in the major water-bearing units 

generally were unavailable; therefore, indirect 

estimates, which were modified during the calibration 

of the numerical model, were required.

Description of Study Area 

The Santa Clara (hydrologic unit 18070102) and 

Calleguas (hydrologic unit 18070103) Basins are 

coastal watersheds that principally drain parts of 

Ventura and Los Angeles Counties; they have a total 

drainage area of 2,010 mi2 (fig. 1). Almost 90 percent 

of the basin surface is characterized by rugged 

topography; the remainder consists of valley floor and 

coastal plain composed of a northeast-trending set of 

anticlinal mountains and synclinal valleys in the 

Transverse Ranges physiographic province. The 

onshore part of the Santa Clara–Calleguas alluvial 

basin is about 32 mi long and includes about 310 mi2. 

The ground-water basin extends as much as 10 miles 

offshore and includes an additional 193 mi2. The 

sloping offshore plain and underlying aquifers are 

truncated by steeply dipping submarine cliffs that are 

dissected by several submarine canyons. 

The Santa Clara–Calleguas Basin is a regional 

ground-water basin that can be divided into 12 onshore 

subbasins (fig. 1). The coastal subbasins extend 

offshore beneath the gently sloping submarine shelf. 

The ground-water subbasins are subareas within the 

surface-water drainage subbasins, and many of their 

boundaries are aligned with known faults and other 

geologic features. The Piru, Fillmore, Santa Paula, and 

Mound subbasins and the northern part of the Oxnard 

Plain known as the Oxnard Plain Forebay subbasin 

compose the Santa Clara River Valley. The Santa Rosa 

Valley, East and South Las Posas Valley, and North and 

South Pleasant Valley subbasins and the southern part 

of the Oxnard Plain subbasin compose the Arroyo 

Simi–Arroyo Las Posas–Conejo Creek–Calleguas 

Creek drainage basin. In the West Las Posas Valley 

subbasin, Arroyo Hondo and Beardsley Wash flow into 

Revolon Slough, which flows along with Calleguas 

Creek into Mugu Lagoon (see figure 4 in the “Surface 

Water” section). These three drainages cross parts of 

the coastal subbasin known as the Oxnard Plain.

The Santa Clara River and the Calleguas Creek 

discharge directly to the Pacific Ocean. The onshore 

ground-water basin is bounded by the Sulfur Mountain 

and the Topatopa Mountains on the north, the Santa 

Susana Mountains and the Simi Hills on the east, and 

the Santa Monica Mountains on the south (fig. 1). 

Mountain peaks, which exceed 6,700 ft in altitude, rise 

above numerous narrow valleys and streams that are 

tributary to the Santa Clara River and Calleguas Creek 

drainage basins. The west-trending Oak Ridge, South 

Mountain, and Santa Susana Mountains separate the 

Santa Clara River Valley from the Las Posas Valley. 

The west-trending Las Posas and Camarillo Hills 

separate Las Posas Valley from Pleasant Valley. These 

intermontane alluvial valleys grade into the coastal 

flood plains in the Oxnard Plain and the Mound 

subbasins. The coastal flood plain continues offshore as 

a gently sloping submarine shelf of the Santa Barbara 

Channel. The submarine shelf is bounded on the west 

by steeply sloping submarine cliffs where the 

water-bearing formations crop out. The shelf is 

dissected by the Hueneme and the Mugu submarine 

canyons and several unnamed smaller submarine 

canyons (fig. 1). The larger submarine canyons dissect 

the submarine shelf to the present-day shoreline.
Introduction 7

Climate

The climate of the basin is of the mediterranean 

type with 85 percent of the rainfall occurring between 

November and April, typical of the southern California 

coastal area. Average annual precipitation is about 

14 in. at Port Hueneme along the coast, about 17 in. 

near Santa Paula in the intermediate altitudes of the 

Santa Clara River Valley, and more than 25 in. in the 

surrounding mountains (Ventura County Public Works 

Agency, 1990, 1993). Daily mean temperatures range 

from as high as 89οF along the coast in late summer 

and early fall to below freezing in the bordering 

mountains during winter. Mean pan-evaporation rates 

range from 59 in/yr at Casitas Dam at Ventura County 

Flood Control District (VCFCD) Station Number 4 to 

73 in/yr at Lake Bard at VCFCD Station Number 227 

(Ventura County Public Works Agency, 1990, 1993).

The climate is seasonally variable and has been 

variable through time (fig. 2). The cumulative 

departure of tree-ring indices and precipitation can be 

used to divide periods of the climatic record into wet 

and dry climatic periods. Wet climatic periods are 

determined using the rising limb of the cumulative 

departure curve, and dry climatic periods are 

determined using the falling limb of the cumulative 

departure curve. The cumulative departure of tree-ring 

indices for southern California for 1458–1966 

(National Atmospheric and Oceanic Administration, 

1994) indicates an apparent shift in the frequency and 

amplitude of wet and dry periods after the early 1700s. 

Prior to the early 1700s, wet and dry periods were 

relatively long (20 to more than 60 years); whereas 

after the early 1700s, wet and dry periods were shorter 

(5 to 20 years) (fig. 2A). The wet and dry periods 

determined from tree-ring indices for 1770–1965 

generally are in agreement with available precipitation 

records for Port Hueneme and Santa Paula and are 

related to periods of major droughts and floods 

(fig. 2B).

Population

The Santa Clara–Calleguas Basin was settled 

and populated by Native American Indians of the 

Shumash Tribes. Spanish missionaries established 

Mission San Buena Ventura in 1787. In the early 1800s, 

Jesuit Fathers from the San Buena Ventura Mission 

established an asistencia (Ventura Mission outpost) 

where the city of Santa Paula is now located (Freeman, 

1968). These colonies and related Spanish land grants 

developed the initial agrarian and ranching industry in 

the river valleys. The town of San Buena Ventura 

(hereinafter referred to as “Ventura”) became the 

county seat. By 1930, Ventura County had a total 

population of 54,976; Ventura and Santa Paula were the 

most populous cities. Ventura, which was largely 

supported by the oil industry, had a population of 

11,603. Santa Paula and Fillmore, which were the 

principal towns in the citrus area, had populations of 

7,452 and 2,890, respectively. Oxnard, the center of the 

beet-sugar industry in Ventura County, had a 

population of 6,285 (California Department of Public 

Works, 1934). By 1970, the population in Ventura 

County increased to 378,497 as various small 

unincorporated settlements grew into towns. The 

population increased to 535,700 by 1980, and to 

686,900 by 1992—a 28 percent increase. Since the 

1960s, a large part of the population increase was 

related to the urbanization of Ventura County. 

Land and Water Use

Prior to the 1900s, most land in the Santa Clara–

Calleguas Basin was used for grazing cattle and 

dry-land farming. In the early 1900s, agricultural and 

petroleum production became the chief economic 

activities. As in all the coastal basins, urbanization 

since the late 1940s resulted in the transfer of 

agricultural lands to residential and commercial uses, 

especially in the Oxnard Plain. In the late 1940s, the 

turbine pump was introduced for pumping ground 

water, and in the early 1950s, the introduction of the 

refrigerated railroad car provided long-range markets 

for fresh produce. As a result, agriculture was 

transformed from predominantly seasonal dry-land 

farming of walnuts and field crops to predominantly 

year-round irrigated farming of citrus, avocados, and 

truck crops, and water use increased to a historical high 

during the 1950s. Currently, about 80 percent of the 

ground-water and surface-water supply is used for 

agriculture. Agricultural land use increased less than 

5 percent and urban land use increased from 39 to 51 

percent between 1969 and 1980. Since 1980, urban 

growth has continued and urban land use has remained 

the dominant land use in the basin. Because of the 

proximity to the Los Angeles metropolitan area, growth 

may continue with further transformation from an 

agriculture-based economy to an urban and industrial 

economy. An excellent summary of the development of 

water in Ventura County is given by Freeman (1968).
8 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California -10 -20 -30 -40

0

40302010

CUMULATIVE DEPARTURE OF PRECIPITATION, IN INCHES

CUMULATIVE DEPARTURE BASED ONSOUTHERN CALIFORNIA TREE-RING INDICESPORT HUENEME PRECIPITATION DATAEL NIÑO YEARS (QUINN AND OTHERS, 1987)

1450 1500 1550 1600 1650 1700 1750 1800 1850 1900 1950 2000

1,000

-1,000-3,000-5,000-7,000

0

7,0005,0003,000

CUMULATIVE DEPARTURE OF TREE-RING INDICES

YEAR

A

dry wet

Historical simulation

period, 1891-1993

Figure 2.

A, Cumulative departure of tree-ring indices for southern California (1458–1966), cumulative departure of precipitation at Port Hueneme (1891–1993), and wet and dry climatic periods for the 

historical simulation period (1891–1993), and 

B, Cumulative departure of tree-ring indices for southern California, cumulative departure of precipitation at Port Hueneme and Santa Paula, wet and dry climatic 

periods, and major climatic events (1770–1965), Ventura County, California.
Introduction 9

CUMULATIVE DEPARTURE

OF TREE-RING INDICES

B

1770

1780

1790

1800

1810

1820

1830

1840

1850

1860

1870

1880

1890

1900

1910

1920

1930

1940

1950

1960

1965

00

2010

-30

+34.40 +32.20 +25.95

+8.56

+15.97

1940-41 WETTEST YEAR

FLOOD 1938

FLOOD 1914

FLOOD 1853

FLOOD 1840

FLOOD 1825

FLOOD 1815

FLOOD 1780

FLOOD 1776

FLOOD 1811

CAPITAL FLOOD 1862

1863-64 SEVERE DROUGHT

1822-23-24 SEVERE

DROUGHT

1797-98 DROUGHT

1794-95 DROUGHT

1843-44-45-46 EXCESSIVE

DROUGHT

1806-07-08 EXCESSIVE DROUGHT

1826-27-28-29 GENERAL

ALLEJO'S DRY YEARS V

FLOOD 1916 LAKE ELSINORE

OVERFLOWED

FLOOD 1890 LAKE ELSINORE

OVERFLOWED

1895 LAKE ELSINORE

OVERFLOWED

1869 LAKE ELSINORE

OVERFLOWED

1862 LAKE ELSINORE

OVERFLOWED

1859 LAKE ELSINORE PRACTICALLY DRY

1830 LAKE ELSINORE PRACTICALLY DRY

1810 LAKE ELSINORE PRACTICALLY DRY

1841 LAKE ELSINORE

OVERFLOWED+47.36

-24.00

-16.65

-36.50

-41.30

-14.58

WETTEST

PERIOD

DRIEST

PERIOD

-10-20

504030

0

2010

-10-20-30

504030

-1,000-3,000-5,000-7,000

0

7,0005,0003,0001,000

10

AVERAGE

16.55

403020

0

40302010

RAINFALL, IN INCHES

ACCUMULATION

ACCUMUL TION A

ACCUMULA ON I T

ACCUMULATION

DEPLETION

DEPLETION

DEPLETION

DEPLETION

DEPLETION

CA UMULATION C

DEPLETION

ACCUMULA ION T

FLOOD 1876

SEVERE DROUGHT 1877

FLOOD 1886

FLOOD 1884 LAKE ELSINORE OVERFLOWED

1883 LAKE ELSINORE WATER LEVEL LOW

HIGHWATER 1907

-30-40

dry dry dry dry dry dry dry wet wet wet wet wet

wet wet wet wet wet wet dry dry dry dry dry dry

wet wet

-10-20

0

40302010

CUMULATIVE DEPARTURE OF

PRECIPITATION, IN INCHES

CUMULATIVE DEPARTURE OF

PRECIPITATION, IN INCHES

CUMULATIVE DEPARTURE BASED ON SOUTHERN CALIFORNIA TREE-RING INDICESPORT HUENEME PRECIPITATION DATAEL NIÑO YEARS (QUINN AND OTHERS, 1987)WET PERIODS - THIS STUDY

ESTIMATED SEASONAL RAINFALL AT SANTA PAULA 1769-70 TO 1871-72

MEASURED RAINFALL AT SANTA PAULA 1872-73 TO 1964-65

AND MAJOR CLIMATIC EVENTS (Freeman, 1968)

WATER YEAR

CALENDAR YEAR

Figure 2. Continued.
10 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Acknowledgments

This study could not have been accomplished 

without the assistance of personnel from the United 

Water Conservation District (UWCD), the hydrology 

section of the Ventura County Public Works 

Department (VCPWD), the Fox Canyon Groundwater 

Management Agency (FGMA), the Calleguas 

Municipal Water District (CMWD), the U.S. 

Department of the Navy, the Pleasant Valley County 

Water District (PVCWD), local municipalities and 

water mutuals, and numerous other well owners that 

provided data and allowed access to their wells for 

sampling and well testing. The authors are also 

indebted to Wesley Danskin, USGS, for assistance with 

the modifications to the diversion component of the 

streamflow-routing package (Appendix 2), and to 

Michael Dettinger, USGS, for assistance with the 

analysis of historical precipitation and estimation of 

future precipitation (Appendix 3). The authors would 

also like to thank Larry Schneider for his extraordinary 

effort in creating the scientific illustrations for this 

report. An additional special thanks also goes out to 

Myrna L. DeBortoli and Mary Gibson for editing this 

large and complicated report.

SURFACE WATER

Runoff from precipitation in the upland areas 

that surround the Santa Clara–Calleguas Basin is the 

predominant source of natural streamflow and 

ground-water recharge. As agriculture developed, some 

streamflow was diverted for irrigation. Since the 1950s, 

imported water from northern California has been 

combined with local surface water and collectively 

used for artificial recharge. Discharge of reclaimed 

sewage effluent, which began in the late 1930s, 

provides an additional source of water to the surfacewater and ground-water systems in parts of the basin.

Precipitation Estimates

Precipitation, and related surface-water flow, has 

been variable through time, and is the major source of 

ground-water recharge. For this study, precipitation and 

streamflow data and statistical relations determined 

from these data were segregated into wet and dry 

seasonal periods to reconstruct historical runoff and 

streamflow. The cumulative departure curve of 

precipitation for Port Hueneme was used to divide 

periods of record into wet and dry climatic periods 

(fig. 2). The wet and dry climatic periods were 

determined using the rising and falling limbs of the 

cumulative departure curve, respectively. 

As noted earlier, for the past few centuries, 

cumulative departure of the tree-ring indices for 

southern California indicates an apparent shift in the 

frequency and amplitude of the wet and dry periods 

after the early 1700s; prior to the early 1700s wet and 

dry periods were relatively long (20 to more than 

60 years) whereas after the early 1700s these periods 

were relatively short (5 to 20 years) (fig. 2A).

Frequency analyses (spectral) of tree rings, 

precipitation, and ground-water levels indicate climatic 

cycles of 22, 5.3, and 2.2–2.9 years for the period of 

record (Appendix 3; Hanson and Dettinger, 1996). 

Collectively, these cycles account for 60 percent of the 

variation in precipitation. Winter and spring rainfall is 

derived largely from arctic-northern frontal storms that 

may be related to the long term (22 year) climatic 

cycles of the Pacific decadal oscillation. Intermediate 

(5.3 year) cycles contribute to fall and winter rainfall 

and may be related to a combination of storms related 

to a northerly flow of moisture from El Niño and 

monsoonal flow from the central Pacific Ocean. 

Additional moisture may be associated with meridianal 

flow of the jetstream and related extracyclonic storms 

that occur during the short-term (2.2–2.9 year) cycles 

of El Niño years in both wet and dry periods (fig. 2A). 

Examples of exceptional storm-type related events that 

may be attributed to subtropical extracyclonic storms 

include a short-lived, intense rain storm, such as 

occurred in September of 1910 during a dry period; a 

relatively wet year, such as 1962, during a dry period; 

and historic flooding, such as in 1853. Freeman (1968) 

originally segregated wet and dry periods on the basis 

of precipitation records from Santa Paula and 

precipitation estimates reconstructed from crop indices 

for 1769 through 1965. Freeman demonstrated a strong 

correlation between the longer term wet and dry 

periods and observed hydrologic events in southern 

California, such as changes in stage of lakes and 

reservoirs, and droughts and floods (fig. 2B).
Surface Water 11

For this study, six alternating climate cycles that 

resulted in six wet and six dry periods between 1891 

and 1993 were identified on the basis of the cumulative 

departure curve for precipitation measured at Port 

Hueneme (fig. 2A). The climate cycles were separated 

into wet-year and dry-year periods as follows:

CYCLE DRY-YEAR PERIOD WET-YEAR PERIOD

 1 1891–1904 1905–1918

 2 1919–1936 1937–1944

 3 1945–1951 1952–1958

 4 1959–1964 1965–1969

 5 1970–1977 1978–1986

 6 1987–1991 1992–1993

This segregation shows good agreement with the 

tree-ring indices and the climate periods delineated by 

Freeman (1968) (fig. 2A,B). Selected coastal 

precipitation stations at Ventura, Oxnard, Port 

Hueneme, and Camarillo were used to assess the 

segregation of data within the wet- and dry-year 

seasons (fig. 3, table 1). Although there are some wet 

years in dry periods and dry years in wet periods, the 

seasonal mean coastal precipitation for these multipleyear wet- and dry-year period groupings is not 

significantly different from the seasonal mean 

precipitation grouped for individual wet and dry years 

(independent of wet- and dry-year periods) but is 

significantly different from the period-of-record mean 

for all seasons except summer (table 1). This general 

segregation of recent historical climatic variability into 

wet- and dry-year periods were used to reconstruct the 

historical estimates of precipitation and streamflow. 

Ground-water recharge and changes in ground-water 

demand measured or estimated from pumpage data 

were categorized on the basis of these wet and dry 

periods.

Kriged estimates of average total seasonal 

precipitation for wet and dry winters, springs, 

summers, and falls were made from available data from 

the Ventura County Flood Control District precipitation 

stations for 1891 to 1991 (fig. 3A–H). Data were not 

available for individual stations for the entire period of 

estimation. The spatial distributions of seasonal 

precipitation for wet and dry periods were similar for 

winter and fall. Spring and summer precipitation 

patterns, however, showed a small shift from relatively 

more precipitation in the northern mountains during 

wet springs and summers to relatively more 

precipitation in the southeastern mountains during dry 

springs and summers (fig. 3C–F). The largest increase 

in seasonal precipitation was between wet and dry 

winters (fig. 3A,B). The ratio of wet- to dry-season 

precipitation was 1.8 for winter, 1.6 for spring, 1.1 for 

summer, and 1.2 for fall.

Streamflow

The Santa Clara River Basin drains the area to 

the north and east of the Santa Clara–Calleguas 

ground-water basin; its major tributaries are Piru, 

Hopper, Pole, Sespe, Santa Paula, and Ellsworth 

Creeks (fig. 4). Calleguas Creek and its major 

tributaries, Conejo Creek and Arroyo Simi–Las Posas, 

drain the areas to the south and east of the alluvial 

basin. Revolon Slough and its major tributaries, Arroyo 

Hondo and Beardsley Wash (fig. 4), drain the western 

part of the Las Posas Valley and the southwestern part 

of the Oxnard Plain. Streamflow represents the major 

natural source ground-water recharge to the basin. The 

steadily increasing use of the surface-water and 

ground-water resources of the Santa Clara–Calleguas 

Basin since the late 1800s has resulted in streamflow 

depletion.

Streamflow measurements were made as early as 

the late 1800s (Grunsky, 1925), but continuous 

measurement at permanent gaging stations was not 

undertaken until 1912 on Piru Creek and not until 1927 

on the Santa Clara River (fig. 4). Gaging stations also 

were established on other Santa Clara River tributaries 

(fig. 4) starting in 1927. Streamflow gaging stations 

were first established on the Arroyo Simi in 1934 and 

on Conejo Creek in the 1970s. Continuous gaging of 

streamflow at downstream sites began at Montalvo on 

the Santa Clara River (11114000) in 1955, on the 

Calleguas Creek above U.S. Highway 101 (11106550) 

in 1971, and at Camarillo (11106000) in 1968 (fig. 4). 
12 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Figure 3. Kriged average total seasonal precipitation 1891–1991 for wet and dry climatic periods by season. A, Wet winter. B, Dry winter. C, Wet spring. D, Dry 

spring. E, Wet summer. F, Dry summer. G, Wet fall. H, Dry fall. Number of seasons available varies between stations.

Boundary of

Santa Clara-Calleguas

Hydrologic Unit

Santa

Castaic

Lake

Calleguas

Clara

River

Creek

Lake

Piru

Pacific Ocean

Ventura

Santa

Paula

Fillmore

Camarillo

Oxnard

Newhall

VENTURA CO.

VENTURA CO.

SANTA BARBARA CO.

LOS ANGELES CO.

KERN CO.

LOS ANGELES CO.

Thousand

Oaks

0 10 20 MILES

0 10 20 KILOMETERS

119°

R24W R22W R20W R18W R16W

T

2

N

T

4

N

T

6

N

T

8

N

118°30'

34°

30'

34°

15'

34°

45'

Ojai

Upper

Ojai

Santa Paula

Ventura

Oxnard

Camarillo

Oxnard

Port

Hueneme

A Wet winter

Precipitation – Numbers show range in

kriged average total seasonal precipitation

for each colored subregion.

Ventura County Flood Control District

precipitation station with selected names

City or town

EXPLANATION

11.0-12.99 inches

9.0-10.99

inches

9.0-10.99

inches

15.0-16.99

inches

15.0-16.99 inches

17.0-18.99

inches

13.0-14.99 inches

19.0-19.99

inches

17.0-

18.99

inches
Surface Water 13

Figure 3—Continued. Kriged average total seasonal precipitation 1891–1991 for wet and dry climatic periods by season. B, Dry winter.

Santa

Castaic

Lake

Calleguas

Clara

River

Creek

Lake

Piru

Pacific Ocean

Ventura

Santa

Paula

Fillmore

Camarillo

Oxnard

Newhall

VENTURA CO.

SANTA BARBARA CO.

VENTURA CO.

LOS ANGELES CO.

KERN CO.

LOS ANGELES CO.

Thousand

Oaks

119° 118°30'

34°

30'

34°

15'

34°

45'

Ojai

Upper

Ojai

Santa Paula

Ventura

Oxnard

Port

Hueneme

B Dry winter 0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

9.0-10.99

inches

3.5-4.99

inches

3.5-4.99

inches

9.0-10.99 inches

5.0-6.99 inches

5.0-6.99 inches

7.0-8.99 inches

7.0-8.99

inches

11.0-11.49

inches

7.0-8.99

inches

3.5-4.99

inches

9.0-10.99

inches

Oxnard Precipitation – Numbers show range in

kriged average total seasonal precipitation

for each colored subregion.

Ventura County Flood Control District

precipitation station with selected names

City or town

Camarillo

R24W R22W R20W R18W R16W

T

2

N

T

4

N

T

6

N

T

8

N
14 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Figure 3—Continued. Kriged average total seasonal precipitation 1891–1991 for wet and dry climatic periods by season. C, Wet spring.

Santa

Castaic

Lake

Calleguas

Clara

River

Creek

Lake

Piru

Pacific Ocean

Ventura

Santa

Paula

Fillmore

Camarillo

Oxnard

Newhall

VENTURA CO.

VENTURA CO.

SANTA BARBARA CO.

KERN CO.

LOS ANGELES CO.

Thousand

Oaks

LOS ANGELES CO.

119° 118°30'

34°

30'

34°

15'

34°

45'

Ojai Upper Ojai

Santa Paula

Ventura

Oxnard

Port

Hueneme

C Wet spring

1.0-1.49

inches

1.0-1.49

inches

1.5-1.99 inches

2.0-2.49 inches

2.5-2.99

inches

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

1.0-1.49

inches

Oxnard Precipitation – Numbers show range in

kriged average total seasonal precipitation

for each colored subregion.

Ventura County Flood Control District

precipitation station with selected names

City or town

Camarillo

R24W R22W R20W R18W R16W

T

2

N

T

4

N

T

6

N

T

8

N
Surface Water 15

Figure 3—Continued. Kriged average total seasonal precipitation 1891–1991 for wet and dry climatic periods by season. D, Dry spring.

Santa

Castaic

Lake

Calleguas

Clara

River

Creek

Lake

Piru

Pacific Ocean

Ventura

Santa

Paula

Fillmore

Camarillo

Oxnard

1-1.49 inches 1.5-1.99 inches

1.5-1.99 inches

1.5-1.99 inches

> 0.5-.99 inch

Newhall

VENTURA CO.

SANTA BARBARA CO.

KERN CO.

LOS ANGELES CO.

LOS ANGELES CO.

VENTURA CO.

119° 118°30'

34°

30'

34°

15'

34°

45'

Ojai Upper Ojai

Santa Paula

Ventura

Oxnard

Port

Hueneme

D Dry spring 0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

2.0-2.5 inches

> 2.5 inches

2.0-2.5

inches

Oxnard Precipitation – Numbers show range in

kriged average total seasonal precipitation

for each colored subregion.

Greater than

Ventura County Flood Control District

precipitation station with selected names

City or town

Camarillo

R24W R22W R20W R18W R16W

T

2

N

T

4

N

T

6

N

T

8

N

>
16 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Figure 3—Continued.Kriged average total seasonal precipitation 1891–1991 for wet and dry climatic periods by season. E, Wet summer.

Santa

Castaic

Lake

Calleguas

Clara

River

Creek

Lake

Piru

Pacific Ocean

Ventura

Santa

Paula

Fillmore

Camarillo

Oxnard

0.5-0.74 inch

0.75-0.99 inch

0.75-0.99 inch

Newhall

VENTURA CO.

SANTA BARBARA CO.

KERN CO.

LOS ANGELES CO.

Thousand

Oaks

LOS ANGELES CO.

VENTURA CO.

119° 118°30'

34°

30'

34°

15'

34°

45'

Ojai Upper Ojai

Santa Paula

Ventura

Oxnard

Port

Hueneme

E Wet summer

0.75-0.99

inches

0 20 MILES

20 KILOMETERS

10

0 10

EXPLANATION

Oxnard Precipitation – Numbers show range in

kriged average total seasonal precipitation

for each colored subregion.

Ventura County Flood Control District

precipitation station with selected names

City or town

Camarillo

R24W R22W R20W R18W R16W

T

2

N

T

4

N

T

6

N

T

8

N
Surface Water 17

Figure 3—Continued. Kriged average total seasonal precipitation 1891–1991 for wet and dry climatic periods by season. F, Dry summer. 

Santa

Castaic

Lake

Calleguas

Clara

River

Creek

Lake

Piru

Pacific Ocean

Ventura

Santa

Paula

Fillmore

Camarillo

Oxnard

Newhall

VENTURA CO.

SANTA BARBARA CO.

KERN CO.

LOS ANGELES CO.

Thousand

Oaks

LOS ANGELES CO.

VENTURA CO.

119° 118°30'

34°

30'

34°

15'

34°

45'

Ojai Upper Ojai

Santa Paula

Ventura

Oxnard

Port

Hueneme

F Dry summer

0.25-0.49

inches

0.75-0.99

inch

0.75-0.99 inch

0.75-0.99

inches

0.5-0.74 inch

0.25-0.49

inch

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Oxnard Precipitation – Numbers show range in

kriged average total seasonal precipitation

for each colored subregion.

Ventura County Flood Control District

precipitation station with selected names

City or town

Camarillo

R24W R22W R20W R18W R16W

T

2

N

T

4

N

T

6

N

T

8

N
18 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Figure 3—Continued. Kriged average total seasonal precipitation 1891–1991 for wet and dry climatic periods by season. G, Wet fall.

Santa

Castaic

Lake

Calleguas

Clara

River

Creek

Lake

Piru

Pacific Ocean

Ventura

Santa

Paula

Fillmore

Camarillo

Oxnard

Newhall

VENTURA CO.

SANTA BARBARA CO.

KERN CO.

LOS ANGELES CO.

Thousand

Oaks

LOS ANGELES CO.

VENTURA CO.

119° 118°30'

34°

30'

34°

15'

34°

45'

Ojai Upper Ojai

Santa Paula

Ventura

Oxnard

Port

Hueneme

G Wet fall

4.5-5.4 inches

4.0-.4.4

inches

4.0-.4.4

inches

6.5-7.4 inches

7.5-7.9

inches

5.5-6.4 inches

5.5-6.4

inches

3.5-3.9

inches

3.5-3.9

inches

8.0

inches

0 20 MILES

20 KILOMETERS

10

0 10

EXPLANATION

Oxnard Precipitation – Numbers show range in

kriged average total seasonal precipitation

for each colored subregion.

Ventura County Flood Control District

precipitation station with selected names

City or town

Camarillo

R24W R22W R20W R18W R16W

T

2

N

T

4

N

T

6

N

T

8

N
Surface Water 19

Figure 3—Continued. Kriged average total seasonal precipitation 1891–1991 for wet and dry climatic periods by season. H, Dry fall.

Santa

Castaic

Lake

Calleguas

Clara

River

Creek

Lake

Piru

Pacific Ocean

Ventura

Santa

Paula

Fillmore

Camarillo

Oxnard

Newhall

VENTURA CO.

SANTA BARBARA CO.

KERN CO.

LOS ANGELES CO.

Thousand

Oaks

LOS ANGELES CO.

VENTURA CO.

119° 118°30'

34°

30'

34°

15'

34°

45'

Ojai Upper Ojai

Santa Paula

Ventura

Oxnard

Port

Hueneme

H Dry fall

3.5-3.9 inches

3.0-3.4

inches

3.0-3.4

inches

5.5-5.9 inches

6.0-6.3

inches

4.5-4.9 inches

4.5-4.9 inches

5.0-5.4 inches

4.0-4.4 inches

4.0-4.4 inches

0 20 MILES

20 KILOMETERS

10

0 10

EXPLANATION

3.5-3.9

inches

Oxnard Precipitation – Numbers show range in

kriged average total seasonal precipitation

for each colored subregion.

Ventura County Flood Control District

precipitation station with selected names

City or town

Camarillo

R24W R22W R20W R18W R16W

T

2

N

T

4

N

T

6

N

T

8

N
20 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Table 1. Summary of coastal precipitation statistics for the Santa Clara–Calleguas Basin, Ventura County, California

[Data from Ventura County, Department of Public Works (Dolores Taylor, written commun., 1992). Grouping: Dry years represent all years in which 

precipitation was less than the mean for the period of record; wet years represent all years in which precipitation was more than the mean for the period of 

record. Dry-year periods are periods of decreasing cumulative departure for precipitation for the period of record and wet-year periods are periods of 

increasing cumulative departure. W is a value of Shapiro–Wilk Statistic normality test where values close to 1 indicate a significant probability of a normally 

distributed group of mean total seasonal precipitation.%, percent; —, reference group]

Precipitation period

(group number) Grouping

Mean/standard deviation, 

 in inches

 (number of samples)

W: Normality test

Significant difference 

in means at 95-percent

 level between groups?

Coastal winter (1) All years 8.37/5.19(101) 0.93 —

Coastal winter (2) Dry years 5.47/2.48(70) .97 (1)–(2):Yes

(2)–(3): No

Coastal winter (3) Dry-year periods 6.24/3.28(58) .93 (1)–(3):Yes

Coastal winter (4) Wet years 14.93/3.20(31) .94 (1)–(4):Yes

(4)–(5): Yes

Coastal winter (5) Wet-year periods 11.19/5.85(43) .96 (1)–(5):Yes

Coastal spring (1) All years 1.15/1.13(100) .83 —

Coastal spring (2) Dry years .31/.73(70) .48 (1)–(2):Yes

(2)–(3): Yes

Coastal spring (3) Dry-year periods 1.05/.96(57) .87 (1)–(3):No

Coastal spring (4) Wet years 1.03/1.10(30) .83 (1)–(4):No

(4)–(5): No

Coastal spring (5) Wet-year periods 1.30/1.33(43) .84 (1)–(5):No

Coastal summer (1) All years .30/.66(100) .53 —

Coastal summer (2) Dry years .30/.73(70) .48 (1)–(2):No

(2)–(3): No

Coastal summer (3) Dry-year periods .26/.68(57) .43 (1)–(3):No

Coastal summer (4) Wet years .28/.48(30) .66 (1)–(4):No

(4)–(5): No

Coastal summer (5) Wet-year periods .36/.65(43) .64 (1)–(5):No

Coastal fall (1) All years 4.11/2.74(99) .94 —

Coastal fall (2) Dry years 4.01/2.68(69) .95 (1)–(2):No

(2)–(3): No

Coastal fall (3) Dry-year periods 3.86/2.63(56) .94 (1)–(3):No

Coastal fall (4) Wet years 4.33/2.90(30) .94 (1)–(4):No

(4)–(5): No

Coastal fall (5) Wet-year periods 4.44/2.87(43) .95 (1)–(5):No
Surface Water 21

Pacific

Ocean

R18W R22W

T

4

N

T

2

N

River

Santa

Clara

Lake Piru

Calleguas

Arroyo Simi

Arroyo

Las Posas

Arroyo

Honda

Revolon

Arundell

Barranca

Slough

Beardsley

Wash

Creek

11114000

(708)

11107000

(-)

11107500

(-)

11113910

(-)

-

(776)

-

(700)

342500

11106550

(805)

11106550

(805)

-

(806)

11106000

(-)

-

(841)

11106400

(800)

11105850, -

(-, 803) -

(802)

11110500

(701)

-

(713)

11113000

(710) 118471701 (-)

11109800

(714)

11110000

(-)

11109000

(-)

11108500

(707)

11113500

(709) D

D

D

D

D

Conejo

Cr

Santa Paula

Ellsworth

Cr

Cr

Sespe

Hopper

ole P

iru P

Cr

Cr

Cr

Hopper

ole P

Cr

Cr

Cr

Sulphur Mountain

Oak Ridge

South Mountain

Camarillo Hills

Las Posas Hills

Simi Val

ey l

Oxnard Plain

Big Mtn

Santa

Paula

Oxnard

El Rio

Montalvo

Camarillo

Fillmore

Saticoy

El Rio

Spreading

Grounds

Saticoy

Spreading

Grounds

Freeman

Diversion

Saticoy

Diversion

Ventura

Santa Paula

Creek

Diversion

Piru Creek

Diversion

Thousand

Oaks

MuguLagoon

EXPLANATION

Santa Clara-CalleguasHydrologic Unit boundary

Selected streamflow gaging station – Upper numberis U.S. Geological Survey gaging station designation;lower number is Ventura County Flood Control Districtgaging station designation (Refer to Appendix 4 forstation names)

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium and unconsolidated depositsShallow alluvium

Consolidated and unconsolidated deposits

Index subdrainage basinfor Hopper and Pole Creeks

Selected diversion

0

0

10 MILES

10 KILOMETERS

118°45' 119°15'

34°22'30"34°07'30"

119°

Subdrainage basin boundary –(California Department ofWater Resources, 1975)

Figure 4. Surface-water drainage features, selected gaging stations, and selected diversions in the Santa Clara–Calleguas ground-water basin, Ventura County, California. 
22 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Hydrographs of daily mean streamflow for eight 

gaging stations in the Santa Clara–Calleguas Basin are 

presented in figure 5. Natural streamflow in all the 

major streams and tributaries in the basin is 

intermittent to ephemeral (fig. 5). Runoff from 

precipitation primarily during December through April 

results in natural streamflow in the winter and spring. 

Most of the streamflow occurs as floodflow. Some of 

the flows recharge the ground-water system and the 

remainder discharges into the Pacific Ocean. Sespe 

Creek is the largest contributor of streamflow to the 

Santa Clara River system and Piru Creek is the second 

largest (table 2). Major streams generally have fewer 

intermittent reaches or become perennial during 

wet-year periods and have more floodflows and larger 

baseflows (fig. 5). The Santa Clara River, Piru Creek, 

Arroyo Simi, and Conejo Creek all have components of 

regulated flow. The average and median streamflow, 

and the number of days of flow for the total period of 

record and for the wet and dry periods defined for this 

study (fig. 2) are summarized in table 2. These 

components of regulated flow increased the mean flow 

and decreased the number of days with no flow 

(table 2).

Major floods generally occur during wet periods 

but can occur during dry-year periods (figs. 2 and 5). In 

1969, the peak discharge for the largest flood for the 

period of record was more than 110,000 ft3/s at the 

Montalvo gage (11114000) on the Santa Clara River 

(not shown in figure 5). In the Santa Clara River and 

most of its major tributaries, multiple-year recession 

periods generally follow wet periods for unregulated 

streamflow (fig. 5). During these subsequent years, the 

gaged outflow at Montalvo can be greater than the 

gaged inflow of the Santa Clara River and its major 

tributaries.

Streamflow-duration curves of gaged streams 

show major differences between wet and dry periods 

(fig. 6). Streamflow on Piru, Pole, Sespe, and Santa 

Paula Creeks is perennial during wet years (fig. 6

C,D,F,G). The magnitude of daily streamflow increases 

by a factor of three to five from dry to wet years for 

streamflows of the same frequency at the seven gaging 

stations in the Santa Clara–Calleguas ground-water 

basin (fig. 6 A–G).

Since the construction of the Santa Felicia Dam 

in 1955, controlled releases of water from Lake Piru 

have resulted in fewer days of no flow in the Santa 

Clara River; however, average annual streamflow in the 

river was reduced by 35 percent during the 21-year 

period (1956–75) after construction of the dam (Taylor 

and others, 1977). Since 1969, discharge owing to the 

release of treated wastewater from Los Angeles County 

and imported water from Castaic Lake has increased 

the minimum flow in the Santa Clara River across the 

Los Angeles–Ventura County line from less than 

10 ft3/s to about 20 ft3/s (fig. 5A). In the Calleguas 

Creek drainage, regulated flow has resulted in 

additional baseflow owing to discharge of treated 

municipal sewage along Arroyo Simi and Conejo 

Creek since about 1970 (fig. 5B) and discharge of 

shallow ground water from dewatering wells. Since 

1962, the release of sewage effluent in Conejo Creek 

has resulted in an increase in baseflow from 0.5 to 

15 ft3/s (fig. 5). The pumping of shallow ground water 

for dewatering upstream in Simi Valley has resulted in 

additional baseflow on the Arroyo Simi at the Madera 

Road Bridge (fig. 5G)—an increase from less than 0.1 

ft3/s to about 4 ft3/s since 1969. Streamflow has 

become more intermittent on the Santa Clara River at 

Montalvo since 1929 owing to diversions at Saticoy 

and Freeman. Based on historical basinwide estimates 

of streamflow and runoff, ungaged tributary runoff 

provides the second (California Department of Water 

Resources, 1975; tables 23 and 24) or third (California 

Department of Public Works, 1934; table 59) largest 

contribution to streamflow. Diversion from Sespe 

Creek, as well as numerous smaller intermittent 

diversions from the Santa Clara River for irrigation, is 

still occurring. Diversions from Piru Creek below Santa 

Felicia Dam and from the Santa Clara River at the 

Freeman Diversion provide water for artificial 

recharge. Controlled releases from Lake Piru Reservoir 

are conveyed down the natural stream channel to these 

artificial-recharge spreading grounds, supplementing 

the intermittent natural streamflow during the generally 

dry summer and fall months.
Surface Water 23

Regulated Flow

Regulated Flow

AB

C

D

SESPE CREEK NEAR FILLMORE AND FILLMORE IRRIGATION COMPANY DIVERSION (11113000)

10,000

1,000

100

10

1

0.1

SANTA CLARA RIVER AT COUNTY LINE (11108500) AND SANTA CLARA RIVER NEAR PIRU (11109000)

PIRU CREEK NEAR PIRU (11110000) AND PIRU CREEK BELOW SANTA FELICIA DAM (11109800)

10,000

1,000

100

10

1

0.1

wet wet wet wet wet wet dry dry dry dry dry

10,000

1,000

100

10

1

0.1

10,000

1,000

100

10

1

0.1

1910

1915

1920

1925

1930

1935

1940

1945

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

SANTA PAULA CREEK NEAR SANTA PAULA (11113500)

YEAR

DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

Figure 5. Daily mean streamflow for wet and dry periods at the major rivers and tributaries in the Santa Clara–Calleguas ground-water basin, Ventura County, California.
24 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California POLE CREEK AT SESPE AVENUE (–) ARROYO SIMI NEAR SIMI (11105850) AND ARROYO SIMI AT MADERA ROAD BRIDGE (–) CONEJO CREEK ABOVE HIGHWAY 101 (11106400)

1910

1915

1920

1925

1930

1935

1940

1945

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

YEAR

1

0.1

10,000

1,000

100

10

1

0.1

10,000

1,000

100

10

1

0.1

10,000

1,000

100

10

1

0.1

DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

10,000

1,000

100

10

HOPPER CREEK NEAR PIRU (11110500)

wet wet wet wet wet wet dry dry dry dry dry

E

F

GH

Regulated Flow

Regulated Flow

Figure 5—Continued.
25 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Table 2. Summary of gaged streamflow data for selected streams in the Santa Clara–Calleguas Basin, Ventura County, California

[Streamflow gaging station (station number): preceding the slash is the U.S. Geological Survey gaging station number and following the slash is the Ventura 

County Flood Control District gaging station number. --, no station number provided; —, no estimate provided]

1Streamflow data combined with streamflow data from Santa Clara River near Piru (11109000) for period 1927–32. Numbers represent the period 

without wastewater flowing into the basin along the Santa Clara River from Los Angeles County for climate periods. 2Values are for the periods with and without wastewater flowing into the basin along the Santa Clara River from Los Angeles County. 3Streamflow data was combined with streamflow data from Fillmore Irrigation Canal diversion (11113001/—) for period 1940–91. 4Streamflow data was combined with streamflow data from Santa Clara River Diversion at Saticoy (11113910/—) for period 1928–92. Values also 

represent the period with releases from Lake Piru. 5Values represent the period without wastewater flowing into the basin along the Santa Clara River from Los Angeles County. 6Values represent the period with and without wastewater flowing into the basin along the Santa Clara River from Los Angeles County, respectively. 

Values also represent the period with releases from Lake Piru. 7Values represent the period without dewatering pumpage flowing into the basin along Arroyo Simi.

Streamflow gaging station

(station No.) 

[period of record]

Arithmetic

average streamflow

(cubic feet per second)

Median/geometric 

mean streamflow

(cubic feet per second)

Number of 

no-flow days

Time averaged

streamflow used 

in predevelopment 

model

(cubic feet 

per second)

Total 

period

Wet 

periods

Dry 

periods

Total

 period

Wet 

periods

Dry 

periods

Total 

period

Wet 

periods

Dry 

periods

Santa Clara River at county line1

(11108500 / 707)

Unregulated flow 

 [1928–32, 1953–71]

32.1 52.5 14.5 2.6/4.3 3.2/4.6 1.9/4.0 801 100 701 2.0

Santa Clara River at county line 

 (11108500 / 707)

Regulated and unregulated flow 

[1953–91]2

48.3 69.3 26.2 17.0/11.4 20.0/ 13.1 14.0/ 9.7 464 100 364 —

Piru Creek near Piru

 (11110000 / —)

 [1912–13, 1927–54]

57.3 100 23.7 12.0/12.8 22.0/23.4 5.1/7.2 1,038 4 1,034 13.0

Piru Creek below Santa Felicia Dam

 (11109800 / 714)

 [1956–92]

42.1 54.6 28.7 12.0/12.2 8.7/14.5 7.2/10.3 544 450 94 —

Hopper Creek near Piru

 (11110500 / 701)

 [1931–90]

6.2 9.7 2.4 .3/1.1 .7/ 1.6 .01/.6 7,765 2,660 4,032 0.3

Pole Creek at Sespe Avenue, 

Fillmore

 (–/ 713)

 [1974–91]

2.3 3.5 .7 .6/.6 1.0/1.0 .3/.3 25 2 23 0.6

Sespe Creek near Fillmore

 (11113000 / 710)3 

 [1940–91]

125.4 179.8 64.2 17.0/20.7 26.0/30.7 10.0/13.2 0 0 0 18.0

Santa Paula Creek near Santa Paula

 (11113500 / 709)

 [1928–91].............

22.5 36.5 10.8 4.5/5.4 7.2/8.7 2.9/3.5 854 0 854 4.5

Santa Clara River at Montalvo 

 (11114000 / 708)4

 [1955–71]5

222.2 319.8 113.0 25.0/47.9 33.9/71.5 18.2/30.7 1,244 669 575 —

Santa Clara River at Montalvo 

 (11114000 / 708)4

 [1955–92]6

257.4 385.4 114.2 46.1/59.7 96.0/106.8 24.5/32.2 1,392 671 721 —

Arroyo Simi near Simi 

 (11105850/—)7 and 

Arroyo Simi at Royal Avenue 

(–1/802) 

 [1934–64].............

1.3 2.1 .5 0/.6 0/.8 0/.3 10,282 4,801 5,481 0

Arroyo Simi near Simi

 (11105850/--)7 and 

Arroyo Simi at Royal Avenue 

(–/802)

 [1934–69]

2.3 3.7 .5 0/.9 0/1.5 0/.3 11,942 6,461 5,481 0
26 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California B. SANTA CLARA RIVER AT MONTALVO AND SATICOY DIVERSION (1955-1971) C. PIRU CREEK NEAR PIRU (1912-13, 1927-54) D. POLE CREEK AT SESPE AVENUE (1974-1991) A. SANTA CLARA RIVER AT COUNTY LINE (1953-1971)

E. HOPPER CREEK NEAR PIRU

(1931-1990)

F. SESPE CREEK NEAR FILLMORE AND

FILLMORE IRRIGATION CANAL

(1940-1991)

H. ARROYO SIMI NEAR SIMI

(1934-1969)

G. SANTA PAULA CREEK NEAR

SANTA PAULA (1928-1991)

DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

CUMULATIVE FRACTION, EXCEEDING OR EQUALING 0.1 1 10 100 1,000 1 1 0.1 10 10 100 100 1,000 1,000 10,000 10,000 1 0.1 0.01 10 100 1,000

1 0.1 0.01 10 100 1,000 10 100 1,000 10,000 1 0.1 1 10 100 1,000 0.1 1 10 100 1,000

DRY YEARS

ALL YEARS

WET YEARS

ALL YEARS

DRY YEARS

WET YEARS

DRY YEARS

ALL YEARS

WET YEARS

DRY YEARS

WET YEARS

ALL YEARS

WET YEARS

ALL YEARS

DRY YEARS

ALL YEARS

WET YEARS

ALL YEARS

WET YEARS

DRY YEARS

WET YEARS

ALL YEARS

DRY YEARS

0.010.020.050.100.200.500.800.900.950.980.99

0.010.020.050.100.200.500.800.900.950.980.99

0.010.020.050.100.200.500.800.900.950.980.99

0.010.020.050.100.200.500.800.900.950.980.99

0.010.020.050.100.200.500.800.900.950.980.99

0.010.020.050.100.20

0.50

0.800.900.950.980.99

0.010.020.050.100.200.500.800.900.950.980.99

0.010.020.050.100.20

0.50

0.800.900.950.980.99

Figure 6. Streamflow duration during wet and dry periods at the major rivers and tributaries in the Santa Clara–Calleguas ground-water basin, Ventura County, California. (See table 2 for gaging station numbers and figure for gaging station locations.)
Surface Water 27

Irrigation Diversions

Diversion of natural streamflow (fig. 4) was the 

first water-resources development for agricultural use 

in the Santa Clara–Calleguas ground-water basin. 

Major diversions from the Santa Clara River and its 

tributaries were constructed in the middle to late 1800s. 

The continued growth of agriculture resulted in 

irrigation and return-flow diversions in the early 1900s 

that captured most of nonflood flows from the Santa 

Clara River. The diversions on the tributaries generally 

were small, permanent structures on bedrock designed 

to capture the low perennial baseflows (less than 1 to 

10 ft3/s) during summer and fall. Mainstem diversions, 

however, commonly were temporary structures that 

were rebuilt within the shifting channel after the 

recession of floodflows. Other historically larger 

diversion canals (not shown on figure 4), such as 

Farmers Ditch, Santa Clara Water and Irrigation 

Company Canal, Camulos Ranch Ditch on the Santa 

Clara River, and Fillmore Land and Water Company 

Canal on Sespe Creek, conveyed diversions of 10 to 40 

ft3/s [shown in Adams (1913, pl. XVI), and Predmore 

and others (1997)]. Most of these diversions operated 

within the subbasins and supplied irrigation water to 

crops on the adjacent flood plain. The larger mainstem 

diversions typically were located where there was 

sustained flow, which generally occurs below the 

confluence with major tributaries where natural 

sediment deposited by inflow causes riffles and 

ponding of streamflow. Some of the mainstem 

diversions along the Santa Clara River were built near 

the upstream side of the constrictions at the subbasin 

boundaries where there is a mixture of streamflow and 

ground-water discharge. The diversions of surface 

water supplied a significant amount of the water used 

for irrigation prior to the early 1930s when irrigation 

demand exceeded the surface-water supplies largely 

owing to the 1923–36 drought.

Imported water

Since 1971, surface water has been imported 

from northern California and routed through a series of 

reservoirs constructed by the UWCD for controlled 

release during the growing season. Water from northern 

California is imported by the UWCD to Pyramid Lake 

and Lake Piru where it periodically is released into Piru 

Creek and the Santa Clara River channels. Water has 

been imported to Castaic Lake since the 1970s where it 

is released into the Santa Clara River channel. This 

imported water, along with treated sewage effluent 

from Los Angeles County, increases the perennial 

baseflow at the streamflow-gaging station on the Santa 

Clara River at the Los Angeles–Ventura County Line 

(fig. 5A). Most of the water brought into the basin since 

1964 was imported by the CMWD using Metropolitan 

Water District (MWD) pipelines—about 1,863,000 

acre-ft of water from 1964 through 1993. The water 

was used primarily for municipal supplies (91 percent), 

and a small part (9 percent) was used for irrigation. 

Some of this water may have entered the ground-water 

flow system as sewage-effluent discharge or as 

percolation of excess applied irrigation water 

(hereinafter referred to as irrigation return flow) in the 

Las Posas Valley and Pleasant Valley subbasins. Even 

though most of the water imported by the CMWD that 

is used for municipal supply becomes treated sewage 

effluent that is discharged to the Pacific Ocean, this 

imported water has helped reduce growth in groundwater pumping in the Oxnard Plain, Pleasant Valley, 

and Las Posas Valley subbasins.

Sewage Effluent

Sewage effluent is discharged directly to the 

Pacific Ocean, the Santa Clara River, Calleguas Creek, 

and Conejo Creek and to percolation ponds for direct 

infiltration or it was reused for irrigation. Most of the 

sewage effluent is either directly discharged to the 

Pacific Ocean or is discharged to stream channels in the 

Oxnard Plain, where low-permeability channels do not 

allow significant infiltration to the regional groundwater flow system. Treated sewage effluent is included 

in the streamflow that enters the basin at the county line 

along the Santa Clara River, Calleguas River, and 

Conejo Creek. These contributions to streamflow are 

part of the gaged streamflow on these rivers.
28 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

GROUND WATER

The Santa Clara–Calleguas drainage basin is part 

of the tectonically active Transverse Ranges 

physiographic province. The mountains are composed 

of a variety of consolidated marine and terrestrial 

sedimentary and volcanic rocks of Late Cretaceous 

through Quaternary age. The subbasins of the Santa 

Clara–Calleguas Basin are filled with a mixture of 

consolidated and unconsolidated marine and terrestrial 

coastal deposits of Tertiary and Quaternary age. These 

basin-fill sediments and consolidated rocks form a 

complex set of aquifer systems that have been the 

primary source of water supplies since the early 1900s. 

Agriculture has been the main user of ground water, 

and in recent years public supply and industry have 

become significant users of ground water. The 

geohydrology of the basin is discussed in detail in 

reports by California Department of Public Works 

(1934), California Department of Water Resources 

(1954, 1958, 1974a,b, and 1975), California State 

Water Resources Board (1956), Mann and Associates 

(1959), and Turner (1975). The reader is referred to 

these reports for a more complete description of the 

geohydrology of the Santa Clara–Calleguas Basin.

Geologic Framework

For this report, the lithologic units mapped by 

Webber and others (1976), Dibblee (1988, 1990a,b, 

1991, 1992a,b,c,d), and Dibblee and Ehrenspeck 

(1990) in the Santa Clara–Calleguas Basin and 

surrounding area were grouped into two general 

categories: (1) upper Cretaceous and Tertiary bedrock, 

and (2) Quaternary unconsolidated deposits. The 

outcrop pattern of these combined units is shown in 

figure 7A and their stratigraphic relations are shown in 

figure 7B.

Consolidated Rocks

The upper Cretaceous and Tertiary consolidated 

rocks include sedimentary, volcanic, igneous, and 

metamorphic rocks. These rocks are virtually 

non-water bearing and form the base of the Santa 

Clara–Calleguas Basin. Although these rocks are not an 

important source of ground water, the erosion and 

subsequent deposition of these rocks are the source of 

the unconsolidated deposits that form the Santa Clara–

Calleguas ground-water basin. The sedimentary rocks 

of Cretaceous age are exposed in the Topatopa 

Mountains north of the ground-water basin and in the 

Simi Hills and Santa Susana Mountains south of the 

basin (California State Water Resources Board, 1956, 

pl. 10). These rocks are generally non-water bearing 

except within the poorly cemented and fractured 

sandstones in the hills near Simi Valley (Turner, 1975, 

p. 3).

The consolidated Tertiary sedimentary rocks 

underlie most of the ground-water basin and compose 

the surrounding mountains and hills. These rocks are 

predominantly marine in origin and are nearly 

impermeable except for the slightly permeable 

sandstones and within fracture zones. Some of these 

Miocene formations contain oil and tar sand beds, 

natural gas, and related methane and brines. The Pico 

Sandstone of Pliocene and Pleistocene epochs 

underlies the unconsolidated deposits throughout most 

of the ground-water basin and crops out in the 

mountains on the north side of the Santa Clara River 

Valley (California State Water Resources Board, 1956, 

pl. 10). These rocks are also considered to be of low 

permeability and non-water bearing. 

Volcanic rocks and related intrusive rocks of 

Miocene age underlie parts of the southern Oxnard 

Plain, South Pleasant Valley, and Santa Rosa Valley 

subbasins (figs. 7 and 8D,E). Although these rocks are 

considered non-water bearing, they have been 

developed for water supply where alluvial deposits are 

absent, such as in the Santa Rosa Valley subbasin. 

These volcanic and intrusive rocks also crop out in the 

Santa Monica Mountains along the southern and 

southeastern boundaries of the ground-water basin 

(California State Water Resources Board, 1956, pl. 10) 

and in the offshore submarine canyons along the 

southwestern boundary of the basin (Kennedy and 

others, 1987, pl. 2A). 
Ground Water 29

Unconsolidated Deposits

The Quaternary unconsolidated deposits consist 

of the Santa Barbara Formation (Weber and others, 

1976), the Las Posas Sand (Dibblee, 1988, 1990a,b, 

1991, 1992a,b,c,d; Dibblee and Ehrenspeck, 1990), the 

San Pedro Formation (Weber and others, 1976), and the 

Saugus Formation (Weber and others, 1976; Dibblee, 

1988, 1990a,b, 1991, 1992 a,b,c,d), all of the 

Pleistocene epoch, and unconsolidated alluvial and 

fluvial deposits of the Pleistocene to Holocene epoch. 

In the Santa Clara–Calleguas Basin, the unconsolidated 

deposits are grouped together into the upper-aquifer 

system and the lower-aquifer system (fig. 7B).

The Santa Barbara Formation, mapped by 

Weber and others (1976), overlies consolidated Tertiary 

rocks in most of the ground-water basin and consists of 

marine sandstone, siltstone, mudstone, and shale. The 

thickness and lithology of the formation varies 

considerably throughout the basin, but the formation is 

thickest, more than 5,000 ft, in the Ventura area (Yerkes 

and others, 1987). The formation is of low permeability 

and generally contains water of poor quality 

throughout most of the basin (Turner, 1975) and, 

therefore, is not considered an important source of 

ground water. In the East Las Posas Valley subbasin, 

the Santa Barbara Formation contains layers of sands 

and gravels that are an important source of water to 

wells in areas where younger unconsolidated deposits 

are absent or are unsaturated. The coarse-grained 

section of the Santa Barbara Formation in the East Las 

Posas Valley subbasin is commonly referred to as the 

“Grimes Canyon” member (California Department of 

Water Resources, 1956).

The Santa Barbara Formation and the lower part 

of the San Pedro Formation mapped by Weber and 

others (1976) consist of shallow marine sand and 

gravel beds that were indicated as a separate formation, 

the Las Posas Sand, by Dibblee (1988, 1990a,b, 1991, 

1992a,b,c,d) and Dibblee and Ehrenspeck (1990). 

These deposits reach a maximum thickness of more 

than 2,000 ft in the Santa Clara River Valley near 

Ventura (Dibblee, 1992a,b,c,d) and consist of a series 

of relatively uniform fine-grained sand layers 100 to 

300 ft thick separated by silt and clay layers 10 to 20 ft 

thick. The upper part of San Pedro Formation consists 

of lenticular layers of sand, gravel, silt, and clay of 

marine and continental origin. The continental fluvial 

silt, sand, and gravel deposits within the upper part of 

the San Pedro Formation are referred to as the Saugus 

Formation by Dibblee (1988, 1990a,b, 1991, 

1992a,b,c,d) and Dibblee and Ehrenspeck (1990). 

These deposits reach a maximum thickness of more 

than 5,000 ft in the Piru subbasin in the Santa Clara 

River Valley (Dibblee, 1991). The sand and gravel 

layers range from 10 to 100 ft thick and are separated 

by silt and clay layers that generally are 10 to 20 ft 

thick. The Santa Barbara and San Pedro Formations are 

absent in the Santa Rosa Valley subbasin east of the 

San Pedro Fault and in the South Pleasant Valley 

subbasin southeast of the Bailey Fault. In the eastern 

part of the Santa Rosa Valley subbasin and in the 

eastern part of the South Pleasant Valley subbasin, 

recent alluvial and terrace deposits were deposited 

unconformably on the marine shale and sandstone beds 

of the Santa Margarita Formation (Late Miocene) or 

rest unconformably on the Conejo Volcanics (Middle 

Miocene). For this study, the Santa Margarita 

Formation in the Santa Rosa Valley subbasin is 

grouped with the unconsolidated sediments of the 

lower system. During the Pleistocene epoch, major 

changes in sea level resulted in cycles of erosion and 

deposition (Dahlen, 1992). The sequence of deposits 

above the erosional unconformities typically starts with 

a basal conglomerate that is laterally extensive, 

relatively more permeable than the underlying 

deposits, and a potential major source of water to wells 

perforated in these deposits. These coarse-grained 

layers of fluvial and beach deposits are interbedded 

with extensive fine-grained layers.
30 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Lake Piru

Clara

Santa

Calleguas

Pacific Ocean

Creek

R18W R22W

T

4

N

T

2

N

Pitas

Point

Fault

McGrath

Fault

0 10 MILES

0 10 KILOMETERS

Shoreline

34°

07'30"

118°45'

34°

22'30"

119° 119°15' 119°30'

Area

not

mapped

?

?

?

?

?

?

Santa

Paula

Camarillo El Rio

Oxnard

Offshore

Oxnard

Plain

Fillmore

Ventura

Thousand

Oaks

A

A'

E

E'

B

C

D

D'

C'

B'

A

Figure 7. A, Generalized surficial geology of the Santa Clara–Calleguas ground-water basin and extents of layers in the numerical ground-water flow model, and 

B, stratigraphic column and 

related aquifer designations of geologic units by source and aquifer-system model layers, Ventura County, California. (Modified from Weber and others, 1976).
Ground Water 31

The Late Pleistocene and Holocene deposits are 

unnamed, consist of relatively flat-lying marine and 

continental unconsolidated deposits, and are regionally 

grouped into the upper system of water-bearing 

deposits (fig. 7B). These deposits, which were derived 

from local sources and from the Santa Clara River and 

Calleguas Creek, were deposited unconformably on the 

older unconsolidated deposits and contain basal 

conglomerates that are laterally extensive and produce 

substantial ground-water supplies. In the Mound and 

Oxnard Plain subbasins, the basal zones are overlain 

with fine-grained deposits of low permeability. Alluvial 

and fluvial sand and gravel deposits with interbedded 

fine-grained deposits of the Holocene epoch 

unconformably overlie the Late Pleistocene deposits. 

The basal deposits of the Holocene epoch consist of 

gravel and sand, which are overlain by fine-grained 

deposits throughout most of the Santa Clara River 

Valley and Oxnard Plain subbasins. These basal 

deposits are relatively more permeable than underlying 

deposits, and are potential major sources of water to 

wells completed in the saturated parts of these deposits. 

Interbedded sand layers occur within the fine-grained 

deposits throughout most of the Oxnard Plain. With the 

exception of recent coarse-grained channel deposits 

along the Santa Clara River and Calleguas Creek, the 

thin layer of Holocene deposits that are not coincident 

with minor tributaries are relatively fine grained and 

relatively low in permeability.

EXPLANATION

Submarine slope outside Santa ClaraCalleguas Basin

Submarine shelf deposits

(Oxnard aquifer)

Upper Pleistocene deposits

(Mugu aquifer)

Sedimentary, igneous, and

metamorphic rocks

(Pliocene-Cretaceous)

Volcanic rocks (Miocene)

BEDROCK (Consolidated or

non-water bearing)

Santa Barbara Formation

(Grimes Canyon)

Alluvium

(Shallow aquifer)

San Pedro Formation

(Upper and Lower Hueneme and Fox

Canyon aquifers) Undifferentiated

Undifferentiated

Lower-Aquifer System –

Ground-water subbasin boundary –

Extent of active flow region in upper

and lower layers (Subbasins are named

in fig. 1)

Onshore upper and lower

Model-layer extents –

USGS multiple-well monitoring

site with geophysical logs

Submarine extent of upper

Submarine extent of lower

UNCONSOLIDATED DEPOSITS

Upper-Aquifer System –

Boundary of Santa Clara-Calleguas

Hydrologic Unit

Quaternary

Holocene Pleistocene

Tertiary

?

Faults – Identified during this study

Line of section – (see figure 8)

Faults – Weber and others (1976);

Green and others (1978)

Approximate boundary of

undifferentiated submarine slope

Other wells with geophysical logs

Other wells with geophysical logs

used for cross sections

Cenozoic Mesozoic

D D'

Figure 7—Continued.
32 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Figure 7B. Stratigraphic column and related aquifer designations of geologic units by source and aquifer system model layers in the ground-water and surface-water flow model of the Santa Clara–Calleguas Basin, Ventura County, California—Continued

Geologic 

era

Geologic

system

Geologic

series

(epoch)

 Weber and others (1976) Dibblee1 Turner (1975) Green and others (1978)2 RASA3 Aquifer system

model layers

Lithologic units and Formations Aquifers

Cenozoic

nary Quater

Holocene Recent Alluvium

(Lagoonal, beach, river and flood plain deposits, artificial fill, and alluvial fan 

deposits)

 Recent alluvial and 

semiperched

Shallo

w Upper-aquifer 

system4, layer 1

Recent Alluvium(Lagoonal, beach, river and flood plain deposits and alluvial fan deposits)

 Oxnard5

Late 

(Upper)Pleistocene6

Older Alluvium(Lagoonal, beach, river and flood plain, alluvial fan, terrace, and marineterrace deposits)

 Mugu2

Saugus Formation7(Terrestrial fluvial sediments) 

Saugus Formation Hueneme Upper Hueneme Lower-aquifer 

system, layer 2

San Pedro Formation8(Marine clays and sands and terrestrial 

fluvial sediments)

Lower Hueneme

Las Posas Sand(Marine shallow regressive sands)

Fox Canyon Fox Canyon

Early

 (Lower) Pleistocene6

Santa Barbara Formation8(Marine shallow regressive sands)

Grimes Canyon9,10 Grimes Canyon

Pico Formation 11(Marine siltstones, sandstones, and conglomerates) 

Formation not included in regional flow model Formation not 

included in regional flow model

Pliocene6 Repetto formation

(Terrestrial conglomerates, sandstones, and shales) 

ertiary T

Miocene

Santa Margarita Formation, Monterey Shale, Rincon Mudstone, Towsley 

Formation

(Terrestrial fluvial sandstones and fine-grained lake deposits)

Not Included Santa Margarita sandstones 

included in northeastern Santa Rosa 

Valley

Lower-aquifer 

system, layer 2

Conejo 

Volcanics

(Terrestrial and marine extrusive and intrusive, felsic-andesites to basalts)

Formation not included in regional flow model Formation not 

included in regional flow model

Lo

wer Topanga Formation, Topanga-Vaqueros Sandstones, Modelo Formation, 

Sisquoc Formation

 (Marine transgressive sands and siltstones)

Oligocene Sespe Formation

(Terrestrial fluvial claystones and sandstones)

Eocene Llajas Formation, Coldwater Sandstone, Cozy Dell Shale, Matilija Sandstone, 

Juncal Formation, Santa Susana Formation

(Marine sandstones, mudstones, and claystones)

Paleocene Martinez Formation

(Terrestrial conglomerate, sandstones, and marine shales)

MesozoicUpper Cretaceous Chico Formation (Sandstones with shales)
Ground Water 33

Figure 7B. Stratigraphic column and related aquifer designations of geologic units by source and aquifer system model layers in the ground-water and surface-water flow model of the Santa Clara–Calleguas Basin, Ventura County, California—Continued

1Formations from Dibblee (1988; 1990a,b; 1991; 1992a,b,c,d) and Dibblee and Ehrenspeck (1990).2Perched aquifer designated in parts of the Oxnard Plain only.3From the current study as part of the Southern California Regional Aquifer-System Analysis Program of the U.S. Geological Survey.4Shallow aquifer included in the Oxnard Plain Forebay and inland subbasins. Semiperched part of Shallow aquifer not included in remainder of Oxnard Plain.5Restricted to the Oxnard Plain and Forebay by Turner (1975).6Modified on the basis of ash-deposit age dates (Yerkes and others, 1987, fig.11.2).7Mapped in eastern Ventura County subbasins of Santa Paula, Fillmore, Piru, and Las Posas Valley and may be time equivalent to parts of the San Pedro and Santa Barbara Formations (Weber and 

others, 1976, fig. 3).

8Mapped in western Ventura County subbasins.9San Pedro Formation everywhere except in Pleasant Valley where the Santa Barbara Formation was assigned to the Grimes Aquifer.10Las Posas and Pleasant Valley subbasins only.11Includes Mud Pit and Claystone Members.
34 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

A

A'

600400200

Sea level

200400600800

1,0001,2001,4001,6001,8002,0002,2002,400

800600400200

Sea level

200400600800

1,0001,2001,4001,6001,8002,0002,2002,400

Pacific Ocean

2N/23W-14N1

2N/23W-13Ka

2N/22W-8L1

2N/22W-9Ma

2N/22W-3Ka

2N/22W-2Ca

3N/22W-36J1

3N/21W-20J3

3N/21W-11Qa

3N/20W-6Aa

3N/20W-5C4

3N/20W-3D7

4N/20W-35Ka

4N/19W-31Ca

4N/19W-32G1

4N/19W-33F1

E-E' Section

4N/18W-31D3-7

(RP1)

4N/18W-20La

4N/18W-28Ea

3N/21W-16H5-8

(SP2)

FEET

FEET

Land

Surface

Land

Surface

Coastline

EXPLANATION

?

?

?

Oak Ridge Fault Country Club Fault Jamaica Fault

Mound

subbasin

Santa Paula

subbasin

Fillmore

subbasin

Piru

subbasin

A

0 10,000 20,000 30,000 FEET

Well location and State well number

Contact -

Number in parentheses is local monitoring wellsite name. Sections bend at well locations

Dashed where inferred.

Queried where doubtful

Indicates truncated depth of well

Fault - Dashed where location and inclination

inferred. Throw indicated by arrows

Shallow aquifer Upperaquifer

system

Loweraquifer

system

Oxnard aquiferMugu aquiferUpper Hueneme aquiferLower Hueneme aquiferFox Canyon aquiferUndifferentiated or non-waterbearing rocks (includes GrimesCanyon aquifer)

?

Vertical exaggeration 15:1

4N/18W-31D3-7

(RP1)

Figure 8. Hydrogeology of the Santa Clara–Calleguas ground-water basin, Ventura County, California. 

A, Section 

A–A′ Santa Clara River. 

B, Section 

B–B′, Los Posas. 

C, Section C–C′, Hueneme. 

D, Section 

D–D′, Pleasant Valley. 

E, Section E–E′, Coastal. (See figure 7

A for location of sections.) 
Ground Water 35

Pacific Ocean

Coastline

B'

B

Sea level

200400600800

1,0001,2001,4001,6001,8002,000

800600400200

Sea level

200400600800

1,0001,2001,4001,6001,8002,000

800600400200

1,4001,2001,000

2N/21W-17F5

2N/21W-15J1 2N/22W-23B3-7

(SG-1)

2N/22W-27Ca

2N/22W-31Ha

1N/23W-1C2 (CM-3)

2N/21W-11J3

(LP1)

2N/21W-12H1

2N/21W-3B1

E-E' Section

3N/19W-19K2 FEET

FEET

3N/20W-35R2

(P7)

Land

Surface

Land

Surface

Oxnard Slope 1 Fault

Arroyo Honda

Oxnard Plain

subbasin

Oxnard Plain

subbasin

Oxnard Forebay

subbasin

West Las Posas

Valley subbasin

East Las Posas

Valley subbasin

Central Las Posas Fault Fairview Fault

Pacific Ocean

Creek

A

B

C

D

D'

C'

Location of Section

34°

07'30"

118°45'

34°

22'30"

119° 119°15' 119°30'

0 10 MILES

0 10 KILOMETERS

E'

A'

B'

E

Lake

Piru

Clara

River

San at

Calleguas

B

0 10,000 20,000 30,000 FEET

Vertical exaggeration 15:1

Port

Hueneme

PointMugu

Figure 8—Continued.
36 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Port Hueneme Point Mugu C'

C

1N/22W-1M3S

2N/21W-31Pa

2N/21W-32E1

Sea level

200400600800

1,0001,2001,4001,600

200

1N/22W-29D1 (CM-2)

1N/22W-20N2

1N/22W-16Ra

Oxnard Plain

subbasin

FEET

Coastline

Pacific Ocean

Clay deposits,

Old Hueneme

Canyon

Tertiary Marine Sediments

(Undifferentiated)

Hueneme Canyon Fault

Revolon Slough

E-E' Section

Southern Springville Fault

Camarillo Fault

Pacific Ocean

Creek

A

B

C

D

D'

C'

Location of Section

34°

07'30"

118°45'

34°

22'30"

119° 119°15' 119°30'

0 10 MILES

0 10 KILOMETERS

E'

A'

B'

E

Lake

Piru

Clara

River

San at

Calleguas

C

0 10,000 20,000 30,000 FEET

Vertical exaggeration 15:1

Land

Surface

Figure 8—Continued.
Ground Water 37

Port

Hueneme

PointMugu

Coastline

Pacific Ocean

1S/21W-8L3 (CM-1)

1S/21W-32Q2 (Q-2)

1N/21W-28N2

1N/21W-21K4

1N/21W-15D2

1N/21W-1B4

2N/20W-30M1

2N/20W-29B1

2N/20W-21L1

2N/20W-23H2

2N/20W-24E1

D D'

E-E' Section

Sea level

400200200400600800

FEET

1,0001,2001,400

Volcanics

Volcanics

Tertiary

Rocks

(Undifferentiated)

Tertiary

Sediments

and

Volcanics

(includes

Santa

Margarita

Formation

and

Conejo

Volcanics)

Tertiary

Marine

Sediments

Mugu Slope 1

Fault

Sycamore Canyon Fault Camarillo Fault San Pedro Fault

Oxnard Plain

subbasin

South

Pleasant

Valley

subbasin

South

Pleasant

Valley

subbasin

North

Pleasant

Valley

subbasin Santa Rosa

subbasin

Revolon Slough

Calleguas Creek Pacific Ocean

Creek

A

B

C

D

D'

C'

Location of Section

34°

07'30"

118°45'

34°

22'30"

119° 119°15' 119°30'

0 10 MILES

0 10 KILOMETERS

E'

A'

B'

E

Lake

Piru

Clara

River

Santa

Calleguas

D

0 10,000 20,000 30,000 FEET

Vertical exaggeration 15:1

Land

Surface

Figure 8—Continued.
38 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Port Hueneme Point Mugu E'

E

Santa Clara-Calleguas

Hydrologic Unit Boundary

1S/22W-1H1 (CM-6)

1N/22W-35E1 (CM-5)

1N/22W-28G1 (CM-4)

1N/22W-29D1 (CM-2)

1N/23W-1C2 (CM-3)

2N/23W-2600a

2N/23W-14N1

2N/23W-10R1

1N/22W-18L2

1N/22W-7Ea

1S/21W-6F1

1S/21W-8L3 (CM-1)

B-B' Section

C-C' Section

D-D' Section

Oxnard Plain

subbasin

Mound

subbasin

Sea level

ELEVATION, IN FEET

400200200400600800

1,0001,2001,4001,6001,800

Mugu

Canyon

? ?

Oak Ridge

Fault

A-A' Section

Ventura

Foothills

Fault

McGrathFault

El RioFault

Santa Clara River

Hueneme Canyon

Fault

Old HuenemeCanyon Fault

North MuguLagoon Fault

Bailey Fault

Pacific Ocean

Creek

A

B

C

D

D'

C'

Location of Section

34°

07'30"

118°45'

34°

22'30"

119° 119°15' 119°30'

0 10 MILES

0 10 KILOMETERS

E'

A'

B'

E

Lake

Piru

Clara River

Santa

Calleguas

E

0 10,000 20,000 30,000 FEET

Vertical exaggeration 15:1

Tertiary

Marine

Sediments

and Volcanics

(Undifferentiated)

Tertiary Rocks

(Undifferentiated)

Land

Surface

Clay deposits,

Old Hueneme

Canyon

Figure 8—Continued.
Ground Water 39

Aquifer Systems

The water-bearing deposits were previously 

divided into six aquifers in the Santa Clara–Calleguas 

Basin within the two regional aquifer systems (Turner, 

1975). Using geophysical and geochemical data 

collected as a part of the USGS RASA Program, the 

aquifer designations were realigned into seven major 

aquifers. The unconsolidated deposits of the late 

Pleistocene and Holocene epochs are grouped into the 

regional upper-aquifer system, which includes the 

Shallow, Oxnard, and Mugu aquifers (fig. 7B). The 

lower-aquifer system is composed of complexly faulted 

and folded unconsolidated deposits of the Pliocene and 

Pleistocene epochs and include the upper and lower 

Hueneme, Fox Canyon, and Grimes Canyon aquifers 

(fig. 7B). The lower aquifer extends to about 1,600 ft 

below sea level in the Oxnard Plain subbasin to more 

than 2,000 ft below sea level in the Mound subbasin 

(fig. 8 A,E). All these aquifers extend offshore within 

the continental shelf (fig. 8); however, the thickness, 

structure, and extent of the submarine outcrops vary 

across the basin for the upper- and lower-aquifer 

systems (figs. 7 and 8).

The onshore part of the Oxnard Plain is 

subdivided into a confined region and an unconfined 

region. The unconfined region includes the Oxnard 

Plain Forebay and the northeastern part of the Oxnard 

Plain. The confined region was subdivided into 

Northwest and South Oxnard Plain model subareas for 

the water-management analysis in this study (fig.1). 

The submarine shelf extends (fig. 7A) southwestward 

from the coastline and is subdivided along the McGrath 

Fault as an extension of the onshore separation between 

the Mound subbasin and the Oxnard Plain (figs. 1

and 7); these subbasins are hereinafter referred to as the 

“offshore Mound” and “offshore Oxnard Plain” 

subbasins. For the water-management analysis in this 

study, the offshore Oxnard Plain was subdivided into 

northern and southern regions separated by the 

Hueneme submarine canyon.

Upper-Aquifer System

Shallow Aquifer—The Shallow aquifer extends 

from land surface to a depth of 60 to 80 ft along the 

Santa Clara and the Arroyo Las Posas flood plains and 

throughout most of the Oxnard Plain and Pleasant 

Valley subbasins (figs. 7 and 8). Along the flood plain 

of the Santa Clara River, the shallow aquifer consists of 

predominantly sand and gravel and is an important 

source of ground water. During prolonged droughts, 

the Shallow aquifer becomes dewatered in the upper 

reaches of the Santa Clara River and Arroyo Las Posas. 

Beneath the Oxnard Plain and Pleasant Valley 

subbasins, the Shallow aquifer consists of fine-tomedium sand with interbedded clay layers and is 

referred to as the “semiperched aquifer”; the clay 

layers separate the Shallow aquifer from the underlying 

Oxnard aquifer. The Shallow aquifer occasionally 

becomes perched locally because of pumping from the 

Oxnard aquifer. Water quality is poor throughout most 

of the Oxnard Plain and Pleasant Valley subbasins and 

consequently few wells are perforated opposite this 

aquifer.

Oxnard Aquifer—The Oxnard aquifer lies at the 

base of the Holocene deposits and consists of sand and 

gravel deposited by the ancestral Santa Clara River and 

the Calleguas Creek and by their major tributaries. The 

coarse-grained basal deposits of the Holocene epoch 

are referred to as the “Oxnard aquifer” (Turner, 1975). 

The base of the aquifer ranges from about 150 to 250 ft 

below land surface throughout most of the Oxnard 

Plain subbasin (fig. 8). The basal deposits range in 

thickness from less than 10 to 200 ft and are a major 

source of water to wells in the Piru, Fillmore, Santa 

Paula, Oxnard Plain Forebay, and Oxnard Plain 

subbasins. Hydraulic conductivity in the Oxnard 

aquifer is about 190 ft/d near Port Hueneme (Neuman 

and Witherspoon, 1972). The Oxnard aquifer is 

relatively fine grained in the Mound, Pleasant Valley, 

Santa Rosa Valley, and Las Posas Valley subbasins; this 

aquifer is not considered an important source of ground 

water in these subbasins. Throughout most of East and 

West Las Posas Valley subbasins, the Oxnard aquifer is 

unsaturated.
40 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

In the Piru and Fillmore subbasins, there are few 

if any clay layers separating the Shallow and Oxnard 

aquifers; therefore, ground water can move freely 

between the two. In the Santa Paula subbasin, the Santa 

Clara River has migrated south of the ancestral river 

that deposited the sediments of the Oxnard aquifer and 

mostly overlies non-water-bearing rocks of Tertiary 

age. As a result, the Santa Clara River does not overlie 

the Oxnard aquifer throughout most of the Santa Paula 

subbasin.

In the Oxnard Plain Forebay subbasin, there are 

relatively few clay layers separating the Shallow and 

Oxnard aquifers. Alluvial fans derived from the 

mountains north of the Mound subbasin pushed the 

Santa Clara River south toward South Mountain. In the 

Oxnard Plain Forebay subbasin, clay layers were 

eroded by the Santa Clara River, and sand and gravel 

were deposited in their place; owing to the absence of 

clay, this subbasin is artificially recharged by surface 

spreading of water diverted from the Santa Clara River. 

The Oxnard aquifer is considered to be unconfined in 

the Oxnard Plain Forebay subbasin.

Throughout the Oxnard Plain and Pleasant 

Valley subbasins, the Shallow and Oxnard aquifers are 

separated by clay layers. These clay layers confine or 

partly confine the Oxnard aquifer throughout most of 

the Oxnard Plain and Pleasant Valley subbasins. 

Previous investigators (California Department of Water 

Resources, 1956; Turner, 1975) reported that the clay 

layers separating the Shallow and Oxnard aquifers in 

the Point Mugu area are thin or absent, allowing free 

interchange of water in this part of the subbasin. 

However, data, collected from several multiple-well 

monitoring sites constructed in the Point Mugu area as 

a part of this study (Densmore, 1996), indicate that 

relatively thick clay layers separate the Shallow and 

Oxnard aquifers.

Mugu aquifer—The Mugu aquifer (Turner, 

1975) is composed of the basal part of the unnamed 

upper Pleistocene deposits. In the Piru, Fillmore, Santa 

Paula, Mound, Oxnard Plain Forebay, and Oxnard 

Plain subbasins, these deposits are similar to those of 

the underlying lower-aquifer system because the Santa 

Clara River was the primary source of sediment for 

both aquifers. The Mugu aquifer is differentiated from 

the lower-aquifer system because it is less indurated 

and relatively undisturbed. However, because of the 

similarities between these deposits, many investigators 

include the upper Pleistocene deposits in the 

lower-aquifer system. In the Pleasant Valley, Santa 

Rosa Valley, East Las Posas Valley, and West Las Posas 

Valley subbasins, the Mugu aquifer sediments were 

derived from South Mountain and the surrounding hills 

and are finer grained than sediments derived from the 

Santa Clara River.

Throughout most of the ground-water basin, the 

Mugu aquifer extends from about 200 to 400 ft below 

land surface (fig. 8) and consists of sand and gravel 

interbedded with silt and clay. The silt and clay layers 

retard the vertical movement of water through the 

Mugu aquifer and confine or partly confine the aquifer. 

Over most of the ground-water basin, the top of the 

aquifer is relatively flat; however, the base of the 

aquifer has a more irregular surface (Turner, 1975) 

owing to a regional uncomformity. This uncomformity, 

which is most pronounced in the Mound and the East 

Las Posas Valley subbasins (fig. 8A,B,E), is due to 

deformation during deposition of older alluvium that 

contains the Mugu aquifer.

Few production wells are perforated solely in the 

Mugu aquifer; most are also perforated in the overlying 

Oxnard aquifer or in the underlying lower-aquifer 

system. In general, wells that are perforated opposite 

both the Oxnard and Mugu aquifers, which are similar 

in thickness, obtain most of their water from the 

Oxnard aquifer because it is significantly more 

permeable. Hydraulic conductivities estimated from 

slug tests at the multiple-well monitoring sites 

constructed for this study range from less than 1 to 

98 ft/d; most, however, are less than 25 ft/d (E.G. 

Reichard, U.S. Geological Survey, written commun., 

1995). When individual wells at the same multiple-well 

monitoring site were tested, the estimated hydraulic 

conductivity of the Oxnard aquifer was almost always 

higher than that estimated for the Mugu aquifer. 
Ground Water 41

In subbasins in which the Mugu aquifer is 

predominantly coarse-grained (the Piru, Fillmore, and 

Santa Paula subbasins), wells perforated in both the 

Mugu aquifer and the underlying lower-aquifer system 

obtain most of their water from the Mugu aquifer. This 

is shown by a wellbore flowmeter test completed on 

well 3N/21W–11J5 in the Santa Paula subbasin (see 

figure A5.1 in Appendix 5). Although this well is 

perforated predominantly in the lower-aquifer system, 

almost all the water yielded by the well is derived from 

the Mugu aquifer. As stated previously, the Mugu 

aquifer is less indurated than the lower-aquifer system, 

which would account for its greater water-yielding 

capacity. In the subbasins where the Mugu aquifer is 

predominantly fine grained, wells yield significant 

quantities of water from the aquifer only if they are 

perforated opposite the basal coarse-grained zone. This 

laterally extensive basal zone, which, as noted earlier, 

is due to a regional unconformity, yields water readily 

to wells. Many wells are not perforated opposite this 

zone, however, because its thickness is 20 ft or less 

throughout many of the subbasins. Results of the 

wellbore flowmeter test for well 1N/21W–15D2 

(figure A5.1 in Appendix 5) in the Pleasant Valley 

subbasin show that the basal zone of the Mugu aquifer 

yields significantly more water per foot of aquifer 

penetrated than does the underlying lower-aquifer 

system.

Lower-Aquifer System

The lower-aquifer system consists of the folded 

and faulted Pleistocene continental and marine deposits 

of the Saugus, San Pedro, and Santa Barbara 

Formations as defined by Weber and others (1976) and 

the Saugus Formation and the Las Posas Sand as 

defined by Dibblee (1988, 1990a,b, 1991, 1992a,b,c,d) 

and by Dibblee and Ehrenspeck (1990). For this study, 

the unconsolidated deposits of the Saugus and the 

upper part of the San Pedro Formations as defined by 

Weber and others (1976) and the Saugus as defined by 

Dibblee were split into the “Upper Hueneme” and 

“Lower Hueneme” aquifers, respectively, for the entire 

Santa Clara–Calleguas Basin (fig. 7B). The lower part 

of the San Pedro Formation as defined by Weber and 

others (1976) and the upper part of the Las Posas Sand 

as defined by Dibblee are referred to as the “Fox 

Canyon aquifer” in the Las Posas, Pleasant Valley, and 

Oxnard Plain subbasins (Turner, 1975). The coarsegrained layers of the Santa Barbara Formation as 

defined by Weber and others (1976) are commonly 

referred to as the “Grimes Canyon aquifer” in the East 

Las Posas Valley subbasin and parts of the Pleasant 

Valley subbasins (Turner, 1975). In most of the other 

subbasins, the Santa Barbara Formation is of low 

permeability, yields poor-quality water, and is not 

considered an important source of water. Regional fault 

systems (figs. 7 and 8) segregate the lower-aquifer 

system into many parts and affect the flow of water 

between and within the subbasins.

Upper and Lower Hueneme Aquifers—The 

Hueneme aquifers constitute the upper part of the San 

Pedro Formation beneath the Oxnard Plain mapped by 

Weber and others (1976), and the Saugus Formation 

beneath the Santa Clara River Valley subbasins mapped 

by Dibblee (1988, 1990a,b, 1991, 1992a,b,c,d) and 

Dibblee and Ehrenspeck (1990). These aquifers consist 

of lenticular layers of sand, gravel, silt, and clay. The 

sediments constituting the aquifers have been subjected 

to considerable folding, faulting, and erosion since 

deposition. These deposits were divided into upper and 

lower aquifers based on data from electric logs which 

show a decrease in electrical resistivity at the contact 

between the aquifers. The decrease is attributed to the 

presence of more fine-grained deposits in the Lower 

Hueneme aquifer than in the Upper Hueneme. The 

Upper Hueneme aquifer reaches a maximum thickness 

of more than 700 ft (fig. 8A) and the Lower Hueneme 

aquifer reaches a thickness of more than 2,000 ft in the 

axis of the Santa Clara syncline in the Santa Paula, 

Fillmore, and Piru subbasins. In areas of the basin that 

have been uplifted since deposition (fig. 8A,D,E), 

much of the sediments constituting Hueneme aquifers 

have been removed by erosion. 
42 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

In the Oxnard Plain subbasin, the Upper 

Hueneme aquifer is predominantly fine grained in two 

areas along the coast line between Port Hueneme and 

Point Mugu (Old Hueneme Canyon on figure 8C,E). 

These fine-grained deposits are more than 200 ft thick 

near the coast, and they extend about 3.5 mi inland. 

Turner (1975) attributed these deposits to a lagoonal or 

embayment depositional environment throughout most 

of the San Pedro Formation deposition. Inspection of 

lithologic and electrical logs collected during the 

drilling of the multiple-well monitoring sites 

constructed for this study indicates that these 

fine-grained deposits are ancestral submarine canyons 

(fig. 8C,E) that were backfilled during a rise in sea 

level. The submarine canyons were carved into the San 

Pedro Formation sometime prior to the deposition of 

the deposits of the upper Pleistocene. These backfilled 

ancestral submarine canyons are important hydrologic 

features because they are low permeable barriers to 

ground-water flow and may contribute to coastal 

subsidence (fig. 9). The hydraulic conductivity of the 

fine-grained deposits in the ancestral submarine 

canyon, estimated from a slug test at the CM-5 

multiple-well monitoring site (fig. 8E), was 0.1 ft/d 

(E.G. Reichard, U.S. Geological Survey, written 

commun, 1995).

Fox Canyon Aquifer—The Fox Canyon aquifer 

constitutes the basal part of the San Pedro Formation 

mapped by Weber and others (1976). The aquifer 

consists of weakly indurated very fine- to mediumgrained fossiliferous sand with occasional gravel and 

clay layers of shallow marine origin. As stated 

previously, Dibblee (1992a,b,c,d) mapped these 

deposits as a separate formation, which he designated 

as the Las Posas Sand. The marine deposition of the 

sediments of the Fox Canyon aquifer resulted in a 

relatively uniform series of layers, which can be 

correlated by the electric logs, over large areas of the 

ground-water basin (Turner, 1975). The Fox Canyon 

aquifer is identified on electric logs by zones of 

relatively high resistivity that are almost identical for 

thicknesses of 100 to more than 300 ft. In contrast, the 

overlying Lower Hueneme aquifer is characterized as a 

series of relatively high resistivity zones 10 to 100 ft in 

thickness separated by relatively low resistivity zones 

10 to 20 ft in thickness. Most of the electric logs 

inspected show there was a significant shift in the 

spontaneous potential curve opposite the Fox Canyon 

aquifer, indicating a change in the aquifer mineralogy 

and (or) a change in the water quality of the aquifer. 

Historically, very few wells tapped the Fox 

Canyon aquifer of the ground-water basin, except in 

the East and West Las Posas Valley and the Pleasant 

Valley subbasins. Because yield is significantly less in 

this aquifer than in the upper aquifer system, few water 

wells were perforated solely in the Fox Canyon aquifer. 

This limited testing of the hydraulic properties of the 

aquifer. For previous investigations, electric logs from 

petroleum wells were used to define the character and 

extent of the aquifer. High-resistivity zones on those 

logs, which indicate possible coarse-grained zones of 

good quality water, led to the belief that the Fox 

Canyon aquifer would be a major source of water to 

wells. 

To help determine the hydraulic properties of the 

Fox Canyon aquifer, at least one piezometer at 13 of 

the 23 multiple-well monitoring sites constructed for 

this study was perforated opposite the aquifer. The 

lithologic and electric logs for these sites indicate 

relatively low permeability; the Fox Canyon aquifer 

consists of predominantly fine- to very fine-grained 

sand that is indurated to slightly indurated (Densmore, 

1996); this is coincident with the high-resistivity zones 

on the electric logs and reflects the low dissolved-solids 

concentration of water in the aquifer and the induration 

of the aquifer sediments. The low permeabilities were 

confirmed by slug tests that indicate hydraulic 

conductivities ranging from 1 to 9 ft/d (E.G. Reichard, 

U.S. Geological Survey, written commun., 1995). 

These hydraulic conductivities are considerably lower 

than those of the overlying aquifers. 
Ground Water 43

Figure 9. Subsidence in Oxnard Plain and Pleasant Valley, Santa Clara–Calleguas ground-water basin, Ventura County, California. A, Geographic 

features. B, Subsidence profile. C, Subsidence of bench marks through time.

Pacific

Ocean

R22W R18W

T

2

N

T

4

N

River

Santa

Clara

Lake

Piru

Calleguas

Revolon

Arundell

Barranca

Slough

Beardsley Wash

Creek

Arroyo S

Arroyo

imi

Las Posas

Arroyo

Honda

Conejo

Cr

Santa Paula

Ellsworth

Cr

Cr

Sespe

Timber

Cr

Hopper

Pole

Piru

Cr

Cr

Cr

Cr

Sulphur Mountain

Oak Ridge

South Mountain

Camarillo Hills

Las Posas Hills

Simi Valley

Oxnard Plain

Big Mtn Santa

Paula

Thousand Oaks

Oxnard

El Rio Camarillo

Fillmore

Saticoy

El Rio

Spreading Grounds

Saticoy Spreading Grounds

Ventura

1920 1940 1960 1980 2000

0

1

2

3

YEAR

TOTAL RELATIVE SUBSIDENCE, IN FEET

0

0 2 4 6 8 10 12

1

1939-78

1939-60

2

3

TOTAL SUBSIDENCE, IN FEET

TIDAL

3

E

584

Z

583

Bend in

traverse

DISTANCE ALONG TRAVERSE, IN MILES

E 584

TIDAL 3

Z 901

Z 583

?

?

?

?

?

?

?

Z 583

TIDAL

3

E

584

Z 901

Z 583

EXPLANATION

Hydrologic unit boundary

Ground-water subbasin

boundary

Bench mark and designation

Oil field

Syncline

Anticline

0

0

10 MILES

10 KILOMETERS

119°15' 118°45'

34°

22'

30"

119°

34°

07'

30"

B C

A

Fault

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium

Shallow alluvium and unconsolidated deposits

Consolidated and unconsolidated deposits

Freeman Diversion
44 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

To determine the relative contribution of water 

from the Fox Canyon aquifer to wells perforated in the 

Fox Canyon and overlying aquifers, available 

flowmeter logs were inspected and additional logs were 

collected (see table 5 in the “Ground-Water Discharge” 

section). The flowmeter log collected at well 2N/22W13N2 in the Oxnard Plain Forebay subbasin (in 

Appendix 5) shows that little, if any, water enters the 

wells from the Fox Canyon aquifer, and almost all the 

water pumped comes from the basal zone of the 

overlying Lower Hueneme aquifer. Flowmeter logs 

collected from wells in the Oxnard Plain and the 

Pleasant Valley subbasins indicate that, in most of the 

wells tested, the flow contribution from the Fox 

Canyon aquifer is less than the flow contribution from 

the overlying aquifers. Data from the flowmeter logs 

from the Pleasant Valley and the Oxnard Plain 

subbasins indicate that the Fox Canyon aquifer is a 

major source of water to wells perforated throughout 

the lower-aquifer system only if the overlying Lower 

Hueneme aquifer is absent or is predominantly fine 

grained. Based on well construction data, this may be 

the case throughout most of the East and West Las 

Posas Valley, Oxnard Plain, and Pleasant Valley 

subbasins. 

Grimes Canyon Aquifer—The Santa Barbara 

Formation (Weber and others, 1976), which consists of 

non-water-bearing marine sandstone, siltstone, 

mudstone, and shale, underlies the Fox Canyon aquifer 

throughout most of the ground-water basin and is 

considered the base of the ground-water system 

throughout most of the basin. However, in parts of the 

ground-water basin, the upper part of the Santa Barbara 

Formation contains water-bearing deposits referred to 

as the “Grimes Canyon aquifer” (Turner, 1975).

In the East Las Posas Valley subbasin, the 

Grimes Canyon aquifer predominantly consists of 

layers of well-indurated sandstones and conglomerate 

with high resistivity as indicated by the electric logs, 

characteristic of well-indurated sandstone and 

conglomerate layers. Because the sediments are well 

indurated, the hydraulic conductivity of the aquifer is 

relatively low. However, the Grimes Canyon aquifer is 

an important source of water in the East Las Posas 

Valley subbasin where the overlying aquifers are absent 

or are unsaturated.

The Grimes Canyon aquifer is also present in 

the southeastern part of the Oxnard Plain subbasin and 

throughout most of the Pleasant Valley subbasins 

(Turner, 1975); many production wells tap this aquifer 

throughout the Pleasant Valley subbasin. Lithologic 

and electric logs collected from multiple-well 

monitoring sites constructed for the RASA study 

indicate that much of the deposits that contain the 

Grimes Canyon aquifer are relatively fine grained and 

water is relatively high in dissolved-solids (Densmore, 

1996). Although deposits similar to those of the Grimes 

Canyon aquifer are present beneath the western part of 

the Oxnard Plain subbasin, few production wells tap 

these deposits owing to their greater depth in that part 

of the subbasin.

Ground-Water Recharge

Sources of recharge to the aquifer systems 

include streamflow infiltration, direct infiltration of 

precipitation on the valley floors of the subbasins and 

on bedrock outcrops in adjacent mountain fronts, 

artificial recharge of diverted streamflow and imported 

surface water, percolation of treated sewage effluent, 

and infiltration of excess irrigation water (irrigation 

return flow) in some agricultural areas. For previous 

studies, recharge was estimated using consumption and 

water-balance methods based on precipitation and 

streamflow data for various historical periods 

(Grunsky, 1925; California Department of Public 

Works, 1934; California State Water Resources Board, 

1956; Mann and Associates, 1959; California 

Department of Water Resources, 1975). 

Streamflow Infiltration

Streamflow infiltration is the largest component 

of ground-water recharge in the Santa Clara–Calleguas 

basin and includes gaged and ungaged streamflow. The 

Santa Clara River and the Calleguas Creek have been 

altered substantially by regulated flow; the construction 

of the Santa Felicia Dam (Lake Piru) transformed flow 

in the Santa Clara River system from predominantly 

winter and spring floodflows to significant summer and 

fall low flows.
Ground Water 45

Gaged Streamflow

Previous estimates of annual subbasin 

streamflow-infiltration rates are summarized in table 3. 

These reported estimates were aggregated into 

averages for the wet and dry periods used in this study 

(fig. 2A). The total estimated gaged streamflow 

infiltration reported by the California Department of 

Water Resources (1975) for 1937–67 ranged from 0 to 

297,700 acre-ft annually (table 3). These estimates 

yield average wet-year and dry-year infiltration rates 

that are 67 and 57 percent of estimated runoff, 

respectively. The ratios of wet-year to dry-year 

infiltration for the Santa Clara River and for the total 

basin during the period were 2.0 and 2.7, respectively 

(table 3). For streamflows less than 250 ft3/s (about 500 

acre-ft/d), the rates of infiltration on the Santa Clara 

River were about 14 percent, and for several dry years 

(such as 1952 and 1958) the rates ranged from 50 to 70 

percent (California Department of Water Resources, 

1975, fig. 15). 

Streamflow loss for the Santa Clara River for wet 

and dry seasonal flows less than 250 ft3/s (about 

500 acre-ft/d) was determined by subtracting 

downstream gaged streamflow (gaging station 

11114000) from the sum of upstream gaged inflows 

(gaging stations 11108500, 11110000, 11110500, 713, 

11113000, 11113500) (fig. 4). Similarly, the 

streamflow loss for Calleguas Creek was estimated as 

the difference between downstream streamflow 

(11106550) and gaged inflows (11106850 and 

11106400) for flows less than 10 ft3/s (20 acre-ft/d) 

(fig. 4). Seasonal streamflow losses in the Santa Clara 

River and the Calleguas Creek varied widely but 

generally show several patterns (fig. 10). Regression of 

seasonal streamflow loss in relation to total gaged 

streamflow indicates an overall loss of 35 percent for 

wet-year seasons (fig. 10A) and 52 percent for dry-year 

seasons (fig. 10B) for the Santa Clara River. Loss from 

the Calleguas Creek during low-flow conditions is 

generally either 0 percent during winter and fall 

seasons or 100 percent during spring and fall seasons 

(fig. 10C). During dry-year summers, 70 to 100 percent 

of the flow in the Santa Clara River is lost to groundwater recharge (fig. 10B). Streamflow loss is low for 

many of the wettest years, such as 1969 and 1984 

(fig. 10A), which may indicate a significant 

contribution of ungaged inflow prior to or during 

periods with relatively low flow (less than 200 ft3/s). 

The annual range of gaged streamflow loss in the Santa 

Clara River for 1956–93 varied from about 2,700 to 

97,800 acre-ft/yr (table 3). On a climatic basis, total 

infiltration for the Santa Clara River was about 34,000 

(22 percent of flow) and 25,100 (37 percent of flow) 

acre-ft/yr for wet- and dry-year periods during 

1956–93, respectively; for the Calleguas Creek above 

Highway 101, it ranged from 0 to 6,100 acre-ft/yr for 

the period of record (1973–93) (table 3). The wide 

range of streamflow loss also was subject to the effects 

of additional inflow from treated municipal sewage 

between gaging stations of about 12 ft3/s (8,700 acreft/yr) and irrigation return flow. 

Streamflow infiltration along the Santa Clara 

River, estimated as part of a sediment-transport study, 

is 23 percent of flow per mile for flows less than 100 

ft3/s, 20 percent of flow per mile for flows from 100 to 

500 ft3/s, 6 percent of flow per mile for flows from 500 

to 1,000 ft3/s, and less than 2 percent of flow per mile 

for floodflows greater than 1,000 ft3/s (Brownlie and 

Taylor, 1981). 

Densmore and others (1992) estimated 

streamflow infiltration for a summer drought under 

conditions of controlled releases from Lake Piru. The 

controlled releases result in an increase in infiltration 

rate with increased channel width in Piru Creek when 

releases exceed 200 ft3/s (Steve Bachman, United 

Water Conservation District, oral commun., 1996). 

These various infiltration estimates collectively 

suggest that infiltration is dependent on antecedent 

conditions, which include antecedent ground-water 

levels; magnitude of the streamflow and related 

properties, such as channel width; and current and 

antecedent regulated flows.
46 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Table 3. Summary of estimated ranges and averages of gaged and ungaged streamflow infiltration in the Santa Clara-Calleguas Basin, Ventura County, California [Time periods are reported in water years except from this study. All estimates are in acre-feet per year. Numbers in parentheses are average infiltration values during dry and wet periods, respectively. — , no estimate was made]

Time period 

of estimate

 Streamflow infiltration

Total infiltration

Basin total

infiltration

Gaged Ungaged

Piru Fillmore Santa Paula

Montalvo 

Forebay of 

Oxnard Plain

Mountain-Front 

Recharge

Santa Clara

River

Calleguas 

Creek

1893 to 

11932

1Estimates from the California Department of Public 

Works (1934, table 59). For Santa Clara River, the net streamflow includes estimates of runoff from ungaged tributaries and valley-floor runoff.

— —

—

— 1,400–56,200 2

( 11,500; 30,200)

2Estimates include only the Santa Clara River Valley.

12,300–138,900(51,300; 65,000) 

— 12,400–143,300 1(62,500; 

99,800)

1937 to 

31951

3Estimates from the California State 

Water Resources Board (1956, tables 12–15).

6,800–68,600(18,500; 38,400)

0–33,600 (0; 0)

0–16,200 (4,100; 9,300)

1,000–145,000 

(23,600; 74,600)

—

—

— —

1937 to 

41957

4Estimates from Mann (1959), Plates 28–31

6,400–68,300(19,100; 36,100)

1,800–49,100 

(20,500; 26,800)

4,200–24,400 

(14,900; 15,700)

1,000–39,300 

(12,100; 22,100)

 — —

—

—

1937 to 

51967

5Estimates from the California Department of 

Water Resources (1975, tables 23 and 24). For basin total the net streamflow includes estimates of runoff from ungaged tributaries.

—

—

—

— 3,600–190,000

(22,400; 80,300)

0–185,800 (26,400; 51,800 )

— 0–297,700 

(40,300; 108,600)

1956 to

61993

6Estimates from this study. Streamflow leakage for Calleguas Creek does not include additional streamflow infiltration from treated waste water or irrigation returnflow.

—

—

—

— 3,800–78,500

 (13,200; 34,200)

2,700–97,800 ( 25,100; 34,000) 

0–6,100

—
Ground Water 47

Figure 10. Estimated seasonal streamflow losses for gaged inflows in the Santa Clara River and Calleguas Creek and tributaries, Ventura County, 

California. A, Santa Clara River streamflow in wet-years seasons. B, Santa Clara River streamflow in dry-year seasons. C, Calleguas Creek streamflow in 

wet-and dry-year seasons. 

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

0

100

200

300

400

500

1956 1957 1957

1957

1958

1959

1965 1965

1968

1968

1968

1981 1985 1982

1986 1987

TOTAL STREAMFLOW IN RELATION TO

STREAMFLOW LOSS FOR WET-YEAR

SEASONS

LINE OF STREAMFLOW LOSS EQUAL

TO TOTAL STREAMFLOW

LINEAR REGRESSION LINE FOR

WET-YEAR SEASON LOSS

1956

1956 1957

1958

1965

1966

1966

1966

1967

1967 1967

1969

1969

1970

1978 1979

1979

1980

1981

1982

1982

1984

1984

1984

1984

1985

1985

1986 1992

1993

1994

1995

SEASON ENDING FOR YEAR

SHOWN

1961

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

0

100

200

300

400

500

STREAMFLOW LOSS, IN ACRE-FEET PER DAY

1959

1960

1960 1960 1961 1962

1963

1963

19631964

1965 1973

1976

1976

1977

1977

1988

1989

1990 1990 1990

1991

TOTAL STREAMFLOW IN RELATION TO

STREAMFLOW LOSS FOR DRY-YEAR

SEASONS

LINE OF STREAMFLOW LOSS EQUAL

TO TOTAL STREAMFLOW

LINEAR REGRESSION LINE FOR

DRY-YEAR SEASON LOSS

SEASON ENDING FOR YEAR

SHOWN WINTER

SPRING

SUMMER

FALL

WINTER

SPRING

SUMMER

FALL

1959

1959

1960 1961

1961

1962

1962

1963

1964

1964

1964

1970

1970

1971

1971

1972

1972

1972

1972

1973

1973

1974

1974

1975

1975

1976 1976

1977 1978

1987

1987

1988

1988

1988

1989

1989

1990

1991

1991

1992

1987

A

B

TOTAL GAGED INFLOW, IN ACRE-FEET PER DAY
48 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Ungaged Streamflow

Infiltration of streamflow in ungaged drainage 

basins at the boundary of an alluvial aquifer system is 

referred to as “mountain-front recharge.” Mountainfront recharge occurs along the arroyos and the small 

tributary stream channels of the 64 ungaged tributary 

drainage basins that drain into the ground-water 

subbasins from the surrounding mountain fronts of the 

Santa Clara–Calleguas Basin. This component of 

streamflow constitutes a small but significant 

contribution to streamflow and ground-water recharge, 

especially during wet years. For this study, it was 

assumed that the streamflow percolates into the 

alluvium and becomes ground-water recharge. This 

assumption may result in an overestimate of recharge 

during floodflows.

Previous estimates of mountain-front recharge 

range from 1,400 to 190,000 acre-ft/yr for 1893–1967 

(table 3). In some wet years such as 1969, 1978, 1979, 

1980, 1983, 1986, and 1993, measured outflow at the 

downstream gaging station at Montalvo (11114000) 

(fig. 4) on the Santa Clara River was greater than gaged 

inflow from the major tributaries. This difference can 

be attributed to the contribution of ungaged streamflow. 

Based on gaging-station data, this ungaged streamflow 

may have ranged from 39,800 to 479,800 acre-ft/yr for 

the Santa Clara River for 1956–93 and from 300 to 

7,800 acre-ft/yr for Calleguas Creek for 1973–93 (the 

period of record).

Figure 10—Continued.

TOTAL GAGED INFLOW, IN ACRE-FEET PER DAY

0 10 20

0

10

20

1989

1989

1990

1990

1990

1991

1991

1991

1992

1992

1993

1993

1994

1994

LINE OF STREAMFLOW LOSS EQUAL TO TOTAL STREAMFLOW

SEASON ENDING FOR YEAR

SHOWN WINTER

SPRING

SUMMER

FALL

1978

1980

1982

1982

1983

1983

1983

1

1984

985

1985

1986

1994

1973

1974

1975

1975

1976

1976

1977

1977

1978

1989

1989

STREAMFLOW IN RELATION TO STREAMFLOW LOSS FOR WET-YEAR SEASONS

STREAMFLOW IN RELATION TO STREAMFLOW LOSS FOR DRY-YEAR SEASONS

CSTREAMFLOW LOSS, IN ACRE-FEET PER DAY
Ground Water 49

For this study, mountain-front recharge was 

estimated by means of a modified rational method 

using gaged streamflow data from two small 

subdrainage basins, Hopper and Pole Creeks (fig. 4), 

referred to as “index” basins. The fraction of 

precipitation assumed to be mountain-front recharge 

was estimated as the ratio of total seasonal streamflow 

for each ungaged subdrainage basin to the average total 

seasonal precipitation for an index basin. To estimate 

mountain-front recharge, estimates of seasonal 

precipitation were required for each of the subdrainage 

basins for each wet year and dry year (fig. 3). It was 

assumed that most of the runoff from the ungaged 

drainage basins infiltrates near the mountain fronts and 

does not contribute significantly to mainstem 

streamflow.

The amount of recharge was estimated as the 

index-basin streamflow fraction of precipitation 

multiplied by the average total volume of seasonal 

precipitation (drainage area multiplied by kriged 

seasonal precipitation) for each of the 64 ungaged 

tributary subdrainage basins. Seasonal (winter, spring, 

summer, and fall) estimates for wet and dry years were 

made for all 64 subdrainage subbasins. The average 

percentages of precipitation that became mountainfront recharge during the period of record for the two 

index subdrainage basins, Pole and Hopper Creeks, 

were 4 and 7.5 percent, respectively. Estimates of 

mountain-front recharge ranged from about 3,800 to 

78,500 acre-ft/yr for 1956–93 (table 3) and averaged 

34,200 and 13,200 acre-ft/yr for wet- and dry-year 

periods, respectively. The estimates of seasonal 

mountain-front recharge ranged from zero for most of 

the Oxnard Plain to as much as 12,000 acre-ft per 

season for the Santa Clara River Valley subbasins 

(figs. 1 and 11A).

Direct Infiltration 

Recharge also occurs as direct infiltration of 

precipitation on the valley floors (hereinafter referred 

to as “valley-floor recharge”) and as direct infiltration 

of precipitation on bedrock outcrops (hereinafter 

referred to as “bedrock recharge”). These components 

of recharge constitute a small but significant 

contribution to streamflow and ground-water recharge, 

especially during wet years.

Previous estimates of direct infiltration for water 

years 1894 through 1957 (California Department of 

Public Works, 1934; Mann and Associates, 1959; 

California Department of Water Resources, 1975) are 

summarized in table 4. The total estimated infiltration 

for the Santa Clara River Valley subbasins ranges from 

0 to 90,800 acre-ft/yr (table 4) and averages 30,400 and 

5,300 acre-ft/yr for wet-year and dry-year periods, 

respectively (Mann and Associates, 1959). 

Direct infiltration was estimated as a percentage 

of precipitation and ranged from no infiltration in the 

confined parts of the Mound, Oxnard Plain, and North 

Pleasant Valley subbasins to as much as 6,238 acreft/yr in the unconfined Fillmore subbasin. The 

percentage of precipitation was based on the modified 

rational method in which the amount of potential 

recharge is the fraction of runoff from the index 

subdrainage basin multiplied by the total volume of 

precipitation for each ground-water subbasin. This 

method may overestimate potential recharge during 

periods of sustained rainfall when soil moisture is 

exceeded and overland runoff to stream channels 

occurs. Total estimated recharge as direct (valley-floor) 

infiltration ranges from 18,300 to 32,700 acre-ft/yr 

(fig. 11A, table 4) during dry- and wet-year periods, 

respectively; this estimate included an additional 

2,200 acre-ft/yr of direct bedrock infiltration along the 

basin margins, which is described in a later section in 

the context of developing estimates of inflow for the 

subareas of the ground-water model.

Artificial Recharge

Artificial recharge is a major contributor to 

ground-water recharge in the Oxnard Plain Forebay 

and the Piru subbasins (fig. 11A). Artificial recharge 

was started in 1929 adjacent to Piru and Santa Paula 

Creeks and the Santa Clara River near Saticoy. The use 

of streamflows for recharge, as well as for agriculture, 

supplemented the growing use of the ground-water 

resources. Additional surface-water storage was 

provided by construction of Santa Felicia Dam on Piru 

Creek in the early 1950s. Major diversions along the 

Piru and Santa Paula Creeks and along the Santa Clara 

River at Saticoy and Freeman have been used for 

artificial recharge of the upper-aquifer system. 
50 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Figure 11. A, Estimates of seasonal ground-water inflows to the subbasins and to the Oxnard Plain subareas of the Santa Clara–Calleguas groundwater basin, Ventura County, California, 1891–1993, and B, Annual estimated and reported ground-water pumpage in the Santa Clara–Calleguas groundwater basin, Ventura County, California, 1891–1993.

1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

YEAR

0

10

20

30

40

50

0

2

4

6

8

10

Oxnard Plain Forebay subarea

1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

Northeast Oxnard Plain subarea

0

2

4

6

8

10

INFLOW, IN THOUSANDS OF ACRE-FEET PER SEASON

Northwest Oxnard Plain subarea

South Oxnard Plain subarea

0

4

8

12

Piru subbasin Fillmore subbasin

0

2

4

6

8

10

Santa Paula subbasin Mound subbasin

16

20

2

6

10

14

18

0

2

4

6

8

10

0

4

8

12

0

2

4

6

8

10

16

20

2

6

10

14

18

A
Ground Water 51

Figure 11—Continued.

1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

YEAR

2

4

6

8

10

North Pleasant Valley subbasin

1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

South Pleasant Valley subbasin

0

0

2

4

6

8

10

INFLOW, IN THOUSANDS ACRE-FEET PER SEASON

West Las Posas Valley subbasin South Las Posas Valley subbasin

0

2

4

6

8

10

2

4

6

8

10

0

0

2

4

6

8

10

0

2

4

6

8

10

East Las Posas Valley subbasin Santa Rosa Valley subbasin

EXPLANATION

Estimated mountain-front recharge

Estimated irrigation return flow recharge

Estimated treated wastewater recharge

Reported artificial recharge

Estimated valley-floor recharge

A
52 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Figure 11—Continued.

1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

YEAR

0

10

20

30

40

50

Oxnard Plain Forebay subarea

1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

Northeast Oxnard Plain subarea

0

10

20

30

40

50

PUMPAGE, IN THOUSANDS OF ACRE-FEET PER YEAR

Northwest Oxnard Plain subarea

South Oxnard Plain subarea

0

10

20

30

40

50

Piru subbasin Fillmore subbasin

0

10

20

30

40

50

Santa Paula subbasin Mound subbasin

TOTAL REPORTED

PUMPED-BACK

RECHARGE

B
Ground Water 53

Figure11—Continued.

YEAR

YEAR

10

20

30

40

50

North Pleasant Valley subbasin

1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

South Pleasant Valley subbasin

0

0

10

20

30

40

50

PUMPAGE, IN THOUSANDS ACRE-FEET PER YEAR

West Las Posas Valley subbasin South Las Posas Valley subbasin

0

10

20

30

40

50

East Las Posas Valley subbasin

Santa Clara-Calleguas ground-water basin

Santa Rosa Valley subbasin

Estimated agricultural pumpage

Reported municipal pumpage

Total reported pumpage

ADJUSTED

TOTAL

BASIN

PUMPAGE

USED FOR

MODEL

Total reported agricultural pumpage

Total reported non-agricultural pumpage

EXPLANATION

1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

40

80

120

160

200

240

280

320

0

B
54 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Table 4. Summary of selected estimates of ranges and averages of direct infiltration of precipitation in subbasins in the Santa Clara–Calleguas Basin, Ventura County, California [All estimates in acre-feet per year. Numbers in parentheses indicate the average infiltration during dry and wet periods, respectively. —, no estimate made] Period of record in water years Piru Fillmore Santa Paula Mound Montalvo Forebay of Oxnard Plain Oxnard Plain East/South Las Posas

West

Las Posas

Pleasant 

Valley/

Santa 

Rosa 

Valley

Total 

bedrock 

recharge

Total direct

infiltration

 recharge

11894 to 1932

1California Department of Public 

Works (1934, table 59). For the Santa Clara River, the net streamflow includes estimates of runoff from ungaged tributaries and valley-floor runoff.

 0–11,300 (2,700; 6,600)

 0– 21,200 (4,400; 11,800)

 0–11,800 

(2,300; 6,600)

0–4,500 (750; 2,200) 

 0–8,000 

(1,600; 4,500)

— — 0–5,400 

(1,000; 3,000)

 —

— 400–62,200 

2(12,900; 34,800)

2Includes only the Santa Clara River Valley and West Las Posas subbasins.

31937 to 1951

3California State 

Water Resources Board (1956, tables 12–15).

0–11,300 (0; 4,800)

0–29,800 ( 0; 11,700)

0–26,200 (0; 9,600)

 — 0–9,500 

(0; 0)

— — — — — —

41937 to 1957

4Mann and Associates (1959, plates 28–31).

1200–15,100 (1,100; 5,600)

2500–37,900 

(2,800; 13,600)

30–25,600 (900; 8,000)

 — 4100–12,200 

(500; 3,200) 

— — — — — —

51937 to 1967

5California Department of 

Water Resources (1975, tables 3, 19, and 20). For basin total, the estimated recharge from direct infiltration of precipitation includes potential runoff.

 — — — — —

— — — — — 4,000–306,400 

(20,200; 108,300)

61891 to 1993

6Final estimates from this study for valley-floor recharge. Period of record is in calendar years.

950–1,900 3,100–6,200 4,500 0 1,700 3,600 800–4,700 

(570; 3,400)

350–2,100 640–2,400 2,200 18,300–32,700
Ground Water 55

Artificial recharge began with offstream 

spreading-works to help provide an adequate and 

dependable water supply for agriculture. Spreadingworks were operated by the Santa Clara Conservation 

District: Santa Clara River streamflow was diverted 

near Saticoy beginning in 1928–29; Piru Creek 

streamflow was diverted near Piru beginning in 

1930–31; and Santa Paula Creek streamflow was 

diverted near Santa Paula beginning in water year 1931 

(Freeman, 1968). The initial capacities of the 

diversions for the Saticoy, Piru, and Santa Paula 

spreading grounds (fig. 4) were 120, 60, and 25 ft3/s, 

respectively (Freeman, 1968). These sites represent 

some of the earliest efforts to divert and artificially 

recharge shallow ground-water aquifers. The Saticoy 

and Piru spreading grounds have been in continuous 

operation since their construction more than 70 years 

ago. The Santa Paula diversion was operated for about 

11 years (1930–41) (fig. 11A), recharging a total of 

26,968 acre-ft.

The UWCD added additional spreading grounds 

at El Rio (fig. 4) in 1955 for diversions at Saticoy and 

added the Pleasant Valley pipeline and reservoir in 

1957 for additional storage capacity. Earthen dikes 

were used to divert as much as 375 ft3/s of streamflow 

from the Santa Clara River at Saticoy. However, 

between 1955 and 1983, there were 81 dike failures at 

the diversion (United Water Conservation District, 

1983). The earthen dike and diversion was relocated 

about 1 mi upstream after the riverbed degraded during 

the large flood of 1969, but the dike remained prone to 

failures at streamflows greater than 1,600 ft3/s. A 

concrete dike and diversion structure, called the 

Freeman Diversion, was constructed in 1991. It is more 

durable and provides a larger diversion capacity of 

460 ft3/s. Natural streamflow during winter and spring 

and controlled releases of combined imported water 

and natural streamflow from Lake Piru during summer 

and fall are diverted at the Freeman Diversion. About 

2,500,000 acre-ft was artificially recharged along the 

Santa Clara River Valley of which 378,054 acre-ft was 

at Piru (October 1931–December 1993), 1,228,615 

acre-ft at Saticoy (October 1928–December 1993), 

868,408 acre-ft at El Rio (December 1955–December 

1993), and 26,968 acre-ft at Santa Paula. Some of the 

surface water diverted at Saticoy and later at the 

Freeman Diversion was directly delivered by pipelines 

for irrigation. About 239,966 acre-ft of the diversions 

was delivered directly through the Pleasant Valley 

pipeline (September 1958–December 1993) from 

surface-water diversions, and an additional 4,161 acreft was delivered to John Lloyd Butler farms (March 

1970–May 1991) for irrigation (Greg Middleton, 

United Water Conservation District, written commun., 

1994). 

Since the 1960s, most artificial recharge at El 

Rio has been pumped back for nearby irrigation or 

delivery by pipeline to adjacent subbasins. During 

October 1955–December 1977, about 389,600 acre-ft 

was recharged at El Rio and about 170,974 acre-ft was 

pumped back. Net recharge during this period was 

about 218,600 acre-ft and the pump-back rate was 

44 percent. During July 1979–December 1993, about 

411,300 acre-ft was recharged and about 231,400 acreft (44 percent) was pumped back at El Rio. The ratio of 

pumpage to recharge for the El Rio artificial storage 

and recovery system (ASR) for 1978–93 ranged from 

0.38 in wet years to 1.5 in dry years.
56 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Irrigation Return Flow

Deep percolation of excess applied irrigation 

water (hereinafter referred to as “irrigation return 

flow”) is an additional source of artificial recharge to 

the ground-water system. However, areally extensive 

confining units retard the recharge of irrigation return 

flow to the upper-aquifer system throughout most of 

the Oxnard Plain and Mound subbasins. Irrigation 

return flow is redirected by drains throughout most of 

the southern part of the Oxnard Plain subbasin to 

streamflow that discharges to the Pacific Ocean through 

Revolon Slough (fig. 4). Increases in nitrate 

concentrations in ground water from wells in the 

upper-aquifer system (Izbicki and others, 1995; Izbicki 

and Martin, 1997) and related increases in groundwater levels may indicate that some irrigation return 

flows are infiltrating back to the upper-aquifer system 

in the Santa Clara River Valley and Las Posas Valley 

subbasins and in the Oxnard Plain Forebay and Santa 

Rosa Valley subbasins. The deep percolation of 

irrigation return flow within these subbasins consists of 

varying amounts of surface water and ground water. 

The amount of return flow was estimated based on a 

70-percent irrigation efficiency of applied water 

(Blaney and Criddle, 1950, 1962) for the areas of 

irrigated agriculture estimated from five land-use maps. 

Estimates by Koczot (1996) were based on areas and 

crop types delineated from land-use maps for 1912 

(Adams, 1913), 1927 (Grunsky, 1925; Koczot, 1996), 

1932 (California Department of Public Works, 1934), 

1950 (California Department of Public Works, 1950), 

and 1969 (California Department of Water Resources, 

1970). The resulting annual estimates were about 

17,900 acre-ft for 1912; 46,100 acre-ft for 1927; 

45,700 acre-ft for 1932; 52,600 acre-ft for 1950; and 

67,900 acre-ft for 1969. When the estimates for the 

Oxnard Plain and Mound subbasins are excluded, the 

annual estimates of irrigation return flow are reduced to 

about 11,800 acre-ft for 1912; 26,900 acre-ft for 1927; 

22,400 acre-ft for 1932; 27,700 acre-ft for 1950; and 

37,900 acre-ft for 1969 (Koczot, 1996). 

Ground-Water Discharge

Discharge of water from the aquifer systems 

includes ground-water discharge as pumpage from 

wells, evapotranspiration along the river flood plains, 

and offshore flow along submarine outcrops. Some 

additional intermittent baseflow to rivers occurs at the 

subbasin boundaries, but the baseflow generally 

infiltrates again in the downstream subbasin and thus is 

not considered a loss to the ground-water flow system. 

During the wet periods, however, ground water 

discharges as stream baseflow to the Pacific Ocean; this 

base-flow component of discharge to the ocean was 

larger prior to the 1930s (Freeman, 1968).

Pumpage

The first wells were drilled on the Oxnard Plain 

in 1870 following the severe drought of 1853–64 and 

during a sustained dry climatic period (1840–83) 

(fig. 2). Although pumping occurred during the late 

1800s and early 1900s, pumpage was minimal and 

therefore was not estimated for this report. These first 

artesian flowing wells typically were drilled to depths 

of 90 to 143 ft, and discharges were about 500 to 

1,000 gal/min (Freeman, 1968). Many wells were 

completed during 1870–71 for irrigation of field crops. 

During the early development of the ground-water 

resources, the drilling of wells diminished the flow of 

the springs and the artesian wells. By 1912, as many as 

42 pumping plants were operating north of the Santa 

Clara River, providing water for irrigation and 

domestic use (Freeman, 1968).

By 1920, a progressive lowering of water levels 

throughout the Santa Clara River Valley and the 

Oxnard Plain subbasins required the replacement of 

many centrifugal pumps with deep turbine pumps. By 

1924, many of the previously undeveloped areas of the 

Santa Clara–Calleguas Basin were being used for 

agriculture (Grunsky, 1925). On the basis of a 1912 

land-use map, estimated agricultural pumpage yields a 

basinwide average rate of withdrawal of about 33,500 

acre-ft/yr, which results in a potential total withdrawal 

of about 267,700 acre-ft for the years 1919–26 of the 

dry-year period 1919–36 (fig. 2).
Ground Water 57

Ground water initially was developed 

predominantly for agricultural use. Agricultural 

ground-water pumpage was estimated indirectly from 

land-use maps for periods prior to the metering of 

pumpage; Koczot (1996) estimated pumpage using 

selected land-use maps and consumptive-use estimates 

for 1912, 1927, 1932, 1950, and 1969. Land-use maps 

were used instead of electrical power records because 

of the labor required to construct pumpage records for 

large timespans and because many wells were not 

powered by electricity. These land-use maps were used 

to delineate agricultural consumptive use which was 

used to estimate pumpage for periods prior to metering 

not represented by land-use maps. The 1912 land-use 

map was used for 1919–26; the 1927 map was used for 

1927–30; the 1932 map was used for 1931–45; the 

1950 map was used for 1946–61; and the 1969 map 

was used for 1962–77. These land-use time periods 

were based on a combination of factors including land 

use, climate, water levels, and historical events. The 

land-use pumpage estimates were used as initial 

agricultural pumpage for the simulation of groundwater flow but were adjusted for some periods during 

model calibration (fig. 11B). Municipal pumpage for 

the cities of Ventura, Camarillo, and Oxnard and for the 

Channel Islands Beach Community Services District 

(near Port Hueneme); pumpage for a fish hatchery in 

the southern end of the Piru subbasin; and pumpage of 

artificial recharge in the Oxnard Plain Forebay 

subbasin were estimated independently and combined 

with the agricultural pumpage for the total estimated 

pumpage prior to 1983.

Ground-water development continued to spread 

in the ground-water basin during the severe drought 

period of 1923–36, tapping deeper aquifers for 

agricultural supplies (fig. 2). As the surface-water 

resources became fully developed in the early 1930s, 

new ground-water development began to provide a 

significant proportion of the water resources. In the 

1930s, the first deep wells were drilled in the Pleasant 

Valley and Las Posas Valley subbasins. Calculated 

agricultural pumpage, estimated from the 1927 landuse map, yields a basinwide average rate of withdrawal 

of about 128,400 acre-ft/yr for 1927 and an estimated 

total withdrawal of about 513,500 acre-ft for 1927–30. 

Calculated pumpage estimated from the 1932 land-use 

map is at about 174,000 acre-ft/yr, yielding an 

estimated total withdrawal of about 2,610,000 acre-ft 

for 1931–45. Estimates of agricultural pumpage, based 

on the 1950 land-use map, yield a basinwide average 

rate of pumpage of 180,000 acre-ft/yr and a total 

withdrawal of about 2,880,000 acre-ft for 1946–61. 

By 1967, about 800 wells equipped with deepwell turbine pumps provided more than 90 percent of 

the water demand in the basin (Freeman, 1968). On the 

basis of 1969 land use, estimates of agricultural 

pumpage yield a basinwide average rate of withdrawal 

of about 201,700 acre-ft/yr, yielding an estimated total 

pumpage of 3,227,200 acre-ft for 1962–77.

Reported pumpage was compiled from the 

technical files of the FGMA and UWCD for July 1979–

December 1993. These data generally were semiannual 

totals of user-reported agricultural, nonagricultural, 

and total pumpage. Early pumpage data were 

incomplete for the Las Posas Valley, Pleasant Valley, 

and Santa Rosa Valley subbasins. For these areas, 1984 

FGMA reported pumpage was used to represent 

pumpage for 1978 through 1983. Estimated and 

reported total annual pumpage were combined for the 

entire Santa Clara–Calleguas Basin and range from 760 

acre-ft for 1912 to as much as 301,400 acre-ft for 1990, 

which was during the last sustained drought.

Reporting of metered pumpage began in the 

1980s; the total reported basinwide pumpage was 

2,468,610 acre-ft during the 10-year period 1984–93 

(Greg Middleton, United Water Conservation District, 

written commun., 1994). Of this reported total 

pumpage, 37 percent was from the Oxnard Plain 

subbasin, 37 percent from the upper Santa Clara River 

Valley subbasins, 13 percent from the Las Posas Valley 

subbasin, 9 percent from Pleasant Valley subbasin, 

3 percent from the Mound subbasin, and 1 percent 

from the Santa Rosa Valley subbasin. 
58 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Evapotranspiration

Evapotranspiration (ET) from the regional 

ground-water flow system is restricted to the river flood 

plains, where ground water and streamflow infiltration 

are within the depths of the root zones of riparian 

vegetation. ET was not calculated for parts of the 

coastal areas of the Oxnard Plain subbasin where the 

Shallow aquifer is “semiperched.”

Previous estimates of annual ET for the Santa 

Clara River Valley subbasins range from 11,700 acreft/yr for 1892–1932 (California Department of Public 

Works, 1934) to 13,724 acre-ft/yr for 1958–59 (Mann 

and Associates, 1959). The estimated average ET for 

the entire Santa Rosa Valley subbasin for 1972–83 is 

about 4,300 acre-ft/yr (Johnson and Yoon, 1987). 

Previous estimates of the ET rate vary widely, ranging 

from 1.1 ft/yr (California Department of Water 

Resources, 1974a,b) to 2.4 ft/yr (California 

Department of Public Works, 1934) to as much as 5.2 

ft/yr (Mann and Associates, 1959).

The total area classified as land with riparian 

vegetation or as a flood plain was estimated from the 

five land-use maps (1912, 1927, 1932, 1950, 1969) 

compiled for the RASA study (Koczot, 1996; 

Predmore and others, 1997). A combination of riparian 

land distributions from the 1912, 1927, 1932, and 1950 

maps of the Conejo Creek area yields an estimated total 

of 14,945 acres of riparian vegetation along the stream 

channels for predevelopment conditions in the basin. 

The 1932 land-use map for the entire basin indicates a 

total riparian area of 11,237 acres. The most detailed 

set of land-use maps (1950) for the entire basin yielded 

a reduction to 6,539 acres of riparian land by 1950. By 

1969, the total was only 2,265 acres. The model, 

developed for this phase of the RASA study, was used 

to simulate the evapotranspiration along the flood plain 

of the Santa Clara River, Calleguas Creek, and its 

major tributaries.

Coastal Flow along Submarine Outcrops

Discharge from the regional ground-water flow 

systems probably occurs as lateral flow to the Pacific 

Ocean through outcrops that are exposed along the 

steep walls of the submarine canyons and that truncate 

the submarine shelf farther offshore. Because of the 

alternating layers of coarse- and fine-grained 

sedimentary deposits in these coastal aquifer systems, 

submarine leakage through the tops of the upper- and 

lower-aquifer systems that crop out along the 

submarine shelf probably is small. Outside of some 

folklore, there are no estimates or evidence, such as 

cold seeps, of submarine discharge in the Ventura area. 

However, the possibility of seawater intrusion along the 

coastal Oxnard Plain subbasin has long been 

recognized (Grunsky, 1925; California Department of 

Public Works, 1934; Freeman, 1968); geochemical 

evidence of seawater intrusion in the upper- and loweraquifer systems (Izbicki, 1991, 1992, 1996a) indicates 

a hydraulic connection to the submarine outcrops of the 

aquifer systems (figs. 7 and 8). Coastal flow was 

estimated using the ground-water flow model 

developed for this study and is described later in the 

report (see Simulation of Ground-Water Flow).

Borehole electromagnetic-induction (EM) logs 

of monitoring wells installed as part of the RASA 

Program indicate that seawater intrusion occurs along 

multiple coarse-grained beds that are commonly, but 

not exclusively, the basal units of the seven major 

aquifers that compose the upper- and lower-aquifer 

systems (figure A5.2 in Appendix 5). These basal units 

commonly occur above regional unconformities that 

are related to the major sea-level changes during the 

Pleistocene epoch. Natural gamma and EM 

geophysical logs collectively indicate that the flow of 

seawater from the ocean occurs laterally through the 

submarine outcrops and remains confined to the most 

transmissive coarse-grained beds that are bounded by 

fine-grained layers (figure A5.2 in Appendix 5). A 

cross-sectional solute transport model developed for 

the Port Hueneme area supports the conceptual 

framework of lateral intrusion, with vertical intrusion 

impeded by shallow fine-grained confining units 

(Nishikawa, 1997). Seawater intrusion forms a 

relatively sharp interface with fresh ground water as it 

enters the basal coarse-grained beds of the aquifer 

systems laterally and remains stratified in the layered 

coastal alluvial-aquifer systems of the Santa Clara–

Calleguas Basin.
Ground Water 59

Ground-Water Levels, Movement, and 

Occurrence

The largest source of discharge from the groundwater flow system in the Santa Clara–Calleguas Basin 

is pumpage. Pumpage has caused water-levels to 

decline below sea level (fig. 12) which has resulted in 

seawater intrusion and changes in ground-water 

quality, altered ground-water vertical-hydraulic 

gradients, reduced streamflow, reduced in ET, and 

caused land subsidence. Long-term hydrographs of 

water levels in production wells (figs. 13 and 14) and in 

the multiple-zone observation wells (fig. 15) show 

fluctuations driven by multiple-year to decadal changes 

in recharge and seasonal to multiple-year changes in 

pumpage.

Upper- and Lower-Aquifer-System Water Levels

Little information exists on predevelopment 

water levels in the upper- or lower-aquifer system 

during the periods of early ground-water development. 

In the 1870s, wells near the coast on the Oxnard Plain 

subbasin were reported to deliver water to the second 

floor of homes under the natural artesian pressures of 

the Oxnard aquifer (Freeman, 1968). Several early 

ground-water-level maps were constructed for parts of 

the basin (Adams, 1913; Grunsky, 1925), but the first 

map of the entire basin was completed for fall 1931 

(California Department of Public Works, 1934), which 

was during a period of agricultural development and a 

severe drought (1923–36, fig. 2). 

As the surface-water resources became fully 

used in the early 1930s, ground-water development 

began to provide a significant part of the water 

resources. If the conditions in 1931 represent, in part, 

conditions prior to major ground-water development, 

then ground water in all the aquifers initially moved 

from the landward recharge areas toward the west or 

southwest to the discharge areas along the submarine 

outcrops offshore in the Pacific Ocean (fig. 12A). By 

the 1930s, water levels had declined as a result of the 

1927–1936 drought (figs. 12A and 13), changing from 

artesian-flowing conditions of the late 1800s to below 

or near land surface in most wells completed in the 

upper-aquifer system in the Oxnard Plain subbasin 

(fig. 13). The effects of ground-water development and 

overdraft first appeared in 1931 when water levels in 

wells in parts of the Oxnard Plain declined below sea 

level (Freeman, 1968). In the 1930s, the first deep wells 

were drilled in the Pleasant Valley and Las Posas Valley 

subbasins. Before development, water levels in the 

lower-aquifer system probably were higher, but the 

water-level patterns probably were similar to the 

patterns shown in figure 12A for 1931.Well owners in 

coastal areas began to recognize the connection 

between the ground-water reservoirs and the ocean 

when they observed that water-level changes in wells 

corresponded with the rising and falling phases of the 

ocean tides (Freeman, 1968). The Santa Clara Water 

Conservation District officially recognized the linkage 

between overdraft and seawater intrusion in their 

annual report of 1931 (Freeman, 1968). 

Ground-water pumpage increased during the 

1940s with the widespread use of the deep turbine 

pump. The effects of permanent overdraft were 

exemplified by the lack of recovery of water levels to 

historical levels after the spring of 1944, which marked 

the end of the wettest climatic period in the 103 years 

of historical rainfall record at Port Hueneme (fig. 2A). 

The effects of overdraft also were recognized landward 

in the Santa Clara River Valley when ground-water 

levels declined about 20 ft in the Fillmore subbasin 

(fig. 14). Water levels in the southern Oxnard Plain and 

Pleasant Valley were below sea level by 1946 

(Freeman, 1968). In 1949, water-level altitudes were 

30 ft below sea level in parts of the Oxnard Plain 

subbasin, and one of the first wells intruded by 

seawater was identified along the coast in the Silver 

Strand well field (north of Port Hueneme) (Freeman, 

1968). The direction of subsurface flow within the 

upper aquifers near the coast has been landward since 

approximately 1947 (California Department of Water 

Resources, 1958).
60 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Pacific

Ocean

118°45' 119°15'

34°22'30"34°07'30"

R18W R22W

119°

0

0

10 MILES

10 KILOMETERS

T4N

T2N

a C ll ge au Cs

reek

Santa Clara

River

Lake

Piru

5

10

20

20

30

40

80

80

60

40

30

30

20

10

2

0

00

300

200

200

100

100

100

200

60

10

01

20

5

5

5

0

01-

0

300

200

300

400

500

600

700

800

900

600

700

04 0

500

5

10

20

20

30

40

80

80

60

40 30

30

20

100

200

300

200

200

100

100

100

200

60

10

10

20

5

5

5

0

-10

0

300

200

300

400

500

600

700

800

900

600

700

400

500

60

80

100

600

200

300

200

400

600

700

800

400

300

10

10

100

80 40

30

20

60

200

300

500

400

600

700

700

60

40

80

20

100

30

200

5

5

0

5

5

500

A

EXPLANATION

Wells

Ground-water subbasin boundary–

Subbasin names are given in figure 1

Water-level contour –

water level for year indicated. Dashedwhere approximately located. Datum issea level. Contour intervals vary

Hydrologic Unit boundary

River and selected streams

Model-gridboundary offlow regionand subareas

Shows altitude of

Simulated water-level contour –

Shows simulated altitude of water-levelfor year indicated. Contour intervals vary

Fall 1931 - Composite of Upper andLower aquifer systems - MeasuredMeasured contours modifiedfrom California Departmentof Public Works (1934).1932 - Upper AquiferSystem - Simulated

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium and unconsolidated depositsShallow alluvium

Consolidated and unconsolidated deposits

Modeled faults

Figure 12. Ground-water level altitudes in the Santa Clara–Calleguas ground-water basin, Ventura County, California. 

A, Fall 1932—Composite measured in upper- and lower-aquifer systems and 

simulated upper-aquifer system. 

B, Spring 1993—lower-aquifer system. 

C, Spring 1993—upper-aquifer system.
Ground Water 61

Pacific

Ocean

118°45'

34°22'30"34°07'30"

R22W R18W

119°

0

0

10 MILES

10 KILOMETERS

T4N

T2N

119°15'

a C ll ge au Cs

reek

Santa Clara

River

Lake

Piru

0

20

40

60

80

200

300

0

0

0

0

0

0

20

20

-20 -20

-20

-60

2- 0

4- 0

04-

-20

-40

40

40

60

60

60

06

40

40

80

80

80

100

100

100

100

100

200

02 0

03 0

300

200

300

80

02

0

0

60

60

40

80

0

0

-20

20

300

200

-20

-20

-40

-60

-40

-60

-80

-40

20

B

Spring 1993 -Lower-Aquifer system

Model fault inlower-aquifer system

Figure 12—Continued.
62 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Pacific

Ocean

118°45'

34°22'30"34°07'30"

R22W R18W

119°

0

0

10 MILES

10 KILOMETERS

T4N

T2N

119°15'

a C ll ge au Cs

reek

Santa Clara

River

Lake

Piru

10

20

02

02

01 10

200

200

400

04 0

600

600

700

500

500

400

300

003

300

100

100

100

200

100

40

80

80

60

60

40

60

80

40

0

0

200

10

10

20

6040

80

100

100

100

200

200

200

200

300

300

300

400

400

500

500

500

700

600

600

0

10

C

Spring 1993 -Upper-Aquifer system

Figure 12—Continued.
Ground Water 63

Figure 13. Measured and simulated water-level altitudes in wells completed in the lower-aquifer system of the Santa Clara–Calleguas ground-water 

basin, Ventura County, California.

118°45'

34°

22'

30"

34°7'30"

R18W

R 22W

T

2

N

T

4

N

119°

Pacific Ocean

0 5 MILES

0 5 KILOMETERS

Lake

Piru

Calleguas Creek

Santa Clara River

200

100

0

-100

MOUND SUBBASIN

2N/23W-14K1 [ ]

2N/23W-24G1 [ ]

100

0

-100

-200

OXNARD PLAIN SUBBASIN

1N/22W-4F4 100

0

-100

-200

OXNARD PLAIN SUBBASIN

1N/22W-20E2 [ ]

1N/22W-21B3 [ ]

100

0

-100

-200

OXNARD PLAIN SUBBASIN

1N/21W-31L1 [ ]

1N/22W-36B2 [ ]

200

100

0

-100

-200

NORTH PLEASANT VALLEY

SUBBASIN

2N/21W-24F1 [ ]

2N/21W-34D1

300

200

100

0

SANTA ROSA VALLEY SUBBASIN

2N/20W-23K1

500

400

300

200

100

0

EAST LAS POSAS VALLEY SUBBASIN

2N/20W-9F1 [ ]

2N/20W-10H1 [ ]

850

750

650

550

450

350

250

150

EAST LAS POSAS VALLEY SUBBASIN

3N/19W-29K4

150

50

-50

-150

-250

OXNARD PLAIN FOREBAY SUBBASIN

2N/22W-26B3

2N/22W-23K4 [ ]

2N/22W-13N2 [ ]

600

500

400

300

PIRU SUBBASIN

4N/19W-33D3

400

300

200

100

0

-100

-200

WEST LAS POSAS VALLEY

SUBBASIN

2N/21W-11J1 [ ]

2N/21W-15M3 [ ]

150

50

-50

-150

OXNARD PLAIN FOREBAY SUBBASIN

2N/21W-7P2
64 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

When ground-water pumpage approached 

recorded maximum levels in 1951, which was at the 

end of a drought, water-level declines reached a new 

historical low in the upper-aquifer system (fig. 14) and 

levels began to decline significantly in the loweraquifer system in the Oxnard Plain subbasin (fig. 13). 

By 1950, water levels had declined below sea level in 

the lower-aquifer system as far inland as the Pleasant 

Valley subbasin (fig. 13). Through 1950, water levels in 

most wells completed in the lower-aquifer system 

remained near land surface (fig. 13). Water levels in 

wells in the West and South Las Posas Valley subbasins 

indicate a water-level recovery in the upper-aquifer 

system beginning in the 1950s (fig. 14) related to 

increased irrigation return flow along Arroyo Simi and 

Beardsley Wash, importation of water which reduced 

local pumpage, discharge of pumped ground water into 

Arroyo Simi to control shallow ground-water levels, 

and discharge of treated municipal sewage into Arroyo 

Las Posas. 

Figure 13—Continued.

Wells with water-level data

Existing well (Unpublished data from

California Department of Water

Resources and County of Ventura Public

Works Agency, written commun., 1993)

WATER-LEVEL ALTITUDE,

IN FEET ABOVE

SEA LEVEL

Water-level hydrograph: colored curves show water levels

for the well with identification number of the same color.

The water-level curves are dashed when the time between

measurements exceeds one year. The dotted line in the

corresponding color represents the land-surface altitude

at the well site. Green curves are simulated water levels

in lower-aquifer system. See figure 1 for locations of

subbasins.

YEAR

300

200

100

0 1920

19351950196519801995

2N/20W-23K1

EXPLANATION

Ground-water subbasin boundary–

Subbasin names are given in figure 1

River and selected streams

Landward model-grid boundary

of flow region and subregions

Hydrologic Unit boundary

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium

Shallow alluvium and unconsolidated deposits

Consolidated and unconsolidated deposits
Ground Water 65

Figure 14. Measured and simulated water-level altitudes in wells completed in the upper-aquifer system of the Santa Clara–Calleguas ground-water 

basin, Ventura County, California.

118°45'

34°22'30"

34°7'30"

R18W

R22W

T2N

T4N

119°

0 5 MILES

0 5 KILOMETERS

Pacific

Ocean

Calleguas Creek

Santa Clara River

Lake

Piru

300

200

100

0

-100

MOUND SUBBASIN

2N/22W-8N1 [ ]

2N/22W-8P1

2N/22W-9J1 [ ]

300

200

100

0

SANTA PAULA SUBBASIN

2N/22W-11A1,2 [ ]

2N/22W-2C1 [ ]

300

200

100

0

SANTA PAULA SUBBASIN

3N/22W-36K2 [ ]

3N/21W-16K2 [ ]

200

100

0

-100

OXNARD PLAIN FOREBAY

SUBBASIN

2N/22W-22R1

OXNARD PLAIN FOREBAY SUBBASIN

2N/22W-12R1

200

100

0

-100

100

0

-100

-200

OXNARD PLAIN SUBBASIN

1N/22W-3F4

100

0

-100

-200

OXNARD PLAIN SUBBASIN

1N/22W-16E1 100

0

-100

-200

OXNARD PLAIN SUBBASIN

1N/21W-7H1

100

0

-100

-200

OXNARD PLAIN SUBBASIN

1N/22W-36L1

100

0

-100

-200

SOUTH PLEASANT VALLEY

SUBBASIN

1N/21W-14A1

450

350

250

150

50

-50

WEST LOS POSAS VALLEY

SUBBASIN

2N/21W-12F1 [ ]

2N/21W-16J1 [ ]

600

500

400

300

200

100

SOUTH LOS POSAS VALLEY SUBBASIN

2N/20W-12G2 [ ]

2N/19W-4K1 [ ]

2N/19W-3A1 [ ]

300

200

100

0

SANTA ROSA VALLEY SUBBASIN

2N/20W-23R1

900

800

700

600

500

EAST LOS POSAS VALLEY

SUBBASIN

3N/19W-29E2 [ ]

3N/19W-29E3

700

600

500

400 PIRU SUBBASIN

4N/19W-25K2

500

400

300

200

FILLMORE SUBBASIN

4N/19W-30R1

500

400

300

200

FILLMORE SUBBASIN

4N/20W-26A2
66 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

The lowering of water levels continued in the 

upper- and lower-aquifer systems in the Oxnard Plain 

subbasin through the next dry period, 1959–64, 

furthering seawater intrusion (figs. 13 and 14). Waterlevel hydrographs (fig. 13) for many wells in the loweraquifer system in the North Pleasant Valley and the Las 

Posas Valley subbasins indicate a monotonic decline 

through the 1950s and 1960s. Water levels started to 

recover in the Santa Rosa Valley subbasin beginning 

around 1965 because of decreased pumpage in the 

upper- and lower-aquifer systems and discharge of 

treated municipal sewage into Conejo Creek (figs. 13

and 14). The hydrographs of wells in the Mound 

subbasin and wells near the Hueneme submarine 

canyon (figs. 13 and 14) show little to no additional 

decline during these decades. By the late 1960s, 

thousands of acres of aquifer had been intruded by 

seawater in the Port Hueneme and Point Mugu areas, 

and coastal farmland had been lowered by land 

subsidence (see “Land Subsidence Effects”) owing to 

several decades of sustained overdraft.

Figure 14—Continued,

Wells with water-level data

Existing well (Unpublished data from

California Department of Water

Resources and County of Ventura Public

Works Agency, written commun., 1993)

Water-level hydrograph: colored curves show water levels

for the well with identification number of the same color.

The water-level curves are dashed when the time between

measurements exceeds one year. The dotted line in the

corresponding color represents the land-surface altitude

at the well site. Green curves are simulated water levels

in upper-aquifer system. See figure 1 for locations of

subbasins.

EXPLANATION

Ground-water subbasin boundary–

Subbasin names are given in figure 1

River and selected streams

Landward model-grid boundary

of flow region and subregions

Hydrologic Unit boundary

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium

Shallow alluvium and unconsolidated deposits

Consolidated and unconsolidated deposits WATER-LEVEL ALTITUDE,

N FEET ABOVE

SEA LEVEL

YEAR

100

0

-100

-200

192019351950196519801995

1N/21W-7H1
Ground Water 67

Figure 15. Measured and simulated water-level altitudes at sites with multiple wells of different depths completed in the Santa Clara–Calleguas 

ground-water basin, Ventura County, California. 

118°45'

34°07'30"

R18W

R22W

T2N

T4N

119°

0 5 MILES

C

0 5 KILOMETERS alleguas Creek

Santa Clara River

Lake

Piru

Pacific

Ocean

1N/22W-35E (1[FC],

2[LH],3[UH],4[M],5[O])

25

-25

-75

-125

1N/22W-27R (3[M],4[O],5[S]) 25

-25

-75

1N/22W-28G (1[FC],

2[LH],3[UH],4[M],5[O]) 25

-25

-75

-125

1N/22W-26J (3[M],4[O],5[S])

25

-25

-75

-125

1N/22W-27C (2[M],3[O],4[S]) 25

-25

-75

1N/23W-1C

(2[FC],3[LH],4[UH],5[O])

25

-25

-75

1N/22W-29D (1[LH],

2[UH],3[UH],4[O]) 25

-25

-75

1N/22W-20M (1[FC],2[LH],

3[UH],4[M],5[O],6[S]) 25

-25

-75

1N/22W-20J

(4[LH],5[UH],6[M],7[M],8[O]) 25

-25

-75

1S/22W-1H

(1[UH],2[M],3[M],4[O])

25

-25

-75

-125

1N/21W-32Q (2[GC],3[FC],

4[UH],5[M],6[M],7[O])

25

-25

-75

-125

1N/22W-36K

(5[FC],6[LH],7[UH],8[M],9[O])

25

-25

-75

-125

1N/21W-19L (10[M],11[M],

12[O],13[O],14[S])

50

100

0

-50

-100

-150

1S/21W-8L (3[UH],4[O],5[O]) 25

-25

-75

-125

2N/21W-34G

(2[FC],3[LH],4[M],5[O]) 50

0

-50

-100

-150

-200

NORTH PLEASANT VALLEY

SUBBASIN

2N/22W-23B

(3[FC],4[LH],5[UH],6[M],7[O])

150

100

50

0

-50

-100

-150

OXNARD PLAIN FOREBAY

SUBBASIN

2N/21W-7L

(3[FC],4[LH],5[M],6[O])

150

100

50

0

-50

OXNARD PLAIN FOREBAY

SUBBASIN

2N/21W-11J

(3[FC],4[LH],5[M])

200

150

100

50

0

-50

-100

WEST LAS POSAS VALLEY

SUBBASIN

2N/20W-16A (2[M],3[O],4[S])

200

250

300

150

EAST LAS POSAS VALLEY

SUBBASIN

3N/20W-35R

(2[FC],3[FC],4[M])

350

300

250

200

150

EAST LAS POSAS VALLEY

SUBBASIN

NS NS

NS

LSE = 378 feet

LSE = 590 feet

34°22'30"

OXNARD PLAIN SUBBASIN

OXNARD PLAIN SUBBASIN

OXNARD PLAIN SUBBASIN OXNARD PLAIN SUBBASIN

OXNARD PLAIN SUBBASIN

OXNARD PLAIN SUBBASIN

OXNARD PLAIN SUBBASIN

OXNARD PLAIN SUBBASIN

OXNARD PLAIN SUBBASIN

OXNARD PLAIN SUBBASIN

OXNARD PLAIN SUBBASIN

OXNARD PLAIN SUBBASIN

OXNARD PLAIN SUBBASIN

OXNARD PLAIN SUBBASIN
68 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Water levels in both aquifer systems in the 

Oxnard Plain subbasin partially recovered in the late 

1960s owing to increased artificial recharge in the 

Oxnard Plain Forebay subbasin and natural recharge 

owing to a wetter climate. The water levels from wells 

in the upper-aquifer system in the Santa Clara River 

Valley subbasins also showed recovery during the late 

1960s and early 1970s. The absence of wells completed 

in the lower-aquifer system in the upper Santa Clara 

River Valley subbasins precluded an assessment of the 

history or distribution of water levels there. Data from 

wells in the East Las Posas Valley subbasin indicate 

that water-levels began to recover in the late 1970s. 

This recovery was related to importation of water that 

reduced local pumpage, discharge of pumped ground 

water into Arroyo Simi to control shallow groundwater levels, and discharge of sewage effluent into 

Arroyo Las Posas. Similar water-level recoveries in the 

Santa Rosa Valley subbasin began in about 1965 

(figs. 13 and 14) owing to decreased pumpage and 

discharge of sewage effluent into Conejo Creek and 

some water-level recovery near stream channels in 

shallower wells. 

Figure 15—Continued.

EXPLANATION

Well and data -

Multiple-well site sampled as a part of this study.

(Densmore, l996)

NS, indicates that data is not shown.

WATER-LEVEL ALTITUDE,

IN FEET ABOVE

SEA LEVEL

Water-level hydrograph and well number with colored curves

showing water levels for the well with sequence number of

the same color. The water-level curves are dashed when the

time between measurements exceeds one year. Letters in

brackets are aquifer designations. The dotted line, when

shown, represents the land-surface altitude (LSA) at the

multiple-well site. Dashed orange curve is simulated water

levels in upper-aquifer system (layer 1) and dashed black

curve is simulated water levels in lower-aquifer system

(layer 2).

YEAR

50

2N/21W-34G

(2[FC],3[LH],4[M],5[O])

0

-50

-100

-150

-200

198919901991199219931994

Ground-water subbasin boundary–

Subbasin names are given in figure 1

River and selected streams

Landward model-grid boundary

of flow region and subregions

Hydrologic Unit boundary

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium

Shallow alluvium and unconsolidated deposits

Consolidated and unconsolidated deposits

[GC] = Grimes Canyon

Aquifer Designation

Upper

Aquifer

Lower

Aquifer [FC] = Fox Canyon

[LH] = Lower Hueneme

[UH] = Upper Hueneme

[M] = Mugu

[O] = Oxnard

[S] = Shallow
Ground Water 69

By the end of the most recent drought (1987–

91), water levels were below sea level throughout the 

Oxnard Plain, Mound, and Pleasant Valley subbasins in 

both aquifer systems and below sea level in the loweraquifer system throughout the West Las Posas Valley 

subbasin. In the inland subbasins, such as the South 

Pleasant Valley and West Las Posas Valley subbasins, 

water levels in many of the wells were near the 

historical lows in 1991 (figs. 13 and 14).

Beginning in 1992, which is the start of the most 

recent wet period, there was an increase in recharge 

owing to, in part, the increased capacity for artificial 

recharge at the Freeman Diversion and to a temporary 

reduction of pumpage from the coastal subbasins 

owing to increased surface-water supplies through 

pipeline deliveries, conservation practices, and new 

irrigation technology that increased irrigation 

efficiency. Pumpage was reduced because of a drilling 

moratorium established by the FGMA in 1983 on new 

wells completed in the upper-aquifer system in the 

Oxnard Plain. A comparison of the water-level maps 

for 1931 and 1993 indicates that by 1993 water levels 

had recovered in the upper-aquifer system and were 

greater than levels in 1931 (fig. 12A,C). Water levels in 

1993 were about 5 ft higher near the coast, more than 

20 ft higher in the Oxnard Plain Forebay than the 1931 

levels, and above sea level throughout most of the 

Oxnard Plain. The water-level map for the loweraquifer system shows that water levels were below sea 

level in the South Oxnard Plain subarea and Pleasant 

Valley subbasins (fig. 12B). Water-level data were not 

available for other inland subbasins for 1931; however, 

the hydrographs of long-term water levels indicate 

subdued fluctuations, or decline and recovery cycles 

(fig. 14), that may indicate that the shallower parts of 

the upper-aquifer system in these ground-water 

subbasins had recharged owing to increased streamflow 

during wet periods or increased discharge of treated 

sewage effluent.

Water-Level Differences Between Aquifers

Differences in water levels occur between the 

different aquifers (fig. 15) in the Santa Clara–Calleguas 

Basin. The water levels in the coastal Oxnard aquifer 

are lower than the water levels in the Shallow aquifer 

during dry-year periods and become higher than the 

water levels in the Shallow aquifer during recoveries 

(fig. 15) in wet-year periods. Large water-level 

differences occur between the Shallow and the 

underlying aquifers during the irrigation season, 

especially within the South Oxnard Plain subarea. 

These differences are primarily due to thick deposits of 

silt and clay in the Shallow aquifer that retard the 

movement of ground water between the Shallow and 

the Oxnard aquifers. Water levels for the RASA 

monitoring wells completed in the Shallow aquifer 

show little seasonal change owing to ground-water 

pumping or precipitation (fig. 15). Other shallow wells 

in the northern part of the Oxnard Plain subbasin show 

rises that are related to precipitation and declines that 

may be related to leakage (Neuman and Gardner, 1989, 

figs. 2 and 3). Previous investigators estimated that 

vertical leakage from the shallow semiperched system 

to the Oxnard aquifer ranges from 6,000 acre-ft/yr 

(California Department of Water Resources, 1971) to 

20,000 acre-ft/yr (Mann and Associates, 1959).

Similarly, wells with depths of less than 50 ft 

completed in the Santa Clara River Valley subbasins 

also have higher water levels than those of nearby wells 

completed deeper in the upper-aquifer system. These 

elevated water levels may indicate some degree of 

hydraulic separation between the Shallow (recent 

alluvium) aquifer and the underlying aquifer along the 

Santa Clara River.
70 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

 Except for those wells tapping the Shallow 

aquifer, water levels in wells in the coastal subareas 

and Santa Clara Valley subbasins indicate spring and 

summer declines followed by recovery during late fall 

and winter of each year. The seasonal fluctuations in 

wells in the upper-aquifer system are comparable with 

the changes in the wells in the lower-aquifer system 

north of the Hueneme submarine canyon. In the 

Oxnard Plain subbasin south of the Hueneme 

submarine canyon and in the Pleasant Valley subbasin, 

seasonal fluctuations in water levels are greater in the 

lower-aquifer system than in the upper-aquifer system. 

The smaller water-level differences and seasonal 

fluctuations near Port Hueneme are partly due to the 

source of water (seawater intrusion) along the nearshore submarine canyon outcrops, which tends to 

subdue the water-level fluctuations and changes in 

water levels between aquifers. In contrast, the larger 

water-level differences near Point Mugu are, in part, 

due to offshore faulting, which creates a barrier to 

ocean inflow for the lower-aquifer system. However, 

wells completed in the Mugu aquifer have water-level 

fluctuations that are similar to those of the loweraquifer system. The similarity in seasonal fluctuations 

in the Mugu aquifer and the lower-aquifer system, in 

part, may be due to well-construction practices; well 

screens typically span the Mugu aquifer and parts of 

the lower-system aquifers. Flowmeter logs of wells 

screened opposite both the Mugu aquifer and the 

lower-aquifer system indicate a significant contribution 

from the Mugu aquifer (table 5). Water levels in the 

Pleasant Valley subbasin are about 50 ft lower in the 

Mugu-equivalent aquifer than water levels in the 

Oxnard-equivalent aquifer. This sustained water-level 

difference, along with water-level responses measured 

during short-term aquifer tests (Hanson and Nishikawa, 

1996) and geophysical data (Densmore, 1996; 

Appendix 6), indicates that these aquifers are separated 

by fine-grained confining beds. The difference in water 

levels between the Oxnard aquifer and the loweraquifer system increases during periods of pumping 

and decreases during seasonal periods of recovery. 

Water levels in the lower-aquifer system were 

consistently more than 100 ft lower than water levels in 

the upper-aquifer system in the inland subbasins of 

Pleasant Valley, West Las Posas Valley, and East Las 

Posas Valley. For the inland Santa Clara River Valley 

subbasins, water-level differences in the Piru and Santa 

Paula subbasins were 10 to 25 ft lower for water levels 

in the lower-aquifer system than for levels in the upperaquifer system.

Inter-Aquifer Flow

Flow between aquifers can be an important 

consideration in the management of water resources. 

Vertical water-level differences (figs. 13–15) indicate 

the potential for upward and downward flow between 

aquifers and aquifer systems. However, these 

differences can result in appreciable leakage only if a 

conductive pathway is present. Vertical flow between 

aquifers can occur as leakage through coarse-grained 

sedimentary layers, through and around fine-grained 

layers, and as vertical flow in and around well bores. 

Vertical flow between the semiperched and the 

upper-aquifer systems also can occur through failed 

and abandoned wells (Stamos and others, 1992). 

Estimates of the number of abandoned and potentially 

failed wells range from 167 (Predmore, 1993) to 238 

(Ventura County Resource Management Agency, 

Environmental Health Department, 1980) in the 

Oxnard Plain and as many as 1,215 wells throughout 

Ventura County (Predmore, 1993). Wellbore heat-pulse 

flowmeter tests in selected wells in the Oxnard Plain 

subbasin indicate that intraborehole flow rates of 3 to 

11 gal/min may occur in some failed wells. This 

suggests a total maximum leakage of about 800 to 

4,220 acre-ft/yr for periods when the hydraulic 

gradients are downward. The hydrographs for the 

multiple-observation well sites show that the heads in 

producing aquifers can vary seasonally and 

climatically (fig. 15). Thus, during wet-year periods or 

during periods of reduced pumpage, heads in the 

aquifer system can result in intraborehole discharge 

from the ground-water flow system to the overlying 

semiperched systems. Conversely, during dry-year 

periods or in areas of increased pumpage, heads in the 

semiperched system could be greater than heads in the 

underlying aquifers and could result in leakage as 

recharge to the ground-water system. For example, 

wellbore leakage of as much as 11 gal/min was 

measured with a heat-pulse flowmeter in failed 

monitoring well 1N/22W-27R2. However, detailed 

chemical sampling at nearby multiple-completion 

monitoring wells 1N/22W-27R3–5 (Izbicki, 1996a) 

indicates that the effects of this wellbore leakage were 

not areally extensive.
Ground Water 71

Vertical flow also can occur from the underlying 

marine sedimentary rocks or from brines related to oil 

deposits. Methane is reported to discharge from some 

production wells that are completed to depths just 

above the oil fields just west of Pleasant Valley in the 

Oxnard Plain subbasin (fig. 9). Geochemical data 

indicate that the amounts of leakage from deeper and 

older formations in the southern part of the Oxnard 

Plain and South Pleasant Valley subbasins probably are 

small (Izbicki, 1991, 1996a, figs. 3 and 5). 

Source of Water to Wells

The relative contribution of water to wells 

completed in multiple aquifer systems is dependent on 

the local stratigraphy and on well construction. The 

vertical distribution of ground-water withdrawals from 

wells was estimated from flowmeter logs of 17 wells 

completed as part of the RASA Program and other 

studies (table 5, fig. 17B presented later in the “Model 

Boundaries” section, figure A5.1 in Appendix 5). 

Where wells are perforated across younger aquifers 

and older aquifers, most of the water is produced from 

the more transmissive younger aquifers [table 5, figure 

A5.1 in Appendix 5]. Combined with the stratigraphy, 

flowmeter logs indicate that the most productive and 

areally extensive water-bearing zones commonly occur 

as basal coarse-grained layers that overlie major 

regional unconformities. However, the relative 

contribution to any particular well from less productive 

aquifers may increase with increased pumping rates 

and decreased water levels in the more productive 

aquifers (table 5).

The most important aspects of well construction 

are the vertical extent of the well screen and the depth 

and location of the pump intake relative to the well 

screen. Wells that are screened across the basal layer of 

the upper-aquifer system can derive as much as 

70 percent of the wellbore inflow from this relatively 

thin layer. Wells that are completed only in the loweraquifer system can derive 100 percent of the wellbore 

inflow from the basal coarse-grained layer in the 

Hueneme aquifer (table 5). Flowmeter logs are not yet 

available for wells throughout most of the Oxnard Plain 

and Las Posas Valley subbasins; for wells in all the 

Piru, Fillmore, and Santa Rosa Valley subbasins; and 

for wells screened only in the upper-aquifer system.

Source, Movement, and Age of Ground Water

The source, movement, and age of ground water 

in the Santa Clara–Calleguas Basin can be inferred 

from the isotopic content of ground-water and surfacewater samples. Based on deuterium isotope samples, 

most of the water in the upper- and lower-aquifer 

systems is derived from streamflow infiltration of 

high-altitude precipitation along the Santa Clara River 

that originated largely as runoff of precipitation falling 

at the higher altitudes of the surrounding mountains 

(Izbicki, 1996b, fig. 3). Isotopic data also suggest a 

local contribution of mountain-front recharge and 

direct infiltration of locally derived precipitation in the 

Las Posas and Pleasant Valleys and along the margins 

of the Santa Clara River Valley (Izbicki, 1996b). 

Although a large component of irrigation return flow 

may contribute to infiltration, no large areas of the 

Oxnard aquifer in the Oxnard Plain had an isotopic 

signature similar to that of evaporated waters. Analysis 

of ground-water samples for the hydrogen isotope 

tritium indicates that recent recharge (since 1952) has 

occurred largely in the Santa Clara River Valley 

subbasin, the Oxnard Plain Forebay subbasin, the 

northwestern part of the Oxnard Plain subbasin, and 

the South Las Posas Valley subbasin (Izbicki, 1996b, 

fig. 5). Tritium data also indicate that the artificial 

recharge from the Oxnard Plain Forebay subbasin has 

largely infiltrated the upper-aquifer system. Ages 

determined by carbon-14 analysis of ground-water 

samples indicate that water in the upper-aquifer system 

directly beneath the Saticoy spreading grounds is 

relatively young (less than 500 years old), but water in 

the lower-aquifer system beneath the El Rio spreading 

grounds ranges from 700 to more than 13,000 years old 

(Izbicki, 1996b, fig. 6). Samples from the lower-aquifer 

system near the coast range from about 7,000 to 23,000 

years old (Izbicki, 1996b, fig. 6). Samples from wells 

in the Las Posas Valley and Pleasant Valley subbasins 

yielded ages of about 700 to 6,000 years old (Izbicki, 

1996b, fig. 7). Collectively, these data indicate that the 

upper-aquifer system is recharged by streamflow 

infiltration and mountain-front recharge; the loweraquifer system has received little recent water; and 

ground water moved relatively slowly under the 

hydraulic gradients present prior to water development.
72 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Table 5. Summary of well-construction data and discharge rates and inflows from flowmeter logs of wells in selected subbasins of the Santa Clara–Calleguas Basin, Ventura County, California [State well No.: See well-numbering diagram in text. Total depth of well and depth to top and bottom of well screen in feet below land surface. —, no data] State well No. Local well name Subbasin

Total drilled 

depth

(feet)

Well

casing 

diameter 

(inches)

Depth to top of 

well screen

(feet)

Depth to

bottom of well 

screen

(feet)

Year of

flowmeter 

log

Well-test

discharge 

rate

(gal/min)

Inflow

Percent from 

upper/lower

aquifer

systems

Percent from 

Oxnard/Mugu/Huen

eme/Fox Canyon/

Grimes Canyon 

system

1N/21W-3K1 PVCWDWELL#04 South Pleasant 

Valley

1,453 18 403 1,433 19801

1991

1,000

1500

17/8330/70

—/17/3/17/63—/30/21/11/38

1N/21W-3R1 PVCWDWELL#01 South Pleasant 

Valley

1,033 18 443 1,013 19801

1991

4,0001,414

9/91

54/46

—/9/91/0/——/54/46/0/—

1N/21W-4D4 PVCWDWELL#03 Oxnard Plain 1,341 18 571 1,321 19801 1991 2,000 1,168 —/100 —/100 —/—/5/36/59 —/—/19/26/551N/21W-4K1 PVCWDWELL#05 South Pleasant 

Valley

1,240 18 400 1,220 19801 2,000 4/96 —/4/5/60/31

1N/21W-8R1 PVCWDWELL#07 Oxnard Plain 1,383 18 603 1,363 19801 1991 2,500 1,128 —/100 —/100 —/—/0/28/72 —/—/0/21/791N/21W-10G1 PVCWDWELL#06 South Pleasant 

Valley

1,020 18 420 1,000 19791

1992

3,1502,000

—/100—/100

—/—/34/53/0—/10/27/37/26

1N/21W-15D2 PVCWDWELL#08 South Pleasant 

Valley

1,103 18 383 1,083 19801

1991

4,0001,121

17/8340/60

—/17/20/58/5—/40/30/23/7

1N/21W-21H2 PVCWDWELL#10 South Pleasant 

Valley

 883 18 503 863 19801 2,500 —/100 —/—/49/51/—

1N/21W-22C1 PVCWDWELL#09 South Pleasant 

Valley

1,023 18 443 1,003 19801

1991

4,0001,440

—/100—/100

—/—/25/53/22—/—/19/37/44

1N/21W-28D1 PVCWDWELL#11 Oxnard Plain 960 9 463 923 19801 1991 2,000 1,100 —/100 —/100 —/—/58/42/— —/—/100/0/—1N/21W-31L1 PTMUGU#03 Oxnard Plain 702 12 350 700 1991 407 12/88 —/12/88/—/—1N/22W-3F5 OX

NARD#20 Oxnard Plain 1,126 18 526 1,106 19842 2,000 —/100 —/—/71/29/—
Ground Water 73

2N/20W-20M5 SAINT JOHNS#6 North Pleasant 

Valley

 700 18 480 700 19923

1992319923

 500

1,0001,500

—/100—/100—/100

—/—/58/42/——/—/58/42/——/—/53/47/—

2N/21W-34G1 PVCWDWELL#02 South Pleasant Valley 1,483 18 403 1,463 19801 1992

1993

4,0002,0652,065

—/100—/100—/100

—/—/25/75/0—/—/25/67/8—/—/18/68/14

2N/22W-8F1 VICTORIAWELL#2 Mound 1,190 18 580 1,180 19944 1994

1994

1,9062,4854,015

50/5044/5633/67

—/50/50/—/——/44/56/—/——/33/67/—/—

2N/22W-13N2 ELRIO#12 Oxnard Forebay 1,112 18 752 1,092 19835 1,960 —/100 —/—/100/—/—3N/21W-11J5 SANTA 

PAULA#12

Santa Paula 700 16 260 700 1991

19911991

1,0001,5002,500

100/073/2772/28

—/100/0/—/——/73/27/—/——/72/28/—/—

1Data from Pleasant Valley Water Conservation District (Lee Miller, written commun.,1991).2Data from Geotechnical Consultants, Inc. (Ted Power, written commun, 1992).3Data from Fugro-McClelland, Inc. (David Gardner, written commun., 1993).4Data from Fugro-McClelland, Inc. (Curtiss Hopkins, written commun., 1994).5Data from United Water Conservation District (Jim Gross, written commun., 1991).

State well No. Local well name Subbasin

Total drilled 

depth

(feet)

Well

casing 

diameter 

(inches)

Depth to top of 

well screen

(feet)

Depth to

bottom of well 

screen

(feet)

Year of

flowmeter 

log

Well-test

discharge 

rate

(gal/min)

Inflow

Percent from 

upper/lower

aquifer

systems

Percent from 

Oxnard/Mugu/Huen

eme/Fox Canyon/

Grimes Canyon 

system

Table 5. Summary of well-construction data and discharge rates and inflows from flowmeter logs of wells in selected subbasins of the Santa Clara–Calleguas Basin, Ventura County, California—Continued
74 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Land-Subsidence Effects

Ground-water withdrawals, oil and gas 

production, and tectonic movement are three potential 

causes of land subsidence in the Oxnard Plain and 

adjacent subbasins (fig. 9) (Hanson, 1995). 

Ground-water levels in the Oxnard Plain subbasin have 

declined steadily since the first wells were completed 

in the 1870s. Ground water, however, has remained a 

primary source of water since the early 1900s. Oil and 

gas has been produced in the Santa Clara–Calleguas 

Basin since the 1920s and in the Oxnard Plain subbasin 

since the 1940s. The basin is a part of the tectonically 

active Transverse Ranges physiographic province. 

Ventura County has delineated a probable subsidencehazard zone that includes parts of the Piru, Fillmore, 

Santa Paula, Mound, Oxnard Plain Forebay, Oxnard 

Plain, and Pleasant Valley subbasins (Ventura County 

Board of Supervisors, 1988). 

Since the early 1900s, water-level declines in the 

upper- and lower-aquifer systems in the Oxnard Plain 

subbasin have ranged from about 50 to 100 ft. Water 

levels in wells at the multiple-well monitoring sites are 

lower in the lower-aquifer system than in the upperaquifer system—by 20 ft near the Hueneme submarine 

canyon along the central coast and by about 80 ft near 

the Mugu submarine canyon along the southern coast 

of the Oxnard Plain subbasin. Because early pumpage 

data are unavailable for the Oxnard Plain subbasin, the 

total quantity of water withdrawn is unknown. 

However, reported pumpage data indicate that during 

1979–91 about 822,000 acre-ft of ground water was 

withdrawn from the Oxnard Plain subbasin at a 

relatively constant rate. This pumpage has resulted in 

water-level declines that, in turn, have increased the 

effective stress on the aquifer-system sediments. An 

increase in the effective stress on aquifer sediments 

beyond their preconsolidation stress results in 

compaction and reduction of pore space and 

mechanically squeezes water from sediments.

 More than 7,900 acre-ft of brines, 8,000 acre-ft 

of oil, and 72 million cubic feet of natural gas were 

withdrawn from oilfields in the Oxnard Plain subbasin 

(fig. 9) between 1943 and 1991 (Steven Fields, 

Operations Engineer, California Department of 

Conservation, Division of Oil and Gas, written 

commun., 1992). Pressure declines equivalent to more 

than 1,100 ft of water-level decline have occurred in 

the Oxnard oilfields since the onset of oil and gas 

production. These declines alone could potentially 

account for local subsidence of 1.5 to 2.0 ft (California 

Division of Oil and Gas, 1977).

Tectonic activity in the form of plate 

convergence and north-south crustal shortening has 

resulted in an average regional horizontal movement in 

the subbasins north of the Oxnard Plain of about 

0.007 ft/yr over the past 200,000 years (Yeats, 1983). 

Vertical movement, as uplift north of the Oxnard Plain 

subbasin and as subsidence in the Oxnard Plain 

subbasin, has been caused by plate convergence and 

related earthquakes throughout the basin. For the 

southern edge of the Oxnard Plain subbasin (fig. 9A), 

elevation data from bench marks (BM) on bedrock (for 

example, BM Z 583) indicate that the 0.17 ft of 

subsidence that occurred during 1939–78 (at a rate of 

about 0.004 ft/yr) may be related to tectonic activity.

Data from a coastal leveling traverse near the 

southeastern edge of the Oxnard Plain (fig. 9A,B)

indicate that as much as 1.6 ft of subsidence occurred 

during 1939–60 at BM E 584 (0.07 ft/yr) and an 

additional 1 ft occurred during 1960–78 (0.06 ft/yr). 

During 1960–92, 0.5 ft of subsidence (0.02 ft/yr) was 

measured at BM Z 901, which is southwest of BM 

E 584 and at the edge of the coastal Oxnard Plain. 

Bench-mark trajectories (fig. 9C) indicate that 

subsidence continues and may be driven by extreme 

water-level declines that occur during drought periods. 

Farther inland, where water-level and oilfield pressure 

declines are largest, greater subsidence might be 

expected.
Simulation of Ground-Water Flow 75

Indirect evidence that subsidence may be related 

to ground-water withdrawals includes water-level 

declines greater than 100 ft, subsurface collapse of well 

casings in the South Pleasant Valley subbasin and 

South Oxnard Plain subarea, required repeated leveling 

of irrigated fields for proper drainage, degraded 

operation of drainage ditches in agricultural areas, and 

lowering of levees along the Calleguas Creek in the 

South Pleasant Valley subbasin. In the Las Posas Valley 

and South Pleasant Valley subbasins, water-level 

declines of 50 to 100 ft have occurred in the upperaquifer system, and declines of about 25 to 300 ft or 

more have occurred in the lower-aquifer system since 

the early 1900s (figs. 13 and 14). Owing to large waterlevel declines, the area of probable subsidence may be 

larger than that delineated by Ventura County and may 

include the Las Posas Valley subbasin and the 

remainder of the Pleasant Valley subbasin. By 1992, 

total subsidence in the Oxnard Plain subbasin could 

exceed the 2.6 ft measured during 1939–78 along the 

coastal traverse. Although the amount of subsidence 

from various sources remains unknown, ground-water 

withdrawals and oil and gas production probably are 

major causes of subsidence in the Oxnard Plain 

subbasin, and tectonic activity probably is a minor 

cause. 

Water released by compaction of layers of finegrained deposits within the upper- and lower-aquifer 

systems can be a significant additional one-time source 

of water to adjacent producing coarse-grained layers in 

the aquifer systems. Geochemistry data (Izbicki, 

1996a, fig. 3) and geophysical data (EM and natural 

gamma logs in Appendix 5) indicate that fine-grained 

beds may be a significant source of the poor-quality 

water in areas such as the South Oxnard Plain subarea 

in the coastal region between the Hueneme and Mugu 

submarine canyons where saline fine-grained layers 

and seasonal pumpage may collectively contribute to 

poor-quality water.

SIMULATION OF GROUND-WATER FLOW

A numerical ground-water flow model of the two 

regional aquifer systems (upper aquifers and lower 

aquifers) in the Santa Clara–Calleguas Basin was 

developed to simulate steady-state predevelopment 

conditions prior to 1891 and transient conditions for 

the development period January 1891–December 1993. 

The model simulations provided information 

concerning predevelopment hydrologic conditions and 

aquifer response to changes in pumpage and recharge 

through time. Simulations were made using the threedimensional finite-difference ground-water flow model 

(MODFLOW) developed by McDonald and Harbaugh 

(1988). Additional packages were incorporated into the 

ground-water flow model to simulate the routing of 

streamflow (Prudic, 1989), land subsidence (Leake and 

Prudic, 1991), and faults as horizontal barriers to 

ground-water flow (Hsieh and Freckleton, 1993).

Transient simulations were calibrated for the 

period of historical systematic data collection, which 

generally spans from the 1920s through 1993. The 

most important period of the calibration spans the 

period of reported pumpage (1984–93). Simulation 

results and model calibration provided insight into the 

conceptual model of the regional flow system, and into 

the limitations and potential future refinements of the 

regional-scale model. The model also was used to 

analyze the distribution of flow and changes in storage 

during 1984–93, to project future ground-water flow, 

and to evaluate alternatives to future projected groundwater flow. The analysis allowed assessment of waterresources management alternatives and of the effect 

that implementation of selected alternatives and 

geologic controls might have on recharge, coastal 

landward flow (seawater intrusion), land subsidence, 

ground-water movement, and overall resource 

management under climatically varying conditions that 

affect supply and demand.
76 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Model Framework

The orientation, areal and temporal 

discretization, vertical layering, areal extent, and 

internal structural boundaries constitute the framework 

of the numerical ground-water flow model developed 

for this study. The model is an extension and 

refinement of the previously developed regional 

models and, as such, represents the RASA Program 

contribution to the continuing effort to evaluate and 

manage the ground- and surface-water resources of the 

Santa Clara–Calleguas Basin. Model attributes and 

related data have been added to the Geographic 

Information System (GIS) completed by the RASA 

Program (Predmore and others, 1997). The metadata 

that describe and document these additional GIS 

coverages are summarized in Appendix 1. The flow of 

information used to estimate and assemble the input 

data for the Recharge Package, Streamflow Package, 

and Well Package of the ground-water model is 

summarized in the flowcharts in Appendix 6.

Previous Models

Previous models of the area include basinwide 

digital Theissan-Weber Polygon superposition 

simulations of historical transient hydraulic and 

water-quality conditions for 1950–67 (California 

Department of Water Resources, 1974a,b, 1975), and 

numerical subregional ground-water flow models of the 

lower-aquifer system in the East and West Las Posas 

Valley subareas (CH2M HILL, 1993) and the upperand lower-aquifer systems in the Santa Rosa Valley 

subarea (Johnson and Yoon, 1987). More recently, 

Reichard (1995) completed an extended and enhanced 

digital model based on the original Theissan-Weber 

Polygon model. Reichard extended this model areally 

to include the offshore coastal areas; like the regional 

model, it simulates the upper- and lower-aquifer 

systems in the Oxnard Plain subareas, the loweraquifer system in the Las Posas Valley and Pleasant 

Valley subareas, and the upper-aquifer system in the 

Santa Clara River Valley subareas. The model uses 

estimates of recharge and pumpage for the historical 

simulation period (1984–89), which is the base period 

used to evaluate the FGMA management goals. 

Reichard’s model was used to simulate the flow of 

ground water and to generate response surfaces for use 

in an optimization model. In turn, the optimization 

model was used to test different ground-water and 

surface-water allocation schemes that would satisfy 

water demands and minimize coastal landward flow 

(seawater intrusion). Nishikawa (1997) completed a 

cross-sectional transport model of a vertical section 

through the Hueneme submarine canyon to test 

alternative conceptual models of seawater intrusion for 

predevelopment conditions and for 1929–93 developed 

conditions. A numerical wellbore hydraulic model of 

an aquifer test in the lower-aquifer system in the South 

Pleasant Valley subarea was completed to test 

alternative conceptual models of the vertical 

distribution of hydraulic properties (Hanson and 

Nishikawa, 1992, 1996). 

Model Grid

The model grid is oriented at N. 27° W. and 

contains 60 rows and 100 columns discretized into 

square cells with sides 0.5 mi in length (figs. 7, 16, and 

A1.4). Average values of aquifer properties and initial 

hydraulic head are assigned to each cell; average initial 

hydraulic head for each cell is assigned at the center, or 

node, of each cell. The model contains two layers, one 

each for the upper- and lower-aquifer systems. The two 

model layers were made identical in areal extent 

everywhere in the landward part of the model domain 

(fig. 16). The top of the upper layer is aligned with the 

bottom of the fine-grained layers that separate the 

semiperched shallow aquifer from the upper-aquifer 

system throughout the Northwest and South Oxnard 

Plain subareas. The top of the upper layer is coincident 

with the land surface throughout the remainder of the 

upper layer. The bottom of the upper layer and the top 

of the lower layer are coincident with the bottom of the 

Mugu aquifer. This boundary generally occurs at a 

depth of 400 ft in the Oxnard Plain subareas. The 

bottom of the lower layer is coincident with the bottom 

of the Fox Canyon aquifer throughout most of the 

model area (figs. 7A and 8).
Simulation of Ground-Water Flow 77

The model was extended offshore farther in the 

northwest corner of the lower layer than previous 

models (California Department of Water Resources, 

1974a,b, 1975; Reichard, 1995). The areal extent of the 

layers was based on the outcrop areas on the geologic 

map (Weber and others, 1976) on land, and the seaward 

extent was based on bathymetry and submarine 

outcrops estimated from geology maps (Kennedy and 

others, 1987). The upper layer (upper-aquifer system) 

(fig. 16A) is an active flow region covering 374 mi2, of 

which about 27 percent is offshore. The lower layer 

(lower-aquifer system) (fig. 16B) is an active flow 

region of 464.5 mi2, of which about 41 percent is 

offshore. 

Temporal Discretization

The model was used to simulate the period from 

January 1891 through December 1993. This 103-year 

historical simulation of ground-water and surfacewater flow was temporally discretized into 3-month 

periods (stress periods) that represent the four seasons 

within a calendar year. For computational purposes, 

streamflow, recharge, and pumpage from wells are 

specified for each season of every year. Each season 

was discretized into 12 equal time steps to estimate 

flow and heads throughout the model.

Model Boundaries

The perimeter of the active flow region within 

the model represents the approximate limit of the 

ground-water flow system. The boundary is 

represented by a combination of no-flow, constant-flux, 

and general-head boundaries. Except where 

mountain-front recharge enters the model along the 

boundaries of the landward active flow region 

(fig. 17A), the landward model cells along this outer 

boundary of both model layers are represented as a 

no-flow boundary. No-flow boundaries occur where 

there is no flow of water between the active flow-region 

model cells and the adjacent areas. The bottom of the 

lower layer is also represented as a no-flow boundary; 

this layer generally is coincident with the base of the 

Fox Canyon aquifer except in the Santa Rosa Valley, 

East Las Posas, and parts of the Pleasant Valley 

subareas. These no-flow boundaries represent the 

contact with non-water-bearing rocks. Mountain-front 

recharge that enters along stream channels in the upper 

layer and at the outcrops of the Santa Barbara 

Formation outside of the active flow system in the 

lower layer are constant-flux boundaries (described 

later in this section). The constant-flux boundaries are 

specified flows that change with every season (stress 

period) of each year for the period of simulation.

The offshore boundary in both layers is 

represented as a strong source-sink boundary; this 

boundary is located at the geographic location of the 

seawater intrusion front identified by Greene and 

others (1978). This boundary is represented in the 

model as a general-head boundary simulating inflow 

(source) of water from outside the model area or 

discharge (sink) of water from the boundary model 

cells to outside the model area. Flow at this boundary is 

proportional to the hydraulic-head difference between 

the equivalent freshwater head of the ocean along the 

submarine outcrops and the head of the model cells that 

are coincident with the boundary (fig. 16). Flow at this 

boundary is also proportional to the hydraulic 

conductance. Hydraulic conductance was determined 

during model calibration and represents the 

impediment to flow at the seawater intrusion boundary 

in each layer. For the purposes of this report, coastal 

inflow along this boundary is termed coastal landward 

flow (a surrogate for seawater intrusion) and outflow is 

termed coastal seaward flow. 

The coastal flow of water through the submarine 

canyon outcrops is, in part, dependent on the equivalent 

freshwater head of seawater and the location of the 

freshwater/seawater interface. On the basis of EM and 

natural gamma logs (figure A5.1 in Appendix 5), the 

intrusion and movement of seawater occurs largely 

along the coarse-grained basal layers above regional 

unconformities. Chloride-concentration data, 

geophysical logs, and cross-section transport modeling 

of the Hueneme submarine canyon (Nishikawa, 1997) 

indicate that seawater intrusion is characterized by a 

relatively sharp front restricted to selected coarsegrained layers. Simulation of the seawater-interface 

boundary in this model assumed a position of the 

interface that is between the submarine outcrop and the 

coast. The interface location for the current model was 

inferred from the location estimated by Green and 

others (1978) for the lower-aquifer system (fig. 16B), 

transport model simulations (Nishikawa, 1997), and 

geochemical data from coastal monitoring wells 

(Izbicki, 1996a). The limitations of this assumption are 

further discussed in the “Model Uncertainty, 

Sensitivity, and Limitations” section.
78 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Pacific

Ocean

118°45'

34°22'30"34°07'30"

R22W R18W

119°

0

0

5 MILES

5 KILOMETERS

T4N

T2N

119°15'

Pitas Point

Pitas Point

0 - 1616 - 4040 - 8080 - 120120 - 160

(0 - 10%)(10 - 25%)(25 - 50%)(50 - 75%)(75 - 100%)

Acres of riparian

vegetation per cell

(percent of cell area)

a C ll ge au Cs

reek

Lake

Piru

-33

-66

-66

-98

-98

-131

-230

-230

-492

-656

Country

Club (1)

Oil Wells (4)

Oak Ridge (0)

Bailey (5)

Bailey (5)

Springville

Extension (4)

FoxCanyon (2)

San Pedro (5)

Camarillo (4)

General-head boundary cells

Boundary head altitude is 3.75 feetBoundary conductance is 1,296feet squared per day

Hydraulic characteristic, in feet per day

(1) 43,200(2) 86.4(3) 43.2(4) 4.32(5) 0.864(6) 0.432

(7) 0.086(8) 0.043(9) 0.0086

(10) 0.00086(11) 0.000086

A

EXPLANATION

Horizontal flow barrierFault name andconductance index

Two colors usedto distinguishadjacent faults

Model-grid boundary offlow region and subareasInferred seawater interface -assigned to Lower-Aquifersystem (Green and others,1978) Fault and selected name

Ground-water subbasin boundary –

Subbasin names are given in figure 1

River and selected streamsSelected bathymetry contours –

Depth below sea level, in feet

Hydrologic Unit boundary Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium and unconsolidated depositsShallow alluvium

Consolidated and unconsolidated deposits

Santa

River

Clara

-66

San

Cayento

Santa

Paula

Figure 16. Location of model grid, location of general-head boundary cells with associated boundary head altitude and conductance, location of horizontal flow barriers and associated hydraulic characteristic, and location of evapotranspiration model cells and percent of riparian vegetation cover per cell in the Santa Clara-Calleguas ground -water basin, Ventura County, California. 

A, Upper-aquifer system. 

B, Lower-aquifer system.
Simulation of Ground-Water Flow 79

 

Pacific

Ocean

118°45'

34°22'30"34°07'30"

R18W

119°

0

0

5 MILES

5 KILOMETERS

T4N

T2N

119°15'

Pitas Point

a C ll ge au Cs

reek

Santa Clara

River

Lake

Piru

General-head boundary cells

Boundary head altitude is 16.67 feetBoundary conductance is 259 feetsquared per day

-33

-66

-66

-98

-98

-131

-230

-230

-492

-656

Ventura (5)

Foothill (5)

Oak Village (5)

Berylwood (5)

CentralLas PosasFault (7)

Conejo NE2 (5)

Fairview (7)

Oak VillageExtension (5) Camulos (5)

Country

Club (1)

Oil Wells (4)

Oak Ridge (10)

Jamaica

(5)

OxnardSlope 2(5)

OxnardSlope 1(5)

MuguSlope 1 (6)

HuenemeSlope 1 (5)

HuenemeCanyon (5) Old

HuenemeCanyon (5)

SouthHuenemeCanyon (5)

Round Mountain (5)

Bailey (5)

Simi (9)

SimiExtension (10)

Springville Extension (6)

Somis (5)

FoxCanyon (5)

La Loma (5)

San Pedro (5)

Little Simi Valley (5)

Camarillo (6)

Sycamore Canyon (8)

Sycamore Extension (8)

McGrath (10)

El Rio (3)

Hydraulic characteristic, in feet per day

(1) 43,200(2) 86.4(3) 43.2(4) 4.32(5) 0.864(6) 0.432

(7) 0.086(8) 0.043(9) 0.0086

(10) 0.00086(11) 0.000086

B

R22W

San

Cayento

Santa

Paula

Figure 16—Continued.
80 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Pacific

Ocean

118°45'

R22W R18W

119°

0

0

5 MILES

5 KILOMETERS

119°15'

a C ll ge au Cs

reek

Santa Clara

River

Lake

Piru Cells with mountain-front recharge, in cubic foot per second

Upper-Aquifer

0.005 - 0.050.05 - 0.10.1 - 0.20.2 - 0.30.3 - 0.812

Lower- Aquifer

0.0160.0310.092

A

B

C

D

Cells with Artifical Recharge

A PiruB Santa PaulaC SaticoyD El Rio

5

4

3 2

1

6

8

7

9

A

EXPLANATION

Subdrainage basin boundary –

(California Department of Water Resources, 1975)

River and selected streams

Fault

Hydrologic Unit boundary

Ground-water subbasin boundary –

Subbasin names are given in figure 1

Model-grid boundary offlow region and subareas

34°

22'30"

34°

07'30"

T

4

N

T

2

N

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium and unconsolidated depositsShallow alluvium

Consolidated and unconsolidated deposits

Cells with recharge

at WastewaterTreatment Plants

1 City of Fillmore2 City of Santa Paula3 Limoneira Assoc.-Olive Lawn4 Limoneira Assoc.-Limoneira5 Saticoy Sanitation District6 Moorpark-Ventura County #197 City of Thousand Oaks8 Camarillo Sanitation District9 Camarillo State Hospital/Camarosa

Wastewater-Treatment Plant

Figure 17.

A, Areal extent of model grid, location of model cells representing mountain-front, artificial, and treated wastewater recharge, and simulated rates of mountain-front recharge in 

the ground-water flow model of the Santa Clara–Calleguas ground-water basin, Ventura County, California.
Simulation of Ground-Water Flow 81

Figure 17—Continued. B, modeled subareas for the upper-and lower-aquifer systems, poercentage of infiltration for seasonal precipitation during wet and dry climatic 

periods, location of wells with flowmeter logs, and the related percentage of pumpage assigned to wells spanning the upper andlower layers.

R22W R18W

T

2

N

T

4

N

34

33

32 30

31

28

27

26

13

12 11

10

9

8

4 6

2

29

20

19

18

17

16

14

25

24

23

22

21

7

3 5

1

15

0 10 MILES

0 10 KILOMETERS

Shoreline

Upper

Layer

Lower

34° Layer

07'

30"

118°45'

34°

22'

30"

119°30' 119°15' 119°

Oxnard Plain Forebay

Northwest Oxnard Plain

Northeast Oxnard Plain

South Oxnard Plain

Piru

Fillmore

Santa Paula

Mound

Offshore Mound

Offshore North Oxnard Plain

Offshore South Oxnard Plain

South Pleasant Valley

Santa Rosa Valley

South Las Posas Valley

West Las Posas Valley

East Las Posas Valley

North Pleasant Valley

(30/30)

(0/0)

(30/30)

(0/0)

(30/15)

(30/15)

(30/30)

(0/0)

(0/0)

(0/0)

(0/0)

(15/5)

(15/5)

(30/5)

(30/5)

(30/5)

(10/0)

1

3

5

7

14

15

16

17

18

19

20

21

22

23

24

25

29

NAME

PERCENTAGE OF

SEASONAL CLIMATIC

PERIOD PRECIPITATION

(WET/DRY)

(100/0)

(90/10)

(90/10)

(100/0)

(100/0)

(30/15)

(70/30)

(50/50)

(0/0)

(0/0)

(0/0)

(90/10)

(100/0)

(100/0)

(50/50)

(20/80)

(70/30)

ASSIGNED PERCENTAGE OF

PUMPAGE FOR WELLS SPANNING

BOTH MODEL LAYERS

(UPPER/LOWER)

3

0

0

0

2

11

1

4

--

--

--

10

9

9

9

9

10

PERCENTAGE CHANGE

IN AGRICULTURAL

PUMPAGE FOR

WET/DRY PERIODS NUMBER

UPPER-AQUIFER SUBAREA LOWER-AQUIFER SUBAREA

Oxnard Plain Forebay

Northwest Oxnard Plain

Northeast Oxnard Plain

South Oxnard Plain

Piru

Fillmore

Santa Paula

Mound

Offshore Mound

Offshore North Oxnard Plain

Offshore South Oxnard Plain

South Pleasant Valley

Santa Rosa Valley

South Las Posas Valley

West Las Posas Valley

East Las Posas Valley

North Pleasant Valley

2

4

6

8

26

27

28

31

32

33

34

9

10

11

12

13

30

NUMBER NAME

2

1

EXPLANATION

Upper-Aquifer system

Subareas with valley-floor recharge

Lower-Aquifer system

(Offshore areas include offshore

areas of upper layer)

Wells with flowmeter logs

(See figure A5.1 for selected examples)

B

8F1

11J5

15D2

13N2

Hydrologic Unit

boundary
82 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

The offshore boundary representing the densitydependent seawater interface was simplified with a 

general-head boundary simulation which may limit the 

accuracy of the model for the simulation of some 

small-scale features in the coastal areas. Since the 

actual location of the boundary through time along the 

entire coast is unknown, the location of the boundary 

was held stationary at an average location for all 

simulations. A general-head boundary represents the 

inflow or outflow of water in a model cell and is 

represented by boundary head and conductance to flow 

between the model cell and the boundary. Flow 

between the boundary and the aquifer is controlled by 

the boundary conductance and by the head gradient, 

which is calculated by the model as the difference 

between the aquifer head in the model cell and the 

specified boundary head. The boundary head that 

represents the equivalent freshwater head of seawater at 

the depth of outcrop was estimated to be equivalent to 

3.75 ft at 46 cells in the upper model layer (fig. 16A)

and 16.67 ft at 65 cells in the lower model layer 

(fig. 16B). The equivalent freshwater head at the 

upper-aquifer boundary was estimated by dividing the 

depth to the submarine outcrop (150 ft below sea level) 

by 40 (density ratio between saltwater and freshwater); 

this outcrop was assumed to represent the basal 

coarse-grained layer in the Oxnard aquifer. In a similar 

manner, the equivalent freshwater head for the loweraquifer boundary was estimated by dividing the depth 

to the submarine outcrop (667 ft below sea level) by 

40; this outcrop was assumed to represent the basal 

coarse-grained layer in the Hueneme aquifer that 

generally occurs at a depth from 400 to 800 ft below 

land surface. 

Boundary conductances initially were based on 

aquifer transmissivity and were modified during model 

calibration. An initial uniform conductance of 

4,320 ft2/d was derived from the assumed values used 

in the extension of a model by Reichard (1995). The 

final distribution of conductances were 1,296 and 259 

ft2/d for the upper- and lower-aquifer systems, 

respectively (fig. 16 A,B).

Faults are simulated as barriers to ground-water 

flow and as such provide peripheral and internal 

boundaries to the ground-water flow system. The 

peripheral faults, however, were not simulated as faults 

because they are coincident with no-flow boundaries. 

The offshore Pitas Point and onshore Ventura, Foothill, 

Santa Paula, and San Cayento (thrust) Faults form the 

northern boundary of the ground-water flow system 

along the northern side of the Santa Clara River Valley 

subareas (fig. 16). The Oak Ridge Fault and South 

Mountain form the southern boundary of the 

ground-water flow system for the Mound (coastal) 

subarea and the inland subareas of the Santa Clara 

River Valley, respectively. 

Internal faults are represented as a horizontalflow barrier (Hsieh and Freckleton, 1993), across 

which the flow of water is proportional to a fault 

hydraulic characteristic determined during model 

calibration. The hydraulic characteristic is defined as 

the transmissivity of the fault divided by the fault width 

for confined aquifers. All faults in the lower-aquifer 

system and a subset of these faults in the upper-aquifer 

system were simulated as flow barriers (fig. 16). The 

most notable boundary occurs at the intersection of the 

Oak Ridge and Country Club (left-lateral reverse) 

Faults (fig. 16A) where the springs at Saticoy seeped 

ground water to the surface under predevelopment 

conditions. Ground-water level differences as great as 

100 ft are reported across the Country Club Fault 

(Turner, 1975); data collected in the spring of 1992 

suggest water-level differences of about 10 to 40 ft 

across this fault (Law/Crandall Inc., 1993). 

Other faults at the subbasin boundaries acting as 

potential barriers to ground-water flow in the loweraquifer system include a previously unmapped fault 

(hereinafter referred to the “Central Las Posas Fault”), 

which separates the lower-aquifer system between the 

West and East Las Posas Valley subbasins, and the 

extension of the Springville Fault, which separates the 

South Las Posas Valley and North Pleasant Valley 

subbasins (fig. 16B). The Camulos Fault, which forms 

the northeastern boundary of the Piru subbasin, also 

was included as a potential barrier to ground-water 

flow in the lower-aquifer system because of the 

extension of the ground-water model to the flanks of 

the mountain front. The Ventura Fault, which is aligned 

with the Pitas Point Fault (fig. 16) near the 

northwestern boundary in the Mound subbasin, also 

was included as a potential interior boundary to 

ground-water flow in the lower-aquifer system 

(fig. 16B).
Simulation of Ground-Water Flow 83

Offshore faults of Pliocene to Miocene (?) age, 

mapped by Green and others (1978) and Kennedy and 

others (1987), were included as barriers to 

ground-water flow in the lower-aquifer system 

(fig. 16B). Some of these offshore faults (figs. 7, 9, and 

16) are curvilinear and generally are subparallel to the 

submarine shelf; their northwest trend is typical of 

structures of the southern Coast Ranges Province. 

Other offshore faults trend west to southwest and are 

subparallel to the axes of the anticlines, synclines, and 

submarine canyons (figs. 7, 9, and 16) typical of 

structures of the Transverse Ranges Province. The 

northwest-trending faults included in the lower-aquifer 

system are an extension of the Sycamore Fault and 

minor fault traces, hereinafter referred to as “Hueneme 

slope 1,” “Mugu slope 1,” “Oxnard slope 1,” and 

“Oxnard slope 2” (fig. 16B). Offshore faults subparallel 

to the fold structures include extensions of the 

McGrath-Jamaica, Bailey, and El Rio Faults, and 

smaller faults coincident with the submarine canyons, 

hereinafter referred to as the “Hueneme Canyon,” “Old 

Hueneme Canyon,” and “South Hueneme Canyon” 

(fig. 16B).

Estimates of the hydraulic characteristics of 

faults were not available from aquifer tests or other 

field data. An initial uniform hydraulic characteristic of 

0.09 ft/d was used to simulate faults as horizontal-flow 

barriers in the lower-aquifer system. The final 

distribution was derived by fitting simulated 

water-level changes near faults and water-level 

differences across faults to measured data; the 

distribution ranges from 43,200 to 8.6 × 10 –5 ft/d 

(figs. 16A and B). On the basis of subsurface 

stratigraphy, mapping, and trenching (California 

Department of Water Resources, 1954; California State 

Water Resources Board, 1956; Weber and others, 1976; 

Jakes, 1979; Dahlen and others, 1990; Association of 

Engineering Geologists, 1991; Dahlen, 1992), selected 

faults were simulated to extend into the upper-aquifer 

system of the model for this study (fig. 16B). These 

faults include Oil Wells, Country Club, Camarillo, Fox 

Canyon, Springville Extension, Oak Ridge, San Pedro, 

and Bailey Faults.

Streamflow Routing and Ground-Water/Surface-Water 

Interactions

Streamflow was simulated using the streamflowrouting package developed by Prudic (1989). As the 

numerical model routes the streamflow from the inflow 

locations through the stream network to the outflow 

locations, the model simulates streamflow infiltration 

to the ground-water flow system, ground-water 

discharge to the streams, streamflow diversions, and 

discharge of streamflow to the ocean. To simulate 

streamflow routing, each cell containing a reach of 

stream channel is assigned a segment number and a 

reach number within the segment. The network of 

streams and diversions contains 233 model cells 

(reaches) that are grouped into 30 segments (fig. 18A). 

The segments are groups of model cells that are 

coincident with the stream channels and represent the 

major parts of the river systems, which are divided at 

the points of confluence (fig. 18B). Streamflow 

entering the headwater segment of each stream and 

major tributary (fig. 18B) is specified for every season 

for the entire historical simulation period. The Santa 

Clara River and Calleguas Creek stream segments were 

linked at the confluence with their major tributaries and 

are shown in figure 18B. The altitude of the stage of the 

stream and streambed conductance for every reach of 

each segment and the altitudes of the top and base of 

the streambed are specified for each model cell.

For this study, streamflow infiltration was 

calculated using measured and estimated streamflow 

and the streamflow-routing program component of the 

ground-water flow model. Streamflow routing required 

construction of streamflow records for the major rivers 

and tributaries in the basin for January 1891 to the 

period of the continuous gaged streamflow record. 

Streamflow was estimated using regression equations 

with seasonal precipitation for wet and dry climatic 

periods (described later in Appendix 4, tables A4.1–

A4.4). Precipitation data from three coastal, one 

intermontane, and two mountain precipitation stations 

were normalized and then used to produce “wet-day” 

nonlinear regression estimates of seasonal streamflow 

(Duell, 1992). Because precipitation data were 

available for coastal stations only for 1891–1905, an 

additional set of nonlinear relations was estimated for 

streamflow reconstruction for this early period of 

water-resources development. Correlations between 

precipitation and streamflow were better for the wettest 

periods (wet winters) than for the driest periods (dry 

summers). Most of the natural streamflow occurs 

during wet winters. Between 51 and 84 percent of the 

variance in natural streamflow during wet winters was 

estimated using the nonlinear relations between 

precipitation and gaged streamflow data.
84 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Pacific

Ocean

R18W R22W

T

4

N

T

2

N

2

2

7 7 7 7 7 7 7 7 7

5 5 5 5 5 5

9 9 9 9 9 9

12

12 12 12

12

19

21 21

21 21 21 21 21 21

21 21 21 23

25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25

25 25

25 25

25

25

25 25

27 27 27

2928

4

8

8

8

8

6

6 6

6

1 1 1 1

10

10

10

10 10

10 10

14

11 11 11 11 11 11

11 11 11 11 11

15

15

15

15 15 15

15

15 15

15 15 15

20

20

20

20

20

20

20 19

24 24 24 24

24 24 24 24

22 22 22 22 22 22

26

26 26 26 26 26 26

26 26 26

26

28

28 28 28 28 28 28

30

30

30 30 30

30 30 30 30 30

30 30 30

30 30

30

30

30

30

30

30

30

30

30

30

30

30

30

30 30

29

29 29 29

29

29 29

29

29

29

29 29

29

29

Conejo

Cr

Santa Paula

Ellsworth

Cr

Cr

Sespe

Hopper

Pole

Pi ur

Cr

r C

r C

Cr

River

Santa

Clara

Lake

Piru

Calleguas

Revolon

Arundell

Barranca

Slough

Beardsley

Wash

Arroyo

Honda

Creek

13

3

3

17

18

Sulphur Mountain

Oak Ridge

South Mountain

Camarillo Hills

Las Posas Hills

Simi Val

ey l

Oxnard

Plain

Big Mtn

Santa

Paula

Thousand Oaks

Oxnard

El Rio Camarillo

Fillmore

Saticoy

El Rio

Spreading

Grounds

Saticoy

Spreading

Grounds

Freeman Diversion

Ventura

EXPLANATION

Hydrologic Unit boundary

Selected streamflow gaging station –

See figure 4 for complete gaging stationnumber. Number indicated is referencedto diagram (figure 18

B)

Model cell reaches within a stream segment –

Number indicates stream segment. Twocolors used to distinguish adjacent segments.(see table (figure 18

B) for model cell group

identification)

Streamflow-diversion cell – Number indicates

diversion stream segment number

Model-grid boundary of flow region andsubareas (see figure 18

B for explanation)

0

0

10 MILES

10 KILOMETERS

118°45' 119°15'

34°22'30"34°07'30"

119°

A

8

7

6

5

4

3

2

1

11 9

9

10

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium and unconsolidated depositsShallow alluvium

Consolidated and unconsolidated deposits

Arroyo Simi

Arroyo Las Posas

Figure 18 Streamflow network as simulated in the numerical model of the Santa Clara–Calleguas ground-water basin, Ventura County, California. 

A, Stream reaches and related gaging 

stations used for inflow and for calibrations of simulated streamflow. 

B, Schematic routing diagram of simulated streamflow network and streambed conductance for stream segments.
Simulation of Ground-Water Flow 85

1. Santa Clara above

Piru

2. Piru Creek above

Piru Diversion

3. Piru Creek Diversion4. Piru Creek above

Santa Clara River

5. Santa Clara River

above Hopper Creek

6. Hopper Creek7. Santa Clara River

above Pole Creek

8. Pole Creek9. Santa Clara River

above Sespe Creek

10. Sespe Creek

11. Santa Clara River above

Santa Paula Creek

.153 13,200

.115/0 9,900/0

0.115 9,900

12. Santa Paula Creek

13. Santa Paula Creek Diversion14. Santa Paula Creek above

Santa Clara River

15. Santa Clara River above

Freeman Diversion(1-4) / (5-13)Santa Clara RiverFreeman Diversion 16.Santa Clara River aboveSaticoy Diversion

17.

Santa Clara RiverSaticoy Diversion 18.Santa Clara River aboveEllsworth Barranca

19.

Ellsworth Barranca(1-4) / (5-8)

20.

Santa Clara River aboveMontalvo (1-9)/(10-11)

21.

Santa Clara River aboveVentura wastewatertreatment plant

22.

Santa Clara River toPacific Ocean

23.

Arundell Barranca toPacific Ocean (1-4)/(5-8)

24.

Arroyo Simi/Arroyo LasPosas/Calleguas Creek aboveConejo Creek (1-18)/(19-30)

25.

Upper Conejo Creek(1-4)/(5-12)

26.

Lower Conejo Creekabove Calleguas Creek

27.28.

29.30.

Calleguas Creek aboveCamarillo (1-4)/(5-8)Calleguas Creek toPacific OceanHonda-Beardsley to RevolonSlough-Pacific Ocean(1-9)/(10-12)/(13-36)

No. Segment descriptions Streambed conductance ft

2/d (rounded) ft

2/s

0 00 0

.153/0 13,200/0

.100/.06 8,600/5,200.138/.06 11,900/5,200

.06 5,200

.06/0 5,200/0

0 0

.01/.003/0 860/260/0

.115 9,900

0 0

.138 11,900.138 11,900.138 11,900.138 11,900.138 11,900.138 11,900.153 13,200

.138 11,900

.138 11,900

0 0

.138/.04 11,900/3,500

0 0

.138 11,900

0 0

.138 11,900

.138/.115 11,900/9,900

No. Segment descriptions Streambed conductance ft

2/d (rounded) ft

2/s No. Segment descriptions Streambed conductance ft

2/d (rounded) ft

2/s

Hopper Cr

Piru Cr ek e

Pole Creek

Sespe Cr

Santa Paula Cr

Ell wo s rth

ar B ranca

Lower

Conejo Cr

Upper

Conejo

Cr

Arroyo

Simi

Arroyo

Las Posas

SANTA

CLARA

RIVER

Calleguas Calleguas Creek

Beardsley

Wash

Revolon

Slou

Arundell Barranca

gh

6

7 9 11 15

16

19

18

20

21 22

23

3029

28

27

26

25

17

15

8

10

12

14 13

2

4 3

5

1

Pacific

Ocean

Ventura

Wastewater

Treatment Plant

Camarillo

Wastewater

Treatment Plant

Specified-streaminflow

Streamflow gaging station usedto obtain stream inflow(see figure 17A)

Stream segment junction

Group of stream segments withsame streambed conductancechangeNo streambed conductancechange (Coastal Plain Group)

Outflow fromstreamflow network

Artificial-rechargediversion (type 3)

Streamflow segmentnumber (See table below)

EXPLANATION (for schematic map)

Diversion to Piru

Spreading

Grounds

1/2 mile

past

Montalvo

Arroyo

Hondo

24

Diversion to Santa Paula

Spreading Grounds

Saticoy

Diversion

Freeman

Diversion

Sespe Creek stream inflow ≥ 200 ft3/s

streambed conductance increased (x2.75)

Sespe Creek stream inflow < 200 ft3/s

streambed conductance decreased (x2.75)

stream inflow ≥ 0.23 ft3/s

streambed conductance increased (x1.25)

stream inflow < 0.23 ft3/s

streambed conductance decreased (x1.25)

Arroyo Hondo Group

stream inflow ≥ 13 ft3/s

streambed conductance increased (x1.25)

stream inflow < 13 ft3/s

streambed conductance decreased (x1.25)

Unregulated Tributary Group

Arroyo Simi Group

Table explanation – Number in parentheses is model cellgroups (numbered on map A in downstream order) withrespective streambed conductance, in ft2/s and ft2/d for

time-averaged predevelopment flow (steady-state)

stream inflow ≥ 50 ft3/s

streambed conductance increased (x2.75)

stream inflow < 50 ft3/s

streambed conductance decreased (x2.75)

Upper Santa Clara River Group

B

9 1

8

11

7

10

6 5 43 2

1

Piru Creek stream inflow ≥ 200 ft3/s

streambed conductance increased (x2.75)

Piru Creek stream inflow < 200 ft3/s

streambed conductance decreased (x2.75)

Release-Diversion Group

Figure 18—Continued.
86 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

The streamflow network represents gaged inflow 

along the Santa Clara River and tributaries, the 

Calleguas Creek and tributaries, Arroyo Hondo, and 

Arundell Barranca. Measured and estimated seasonal 

streamflow was used to simulate streamflow from 

11 inflow points on the Santa Clara River, Piru Creek, 

Hopper Creek, Pole Creek, Sespe Creek, Santa Paula 

Creek, Ellsworth Barranca, Arrundell Barranca, 

Arroyo Hondo, Arroyo Simi, and Upper Conejo Creek 

(fig. 18B). Seasonal inflow rates were specified as the 

total seasonal flow volume divided by the number of 

days in the season for the period of record of each 

inflow site. For the period prior to historical records, 

nonlinear regressions of flow as a function of 

precipitation were used to estimate wet- and dry-period 

seasonal flows for the Santa Clara River, Piru, Hopper, 

Pole, Sespe, and Santa Paula Creeks and Arroyo Simi 

(Appendix 4, table A4.1–A4.4). Streamflow estimates 

for Ellsworth and Arrundell Barrancas, Arroyo Hondo, 

and Conejo Creek were based on seasonal ratios of 

gaged runoff to precipitation (modified rational 

method) for Pole and Hopper Creeks. The modified 

rational method was used for the period prior to the 

period for which streamflow-gaging data are available 

because there was no period of unregulated gaged 

streamflow that could be used to establish regression 

relations between streamflow and precipitation. 

Streamflow between the segments is the simulated 

streamflow routed from all upstream segments 

connected to a given segment. The simulation of 

predevelopment conditions used time-averaged 

streamflow estimates based on the geometric means 

and median streamflow values for the gaged 

streamflow (table 2) and the geometric-mean values of 

long-term runoff for ungaged tributaries.

The diversions at Piru, Santa Paula, and Saticoy 

and at the Freeman Diversion, which provide surface 

water for irrigation and artificial recharge, were 

simulated as losses from the stream network 

(fig. 18 A,B). The streamflow-routing package of this 

model was altered to offer additional types of diversion 

(Appendix 2). The modified diversion type used for all 

four simulated diversions is referred to as an “artificial 

recharge diversion” [type 3 (Appendix 2)]; it will 

accept all streamflow available up to the specified 

amount of diversion. The seasonal amounts of 

diversion were based on the UWCD’s reported total 

monthly diversions (Greg Middleton, United Water 

Conservation District, written commun., 1993).

Streamflow stages for all the reaches were 

estimated from relations between stream stage and 

streamflow at the inflow-gaging stations. The stream 

stage was held constant for all reaches in all segments 

for all simulations. Stream stage was initially estimated 

using extrapolated gaged height at the estimated 

predevelopment flow, which ranged from 0.3 to 4.5 ft 

for the steady-state flow rate at the inflow-gaging 

stations. However, stream stages were simplified and 

finally held to a constant value of 2.5 ft above the top of 

the streambed for all simulation periods and for all 

river reaches. The altitude of the top of the streambed 

was estimated from the arithmetic average of landsurface altitudes for the entire extent of the stream 

channel in each reach, which was estimated from 

digital altitude model data, 1:24,000-scale topographic 

maps, and gaging-station altitudes. The altitude of the 

base of the streambed was assumed to be 10 ft below 

the altitude of the top of the streambed for all the 

reaches for all time periods.

As water flows down the channels of the Santa 

Clara River and Calleguas Creek and their tributaries, 

some of the water infiltrates through the streambed and 

becomes ground-water recharge. In a few places, 

however, shallow ground water discharges to streams. 

In the model, this vertical flow between the stream and 

the aquifer is controlled by the streambed conductance 

and a vertical gradient that is driven by the difference 

between the specified stream stage and the simulated 

ground-water level. Stream stage for each stream reach 

was specified and was not changed for the entire 

simulation time. Streambed conductance initially was 

estimated as the product of the assumed channel width, 

channel length, and vertical hydraulic conductivity of 

the streambed deposits divided by the streambed 

thickness. Streambed conductance also can be 

estimated as the product of the streamflow and the 

fraction of streamflow loss divided by the streambed 

thickness. Although the actual stream channel width 

and streambed thickness vary spatially and with flow 

within many of the model cells, the streambed 

conductances were simplified into groups of segments 

with the same streambed conductance values 

(fig. 18B).
Simulation of Ground-Water Flow 87

Initial estimates of streambed conductance were 

based on streamflow-loss estimates made in the early 

1930s (California Department of Public Works, 1934) 

and in 1991 (Densmore and others, 1992); however, 

these direct estimates of streamflow losses vary 

widely—from 1 to 100 percent. Various mass-balance 

estimates for the Santa Clara River (Taylor and others, 

1977; Dal Pozzo, 1992; Law/Crandall Inc., 1993) also 

have been made; these estimates also vary widely, 

ranging from 0 to 100 percent, with an average loss of 

about 22 percent. A water-balance approach yielded an 

estimate of streambed hydraulic conductivity of about 

2 ft/d for the Santa Paula subarea (Law/Crandall Inc., 

1993). The simulation of streamflow in the Santa Rosa 

Valley subarea model used vertical hydraulic 

conductivities of 3 ft/d for Arroyo Conejo and Conejo 

Creek and 1 ft/d for Arroyo Simi and an assumed 

streambed thickness of 1 ft (Johnson and Yoon, 1987). 

The assumed width is 50 ft, and the assumed streambed 

length was assumed to be the length of the cell 

(2,640 ft). Using values from Johnson and Yoon 

(1987), estimated streambed conductance is 

13,200 ft2/d for Arroyo Simi and 39,600 ft2/d for 

Conejo Creek.

For the regional-scale model, the stream channel 

width initially was assumed to range from 50 to 200 ft, 

the length of the reach was assumed to be the length of 

the cell (2,640 ft), and the streambed thickness was 

assumed to be 10 ft. The streambed conductances were 

then put into six groups: the coastal plain group for 

which segments and reaches were set to a streambed 

conductance of zero, the upper Santa Clara River 

group, the release-diversion group, the unregulated 

tributary group, the Arroyo Simi group, and the Arroyo 

Hondo group (fig. 18B). Streambed conductances for 

each group were increased and decreased from the 

predevelopment values and were changed on the basis 

of threshold values of stream inflows (fig. 18B). 

Results of model calibration indicate that the three 

groups of streambed conductances for the Santa Clara 

River system were increased when streamflows were 

greater than the flow threshold and decreased when 

they were less than the flow threshold by a factor of 

2.75 with respect to conductances used to simulate 

time-averaged predevelopment conditions. The Arroyo 

Hondo and Arroyo Simi groups were increased when 

streamflows were greater than the flow threshold and 

decreased when streamflows were less than the flow 

threshold by a factor of 1.25 with respect to 

conductances used to simulate time-averaged 

predevelopment conditions. This change in 

conductance is believed to reflect the change in channel 

width and is similar to the factors of 1.2 to 2.0 used for 

the simulation of the streamflow routing of the Little 

Humboldt River, Nevada (Prudic and Herman, 1996). 

The final distribution of streambed conductances 

ranges from 0 to 13,200 ft2/d (fig. 18B) for timeaveraged predevelopment conditions. These final 

values are the product of model calibration for timeaveraged predevelopment (steady-state) conditions and 

of comparisons of the streamflow hydrographs for 

historical downstream streamflow and diversions.
88 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Mountain-Front Recharge

Natural recharge along the model boundaries, 

mountain-front recharge, was simulated as a constantflux inflow for each season (fig. 17A,B). Mountainfront recharge was simulated as a seasonally varying 

estimate of runoff specified as infiltration at the 

mountain front for 64 ungaged surface-water 

subdrainage basins (California Department of Water 

Resources, 1975, plate 2) that surround and drain into 

the 12 ground-water subbasins of the Santa Clara–

Calleguas Basin (figs. 1 and 17A). The average for total 

wet- and dry-seasonal precipitation was estimated for 

each ungaged subdrainage basin. The modified rational 

method was used to estimate the seasonal runoff for 

each of the 412 seasons in the simulated historical 

period January 1891–December 1993. The ratio of 

runoff from Pole or Hopper Creeks to the total seasonal 

precipitation for these two index subdrainage basins 

ranged from 0 to 7, but most of the ratios were less than 

0.25. These ratios were comparable to the fraction of 

precipitation as ground-water recharge estimated from 

detailed water-balance studies completed by Blaney 

(California Department of Public Works, 1934) for 

water years 1928–32. Blaney estimated annual 

fractions of rainfall penetration ranging from 0.01 to 

0.17 for dry years and from 0.06 to 0.34 for wet years. 

Using the modified rational method, estimated ratios 

greater than 1 would result in a runoff total that is 

greater than the average precipitation. On the basis of 

previous infiltration studies in the Santa Clara–

Calleguas Basin (California Department of Public 

Works, 1934; Taylor and others, 1977; Densmore and 

others, 1992), most fractions of runoff that infiltrate are 

less than 0.9. The ratios selected for estimating 

recharge were from Pole Creek for winter and fall 

seasons and from Hopper Creek for spring and summer 

seasons. When any ratio exceeded 0.9, the ratio from 

the other index subarea was used. When both ratios 

exceeded 0.9, the ratios were replaced with the 

geometric mean of ratios less than or equal to 0.9 for 

that respective wet or dry climatic season. The 

estimated mountain-front recharge for each 

subdrainage basin was then equally distributed to one 

or more cells that are coincident with the stream 

channels at the model boundary (fig. 17A). The 

resulting recharge estimates for an individual cell was 

reduced to 3.4 ft3/s if the estimated recharge value 

exceeded that amount. This value was determined from 

streamflow seepage measurements of low flows on 

Santa Paula Creek (Dal Pozo, 1992). 

The estimated total time-averaged mountainfront recharge rate used for the steady-state simulation 

of predevelopment conditions was 12,500 acre-ft/yr. 

The constant rate of recharge for the steady-state 

simulation, which was based on the geometric-mean 

ratios, was used to estimate the time-averaged runoff 

from each mountain-front subdrainage basin. The 

estimated total time-varying mountain-front recharge 

rate used for transient-state simulation of historical 

conditions ranged from 6,000 acre-ft/yr in 1923 to 

80,600 acre-ft/yr in 1993. Mountain-front recharge was 

simulated as injection wells, with a constant rate of 

recharge per season, for 119 model cells in the 

uppermost active layer that coincide with the stream 

channels in the ungaged-tributary drainage basins 

(fig. 17A).

Additional recharge as direct infiltration on the 

outcrops of the San Pedro Formation (fig. 7A) was 

estimated based on wet-period average winter 

precipitation for 54 model cells that coincide with the 

San Pedro Formation in the Fillmore, Santa Paula, and 

Las Posas Valley subareas (figs. 7A and 17A). The 

recharge rate representing deep infiltration over the 

outcrops was estimated using the modified equation 

developed by the Santa Barbara County Water Agency 

(1977): 

Recharge = (Pwet - 17 inches)/1.55,

where

Recharge is average recharge rate, in inches per year, 

and Pwet is wet-period total annual precipitation of 

20.75 in. for outcrops surrounding the Las Posas Valley 

subareas and 21.25 in. for outcrops on the north side of 

the Santa Clara River Valley subareas.

This method assumes uniform temporal and 

areal distributions of rainfall without regard to the 

intensity of individual storms. The resulting recharge 

rate is reduced by the fraction of wet years (32 years) in 

the total period of historical simulation (103 years). 

The resulting estimates for a constant average recharge 

were 470 acre-ft/yr for East Las Posas Valley subarea, 

740 acre-ft/yr for South Las Posas Valley subarea, 

400 acre-ft/yr for West Las Posas Valley subarea, 

240 acre-ft/yr for Fillmore subarea, and 320 acre-ft/yr 

for Santa Paula subarea. Thus, the long-term average 

recharge to the lower-aquifer system for a total bedrock 

recharge was about 2,200 acre-ft/yr (table 4).
Simulation of Ground-Water Flow 89

Valley-Floor Recharge

Direct infiltration of precipitation on the valley 

floors, hereinafter referred to as “valley-floor 

recharge,” was simulated using the model recharge 

package and was distributed equally to all cells in each 

valley floor of the Piru, Fillmore, Santa Paula, Las 

Posas Valley (East, West, and South), Pleasant Valley 

(North and South), Oxnard Plain Forebay, and Santa 

Rosa Valley, and the Northeast Oxnard Plain subareas 

(fig. 17B). The estimated total time-averaged recharge 

rate used for the steady-state simulation of 

predevelopment conditions was 4,800 acre-ft/yr, which 

is based on the geometric-mean ratios of runoff to 

precipitation at Pole and Hopper Creeks. The total 

time-varying valley-floor recharge used for the 

transient-state simulation of historical conditions was 

varied seasonally using the same percentages of 

infiltration of irrigation based on model calibration 

(fig. 17B). The recharge rates ranged from 18,300 

acre-ft/yr for dry-year periods to 32,700 acre-ft/yr for 

wet-year periods (table 4).

Artificial Recharge

Recharge of infiltration of diverted streamflow, 

discharge of treated sewage effluent, and irrigation 

return flow were simulated as a constant-flux inflow 

using the MODFLOW well package. No artificial 

recharge was applied to predevelopment (steady-state) 

conditions. For developed (transient-state) conditions, 

infiltration of diverted streamflow was applied for the 

period 1928–93, infiltration of irrigation was applied 

for the period 1891–1993, and infiltration of treated 

sewage effluent was applied for the period 1936–93.

Recharge of diverted streamflow was simulated 

at the artificial-recharge spreading grounds (basins) 

operated by the UWCD in the Piru and Santa Paula 

subareas and in the Oxnard Plain Forebay subarea 

(figs. 4 and 18A). The quantity of artificial recharge 

simulated in the model (fig. 11A) was based on 

reported annual and seasonal amounts of recharge 

(United Water Conservation District, 1986, plate 5a,b; 

Greg Middleton, United Water Conservation District, 

written commun., 1993).

Recharge of treated sewage effluent was 

simulated as constant-flux inflows using the 

MODFLOW well package. This recharge was based on 

reported and extrapolated annual amounts of treated 

sewage discharge (California Department of Water 

Resources, 1975; W.D. Jesena, California Regional 

Water Quality Control Board, written commun., 1991; 

E.G. Reichard, U.S. Geological Survey, written 

commun., 1993; Mitri Muna, Ventura County 

Waterworks, written commun., 1995) and was assigned 

to nine model cells (fig. 17A) at a rate reduced to 

74 percent (Farnsworth and others, 1982) of the 

reported or interpolated annual rate of discharge to 

account for the free-water surface evaporation while in 

percolation ponds and streambeds. The treated sewage 

effluent represents discharge from the city of Fillmore 

during 1958–93, the city of Santa Paula during 

1937–93, the Limoneira Association at Olive Lawn 

Farm and Limoneira Farm during 1975–93, the Saticoy 

Sanitation District during 1960–93, the Camarillo 

Sanitation District during 1959–93, the city of 

Thousand Oaks during 1962–72, the Camarillo State 

Hospital during 1960–80, the Camarosa wastewatertreatment plant during 1981–93, and the MoorparkVentura County wastewater-treatment plant No. 19 

during 1973–93. Additional sewage effluent discharged 

from the city of Thousand Oaks Hill Canyon Plant is 

represented as streamflow during 1973–93. Treated 

sewage effluent from the percolation ponds near the 

Santa Clara River which was used by the city of Piru 

during 1975–93 was not included because of the small 

volumes of discharge (Charles Rogers, city of Piru, oral 

commun., 1995). Total treated-sewage effluent that 

becomes ground-water recharge was applied at a 

constant rate for all four seasons of every year; the rate 

increased from 20 acre-ft/yr in 1936 to 9,000 acre-ft/yr 

in 1993 (fig. 11A).
90 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Irrigation return flow was estimated as a 

percentage of the total applied water and included 

ground-water and surface-water components for many 

of the subareas. This recharge was simulated as a 

constant-flux inflow using the MODFLOW well 

package for the uppermost layer of the model. 

Irrigation return flow was estimated for each of the 

land-use periods and held constant for the same periods 

used to estimate ground-water pumpage (fig. 11A,B). 

The irrigation return flow was applied over a 245-day 

growing period prior to 1927 and applied uniformly for 

the entire year for the remainder of the simulation 

period. It was applied over the entire year because 

infiltration through the unsaturated zone tends to 

extend the period of infiltration. The 1969 land-use 

map was used to estimate the distribution of irrigation 

return flow for the period of reported pumpage, 

1973–93. The assumed infiltration ranged from 5 to 

30 percent of applied irrigation water for all subareas 

and was varied for wet- and dry-year periods (fig. 17B). 

The percentage of irrigation return flow was estimated 

during model calibration. Irrigation return flow ranged 

from less than a few hundred acre-feet per season for 

the Mound and North Peasant Valley subareas to about 

1,400 acre-ft per season for the Santa Paula subarea 

(fig. 11). Total irrigation return flow ranged from 

14,600 acre-ft/yr for the 1890s to 51,500 acre-ft/yr for 

the drought period 1987–91.

Other Sources of Recharge

Other sources of recharge include flow of water 

along some fault zones from older (Miocene age) 

marine sedimentary rocks and brines related to oil 

deposits. Some of these potential sources of water may 

yield water of poor quality or water of different 

chemical composition. Water-chemistry data indicate 

that the amount of leakage from the deeper, older 

formations in the South Oxnard Plain subarea and the 

South Pleasant Valley subarea probably is small 

(Izbicki, 1991, 1996a); therefore, it was not included in 

the current regional simulations.

Another source of potential recharge is leakage 

of the semiperched water to the upper-aquifer system. 

Leakage of semiperched ground water may enter the 

upper- and lower-aquifer systems through failed and 

abandoned wells. Because the initial water-chemistry 

data indicate a potentially small effect and because 

water-level hydrographs indicate a potentially 

complicated relation, this element was not included in 

the current regional simulation. Any potential leakage 

through intraborehole flow or failed wells was included 

collectively and simulated in the irrigation-return-flow 

component.

Natural Discharge

Natural discharge is simulated as seaward 

coastal flow through submarine outcrops and as 

evapotranspiration (ET) along the flood plains of the 

Santa Clara River and Calleguas Creek. The coastal 

flow of water to the ocean was determined through 

model simulation and calibration; it is described in the 

“Model Boundaries” section. 

ET by riparian vegetation (phreatophytes) and 

evaporation from bare soil were simulated at 

306 model cells of layer 1 (upper-aquifer system) 

(fig. 16A) using the MODFLOW evapotranspiration 

package. Using previous estimates (California 

Department of Public Works, 1934), a maximum ET 

rate of 2.4 ft/yr was assumed when the water table is at 

land surface, and ET was assumed to decrease linearly 

to zero when the water table reaches a depth of 10 ft or 

more below land surface. The ET rate was multiplied 

by the ratio of riparian vegetation area to total modelcell area to account for the riparian vegetation density 

in each model cell. The weighting factor is the number 

of acres of riparian vegetation, estimated from the 

1912, 1927, 1932, and 1950 land-use maps, for each 

cell divided by the total number of acres (160) in a 

model cell. The composite ET rates and the model cells 

with the potential for ET in 1912, 1927, 1932, and 

1950 (Conejo Creek area) were used for the 

predevelopment and historical simulation for 1891–

1926. The ET surface remained the same for the 

remainder of the simulation periods, but the ET rates 

were updated to reflect changing ET acreage. Thus, 

acreage for riparian vegetation was updated using the 

1932 acreage for 1927–46 and the 1950 acreage for the 

remainder of the simulation period.
Simulation of Ground-Water Flow 91

Pumpage

The simulation of ground-water withdrawal from 

wells as pumpage required a compilation of historical 

estimates that include indirect estimates of agricultural 

pumpage based on land use (1891–1977), reported 

municipal pumpage (1914–77), and metered 

agricultural and municipal pumpage (1978–93) 

reported to and compiled by the UWCD and the 

FGMA. Estimated pumpage ranged from 34,800 acreft for the drought years of the 1920s to a maximum 

pumpage of 301,400 acre-ft for the 1990 drought year. 

Estimated pumpage is shown in figure 11B for the 

period of simulation. The annual and biannual 

pumpage estimates were temporally distributed for 

model input to the seasonal intervals on a well-by-well 

basis. The initial vertical distribution of pumpage 

between aquifer systems was based on well 

construction (Predmore and others, 1997) and wellbore 

flowmeter studies completed as part of the RASA 

studies (table 5). For wells completed only in the 

upper-aquifer system, all water was derived from the 

upper model layer, and for wells completed only in the 

lower-aquifer system, all water was derived from the 

lower model layer. For wells that were completed in 

both the upper- and lower-aquifer systems, a 

percentage of total well pumpage was assigned to the 

upper and lower layers on the basis of wellbore 

flowmeter data, slug tests, and model calibration (fig. 

17B). Pumpage from wells with no construction data 

was distributed using these same assumed percentages 

of pumpage. The distribution of pumpage from the 

upper- and lower-aquifer systems, estimated from the 

land-use map for agricultural pumpage, also used these 

same percentages for all the subareas. 

Indirect estimates of agricultural pumpage were 

compiled for five land-use periods that span from 1912 

to 1977 (Koczot, 1996). The compilation was based on 

land-use maps for 1912, 1932, 1950, and 1969 and on a 

mosaic of areal photos from 1927 (Predmore and 

others, 1997). The distribution of estimated agricultural 

pumpage was based on well locations reported in 1987 

and on percentages of pumpage within each subarea. 

The estimates of agricultural pumpage were distributed 

over time on the basis of major changes in crop types 

and climatic periods. Because the growing periods of 

the various crop types spanned an 8-month period, 

pumpage was estimated and distributed using a 

245-day growing season (Koczot, 1996) spanning 

March through October for the period 1912–26. The 

growing season was extended to 275 days, spanning 

from March through November for the period 

1927–77. The extension of the growing period was 

based on inspection of water-level hydrographs and the 

wider variety of truck and orchard crops introduced 

during this period. The magnitude of pumpage was 

reduced during wet climatic periods and increased 

during dry climatic periods. The percentage change in 

agricultural pumpage was based on the ratios of 

wet-year to average-annual reported pumpage for each 

subarea and dry-year to average-annual reported 

pumpage (fig. 17B). The reported municipal pumpage 

for the cities of Ventura, Camarillo, and Oxnard and the 

Channel Island Community Services District; pumpage 

for the fish hatchery in the southern end of the Piru 

subarea; and pumpage of artificial recharge in the 

Oxnard Plain Forebay were estimated independently 

and combined with agricultural pumpage for input to 

the ground-water flow model for the period of 

simulation prior to 1983 (fig. 11B).
92 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Regional management of ground-water resources 

was implemented by the State of California in 1983 

with the creation of the Fox Canyon Groundwater 

Management Agency (FGMA) for controlling seawater 

intrusion. The FGMA jurisdiction covers part of the 

Santa Clara–Calleguas Basin and includes the Oxnard 

Plain, Oxnard Plain Forebay, Pleasant Valley, and Las 

Posas Valley subareas (figure 26 presented later in the 

section “Analysis of Ground-Water Flow”). Reported 

pumpage was compiled from the technical files of the 

FGMA and the UWCD for the period July 1979–

December 1993. These data generally consist of 

semiannual totals of user-reported agricultural, 

nonagricultural (municipal, industrial, and domestic), 

and total pumpage. Agricultural pumpage was 

distributed based on a 275-day growing period and the 

nonagricultural pumpage was distributed equally over 

seasonal periods of the flow model. Pumpage for 1980, 

which was based on water-level hydrographs and on 

climate data, was used for the period 1978 through 

1980. When only total pumpage was reported, that 

pumpage was assumed to be for agricultural use. Early 

pumpage data were incomplete for the Las Posas 

Valley, the eastern part of the Pleasant Valley, and the 

Santa Rosa Valley subareas. For these areas, 1984 

FGMA-reported pumpage was used to represent 

pumpage for 1978 through 1983. Total reported annual 

pumpage ranged from as little as 850 acre-ft in the 

South Las Posas Valley subarea during 1992 to as much 

as 107,300 acre-ft in the Oxnard Plain and Oxnard 

Plain Forebay subareas during 1990.

Hydraulic Properties

Estimates of transmissivities and storage 

coefficients for both model layers and estimates of 

coefficients of vertical leakance between layers are 

required to simulate the flow of ground water. 

Estimates of the horizontal conductance of faults are 

required to simulate potential barriers to ground-water 

flow, and the vertical conductance of streambeds is 

required to simulate the flow of water between shallow 

ground water and streamflow. The average values for 

these parameters are used in the model and represent 

the hydraulic properties which are the spatial averages 

over individual model cells. They generally are held 

constant through time. Except for fault hydraulic 

characteristics, vertical conductances of the streambed, 

subsidence parameters, and areas where model layers 

were extended, the initial estimates for all the model 

parameters were derived largely from the spatial 

estimates used in previous ground-water flow models 

of the basin (California Department of Water 

Resources, 1974a,b, 1975; Johnson and Yoon, 1987; 

CH2M HILL, 1993; Reichard, 1995).

Transmissivity

Transmissivity is the product of the hydraulic 

conductivity and saturated thickness of the aquifers; 

therefore, transmissivity values may be affected by 

changes in saturated thickness. Transmissivity 

throughout much of the modeled area is associated 

with the basal coarse-grained layers of the aquifers that 

remain saturated; many parts of the aquifers are 

confined or show water-level changes that are a 

relatively small percentage of the saturated thickness. 

Because the effective saturated thickness is relatively 

constant over most of the model area, this model uses 

constant transmissivities for the entire period of 

simulation. Transmissivities estimated from specificcapacity tests were used to simulate ground-water flow 

using the Theissan-Weber Polygon model (California 

Department of Water Resources, 1975). Estimates for 

the upper-aquifer system range from 650 ft2/d along 

the northern edge of the Santa Paula subarea to more 

than 53,000 ft2/d in the northern Oxnard Plain and 

67,000 ft2/d north of the Mugu submarine canyon 

(California Department of Water Resources, 1975, 

pl. 8). Estimates for the lower-aquifer system range 

from about 1,300 ft2/d near Moorpark to 53,000 ft2/d 

north of Port Hueneme (California Department of 

Water Resources, 1975, pl. 8). The coastal estimates 

from the Theissan-Weber Polygon model were 

extended as constant values to the adjacent offshore 

regions by Reichard (1995, fig. 10). 
Simulation of Ground-Water Flow 93

The current model modified these estimated 

transmissivities and used additional estimates beyond 

the areal extent of the previous models for the upperaquifer system (layer 1) in the Las Posas Valley, 

Pleasant Valley, and Santa Rosa Valley subareas and for 

the lower layer in the Santa Clara River Valley subareas 

(fig. 17B and 19A). The estimated transmissivities for 

the upper-aquifer system (layer 1) ranged from 1.3 ft2/d 

for the Las Posas Valley subarea to about 73,800 ft2/d 

for the Oxnard Plain Forebay (fig. 19A); the estimated 

transmissivities for the lower-aquifer system (layer 2) 

ranged from about 38 to 26,500 ft2/d. A constant 

transmissivity of about 4,700 ft2/d was assigned to the 

lower-aquifer system (layer 2) for the offshore part of 

the Mound subarea on the basis of the estimated 

thicknesses and the hydraulic conductivities used 

onshore (fig. 19A).

The final estimates of transmissivities in the 

calibrated model for both model layers were refined for 

each subarea using the sum of transmissivities for the 

aggregate thicknesses of the coarse-grained and finegrained deposits in each model cell (fig. 20). The 

transmissivity of the coarse-grained deposits was 

determined as the product of the thickness of the 

coarse-grained deposits (estimated from resistivity 

logs) and a geometric-mean hydraulic conductivity 

(estimated from slug tests). The transmissivity of the 

fine-grained deposits is the product of the thickness of 

fine-grained deposits and an assumed hydraulic 

conductivity of 0.1 ft/d.

 Some of the transmissivities from previous 

regional models for the upper-aquifer system were 

reestimated using estimates of a geometric-mean 

hydraulic conductivity from the slug tests and the 

aggregate thicknesses of the coarse- and fine-grained 

deposits (fig. 20A). Transmissivity estimates were 

made using a hydraulic conductivity of 35.1 ft/d for the 

coarse-grained deposits in the Piru and Santa Paula 

subareas; these values were based on slug-test values 

that range from 18 to 88 ft/d in monitoring wells 

completed in these subareas (E.G. Reichard, U.S. 

Geological Survey, written commun., 1995). 

Transmissivities for the upper-aquifer systems 

(layer 1) of the Las Posas Valley, Santa Rosa Valley, 

and Pleasant Valley subareas were needed to extend the 

upper model layer of the previous models for all the 

subareas (figs. 17B and 19A). The transmissivities of 

the coarse-grained deposits of the East Las Posas 

Valley subarea were estimated using a geometric-mean 

hydraulic conductivity of 0.3 ft/d, which was based on 

slug-test values that range from 0.21 to 0.47 ft/d in 

monitoring wells completed in this subarea. 

Transmissivities of the coarse-grained deposits of the 

West Las Posas Valley subarea were estimated using a 

geometric-mean hydraulic conductivity of 0.19 ft/d, 

which was based on slug-test values that range from 

0.14 to 0.27 ft/d in monitoring wells completed near 

Arroyo Hondo. Transmissivities of the coarse-grained 

deposits of the South Las Posas Valley subarea were 

estimated using a geometric-mean hydraulic 

conductivity of 1.58 ft/d, which was based on slug-test 

values that range from 0.48 to 3.49 ft/d in monitoring 

wells completed in the subarea. The transmissivities of 

the coarse-grained deposits of the Santa Rosa Valley 

subarea (fig. 20) were based on two sets of hydraulic 

conductivities: A reported value of 80 ft/d for the 

Saugus Formation (Johnson and Yoon, 1987) was used 

to represent the upper and lower aquifers on the west 

side of the San Pedro Fault; reported values of 150 and 

120 ft/d for the alluvium and the Santa Margarita 

Formation, respectively, were used for the east side of 

the San Pedro Fault (Johnson and Yoon, 1987). 

Transmissivity for the Pleasant Valley subarea was 

estimated using a geometric-mean hydraulic 

conductivity of 8.8 ft/d for the coarse-grained deposits, 

which is based on slug-test values that range from 0.13 

to 11.8 ft/d in monitoring wells in this subarea. 

Estimates of hydraulic conductivities for the 

lower-aquifer system (layer 2) deposits range from 1 to 

8 ft/d for monitoring wells completed in the northern 

part of the Oxnard Plain subarea and from 5.5 to 44 ft/d 

for monitoring wells completed in the Piru and Santa 

Paula subbasins (E.G. Reichard, U.S. Geological 

Survey, written commun., 1992). Transmissivities for 

the coarse-grained deposits within layer 2 of the Santa 

Clara River Valley subareas were estimated using a 

geometric-mean hydraulic conductivity of 15.4 ft/d. 
94 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Pacific Ocean

Pacific Ocean

Pacific Ocean

Vertical leakance (VC) x

10-6 per day

Total range 1 - 303

1 ≤ VC ≤ 55 < VC ≤ 1515 < VC ≤ 3030 < VC ≤ 5050 < VC ≤ 303

Transmissivity (T), in feet squared

per day

Total range 38 - 26,474

38 ≤ T ≤ 1,0001,000 < T ≤ 5,0005,000 < T ≤ 10,00010,000 < T ≤ 15,00015,000 < T ≤ 26,474

Transmissivity (T), in feet squared

per day

Total range 1.3 - 73,767

1.3 ≤ T ≤ 1,0001,000 < T ≤ 5,0005,000 < T ≤ 15,00015,000 < T ≤ 45,00045,000 < T ≤ 73,767

118°45'

R22W R18W

119°

0

0

10 MILES

10 KILOMETERS

119°15'

118°45'

R22W R18W

119°

0

0

10 MILES

10 KILOMETERS

119°15'

118°45' 119°

0

0

10 MILES

10 KILOMETERS

119°15'

a C ll ge au Cs reek

Santa Clara River

Lake

Piru

a C ll ge au Cs reek

Santa Clara River

Lake

Piru

a C ll ge au Cs reek

Santa

Clara River

Lake

Piru

R18W R22W

T

4

N

T

2

N

A

EXPLANATION

Layer 1

Between

layer 1

and 2

Layer 2

Hydrologic Unit boundary

River and selected streams

Model-grid boundary offlow region and subareas

Fault

34°

22'30"

34°

07'30"

T

4

N

T

2

N

34°

22'30"

34°

07'30"

T

4

N

T

2

N

34°

22'30"

34°

07'30"

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium and unconsolidated depositsShallow alluvium

Consolidated and unconsolidated deposits

Figure 19. Distribution of hydraulic properties in layers 1 and 2 of the model of the Santa Clara–Calleguas ground-water basin, Ventura County, California. 

A, Distribution of transmissivity 

in model layers 1 and 2, and vertical leakance between model layers 1 and 2. 

B, Storage coefficient for model layers 1 and 2.
Simulation of Ground-Water Flow 95

Pacific Ocean

Pacific Ocean

Pacific Ocean

Pacific Ocean

Elastic Skeletal Storage

Coefficient (Ske)

Total range 0.00015 - 0.097

0.00015 ≤ Ske ≤ 0.0050.005 < Ske ≤ 0.010.01 < Ske ≤ 0.030.03 < Ske ≤ 0.050.05 < Ske ≤ 0.097

Elastic Skeletal Storage

Coefficient (Ske)

Total range 0.0001 - 0.0146

0.0001 ≤ Ske ≤ 0.0010.001 < Ske ≤ 0.0030.003 < Ske ≤ 0.0050.005 < Ske ≤ 0.010.01 < Ske ≤ 0.0146

Storage Coefficient (S)Total range 0.0000005 - 0.07

0.0000005 ≤ S ≤ 0.00010.0001 < S ≤ 0.00020.0002 < S ≤ 0.00030.0003 < S ≤ 0.00070.0007 < S ≤ 0.07

Storage Coefficient (S)Total range 0.00002 - 0.19

0.00002 ≤ S ≤ 0.010.01 < S ≤ 0.050.05 < S ≤ 0.10.1 < S ≤ 0.150.15 < S ≤ 0.19

118°45'

R22W R18W

119°

0

0

10 MILES

10 KILOMETERS

119°15' 118°45' 119°

0

0

10 MILES

10 KILOMETERS

119°15'

118°45' 119°

0

0

10 MILES

10 KILOMETERS

119°15' 118°45'

R22W R18W

119°

0

0

10 MILES

10 KILOMETERS

119°15'

a C ll ge au Cs reek

Santa Clara River

Lake

Piru

a C ll ge au Cs reek

Santa Clara River

Lake

Piru

a C ll ge au Cs reek

Santa Clara River

Lake

Piru

a C ll ge au Cs reek

Santa Clara

River

Lake

Piru

T

4

N

R18WR18W

T

2

N

T

2

N

R22WR22W

B

Layer 1Layer 1

Layer 2Layer 2

34°

22'30"

34°

07'30"

T

4

N

T

2

N

34°22'30"34°07'30"

T

4

N 34°

22'30"

34°

07'30"

T

4

N

T

2

N

34°

22'30"

34°

07'30"

Figure 19—Continued.
96 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Figure 20. Distribution of estimated total thickness of coarse-grained and fine-grained interbeds used to estimate hydraulic properties and storage 

properties for the model of the Santa Clara–Calleguas ground-water basin, Ventura County, California. A, Upper-aquifer system (model layer 1). B, Loweraquifer system model (layer 2).

Pacific

Ocean

Pacific

Ocean

Thickness of fine-grained

deposits (b)

Total range2- 406 feet

2 ≤ b ≤ 25

25 < b ≤ 50

50 < b ≤ 100

100 < b ≤ 200

200 < b ≤ 406

Thickness of coarsed-grained

deposits (b)

Total range 5.5 - 589 feet

5.5 ≤ b ≤ 100

100<b ≤ 200

200<b ≤ 300

300<b ≤ 400

400<b ≤ 589

118°45'

R22W R18W

119°

0

0

10 MILES

10 KILOMETERS

119°15'

118°45'

34°

22'

30"

34°

07'

30"

R22W R18W

119°

0

0

10 MILES

10 KILOMETERS

T

2

N

T

4

N

34°

22'

30"

34°

07'

30"

T

2

N

T

4

N

119°15'

Calleguas Creek

Santa Clara River

Lake

Piru

Calleguas Creek

Santa Clara River

Lake

Piru

A

EXPLANATION

River and selected streams

Hydrologic Unit boundary Fault

Model-grid boundary of flow

region and subareas

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium

Shallow alluvium and unconsolidated deposits

Consolidated and unconsolidated deposits
Simulation of Ground-Water Flow 97

Figure 20—Continued.

Pacific

Ocean

Pacific Ocean

Thickness of fine-grained

deposits (b)

Total range 0.5 - 462 feet

0.5 ≤ b ≤ 25

25<b ≤ 50

50<b ≤ 100

100<b ≤ 200

200<b ≤ 462

Thickness of coarsed-grained

deposits (b)

Total range 0.3 - 803 feet

0.3 ≤ b ≤ 100

100<b ≤ 200

200<b ≤ 300

300<b ≤ 400

400<b ≤ 803

0

0

10 MILES

10 KILOMETERS

0

0

10 MILES

10 KILOMETERS

Calleguas Creek

Santa Clara River

Lake

Piru

Calleguas Creek

Santa Clara River

Lake

Piru

R22W

R22W

118°45'

R18W

119°15' 119°

118°45'

R18W

119°15' 119°

B

34°

07'

30"

34°

22'

30"

T

2

N

T

4

N

34°

07'

30"

34°

22'

30"

T

2

N

T

4

N
98 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Storage Properties

The hydraulic properties used to simulate the 

changes in storage of water within the aquifer systems 

consist of three components (Hanson, 1989). The first 

two components are specific yield and the elastic 

storage coefficient of the aquifer system, and the third 

component is the inelastic storage coefficient, which 

governs the irreversible release of water from the 

inelastic compaction of the fine-grained deposits. The 

specific yield and the elastic storage coefficients 

represent and govern the reversible release and uptake 

of water from storage. The elastic and inelastic storage 

coefficient represents the sum of storage owing to the 

compressibility of water and to the compressibility of 

the matrix or the skeleton of the aquifer system.

Storage owing to the compressibility of water 

was estimated as the product of the compressibility and 

the specific weight of water, the porosity, and the total 

thicknesses of the coarse- and fine-grained deposits in 

the aquifer (fig. 20). The assumed porosities were 35 

and 25 percent for fine- and coarse-grained deposits, 

respectively; they were estimated from transport 

modeling of seawater intrusion along the Hueneme 

submarine canyon (Tracy Nishikawa, U.S. Geological 

Survey, written commun., 1994) and range from 1.8 ×

10–5 to 2.5 × 10–4 for the upper-aquifer system (layer 

1) and from less than 1 × 10–6 to 4.5 × 10–4 for the 

lower-aquifer system (layer 2). The ranges were 

specified within MODFLOW as the aquifer-system 

storage coefficients.

The upper-aquifer system (layer 1) was 

simulated as unconfined in the Santa Clara Valley, the 

Las Posas Valley, parts of the Santa Rosa Valley 

subareas, the Oxnard Plain Forebay subarea, and the 

Northeast Oxnard Plain subareas (fig. 19B). In the 

remainder of the Oxnard Plain and the Mound 

subareas, the upper-aquifer system was simulated as 

confined. Storage coefficients, estimated from specific 

yields from previous models, range from 0.01 to 0.19 

in the Santa Clara River subareas; the estimate was 

0.12 along Conejo Creek in the Santa Rosa Valley 

subarea. The storage coefficients (specific yields) were 

assumed to range from 0.02 to 0.19 in the Las Posas 

Valley subareas (fig. 19B). 

The elastic and inelastic skeletal storage 

coefficients were simulated using the interbed storage 

package (Leake and Prudic, 1991). The elastic skeletal 

storage coefficient of the coarse-grained deposits was 

estimated from the difference between an estimated 

aquifer specific storage and the specific storage 

representing the compressibility of water (Hanson, 

1989). Specific storage is the ratio of the storage 

coefficient to the thickness of the sediments, in this 

case the aggregate thickness of the coarse-grained 

deposits. Reported values for aquifer specific storage 

determined from local aquifer tests in the upper- and 

lower-aquifer systems range from 1.2 × 10–6 to 2 ×

10–6 ft–1 (Neuman and Witherspoon, 1972; Hanson and 

Nishikawa, 1996). An initial elastic specific storage of 

3 × 10–6 ft–1 was assumed from other reported values 

for alluvial sediments (Ireland and others, 1984; 

Hanson, 1989). The aquifer elastic skeletal storage 

coefficient was estimated as the product of the aquifer 

skeletal specific storage and the aggregate cell-by-cell 

thickness of the coarse-grained deposits for each model 

layer (fig. 20). In a similar manner, the elastic skeletal 

storage coefficient of the fine-grained deposits was 

estimated from the difference between a specific 

storage for the fine-grained deposits and the specific 

storage representing the compressibility of water 

(Hanson, 1989). The elastic storage coefficient for the 

fine-grained deposits was estimated as the product of 

the elastic skeletal specific storage of the fine-grained 

deposits and the aggregate cell-by-cell thickness of 

fine-grained deposits for each model layer (fig. 20). 

The composite aquifer-system elastic skeletal storage 

coefficient was the sum of the elastic skeletal storage 

coefficients for the coarse-grained and fine-grained 

deposits for each cell in each model layer (fig. 19B).

The third component of storage, owing to the 

inelastic compaction of the fine-grained deposits, was 

estimated as the product of the inelastic specific storage 

and the aggregate cell-by-cell thickness of the finegrained deposits for each model layer (fig. 20). An 

initial inelastic skeletal specific storage of 2 × 10–4 ft–1

was based on the estimates from a consolidation test 

performed on the cores of fine-grained deposits from 

the Oxnard and Mugu aquifers (California Department 

of Water Resources, 1971, figs. VI–12 and VI–13) and 

aquifer-test analyses (Neuman and Witherspoon, 1972; 

Neuman and Gardner, 1989); these estimated range 

from 1.3 × 10–4 to 4.3 × 10–4 ft–1.
Simulation of Ground-Water Flow 99

The transition from elastic to inelastic storage is 

controlled by the preconsolidation stress—the 

maximum previous load that has been put on each 

sedimentary layer. The preconsolidation-stress 

threshold, expressed in terms of equivalent hydraulic 

head, can range from 50 ft of water-level decline in 

some well-sorted, fine-grained deposits that have had 

minimal sedimentary loading or lithification to more 

than 150 ft of water-level decline in some lithified, 

compressed, poorly sorted, or coarse-grained deposits 

(Holzer, 1981). The transition from elastic to inelastic 

storage was estimated to be 150 ft of water-level 

decline from predevelopment conditions throughout 

the lower-aquifer system and 100 ft of water-level 

decline throughout the upper-aquifer system, with the 

exception of 50 ft of water-level decline in the 

upper-aquifer system in the South Oxnard Plain 

subarea. These estimates were based, in part, on 

consolidation tests (California Department of Water 

Resources, 1971), water-level hydrographs (figs. 13

and 14), subsidence trajectories (fig. 9C), and 

lithologic data (Densmore, 1996).

Vertical Leakance

Vertical leakance controls vertical flow between 

the upper- and lower-aquifer systems. Vertical leakance 

was calculated for this model (fig. 19A) as the 

estimated vertical hydraulic conductivity divided by 

the combined half-thicknesses of each adjacent model 

layer for the estimated fine-grained deposits (fig. 20) in 

the upper- and lower-aquifer systems (McDonald and 

Harbaugh, 1988, eq. 5). Estimates of vertical leakance 

of flow between the upper and lower aquifers used in 

previous regional models range from less than 9 × 10–6 

to 0.002 (ft/d)/ft for the Oxnard Plain subarea 

(California Department of Water Resources, 1975; 

Reichard, 1995, fig. 12). A subregional model 

developed for the Santa Rosa Valley subarea (Johnson 

and Yoon, 1987) yielded estimates that range from 1.5 

× 10–3 (ft/d)/ft between the alluvium and the 

underlying Santa Margarita Formation to 3 × 10–5

(ft/d)/ft between the Santa Margarita and Saugus 

Formations and the underlying Conejo Volcanics. A 

subregional model developed for Las Posas Valley 

(CH2M HILL, 1993) used a uniform value of vertical 

hydraulic conductivity of 0.05 ft/d to simulate flow 

across the aquitards separating the Fox Canyon and 

Grimes Canyon aquifers in the Las Posas Valley 

subareas. Published values of vertical hydraulic 

conductivity range from 0.01 to 1 × 10–4 ft/d for the 

Oxnard Plain subarea (California Department of Water 

Resources, 1975; Neuman and Gardner, 1989) and 

from 24. to 6 × 10–4 ft/d for the Pleasant Valley subarea 

(Hanson and Nishikawa, 1996). 

The initial estimates of vertical leakance were 

from previous ground-water flow models. For the 

extensions of the two model layers, the initial values 

used were 1 × 10–6 (ft/d)/ft for the Mound, the Santa 

Clara River Valley, the Pleasant Valley, and the Santa 

Rosa Valley subareas and for the offshore regions, and 

1 × 10–5 (ft/d)/ft for the Las Posas Valley subareas. 

These are largely assumed values. The final distribution 

of vertical leakance was based on fitting simulated 

head differences to those measured at multiple-well 

completion sites (fig. 15). All vertical leakance values 

were held constant for the period of simulation.

Model Calibration

Calibration of the transient-state simulations was 

done for 1891–1993 and was based on matching water 

levels (fig. 13, 14, 15, and 21) and streamflows (fig. 

22). Predevelopment conditions (steady-state) were 

used as the initial conditions for the transient-state 

calibration. The long period of transient simulation was 

required because features of development, such as 

coastal landward flow (seawater intrusion) and 

subsidence, are dependent on the initial state of the 

aquifer systems.

Calibration Summary

Calibration was achieved through trial-and-error 

adjustments to recharge, hydraulic properties, and 

pumpage to achieve a good fit within each subarea over 

the historical period of record. These adjustments were 

made as systematically as possible, starting with 

recharge and streamflow, then hydraulic properties, and 

finally indirect agricultural pumpage estimates. 

Calibration and model development began using the 

extended model developed by Reichard (1995).
100 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Figure 21. Relation between measured and simulated water-level altitudes for selected years for the transient simulation of developed conditions 

(1927, 1932, 1950, 1991, and 1993) in the Santa Clara–Calleguas ground-water basin, Ventura County, California. A, Upper-aquifer system (model layer 1). 

B, Lower-aquifer system (model layer 2). C, Oxnard Plain.

B

-200 200 400 600 800 0

0

200

-200

400

600

800

-200

200

-200

0

400

600

800

SIMULATED WATER-LEVEL ALTITUDE, IN FEET ABOVE MEAN SEA LEVEL

One-to-one correlation line

1950 Water-level altitude

1991 Water-level altitude

1993 Water-level altitude

MEASURED WATER-LEVEL ALTITUDE, IN FEET ABOVE MEAN SEA LEVEL

0 200 400 600 800

One-to-one correlation line

1927 Composite measured water-level altitude

1932 Composite measured water-level altitude

1950 Water-level altitude

1991 Water-level altitude

1993 Water-level altitude

A

UPPER-AQUIFER SYSTEM

(MODEL LAYER 1)

LOWER-AQUIFER SYSTEM

(MODEL LAYER 2)
Simulation of Ground-Water Flow 101

Predevelopment Initial Conditions

Calibrating the model was an iterative process 

between the steady-state and transient-state 

simulations. The steady-state simulation provided 

initial conditions for the transient-state calibration. 

After each transient-state calibration, the updated 

model parameters were used to simulate updated 

steady-state conditions prior to additional calibration. 

The steady-state conditions were dependent on 

recharge (streamflow, mountain-front recharge, and 

valley-floor recharge) and discharge (streamflow and 

ET) from the aquifer system, transmissivity, vertical 

leakance between layers, fault hydraulic characteristic, 

and general-head boundary conductance. Because 

water levels are constant under steady-state conditions, 

storage is not required to simulate steady-state 

conditions. Initial recharge was based on the long-term 

seasonal geometric-mean ratios of runoff to wet-period 

winter precipitation. Streamflows were simulated as 

median streamflows. The composite ET rates and the 

model cells with the potential for ET for the years 

1912, 1927, 1932, and 1950 (Conejo Creek area) were 

used for the predevelopment simulation (fig. 23). The 

initial hydraulic properties were based on Reichard’s 

(1995) values and were adjusted during transient-state 

calibration. Few data were available for comparison of 

steady-state conditions. However, the simulated initial 

conditions are considered adequate if water levels are 

40 to 50 ft above sea level near the coast along the 

Oxnard Plain subareas. This requirement was based on 

a report of early hydraulic conditions (Freeman, 1968). 

Figure 21—Continued.

C

-100 50 100 150 200 -50 0

One-to-one correlation line

1993 Upper water-level altitude

OXNARD PLAIN

(MODEL LAYER 1)

MEASURED WATER-LEVEL ALTITUDE, IN FEET ABOVE MEAN SEA LEVEL

-50

-100

0

50

100

150

200

SIMULATED WATER-LEVEL ALTITUDE, IN FEET ABOVE MEAN SEA LEVEL
102 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Figure 22. Measured and simulated seasonal streamflows or diversion rates for the Santa Clara–Calleguas ground-water basin, Ventura County, 

California.

118°45'

34°22'30"

34°7'30"

R18W

R22W

T2N

T4N

119°

0 5 MILES

0 5 KILOMETERS

Calleguas Creek

Santa Clara River

Lake

Piru

Pacific

Ocean

0.1

1

10

100

1,000

PIRU CREEK DIVERSION (1931 – 1993)

[UWCD]

192019351950196519801995

192019351950

0.1

1

10

100

1,000

SANTA PAULA CREEK

DIVERSION (1930 – 1941)

[UWCD]

0.1

1

10

100

1,000

SANTA CLARA RIVER DIVERSION–SATICOY [ USGS-11113910]

AND AT FREEMAN [UWCD] 1920 1935 1950

196519801995

0.1

1

10

100

1,000

10,000 1890

1905192019351950196519801995

SANTA CLARA RIVER AT MONTALVO

[USGS-11114000]

STREAMFLOW

0.1

1

10

100

1,000

10,000

CALLEGUAS CREEK AT CAMARILLO

[VCFCD-805] 1890 1905

192019351950196519801995

STREAMFLOW

0.1

1

10

100

1,000

10,000

CALLEGUAS CREEK ABOVE HWY. 101

[VCFCD-806]

18901905192019351950196519801995

STREAMFLOW

EXPLANATION 1920

19351950

0.1

1

10

100

1,000

SANTA PAULA CREEK

DIVERSION (1930 – 1941)

[UWCD]

FLOW RATE, IN CUBIC FEET

PER SECOND

Streamflow- and diversion-gaging station - Name on graph title indicates

operating agency

Streamflow and diversion hydrographs with colored

curves showing flow rate for the gaging station

number of the same color. Blue and gray curves

are reported and measured streamflow or diversion

rates. Red curves are simulated streamflow or

diversion rates. Gaging station number appears in

brackets of graph title. Diversion time period

appears in parenthesis following graph title.

TIME, IN YEARS

Ground-water subbasin boundary–

Subbasin names are given in figure 1

River and selected streams

Hydrologic Unit boundary

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium

Shallow alluvium and unconsolidated deposits

Consolidated and unconsolidated deposits

USGS - U.S. Geological Survey

VCFCD - Ventura County Flood Control District

UWCD - United Water Conservation District
Simulation of Ground-Water Flow 103

Pacific Ocean

118°45'

34°34

07'30"

°

22'30"

R22W R18W

119°

0

0

5 MILES

5 KILOMETERS

T

4

N

T

2

N

119°15'

a C

ll

ge

au

C

s

reek

Lake

Piru

239

4

171

169

169

227

EXPLANATION

Fault

Evapotranspiration(ET) Model Cells

Cells Simulating ET

Cells Not Simulating ET

1891 (Predevelopment)

1993

Hydrologic Unit boundary

Ground-water subbasin boundary –

River and selected streams

Subbasin names are given in figure 1

Model-grid boundary offlow region and subareasVentura Flood Control DistrictEvaporation Station - Number isstation number

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium and unconsolidated depositsShallow alluvium

Consolidated and unconsolidated deposits

Santa

River

Clara

19321950

Figure 23. Model cells with simulated evapotranspiration in 1891 (predevelopment), 1932, 1950, and 1993 in the upper layer of the model of the Santa Clara-Calleguas ground-water basin, Ventura County, California.
104 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Transient-State Calibration Parameters

Transient-state conditions were dependent on 

recharge (streamflow, mountain-front recharge, valleyfloor recharge, and artificial recharge) to and discharge 

(pumpage, streamflow, and ET) from the aquifer 

system and on transmissivity, storage, vertical leakance 

between layers, fault hydraulic characteristics, and 

general-head boundary conductance. Because of the 

large head differences within some parts of the aquifer 

systems, water-level maps were used for comparisons 

but are considered less reliable than time-series data. 

Estimates of spatial fit were made for selected times of 

the transient simulation (fig. 21). Calibration was 

primarily based on temporal comparisons, instead of 

spatial comparisons, using long-term water-level 

hydrographs (figs. 13, 14, and 15), streamflow 

hydrographs (fig. 22) and time-series of bench-mark 

land-surface altitudes (subsidence trajectories) (fig. 9).

Recharge was adjusted to reduce the 

overestimation of mountain-front recharge, valley-floor 

infiltration, and streamflow infiltration. The modified 

rational method of estimating infiltration tended to 

overestimate the water available during the wettest 

seasons; therefore, the upper limit of runoff available 

for mountain-front recharge was limited to less than 

90 percent of average precipitation.

Simulated streamflow infiltration initially was 

too large when floodflows or intermittent flows were 

spread over an entire season, and it did not reflect the 

observed and measured changes in streamflow during 

low-flow and high-flow conditions. The flowdependent changes in streambed conductance are 

believed to be related mostly to changes in channel 

width. Grouping and varying streambed conductance 

with flow were critical for accurately depicting waterlevel declines and recoveries in wells during wet and 

dry periods (figs. 13 and 14). Grouping and varying 

streambed conductance for the dry periods helped to 

simulate a more accurate depiction of the conveyance 

(delivery) of controlled releases from Lake Piru that are 

routed down the Santa Clara River and are simulated as 

the total reported diversions at Piru, Santa Paula, 

Saticoy, and Freeman (fig. 22). Segments of the 

streamflow network in the coastal plain (segments 22, 

23, 29, and part of 30) (fig. 18B) are not in direct 

connection with the upper-aquifer system and therefore 

were assigned a streambed conductance of zero. This 

allowed the simulated water levels for predevelopment 

conditions and the recovery periods for development 

conditions to rebound to the measured water levels 

(figs. 13 and 14). Streamflow was increased from about 

1.5 to 14 ft3/s for Arroyo Simi to account for treatedsewage effluent discharged between 1964 and 1993. On 

the basis of streamflow data from the hydrographs for 

Calleguas Creek at Camarillo (fig. 22, VCFCD station 

805), the initial discharge (1964–79) was estimated to 

start at 1.5 ft3/s and increase linearly to 10 ft3/s.

The hydraulic properties estimated by Reichard 

(1995) were adjusted during model calibration; they 

include transmissivity, storage properties, and vertical 

leakance. The initial estimates were described earlier 

(see section on “Hydraulic Properties”). The only 

change to the storage properties was the transformation 

of Reichard’s (1995) initial estimates to cell-by-cell 

estimates, as was described earlier. Additional 

calibration also was done for fault hydraulic 

characteristics and offshore general-head boundary 

conductance. These properties were adjusted for the 

period of reported pumpage largely on the basis of the 

water levels in the hydrographs.

Transmissivity values were reduced by a factor 

of 0.55 for the lower layer of the Port Hueneme area 

and were increased by a factor of 1.5 for the lower 

layer of the East Las Posas Valley subarea (figs.17B

and 19A) compared with the values used by Reichard 

(1995). The decrease in transmissivities in the lower 

layer brought the values closer to those in the transport 

model of the Port Hueneme area (Nishikawa, 1997) 

and to those estimated from aquifer tests completed in 

the East Las Posas Valley area (CH2M HILL, 1992). 

The transmissivities of the aquifer layer underlying the 

major streams and tributaries also were increased 

during model calibration.
Simulation of Ground-Water Flow 105

Adjustments in vertical leakance were made on 

the basis of water-level differences at multiple-well 

observation sites and, for some areas, on the basis of 

data from the hydrographs of selected production 

wells. Recall that the vertical leakances were calculated 

as the estimated vertical hydraulic conductivity divided 

by the combined half-thicknesses of the estimated finegrained deposits in the upper- and lower-aquifer 

systems (McDonald and Harbaugh, 1988, eq. 51). 

Cell-by-cell estimates for the West Las Posas Valley 

subarea were based on a vertical hydraulic conductivity 

of 0.0005 ft/d. Cell-by-cell estimates for the Forebay 

region of the Oxnard Plain were based on a vertical 

hydraulic conductivity of 0.001 ft/d for all but five cells 

in the Saticoy area, for which a value of 0.01 ft/d was 

used. The final distribution of vertical leakances ranged 

from 1 × 10–7 to 3.03 × 10–5 (ft/d)/ft (fig. 19A). 

Cell-by-cell estimates initially were made for all the 

subareas, but the estimates did not improve model fit 

for the East and South Las Posas Valley subareas. For 

these two subareas, estimates were not based on the 

thickness of the fine-grained deposits; the final 

calibrated vertical leakances align with the underlying 

syncline-anticline structures within the lower-aquifer 

system (figs. 9 and 20).

Although pumpage was the largest stress in the 

model, some uncertainty remained about the accuracy 

of the land-use estimates of pumpage. Some 

adjustments in the magnitude and distribution of the 

pumpage estimated from land use were made during 

the calibration of the flow model in order to have the 

model enter the final 10-years of reported pumpage at 

the correct water-level altitudes. These changes were 

largely based on the measured temporal variations in 

ground-water levels in the subareas and on the 

magnitude and changes of pumpage for the 1983–93 

period of reported pumpage. Changes to land-use 

estimates of historical pumpage include elimination of 

pumpage from the Santa Clara River Valley subareas 

and the Oxnard Plain Forebay subarea for 1891–1918 

so that the first significant ground-water pumpage 

began with the dry period of 1919–36. The 1950 and 

1969 estimates of the land-use-based pumpage also had 

to be modified for selected subareas. The changes in 

the 1950 estimate of agricultural pumpage applied over 

the period 1946–61 ranged from a 34-percent reduction 

in pumpage for the Mound subbasin to an approximate 

300-percent increase for the Piru subbasin; the changes 

in the 1969 estimate applied over the period 1962–77 

ranged from a 34-percent reduction for the Mound 

subarea to an approximate 100-percent increase in the 

North and South Pleasant Valley and the Piru subareas. 

These changes brought the estimated historical 

agricultural pumpage into alignment with the reported 

agricultural pumpage (fig. 11B) and improved the 

alignment between the measured and simulated 

ground-water levels and the land-use changes in 

various subareas for these two periods. Pumpage was 

reduced to 40 percent of the 1932 estimate for the years 

1935–45, which span the post-Great Depression and 

World War II period, as well as a severe drought that 

was followed by one of the wettest periods on record 

(fig. 2). This reduction was the only way to achieve the 

record water-level recoveries that have been equaled 

only during predevelopment conditions and more 

recently during 1993. These adjustments did not affect 

calibration of hydraulic properties or recharge during 

the period of reported pumpage.
106 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

The percentage of pumpage between layers was 

changed during model calibration. The final vertical 

distribution of pumpage between the model layers for 

wells spanning both model layers is summarized in 

figure 7B for all the subareas.

The general-head boundaries that initially were 

placed at the submarine outcrops were moved landward 

to better represent the average location of the 

freshwater-saltwater interface. The values of the 

boundary heads were aligned with the top of the basal 

coarse-grained layers in the Oxnard and Hueneme 

aquifers for the upper- and lower-aquifer systems, 

respectively. The boundary conductances were grouped 

into several coastal subreaches with different values, 

grouping the conductances, however, did not improve 

model fit. The final configuration consisted of a single 

value for each model layer, which was the simplest 

approach without additional data and was adequate for 

matching water levels along the coast. The flows at the 

general-head boundaries were monitored to verify that 

simulated outflow was occurring during wet periods 

when recovery of water levels exceeded the specified 

heads of the seawater at the general-head boundary. To 

be consistent, the model should simulate coastal 

landward flow (seawater intrusion) during the major 

droughts when water levels decline below the heads of 

the denser seawater. The current model is consistent 

with the concept of the wet-period outflow, as shown 

by the outflows of 1984–93 (figure 25B in the section 

“Transient-State Model Comparisons”), and with the 

concept of coastal landward flow (seawater intrusion) 

during droughts, such as the drought of 1987–91.

Transient-State Model Comparisons

Calibration and goodness-of-fit of the transientstate model were determined by comparing simulated 

values with measured values for ground-water levels, 

streamflow, and land subsidence. The simulated water 

levels were compared with water-level maps for 1932 

and 1993 (fig. 12) and correlated with the water-level 

data for 1927, 1932, 1950, 1991, and 1993 (fig. 21) and 

the water-level hydrographs of selected production 

wells (figs. 13 and 14) and multiple-well observation 

sites (fig. 15). A comparison of simulated streamflow 

was made for the downstream gaging stations and the 

streamflow-diversion sites (fig. 22). The spatial 

distribution of potential ET, based on riparian 

vegetation, and the spatial distribution of simulated ET 

for predevelopment and developed conditions in 1932, 

1950, and 1993 were also compared (fig. 23). 

Measured and simulated subsidence for selected bench 

marks (fig. 24) were used to compare the potential 

effects of water-level declines on simulated subsidence 

in the South Oxnard Plain subarea. And, finally, 

selected comparisons of ground-water flows were used 

to confirm that flows within the model (fig. 25) were 

conceptually consistent with the framework provided 

by geohydrologic and geochemical analyses.

The best and primary comparison period is the 

10-year period of reported pumpage, 1984–93, which 

represents one dry period and parts of two wet periods 

(fig. 2A). Within this period is a 4-year period 

(1990–93) for which measured water levels and 

water-level differences between aquifer systems 

measured at the multiple-well monitoring sites 

(fig. 15) can be compared with model results.

The model generally matched the measured 

water-level, streamflow, and bench-mark data for the 

calibration period (figs. 12, 13–15, 21; 22, and 24, 

respectively). Comparisons of the simulated and 

measured water levels estimated from land-use maps 

have some uncertainty because the measured groundwater levels reflect a wide variety of screened intervals 

in wells, and the “synoptic” measured water levels 

reflect water levels measured over spans of several 

months over a season (fig. 12A,B). The model slightly 

overestimates historical water-level altitudes for the 

early period of development (fig. 12A). The correlation 

diagram on figure 12A shows no systematic 

discrepancies between measured and simulated water 

levels in the upper-aquifer system. Measured minus 

simulated water levels have a mean error (ME) for the 

upper aquifer system of –22.8 ft and a root mean 

square error (RMSE) of 35.2 ft for 1927 (number of 

comparison wells: N = 169), and a ME of 7.29 ft and a 

RMSE of 42.2 ft for 1932 (N = 354). A comparison of 

the measured and simulated water levels for 1932 (fig. 

12A) indicate similar patterns. Water-level differences 

between simulated and measured data range from less 

than 5 ft near the coast to about 40 ft in the Forebay, 

and they are less than 20 to 40 ft in the Santa Clara 

River Valley and Pleasant Valley subareas. 
Simulation of Ground-Water Flow 107

Pacific

Ocean

118°45'

34°22'30"

34°7'30"

R22W

R18W

119°

0

0

5 MILES

5 KILOMETERS

T

4

N

T

2

N

a C ll ge au Cs

reek

Santa Clara

River

Lake

Piru

2

3

3

5

5

5

5

3

1

1

1

0.5

0.5

0.5

5. 0

0 5. 0 5.

0.5

0 2. 5

0 2. 5

2. 0 5

0.25

0 2. 5

0.5

0.5

0.5

0.25

2. 0 5

0 1.

0 1.

0 1.

0 1.

0.1

0 1.

5

2

2

3

1

1

1

1

1

0.1

0.1 0.1

0

1

1

1

0 5.

0 2. 5

0.25

1920 1940 1960

YEAR

1980 2000

0.00.51.01.52.02.53.03.5

Layer 1

Total Simulated

Subsidence

(1939-93)

Total Simulated

Subsidence

(1923-93)

Layer 2

E584

Measured

Subsidence

(1939-78)

1920 1940 1960

YEAR

1980 2000

0.00.51.01.52.02.53.0

Layer 1 Total Simulated

Subsidence

(1939-93)

Total Simulated

Subsidence

(1923-93)

Layer 2

TIDAL3

Measured

Subsidence

1920 1940 1960

YEAR

1980 2000

0.00.5

0.0

0.5

1.01.5

COMPACTION OR

SUBSIDENCE, IN FEET

COMPACTION OR

SUBSIDENCE, IN FEET

COMPACTION OR

SUBSIDENCE, IN FEET

2.02.53.0

Layer 1

Total Simulated

Subsidence

(1959-93)

Total Simulated

Subsidence

(1923-93)

Measured

Subsidence

Layer 2

Z901

EXPLANATION

Gound-water subbasin boundary–

Subbasin names are given in figure 1

Total simulated subsidence from withdrawal ofground water (1891-1993) - Contour interval variesBench mark

Hydrologic Unit boundary

River and selected streams

Model-grid boundary offlow region and subareas

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium and unconsolidated depositsShallow alluvium

Consolidated and unconsolidated deposits

Port

Hueneme

Point Mugu

Figure 24. Simulated compaction owing to withdrawal of ground water, 1891–1993, in the Santa Clara–Calleguas ground-water basin, Ventura County, California, and locations of selected bench marks and related measured and simulated bench-mark trajectories.
108 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Figure 25. A, Simulated water-altitudes (December 1992), decline in ground-water levels from 1984 to 1994, and mean ground-water flow in the Santa 

Clara–Calleguas ground-water basin, Ventura County, California. B, Cumulative changes in ground-water storage and ground-water flow for selected 

subareas during 1984–93. C, Hydrologic budgets for predevelopment conditions. D, Hydrologic budgets for 1984–93 period.

Pacific Ocean

Pacific Ocean

0

0

10 MILES

10 KILOMETERS

0

0

10 MILES

10 KILOMETERS

118°45'

34°

22'

30"

R22W R18W

119°

T

2

N

T

4

N

34°

07'

30"

119°15'

118°45'

34°

22'

30"

119°

T

2

N

T

4

N

34°

07'

30"

119°15'

Calleguas Creek

Santa Clara River

Lake

Piru

Calleguas Creek

Santa Clara River

Lake

Piru

20

40

40

40

20

40

60

100

40

100

200

300

400

500

600

400

300 200

500 600

700

0

0 0

0

100

0

10

20

10

1040

40

100

125

150

300

400

300

20

10

0

100

0

500

600

600

700

200

400

500

600

400

125

0

0

0

4

3

20

19

18

17

7

5

1

34

33

32

31

8

2

30

28

27 26

13

12

11

10

9

6

29

16

14

25

24

23

22

21

15

2,804

2,354

1,869

109

29

29

2,021

949

420

558

5,296

3,477

594

383

43

138

13,692

16,452

2,818

746

159

4,202

2,166

877

1,702

0

0

0

0

130

A

EXPLANATION

River and selected streams

Water-level altitude

(Simulated December, 1993) –

in feet. Contour interval varies

Hydrologic Unit boundary Subarea number –

See figure 17B for subarea names

Upper-aquifer system

Lower-aquifer system Model-grid boundary of

flow region and subareas

Mean simulated ground-water

underflow and coastal flow –

in acre-feet per year

Difference in water-level altitude (∆h)

from 1984 - 93, in feet

Total Range -92 to 48 feet

Difference in water-level altitude (∆h)

from 1984 - 93, in feet

Total Range -195 to 251 feet

-92 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 30

30 < ∆h ≤ 48

-195 ≤ ∆h ≤ -50

-50 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 50

50 < ∆h ≤ 251

R22W R18W

100

10

22

949

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium

Shallow alluvium and unconsolidated deposits

Consolidated and unconsolidated deposits

Upper-Aquifer

System

Lower-Aquifer

System

Rise

Decline

Rise

Decline
Simulation of Ground-Water Flow 109

 Figure 25—Continued.

0

50

-50

-100

-150

1984 1988 1992 1996

0

50

-50

-100

-150

50

100

0

-50

-100

CUMULATIVE STORAGE, IN THOUSANDS OF ACRE FEET

CUMULATIVE STORAGE, IN THOUSANDS OF ACRE FEET

0

50

-50

-100

-150

50

100

0

-50

-100

CUMULATIVE FLOW TO OCEAN, IN THOUSANDS OF ACRE FEET

0 50

50

-50

-100

-150

100

0

-50

-100

0

50

-50

-100

-150

0

50

-50

-100

-150

0

50

-50

-100

-150

YEAR

1984 1988 1992 1996

YEAR

1,000

2,000

0

-1,000

-2,000

SEASONAL FLOW TO OCEAN, IN ACRE-FEET PER SEASON

1,000

2,000

0

-1,000

-2,000

1,000

2,000

0

-1,000

-2,000

1984 1988 1992 1996

YEAR

SEAWARD

FLOW

LANDWARD

FLOW

SEAWARD

FLOW

LANDWARD

FLOW

SEAWARD

FLOW

LANDWARD

FLOW

wet dry

wet dry

wet dry

Subarea number –

See figure 17B for subarea names

EXPLANATION

B

Upper-aquifer system

Lower-aquifer system

Upper-aquifer system

Lower-aquifer system
110 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Figure 25—Continued.

C

Valley-Floor Recharge

4,770 acre-feet per year

Mound 0% Santa Rosa 2%

Oxnard Plain

15% Pleasant

Valley

16%

Las

Posas

30% Santa Clara

37%

Mountain-Front and Bedrock Recharge

14,600 acre-feet per year

Santa Clara

62%

Las

Posas

20%

Pleasant

Valley

8%

Mound Santa Rosa 3%

5%

Net Streamflow Recharge

14,300 acre-feet per year

Mound 1%

Santa Rosa 0%

Oxnard Plain

40%

Pleasant Valley

5%

Las Posas

4%

Santa

Clara

50%

Evapotranspiration

14,800 acre-feet per year

Las

Posas

5%

South Pleasant

Valley

21%

Mound 0% Santa Rosa 0%

Oxnard Plain

13%

Santa Clara

61%

Total Discharge

59,900 acre-feet per year

Coastal

Outflow

Mound

8%

Coastal Outflow

Oxnard Plain

23% ET

ET = Evapotranspiration

V.F. Rech. = Valley-Floor Recharge

M.F. Rech. = Mountain-Front and Bedrock Recharge

25%

Stream

Baseflow 44%

Oxnard Plain

2%

Total Recharge

59,900 acre-feet per year

M.F. Rech.

21%

V.F.Rech. 8%

Bedrock

recharge

4%

Streamflow

Infiltration

67%

Total Net Recharge

33,650 acre-feet per year

Bedrock

Recharge 6%

Net Streamflow

43%

V.F.Rech. 14%

M.F. Rech.

37%

Predevelopment Conditions
Simulation of Ground-Water Flow 111

Figure 25—Continued.

Valley-Floor Recharge

24,100 acre-feet per year

Mound

0%

Santa Rosa 1%

Oxnard

Plain 22%

Pleasant

Valley 7%

Las

Posas

26%

Santa Clara

44%

Mountain-Front and Bedrock Recharge

27,760 acre-feet per year

Mound

5%

Santa Rosa

3% Oxnard Plain

2%

Pleasant

Valley

9%

Las Posas

23%

Santa Clara

58%

Evapotranspiration

1,060 acre-feet per year

3%

Pleasant

Valley 9% 1%

Mound 0%

Oxnard Plain

11%

76%

Subsidence

4,100 acre-feet per year

Mound 10%

Pleasant

Valley

7%

34%

2%

Oxnard

Plain

47%

Pumpage

247,000 acre-feet per year

13%

Pleasant

Valley

9%

1%

Mound

3%

Oxnard

Plain

37%

37%

Downward Flow

67,010 acre-feet per year

34%

Pleasant Valley

22%

2%

Mound 1%

Oxnard Plain

34%

7%

Total Recharge

227,700 acre-feet per year

Santa Clara

River Valley

Santa Clara

River Valley

Santa Clara

River Valley

Santa Clara

River Valley

Santa Clara

River Valley

37%

Oxnard

Plain

36%

Mound

2% Santa Rosa

Valley

Santa Rosa

Valley

Santa Rosa

Valley

Santa Rosa

Valley

3%

Pleasant

Valley

10%

Las Posas

Valley

Las Posas

Valley

Las Posas

Valley

Las Posas

Valley

Las Posas

Valley

12%

Coastal Inflow

6,450 acre-feet per year

NW Oxnard

Plain(2)

43%

S. Oxnard

Plain(1)

1%

S. Oxnard

Plain(2)

54%

Mound(2) 2%

Net Streamflow Recharge

62,200 acre-feet per year

W.L.P(1)

0%

E.L.P.(1) 3%

S.L.P.(1) 5% N.P.V.(1) 6%

Mound(1)

4%

S.P.V.(1)

8%

S.R.V(1)

6%

Santa Paula(1)

15%

Fillmore(1) 22%

Oxnard Plain

Forebay(1) 5%

Piru(1) 26%

Historical Period 1984-93

Total Inflow By Component

364,600 acre-feet per year

Subsidence

3%

Coastal Flow

2%

Storage

30% Stream Inflow

20%

Wastewater

Effluent 2%

Irrigation

Return

14% Artificial Recharge

15%

V.F.Rch

6% M.F. Rch

8%

Total Outflow By Component

364,600 acre-feet per year

Stream

Baseflow

2.4%

Storage

26.9%

Coastal Flow

0.2%

Subsidence

2.5%

Pumpage

67.7%

ET

0.3%

ET = Evapotranspiration

V.F. Rech. = Valley-Floor Recharge

M.F. Rech. = Mountain-Front and Bedrock Recharge

S.P.V.(1) = South Pleasant Valley

(Upper-aquifer system)

N.P.V.(1) = North Pleasant Valley

(Upper-aquifer system)

S.R.V.(1) = Santa Rosa Valley

(Upper-aquifer system)

S.L.P.(1) = South Los Posas Valley

(Upper-aquifer system)

W.L.P.(1) = West Los Posas Valley

(Upper-aquifer system)

E.L.P.(1) = East Los Posas Valley

(Upper-aquifer system)

S. Oxnard Plain(1) = South Oxnard Plain

(Upper-aquifer system)

S. Oxnard Plain(2) = South Oxnard Plain

(Lower-aquifer system)

NW Oxnard Plain(2) = Northwest Oxnard Plain

(Lower-aquifer system)

Mound(2) = Mound

(Lower-aquifer system)

D
112 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

The simulated water levels were lower than the 

measured water levels for 1950, the first period of 

substantial ground-water development in both aquifer 

systems [ME and RMSE are 9.95 and 52.7 ft, 

respectively, for the upper-aquifer system (N = 297) 

(fig. 21A), and 8.39 and 39.3 ft, respectively, for the 

lower-aquifer system (N = 31) (fig. 21B)]. The 

simulated water levels for the 1987–91 drought were 

lower than the measured water levels in the upperaquifer system [ME and RMSE are 1.96 ft and 26.1 ft, 

respectively, (N = 130) (fig. 21A)] and higher than the 

measured water levels in the lower-aquifer system [ME 

and RMSE are –89.8 ft and 110.4 ft, respectively 

(N = 101) (fig. 21B)]. The differences between the 

measured and simulated water levels in the loweraquifer system are, in part, due to the many wells used 

for the calibration which are completed solely in the 

Fox Canyon or Grimes Canyon aquifer in parts of the 

Pleasant Valley and Las Posas Valley subareas. These 

aquifers were not simulated as separate aquifer layers 

in the current model and therefore the simulation 

represents the average water level for the entire lower 

aquifer system. The Fox Canyon and Grimes Canyon 

aquifers are relatively low-permeability aquifers; 

pumpage from these aquifers resulted in large 

water-level declines. The overlying Hueneme aquifer is 

relatively more permeable; pumpage from this aquifer 

resulted in smaller water-level declines. Measured 

water levels for the multiple-well monitoring sites 

indicate water-level differences within the 

lower-aquifer system of as much as 75 ft between the 

Hueneme aquifer and the Fox Canyon and Grimes 

aquifers (fig. 15). The model was calibrated to the 

Hueneme aquifer and would have required additional 

layers to simulate the water-level differences for the 

lower aquifers. Some water-level measurements also 

may have been affected by pumping, which resulted in 

measured water levels being lower than the simulated 

levels. Another reason for the water-level differences 

may be that instantaneous water-level measurements 

were compared with simulated water levels controlled 

by average seasonal pumpage. 

Measured water levels for the 1992–93 

wet-period recovered; the simulated water levels were 

lower than the measured water levels for the 

upper-aquifer system [ME and RMSE are 9.68 ft and 

20.5 ft, respectively (N = 161) (fig. 21A)] and higher 

than the measured water levels in the lower-aquifer 

system [ME and RMSE are –42.3 ft and 66.9 ft, 

respectively (N = 94) (fig. 21B)]. When the comparison 

was restricted to the upper-aquifer system of the 

Oxnard Plain for spring 1993, the simulated water 

levels were only slightly lower than the measured water 

levels [ME and RMSE are 1.61 ft and 10.7 ft, 

respectively (N = 90) (fig. 21C)]. 

In general, the long-term water-level 

hydrographs (figs. 13 and 14) indicate that the match 

between measured and simulated water-level altitudes 

is good for the entire period of simulation, especially 

those for the Oxnard Plain subbasin. However, some 

hydrographs show large discrepancies between the 

simulated and measured water levels; examples of 

these discrepancies can be seen on the hydrographs of 

wells along Beardsley Wash, such as well 2N/21W16J1 in the West Las Posas Valley subarea and wells 

along the Santa Clara River, wells 2N/22W-2C1 and 

3N/22W-36K2 in the Santa Paula subarea, well 

2N/22W-9J1 in the Mound subarea, and well 3N/19W29E2 in the East Las Posas Valley subarea (fig. 14). A 

comparison of the short-term hydrographs for the 

RASA multiple-well monitoring sites shows good 

agreement between the simulated and measured water 

levels (fig. 15). The simulated water-level differences 

between the upper and lower layers closely match the 

measured seasonal and multiple-year patterns of waterlevel differences (fig. 15). This indicates that the 

collective estimates of vertical leakance, vertical 

distribution of pumpage, and recharge are reasonable. 

Water-level differences between wells across 

faults were calibrated by adjusting fault hydraulic 

characteristics; for example, the water-level differences 

between well 2N/20W-23K1 (fig. 13) and well 

2N/20W-23R1 (fig.14) across the San Pedro (Bailey) 

Fault in the Santa Rosa Valley subarea. 
Simulation of Ground-Water Flow 113

Seasonal water-level variations in the 

upper-aquifer system are controlled largely by 

streamflow infiltration and related streambed 

conductance; these factors, when combined with 

seasonally and climatically variable pumpage, resulted 

in water-level fluctuations of tens to a hundred feet in 

wells in the Santa Clara River Valley subareas [wells 

4N/19W-25K2, 30R1; 22N/22W-11A1,2 (fig. 14)]. 

Water-level fluctuations in the Oxnard Plain Forebay 

subareas include the effects of artificial recharge and 

pumping back artificially recharged water [wells 

2N/22W-12R1, 22R1 (fig. 14); wells 2N/22W-23B3–7, 

2N/21W-7L3–6 (fig. 15)].

Simulated streamflows for Montalvo and for the 

Piru, Santa Paula, Saticoy, and Freeman diversions 

closely match measured streamflow along the Santa 

Clara River system. Simulated streamflows also match 

many of the historical high flow events (figs. 2B and 

22); however, they overestimate low streamflow 

conditions (less than 10 ft3/s) for some dry-year 

periods at Montalvo on the Santa Clara River (fig. 22). 

The simulations underestimated the diversions for 

some dry-year periods when flows were less than 2 to 

10 ft3/s at Saticoy and less than 2 ft3/s at the Santa 

Paula and Piru diversions (fig. 22). Simulated 

streamflows for Camarillo and above Highway 101 in 

Calleguas Creek match measured streamflow; the 

simulated streamflow is intermittent in character after 

the onset of ground-water development in the late 

1920s (fig. 22).

Simulation results indicate that land subsidence 

started as early as the 1920s and continued through 

1984–93, the period when water levels declined below 

the water levels of the 1950s and 1960s. Results also 

indicate that preconsolidation may vary considerably 

and that subsidence occurred primarily during dry-year 

periods when seasonal and multiple-year water-level 

declines exceeded past declines in the South Oxnard 

Plain, Las Posas Valley, and Pleasant Valley subareas 

(figs. 24 and 25B). Subsidence started in the upperaquifer system in the South Oxnard Plain subarea 

during the early period of development (1939–60) 

(fig. 24). Subsidence has continued, in part, because of 

the development of the lower-aquifer system, which 

has contributed most of the subsidence in recent 

decades (1959–93) (fig. 24). 

Simulated subsidence generally matches total 

measured subsidence in the South Oxnard Plain 

subarea (fig. 24). The time-series comparisons of 

subsidence from bench-mark measurements are similar 

in trend but underestimate subsidence at BM Z 901 

near Point Mugu and overestimate subsidence at BM 

TIDAL 3 near Port Hueneme (fig. 24). The extent of 

subsidence generally is not well known for areas 

outside the South Oxnard Plain subarea but may be 

overestimated for parts of the Pleasant and Las Posas 

Valley subareas. Field inspections throughout West and 

East Las Posas subareas did not reveal any surface 

expressions of land subsidence that would be expected 

for the amount of simulated subsidence. This 

overestimation may be caused by overestimation of 

inelastic skeletal specific storage, overestimation of the 

aggregate thickness of fine-grained material that is 

actually subject to loading from water-level declines, 

and a lack of separate model layers within the loweraquifer system for the Pleasant and Las Posas Valley 

subareas. A detailed land survey or Interferometric 

Synthetic Aperture Radar (InSAR) imagery analysis 

would be needed to resolve this issue.

Model Uncertainty, Sensitivity, and Limitations

Numerical models of ground-water flow are 

useful tools for assessing the response of an aquifer 

system to changing natural and human-induced 

stresses. Regional-scale models are especially useful 

for assessing many of the components in the 

hydrologic cycle and the collective effect of groundwater development in separate subareas of a regional 

ground-water system. Models, however, are only an 

approximation of actual systems and typically are 

based on average and estimated conditions. The 

reliability or certainty with which a model can simulate 

aquifer response is directly related to the accuracy of 

the input data, the amount of detail that can be 

simulated at the scale of the model, and the model 

discretization of time and space. Hence, the regional 

models can be useful for simulating subregional and 

regional performance of a flow system and for 

providing boundary information for more detailed 

local-scale models even though the results of the 

regional model for a local scale may not be appropriate 

for site-specific problems such as the performance at a 

particular well. 
114 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

The certainty of a model is inversely related to 

the duration, magnitude, and distribution of simulated 

inflows and outflows. Thus, better time-varying 

estimates of pumpage, recharge, irrigation return flow, 

streamflow, and coastal landward flow (seawater 

intrusion) could improve simulation of historical 

development. Additionally, the trial-and-error 

calibration process is inexact, and this problem is 

compounded by uncertainty of the variables and by 

sensitivity of the aquifer-parameter and boundarycondition estimates. Uncertainty in model attributes 

results in a broader range of possible aquifer-parameter 

and boundary-condition estimates used to constrain 

calibration of the ground-water flow model. 

Uncertainty in water levels in wells, streamflows, and 

altitudes of bench marks used for model comparison 

during calibration can affect the degree of fit achieved. 

Sensitivity to changes in model parameters and 

boundary conditions during calibration also can affect 

the degree of fit and the possible range of values used 

to simulate historical ground-water flow. 

An exhaustive analysis of the uncertainty and 

sensitivity of every model parameter and boundary 

condition is beyond the purpose and scope of this 

report. However, a summary can yield insight into the 

capabilities and limitations of the model, and specific 

insight into its performance with respect to groundwater management. The combination of the uncertainty 

in the model-input and comparison data and the 

sensitivity of the model to changes in model input yield 

a qualitative measure of the importance of various 

model attributes. For example, uncertainties in the 

measurement of streamflows may contribute to 

uncertainties in the simulation of streamflows and 

affect the comparison between measured and simulated 

streamflows. Based on gaging-station ratings, 

inaccuracy in streamflow measurements can range 

from 5 to 20 percent. For high flows, this inaccuracy 

may result in an uncertainty of hundreds to thousands 

of acre-feet in potential recharge for some wet years. 

Other sources of uncertainty include estimates of 

precipitation, which may have estimation errors 

(kriging errors) ranging from 5 to 10 percent which can 

result in thousands of acre-feet of uncertainty for wetyear seasons; estimates of irrigation return flow, which 

may have estimation errors ranging from 10 to 

20 percent owing to the uncertainty and the variability 

of the estimates of applied water and irrigation 

efficiency (Koczot, 1996); and errors in the assignment 

of percentages of pumpage for wells completed across 

both aquifer systems, which may range from 10 to 20 

percent.

Additional uncertainties also may exist with 

respect to boundary conditions such as the average 

location of the seawater front, which is represented by 

the general-head boundary cells; horizontal-flow 

barriers, some of which may be of inferred extent; and 

the conductance of some faults. The importance of 

some faults remains uncertain; for example, faults 

whose traces generally are parallel to the hydraulic 

gradient, such as the Oak Ridge and McGrath Faults in 

the upper-aquifer system, or faults that are adjacent to a 

spatial contrast in transmissivity, such as the Country 

Club Fault. Considerable testing of these boundaries 

was done during model calibration; the resulting 

estimates for boundary locations and conductance 

satisfy the conceptual framework and the measured 

comparison data.
Analysis of Ground-Water Flow 115

The simulated ground-water levels in the 

regional model were most sensitive to the location of 

the freshwater–saltwater interface, the amount of 

recharge and irrigation return flow applied to the 

Oxnard Plain, the vertical distribution of pumpage, the 

variation in streambed conductance, and the 

conductance of faults at subarea boundaries where the 

hydraulic gradient is approximately perpendicular to 

the fault trace. The model also was sensitive to 

estimates of vertical conductivity in areas where there 

are large differences between heads in the two aquifer 

systems. For the most part, a group of model 

parameters, such as the vertical distribution of 

pumpage; vertical leakance; general-head boundary 

conductance; and irrigation return flow controlled the 

goodness-of-fit for the Oxnard Plain. The model was 

relatively insensitive to ET, valley-floor infiltration, and 

some aquifer parameters such as transmissivity. As in 

most models, changes in water levels and ground-water 

flow were most sensitive to changes in the recharge and 

discharge boundary conditions near basin margins. 

Changes in pumpage, vertical leakance, and storage 

properties were more important to changes in head and 

ground-water flow in areas away from basin margins. 

Pumpage, and its vertical distribution, was the most 

sensitive parameter in this regional ground-water flow 

model. As in previous simulations of regional 

subsidence (Hanson, 1989; Hanson and Benedict, 

1994), matching the timing and the amount of land 

subsidence was most sensitive to changes in the initial 

preconsolidation stress thresholds.

This current model adequately reproduces longterm historical changes in flows and in ground-water 

levels on a regional scale, but the ability of the model to 

simulate the specific water-level histories of some 

wells is limited because the aquifers were grouped into 

only two layers. Because the ocean boundary greatly 

simplifies the mobile freshwater–saltwater interface, 

the simulation of coastal inflow and outflow is only a 

crude approximation of the actual process of seawater 

intrusion; therefore, caution should be taken in using 

this model to simulate relatively small-scale flows near 

the coast. Inflows and outflows over seasonal time 

periods were combined in the model; this may have 

had some effect on the ability of the model to simulate 

rapidly changing streamflow conditions during natural 

floodflows or during releases from Lake Piru in lowflow summer and fall months. The complex processes 

of irrigation return flow and related vertical leakage to 

the upper-aquifer system in the Oxnard Plain were 

further simplified by the exclusion of the semiperched 

system. The exclusion of the shallow fluvial deposits as 

a separate layer precluded the assessment of some 

ground-water/surface-water exchanges along the Santa 

Clara River and Calleguas Creek. However, even with 

these significant limitations, this model provides a 

framework for assessing regional water-resources 

management issues and a basis for further model 

development and refinement. This model also can be 

used to assess future water-supply projects and the 

relative importance of various flow components on a 

regional scale.

ANALYSIS OF GROUND-WATER FLOW

The calibrated ground-water flow model was 

used to analyze the distribution and magnitude of 

ground-water flow within the entire Santa Clara–

Calleguas Basin. The flow analysis in this report 

includes a summary of flow under predevelopment and 

historical conditions, the period of reported pumpage 

1984–93, projected future ground-water flow 

conditions in relation to planned water-supply projects, 

and projected future ground-water flow conditions for 

possible alternative water-supply projects. Formulation 

of planned future and alternative future water-supply 

projects was done jointly by the FGMA, the UWCD, 

and the CMWD.
116 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

The summaries of the flow analysis are grouped 

into categories of recharge, coastal flow, inland flow, 

and subsidence. These summaries describe the major 

inflow and outflow components driving the changes in 

supply and the effects of ground-water overdraft 

(demand). For budgetary-flow analysis, the regional 

ground-water flow system was divided into 34 subareas 

(fig. 17B) that represent the upper- and lower-aquifer 

systems in the 12 landward subbasins and offshore 

subareas of the Santa Clara–Calleguas Basin (fig. 1). 

Total flows, relative percentages of flow, and mean 

flows for the simulation period were used for the 

analysis of the long-term ground-water conditions. The 

mean flows were based on the flows from the last time 

step of every season; therefore, the mean flows of headdependent boundary conditions used to describe flows 

closely approximate but may not equal the average 

total flow over a simulation period. The mean flows 

should be considered with some caution because they 

may not adequately represent the true variability or the 

cumulative magnitude of a particular flow component. 

Net flow represents the difference between groundwater inflow and outflow for a particular boundary 

flow, such as coastal flow accross subarea boundaries.

The basin is partially under the management 

authority of the FGMA; other water purveyors include 

the UWCD and the CMWD water districts—all of 

which provide water and water-related services to 

different parts of the basin (fig. 26). The Oxnard Plain 

subbasin is subdivided into four model subareas: the 

Oxnard Plain Forebay, the Northwest Oxnard Plain, the 

Northeast Oxnard Plain, and the South Oxnard Plain 

(fig. 17B). These subareas are roughly in alignment 

with surface-water pipeline service areas and are 

coincident with the areal extent of the fluvial deposits 

within the two river drainages that cross the Oxnard 

Plain. The offshore part of the model is subdivided into 

three subareas that represent extensions of the Mound 

subbasin (Offshore Mound subarea), the northwestern 

Oxnard Plain north of the Hueneme submarine canyon 

(Offshore North Oxnard Plain), and the southern 

Oxnard Plain south of the Hueneme submarine canyon 

(Offshore South Oxnard Plain) (fig. 17B). For the 

purposes of this discussion, the Santa Clara River 

Valley consists of the Piru, Fillmore, Santa Paula, and 

Mound subareas, and the non-FGMA area consists of 

these same subareas plus the Santa Rosa Valley 

subarea. The FGMA areas are composed of the Oxnard 

Plain model subareas, referred to as the coastal FGMA 

subareas, and the Pleasant Valley and the Las Posas 

Valley model subareas, referred to as the inland FGMA 

subareas (fig. 17B).

Total flows, relative percentages of flow, and 

mean flows for the simulation period were used to 

analyze long-term ground-water conditions. Mean 

flows were based on flows from the last time step of 

every season. Therefore, mean flows of head-dependent 

boundary conditions, used to describe flows, closely 

approximate but may not exactly equal the average 

total flow over a simulation period. Because the 

regional model does not simulate transport or densitydependent flow, the summaries on coastal landward 

flow (seawater intrusion) (fig. 25B) are meant to give 

some regional approximation of potential flow along 

the coastal boundary of the regional aquifer system. 

Thus, using the reference to seawater intrusion implies 

that gradients above the equivalent freshwater head at 

the approximate average location of the seawater 

interface represent the inflow of seawater into the 

coarse-grained layers of the aquifer systems. Without 

density-dependent or transport modeling, such as that 

described by Nishikawa (1997), or some surrogate for 

advective flow, such as particle tracking, the reference 

to outflow at the coast could include moving the 

seawater front seaward or actually discharging 

freshwater into the ocean.
Anal

ysis of Ground-Water Flow 117

Pacific

Ocean

R18W R22W

T

4

N

T

2

N

Water Authorities

Fox Canyon GroundwaterManagement Agency (FGMA)United Water ConservationDistrict (UWCD)Calleguas MunicipalWater District (CMWD) River

Santa

Clara

Lake

Piru

Cal eguas l

Creek

EXPLANATION

Hydrologic UnitboundaryGround-watersubbasin boundary –Subbasin names aregiven in figure 1

Model-grid boundary offlow region and subareas

OxnardHueneme

pipeline

Pumping- Trough pipeline

Channel Islands wellsafter 1997 (5)City of Oxnard continuedpumpage after 1996 (1)Port Hueneme wellsafter 1997 (5)

Pleasant Valley wells – Pumpagereduced 5,000 acre-feet per yearwith Hill Canyon Deliveries (6)

CMWD Project – Injection andpump-back variable based on climate (3)

Increased recharge atSaticoy (4)

Increased recharge at El Rio (4)Del Norte wells – Pumpagereduced by 900 acre-feet per year (2)

Fault

Model cells used to simulate effects of proposed projects

(Project number)

00

10 MILES

10 KILOMETERS

A

118°45' 119°15'

34°

34

07'30"

°

22'30"

119°

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water

basin

Shallow alluvium and unconsolidateddepositsShallow alluvium

Consolidated and unconsolidateddeposits

Figure 26. Location of model cells used to simulate (A) proposed water-supply projects for the existing management plan, Santa Clara–Calleguas ground-water basin, Ventura County, California.
118 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Pacific

Ocean

R18W R22W

T

4

N

T

2

N

Water Authorities

Fox Canyon GroundwaterManagement Agency (FGMA)United Water ConservationDistrict (UWCD)Calleguas MunicipalWater District (CMWD)

River

Santa

Clara

Lake

Piru

Calleguas

Creek

B

EXPLANATION

Hydrologic Unit boundaryGround-water subbasin boundary –Subbasin names are given in figure 1Model-grid boundary offlow region and subareas

OxnardHueneme

pipeline

Pumping- Trough pipeline

Southern Oxnard Plain (3,6)Injection Southern Oxnard

Plain (1)

Northeast Oxnard Plain (5)

Pumping Trough Pipeline

Wells (4)

OH-Well pump-back (1)

(20,000 acre-feet per year)

Pleasant Valley (7)Happy Camp Canyon

Recharge (2)

Happy CampCanyon

Additional Components

(See figure 26A) Fault

0

0

10 MILES

10 KILOMETERS

118°45' 119°15'

34°3407'30"

°

22'30"

119°

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-waterbasin

Shallow alluvium and unconsolidateddepositsShallow alluvium

Consolidated and unconsolidateddeposits

Model cells used to simulate effects of alternative projects

(Potential-case number)

Figure 26—Continued. (

B) alternative water-supply projects in the model of the Santa Clara–Calleguas ground-water basin, Ventura County, California. 
Analysis of Ground-Water Flow 119

Predevelopment Ground-Water Flow

Simulated water levels for predevelopment 

conditions were about 40 and 50 ft above land surface 

in the upper- and lower-aquifer systems along the 

coast, respectively, which is consistent with early 

reports of artesian water levels during 1870–90. The 

total simulated recharge for predevelopment conditions 

was 59,900 acre-ft/yr, and the total net recharge was 

33,650 acre-ft/yr. Streamflow accounted for 68 percent 

of the total recharge and nonstreamflow recharge 

accounted for 32 percent (fig. 25C). Simulated 

streamflow resulted in 40,600 acre-ft/yr of infiltration 

and 26,300 acre-ft/yr of ground-water discharge back 

into the stream channels for a net streamflow recharge 

of about 14,300 acre-ft/yr (table 6). Net streamflow 

recharge was largest in the Piru (32 percent) and 

Fillmore (22 percent) subareas and the Oxnard Plain 

Forebay (18 percent). Ground-water discharge to the 

Santa Clara River was largest in the Fillmore subarea 

(41 percent) and was concentrated near the narrow 

boundary with the Santa Paula subarea. Streamflow 

discharge also occurs in the Piru subarea at the narrow 

boundary with the Fillmore subarea and in the Santa 

Paula and South Las Posas Valley subareas. Total mean 

nonstreamflow recharge for the entire regional flow 

system was about 19,400 acre-ft/yr (table 6) of which 

about 4,800 acre-ft/yr is valley-floor recharge and 

about 14,600 acre-ft/yr is mountain-front and bedrock 

recharge (fig. 25C). 

Total simulated natural discharge was 59,900 

acre-ft/yr (fig. 25C) and the total net discharge, which 

equals net recharge, was about 33,650 acre-ft/yr. 

Coastal outflow accounts for about 18,900 acre-ft/yr 

which is 31 percent of the total discharge (fig. 25C) and 

56 percent of the net discharge. ET accounts for about 

14,800 acre-ft/yr which is 25 percent of the total 

discharge (fig. 25C) and 44 percent of the net 

discharge. The largest amounts of ground-water 

discharge as ET occur in the Fillmore (38 percent), 

South Pleasant Valley (21 percent), and Santa Paula 

subareas (17 percent). 

Net underflow from the Santa Clara River Valley 

subareas to the Oxnard Plain subareas was simulated as 

about 6,890 acre-ft/yr for time-averaged 

predevelopment conditions. A net downward leakage 

between aquifer systems of about 450 acre-ft/yr was 

simulated for the entire Oxnard Plain subareas. The 

largest downward flow, about 1,330 acre-ft/yr, was 

simulated in the Oxnard Plain Forebay. This relatively 

small net leakage includes downward leakage in the 

Oxnard Plain Forebay and strictly upward leakage in 

the South Oxnard Plain subarea and most of the 

Northeast and Northwest Oxnard Plain subareas, which 

is consistent with the upward vertical head gradient in 

these areas.

Historical Ground-Water Flow, 1984–93

The analysis of historical ground-water flow was 

restricted to 1984–93, the period when estimates of 

pumpage were the most complete and were largely 

based on reported values of metered pumpage. This 

period contains an equal number of wet and dry years; 

3 wet years, followed by 5 dry years, followed by 2 wet 

years. This period also was one of increasing groundwater management related actions: increasing 

streamflow diversions for artificial recharge at the 

Freeman Diversion, increasing discharges of treated 

sewage effluent, and increasing pumpage. As a result of 

these increases in supply and demand, there was an 

increase in recharge, seawater intrusion, subsidence, 

leakage between aquifers, and ground-water flow 

between subareas, as well as reduced ET and a 

reduction in ground water in storage. The policies of 

the FGMA resulted in a moratorium on new wells in 

the upper-aquifer system in the northwestern part of the 

Oxnard Plain subareas and on the drilling of new wells 

and increased pumpage in the lower-aquifer system 

(Rick Farnsworth, Fox Canyon Ground-Water 

Management Agency, oral commun., 1991). This has 

resulted in additional seawater intrusion in the lower 

aquifer system and additional subsidence.

Summary of Ground-Water Conditions

The total simulated pumpage for 1984–93 is 

2,468,600 acre-ft, which is an average of about 

247,000 acre-ft/yr. About 37 percent of the pumpage 

was from the Oxnard Plain subareas, 37 percent from 

the Santa Clara River Valley subareas, 13 percent from 

the Las Posas Valley subareas, 9 percent from the 

Pleasant Valley subareas, 3 percent from the Mound 

subarea, and 1 percent from the Santa Rosa Valley 

subarea (fig. 25D). The distribution of pumpage for the 

ground-water management area is 59 percent for the 

FGMA-managed areas and 41 percent for the 

non-FGMA-managed areas.
120 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Overall, pumpage during the 1984–93 sequence 

of wet and dry years resulted in overdraft of the 

ground-water flow system. The combination of water 

from storage, coastal landward flow (seawater 

intrusion), and subsidence represents an estimated 

23,830 acre-ft/yr of average overdraft for the 1984–93 

period, which is about 10 percent of the average annual 

pumpage. Of the total overdraft, 60 percent is from 

aquifer storage depletion, 31 percent is from coastal 

landward flow (seawater intrusion), and 17 percent is 

from subsidence. The mean rate of water extracted 

from aquifer storage is 14,260 acre-ft/yr. Results of the 

model simulations indicate that a relatively large 

contribution of aquifer storage is from the loweraquifer system (layer 2) of the Oxnard Plain subarea 

and of the inland FGMA-managed subareas, and from 

the upper-aquifer system in the Santa Clara River 

Valley subareas. The contribution of ground water from 

subsidence (interbed storage) was about 4,100 acreft/yr (table 6) and represent about 17 percent of the 

average annual overdraft (23,830 acre-ft/yr). Recall 

that water derived from subsidence is, in part, a onetime source of water because the inelastic component 

of interbed storage is irreversible. Simulation results 

show that most of water derived from subsidence is 

from the Oxnard Plain subareas (47 percent) and the 

Las Posas Valley subareas (34 percent) (fig. 25D). 

Ground-water pumpage resulted in a decrease in 

ET and stream baseflow in the inland subareas. Both 

ET and stream baseflow remain concentrated at the 

basin narrows of the Santa Clara River Valley and the 

Las Posas Valley subareas. Most of the simulated ET 

occurs in the Santa Clara Valley subareas (76 percent or 

810 acre-ft/yr) (fig. 25D). The simulated ET for 

1984–93 averaged 1,060 acre-ft/yr and is 7 percent of 

the simulated annual ET for the predevelopment 

period. Baseflow averaged about 8,250 acre-ft/yr for 

the period 1984–93 and is about 13 percent of the total 

streamflow infiltration and about 33 percent of the 

simulated predevelopment baseflow. 

Recharge

Hydrological, geophysical, and geochemical 

data and ground-water simulations indicate that the 

upper-aquifer system is the recipient of most of the 

natural and artificial recharge and, thus, is a relatively 

more dynamic flow system than is the lower-aquifer 

system. Simulated total recharge (natural and artificial) 

for 1984–93 was 228,500 acre-ft/yr, which is about 

93 percent of the average pumpage for this period. 

Most of the recharge occurred in the upper-aquifer 

system of the Santa Clara River Valley and the Oxnard 

Plain Forebay subareas. The total simulated natural 

recharge was about 114,100 acre-ft/yr: 27,800 acreft/yr of mountain-front and bedrock recharge, 

24,100 acre-ft/yr of valley-floor recharge, and 

62,200 acre-ft/yr of net streamflow infiltration. The 

distributions of natural recharge show that most of the 

mountain-front and bedrock recharge occurs in the 

Santa Clara River Valley subareas, most of the 

streamflow recharge occurs in the Piru and Fillmore 

subareas, and most of the valley-floor infiltration 

occurs in the Santa Clara subareas (fig. 25D). 

Simulated natural recharge and streamflow infiltration 

were 21 and 25 percent, respectively, of the total 

pumpage for 1984–93. The model simulated 

54,400 acre-ft/yr of artificial recharge; 51,000 acreft/yr of irrigation return flow; and 9,000 acre-ft/yr of 

treated sewage effluent. About 93 percent of the total 

distribution of artificial recharge occurs in the Oxnard 

Plain Forebay and 7 percent occurs in the Piru subarea. 

Simulated irrigation return flow is greatest in the 

Oxnard Plain, and infiltration of treated sewage effluent 

is greatest in the Pleasant Valley subareas. 

A comparison of the 1984–93 conditions with 

predevelopment conditions indicated a large increase in 

the rate of valley-floor recharge and streamflow 

recharge (fig. 25C,D). The largest increases were in the 

Santa Clara River Valley subareas. The net streamflow 

recharge increased from 14,300 acre-ft/yr to 62,200 

acre-ft/yr.
Analysis of Ground-Water Flow 121

Coastal Flow

Net coastal landward flow occurred in both 

aquifer systems throughout the Oxnard Plain subareas 

during parts of the 1984–93 period (fig. 22A,B). The 

total simulated net seaward flow in the upper-aquifer 

system (layer 1) was 9,500 acre-ft, which is 

considerably less than the seaward flow simulated for 

steady-state conditions. Flow was seaward in 1984 but 

reversed to landward in 1985; landward flow increased 

during the 1987–91 dry-year period (fig. 25B). By the 

end of 1993, the measured and simulated water levels 

had recovered and were above the equivalent 

freshwater head in the upper-aquifer system of the 

submarine outcrops (fig. 25A) resulting in seaward 

flow and artesian conditions and flowing wells in parts 

of the Oxnard Plain subareas. This change in coastal 

flow in the upper-aquifer system is supported by 

reduced chloride concentrations and reduced EM 

conductivities in many of the coastal monitoring wells 

(figure A5.2 in Appendix 5).

The simulated total coastal landward flow for 

1984–93 was 64,200 acre-ft; the landward flow was 

due to declining water levels in the lower-aquifer 

system (fig. 25A,B). This sustained coastal landward 

flow (fig. 25B) is supported by increased chloride 

concentrations and increased EM conductivities in 

many of the coastal monitoring wells (figure A5.2 in 

Appendix 5). 

The model simulations indicate that total coastal 

landward flow occurs during seasonal and climatic 

cycles and during periods of long-term storage 

depletion (fig. 25B). Simulated coastal landward flow 

began in the lower-aquifer system of the South Oxnard 

Plain subarea in about 1928, in the Northwest Oxnard 

Plain subarea in about 1930, and in the Mound subarea 

as early as 1919. Coastal flow was landward in the 

upper-aquifer system during the droughts of the 1930s 

and from the mid-1940s through the last drought 

(1987–91), and was seaward during the intervening wet 

periods. Coastal flow was consistently landward in the 

lower-aquifer system of the south Oxnard Plain subarea 

for the entire period 1928–94. The general timing of 

the simulated coastal landward flow is consistent with 

observed increases in salinity, which were due to 

seawater intrusion into the water-supply wells. The 

earliest documented seawater intrusion in the upper 

aquifer occurred in the Oxnard Plain subareas during 

1930–40 followed by increases in seawater intrusion 

between 1946 and the late 1970s. 

Of the total simulated coastal landward flow, 

about 54 percent entered the South Oxnard Plain 

subarea, most of which entered the lower-aquifer 

system (fig. 25D). The mean net coastal seaward flow 

was about 950 acre-ft/yr for the upper-aquifer system 

and the mean net coastal landward flow (seawater 

intrusion) was about 6,420 acre-ft/yr for the loweraquifer system (table 6). Seawater intrusion, however, 

has a cumulative effect, contributing to long-term 

overdraft and loss of storage for potable water. The 

long-term simulation of coastal landward flow 

indicates that seawater intrusion started as early as the 

summer of 1927; by 1932, about 1,957 acre-ft/yr was 

intruding the offshore parts of the upper-aquifer 

system. This is consistent with the early accounts of 

increased salinity in some of the shallow coastal wells. 

Model simulations show that the total coastal landward 

flow during 1984–93 was about 12 percent of the 

526,600 acre-ft of total coastal landward flow (seawater 

intrusion) simulated for the summer of 1927 through 

the winter of 1993.

Flow Between Subareas and Aquifer Systems

The direction and mean flow for the simulated 

historical period 1984–93 are shown in figure 25A. 

Faults are an important factor in the distribution of 

ground-water and water levels in the lower-aquifer 

system and, to a lesser extent, in the upper-aquifer 

system. For the upper-aquifer system, ground-water 

underflow to the Oxnard Plain subareas averaged about 

4,200 acre-ft/yr of inflow from the Santa Paula subarea 

and about 2,770 acre-ft/yr of outflow into the South 

Pleasant Valley subarea (fig. 25A). For the loweraquifer system, the simulated flow averaged less than 

140 acre-ft/yr out of the Oxnard Plain Forebay toward 

the Santa Paula subarea and about 3,000 acre-ft/yr into 

the region of lower water levels in the South Pleasant 

Valley subarea (fig. 25A).
122 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Table 6. Summary of simulated ground−water flow components for the Santa Clara–Calleguas Basin, Ventura County, California [All flows in acre−feet per year. The precision of the numbers do not reflect the variable accuracy of the estimates. FGMA, Fox Canyon Ground−water Management Agency] Simulation case and time period1 1) Total−mean nonstreamflow recharge2 2) Mean spreading grounds recharge3 3) Las Posas recharge4 Mean ground−water inflow to Oxnard Plain from adjacent subbasins5: 1) Santa Clara River Valley 2) West Las Posas 3) South Pleasant 

Valley

Mean−net 

streamflow 

recharge6

Mean

coastal flow7

1) Layer 12) Layer 2

 Mean 

vertical flow 

between 

aquifer 

systems in 

Oxnard 

Plain8

Mean 

evapotranspiration

Mean pumpage9

1) FGMA area

2) Outside FGMA area

3) Total Basin area

Mean 

flow from 

interbed 

storage 

(subsidence)

Mean

change in

 aquifer

storage10

Predevelopment 

(time−averaged)

1) 19,400

2) 03) 0

1) 6,9002) 2,1803) 1,720

14,300 1) −16,000

2) −2,900

450 14,800 1) 0

2) 03) 0

 0 0

Reported 

pumpage period: 

1984−1993

1) 166,000

2) 54,400

3) 0

1) 5,7702) 1,2603) 5,720

62,200 1) −950

2) 6,420

22,700 1,060 1) 146,000

2) 101,0003) 247,000

4,100 14,260

Base−Case 1: 1994−2017

1) 179,000

2) 63,500

3) 3,750

1) 2,900

2) −80

3) −2,740

58,700 1) −3,970

2) 4,770

20,900 950 1) 141,000

2) 100,0003) 241,000

1,500 2,700

Base−Case 2: 1994−2017

1) 179,000

2) 63,500

3) 3,750

1) 2,540

2) 20

3) −1,840

56,000 1) −5,750

2) 3,420

18,600 1,030 1) 129,000

2) 100,0003) 229,000

 420 −2,400

Base−Case 3: 1994−2017

1) 179,000

2) 63,500

3) 3,750

1) 2,000

2) 30

3) −1,360

50,300 1) −9,430

2) 2,130

17,400 1,180 1) 113,600

2) 99,700

3) 213,000

−330 −7,000

Base−Case 4: 1994−2037

1) 159,000

2) 55,300

3) 3,750

1) 3,490

2) −110

3) −2,500

80,800 1) −2,680

2) 4,950

 20,500 2,030 1) 140,000

2) 100,0003) 240,000

1,070 −1,750

Potential−Case 1: 1994−2017

1) 194,000

2) 63,500

3) 3,750

1) 3,500

2) −100

3) −2,950

60,600 1) −8,000

2) 4,900

20,700 900 1) 140,000

2) 101,0003) 241,000

1,500 950

Potential−Case 2: 1994−2017

1) 190,000

2) 63,500

3) 3,750

1) 2,900

2) 80

3) −2,740

58,600 1) −3,970

2) 4,770

20,900 950 1) 141,000

2) 100,0003) 241,000

1,500 −8,500

Potential−Case 3: 1994−2017

1) 179,000

2) 63,500

3) 3,750

1) 2,540

2) 290

3) −4,580

56,100 1) −7,330

2) 2,470

17,800 1,030 1) 129,000

2) 100,0003) 229,000

790 −420
Analysis of Ground-Water Flow 123

Potential−Case 4: 1994−2017

1) 179,000

2) 63,500

3) 3,750

1) 2,910

2) 80

3) −2,880

58,700 1) −3,640

2) 4,480

19,600 950 1) 141,000

2) 100,0003) 241,000

1,360 −40

Potential−Case 5: 1994−2017

1) 179,000

(2) 63,500

3) 3,750

1) 3,000

2) 110

3) −3,380

58,700 1) −2,880

2) 3,590

14,800 950 1) 141,000

2) 100,0003) 241,000

1,250 3,130

Potential−Case 6: 1994−2017

1) 179,000

2) 63,500

3) 3,750

1) 2,910

2) 80

3) −3,260

58,500 1) −2,220

2) 3,180

16,600 950 1) 141,000

2) 100,0003) 241,000

1,270 2,800

Potential−Case 7: 1994−2017

1) 179,000

2) 63,500

3) 3,750

1) 2,860

2) 110

3) −1,600

57,900 1) −3,950

2) 4,040

18,100 970 1) 141,000

2) 100,0003) 241,000

1,070 4,650

1 All time periods are in calendar years and all simulated mean flows are in acre-feet per year.2 Recharge includes mountain-front recharge, valley-floor infiltration, bedrock infiltration, irrigation returnflow, sewage effluent, and artificial recharge.3 Spreading-grounds recharge is the sum of infiltration of surface spreading at the Piru, El Rio, and Saticoy spreading grounds operated by United Water Conservation District. 4 Number is the additional potential recharge at the aquifer storage and recovery facility planned for operation by Calleguas Municipal Water District in the East Las Posas subarea in the loweraquifer system.5 Simulated mean underflow to the Oxnard Plain subareas with top number net underflow from Santa Clara Valley subareas, middle number is mean underflow from West Las Posas subarea, and 

bottom number is mean underflow from the South Pleasant Valley subarea.

6 Total net mean streamflow loss to all simulated rivers and tributaries.7 Mean coastal flow is the total flow between all coastal subareas and the adjacent offshore subareas and is used in this study as a surrogate for seawater intrusion for the total time period simulated. 

A positive number is coastal landward flow (seawater intrusion) from offshore subareas and a negative number is coastal seaward flow to offshore subareas.

8 Total net mean flow from upper-aquifer system (model layer 1) to lower-aquifer system. Positive number is downward flow and negative number is upward flow.9 Top number indicates simulated mean pumpage for FGMA area, middle number is pumpage outside of GMA areas, and bottom number is pumpage in total Santa Clara−Calleguas Basin.10 Mean change in storage is the total for the upper- and the lower-aquifer system divided by the number of years for the period of simulation for the entire model. Positive number indicates water 

coming from ground-water storage and negative number indicates water returning to ground-water storage.

Simulation

case and 

time period1

1) Total−mean 

nonstreamflow 

recharge2

2) Mean spreading 

grounds recharge3

3) Las Posas recharge4

Mean ground−water 

inflow to Oxnard Plain 

from adjacent 

subbasins5:

1) Santa Clara

River Valley

2) West Las Posas

3) South Pleasant 

Valley

Mean−net 

streamflow 

recharge6

Mean

coastal flow7

1) Layer 12) Layer 2

 Mean 

vertical flow 

between 

aquifer 

systems in 

Oxnard 

Plain8

Mean 

evapotranspiration

Mean pumpage9

1) FGMA area

2) Outside FGMA area

3) Total Basin area

Mean 

flow from 

interbed 

storage 

(subsidence)

Mean

change in

 aquifer

storage10

Table 6. Summary of simulated ground−water flow components for the Santa Clara–Calleguas Basin, Ventura County, California
124 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

The simulated flow across the Oak Ridge and 

McGrath Faults from the Mound and Santa Paula to the 

Oxnard Plain subareas for 1984–93 was about 

5,800 acre-ft/yr, of which about 73 percent flowed to 

the upper-aquifer system, in the narrow swath of the 

Santa Clara River flood plain where the fault was not 

simulated. Almost no flow occurred across the Oak 

Ridge and McGrath Faults into the lower-aquifer 

system. The simulated mean flow across the Country 

Club Fault from the Santa Paula subarea to the Mound 

subarea was about 4,200 acre-ft/yr, resulting in a net 

inflow to the Mound subarea of about 2,500 acre-ft/yr. 

Three other fault-related flow barriers between the 

subareas control underflow: the Central Las Posas 

Fault, the extension of the Springville Fault and the 

Somis Fault, and the Camarillo Fault. The Central Las 

Posas Fault controls flow between the East and West 

Las Posas Valley subarea; the simulated mean flow 

across this fault toward the West Las Posas Valley 

subarea was 920 acre-ft/yr. The extension of the 

Springville Fault and the Somis Fault control flow 

between the East Las Posas Valley and North Pleasant 

Valley subareas; the simulated mean flow toward North 

Pleasant Valley subarea was 1,500 acre-ft/yr in the 

lower-aquifer system and about 196 acre-ft/yr in the 

upper-aquifer system. The Camarillo Fault controls 

flow between the North and South Pleasant Valley 

subareas; the simulated mean flow across this fault 

from the South to the North Pleasant Valley subareas 

was 3,600 acre-ft/yr (figs. 12 and 16). Coastal and 

offshore faults, such as the Bailey Fault and the 

extension of the Sycamore Fault, are effective barriers 

that have contributed to water-level declines of more 

than 100 ft below sea level at the coast and prevent 

seawater intrusion into the lower-aquifer system in the 

southern Oxnard Plain near Mugu submarine canyon 

(fig. 16). The Hueneme Canyon, the Old Hueneme 

Canyon, and the South Hueneme Canyon Faults reduce 

flow along the southern exposures of the submarine 

canyons and retard the northwestern propagation of 

water-level declines caused by pumping in the 

lower-aquifer system of the South Oxnard Plain 

subarea (figs. 16 and 25A).

The simulated downward flow from the upper- to 

the lower-aquifer system during 1984–93 for groups of 

subareas (fig. 25D) averaged about 67,000 acre-ft/yr. 

The downward flow was greatest in the Las Posas 

Valley (34 percent, or 22,800 acre-ft/yr), Oxnard Plain 

(34 percent, or 22,700 acre-ft/yr), and Pleasant Valley 

(22 percent, or 14,700 acre-ft/yr) subareas (fig. 25D). 

The simulated average downward flow in the Oxnard 

Plain is similar to previous estimates (Mann and 

Associates, 1959; California Department of Water 

Resources, 1971). The downward flow between aquifer 

systems increased during the dry years owing to 

increases in water-level differences (fig. 15). 

Water-level differences between the upper- and 

lower-aquifer systems were more than 100 ft in the 

East and West Las Posas Valley and the Pleasant Valley 

subareas, more than 30 ft in the Oxnard Plain subareas, 

and more than 10 ft in the Santa Clara River Valley 

subareas. 

Land Subsidence

Simulation results indicate that the total quantity 

of water derived from subsidence during 1984–93 was 

35,700 acre-ft, for an average net rate of subsidence of 

3,570 acre-ft/yr. The largest contributions were from 

the Oxnard Plain (47 percent) and the Las Posas Valley 

(34 percent) subareas; smaller contributions were from 

the Mound subarea (10 percent), the Pleasant Valley 

subareas (7 percent), and the Santa Clara River Valley 

subareas (2 percent) (fig. 25D). Water derived from 

compaction is about 20 percent of the mean annual 

overdraft, which is comparable to previous regional 

estimates (Hanson and Benedict, 1994). 

Simulation results for the 1984–96 period show 

that 96 percent of the water was derived from 

compaction of the lower-aquifer system. This may 

reflect, in part, the additional development of ground 

water from the lower-aquifer system and, in part, the 

moratorium of the 1980s on new wells in the upperaquifer system throughout the Oxnard Plain subareas. 

Collectively, these resulted in increased water-level 

declines in the lower-aquifer system during the 

1987–91 drought. Thus, overdraft appears to have a 

significant effect on subsidence in the coastal regionalaquifer systems. Overdraft and land subsidence will 

continue during dry-year periods when water levels 

drop below previous maximum declines.
Analysis of Ground-Water Flow 125

Projected Future Ground-Water Flow for Existing 

Management Plan

The model was used to assess future groundwater conditions based on the implementation of 

proposed water-supply projects included in the existing 

management plan for the Santa Clara–Calleguas 

ground-water basin. These plans assume the current 

water demands plus the addition of proposed watersupply projects. Testing of projects included assessing 

long-term conditions of the ground and surface water 

through periods of climatic extremes; for example, the 

ability to recharge aquifers during wet periods and to 

arrest seawater intrusion and subsidence during dry 

periods. 

Using the model to cycle the average water 

demand through a wet and dry period, simulated 

natural and artificial recharge were varied to reflect the 

changing and extreme conditions typical of the 

southern California coast. Two approaches were used 

to estimate future recharge, streamflow, and climaterelated water-demand: a 24-year projection (1994–

2017) using historical estimates of recharge and 

measured streamflow, and a 44-year spectral projection 

(1994–2037) of future precipitation.

The primary approach used to project future 

ground-water flow was to simulate the 24-year period 

1994–2017. The historical inflow conditions for 

1970–93 were used for these simulations; this period 

cycles through a combination of 13 dry and 11 wet 

years (fig. 2A). This record was used to simulate the 

extremes in recharge, streamflow, and pumping 

demand that may be typical of future interdecadal 

climate variation. The 1970–93 data series, although 

not a correlated projection of probable future 

conditions, does capture the complete variation of 

recent climate, recharge, and streamflow and the 

beginning of regulated streamflow (1970) in the Santa 

Clara–Calleguas Basin.

The alternative approach to project future 

ground-water flow was to simulate recharge, 

streamflow, and climate-related demand based on 

spectral estimates of future precipitation. For this 

approach, precipitation was estimated for the next 

50 years (see Appendix 3 for a description of this 

approach). The first 44 years, 1994–2037, represent a 

total of 21 wet years and 23 dry years. The 

precipitation estimates are an autocorrelated series of 

probable future conditions that include three climatic 

cycles of intradecadal (2.9 and 5.3 yr) to decadal 

(22 yr) length; they represent 60 percent of the 

variation of typical changes in rainfall. The 44-year 

period approximately represents two decadal cycles of 

climate variability. The advantages of the spectral 

approach are a longer period of projection and a 

seamless transition from historical climatic and aquifer 

conditions to probable future conditions of supply and 

demand (Hanson and Dettinger, 1996). The spectral 

approach uses a moving autocorrelation with historical 

rainfall data that closely approximates rainfall for the 

years 1994–96 (figure A3.3 in Appendix 3). The 

autocorrelation with historical data provides a seamless 

transition with high correlation for about the first 

7 years into the future and decreasing correlation 

further into the future.

The simulations for both projection approaches 

included adjusting average ground-water pumpage on a 

well-by-well basis for the period of reported pumpage 

(1984–93), estimating irrigation return flow from the 

1969 land-use distribution, and varying recharge and 

streamflow climatically. Average pumpage and 

irrigation return flow were adjusted climatically using 

ratios of wet or dry pumpage to average historical 

reported pumpage for each subarea. The following six 

proposed water-supply projects (fig. 26B) were 

included in assessing the potential for continued 

overdraft conditions: 

(1) Cessation of pumping of well in the city of 

Oxnard from July 1995 through December 1996, and a 

restart of pumping in January 1997;

(2) UWCD surface-water deliveries of 900 acreft/yr to Del Norte in lieu of pumpage from the upperaquifer system starting in January 1997;

(3) CMWD aquifer storage and recovery (ASR) 

project in the East Las Posas Valley subarea from 

January 1997 to December 2001, using a proposed 

injection rate of 5,000 acre-ft/yr for wet years, 

1,250 acre-ft/yr for average years, and a pump-back 

recovery of 2,500 acre-ft/y for dry years. In 2002, the 

proposed injection rate was increased to 10,000 acreft/yr for wet years; 2,500 acre-ft/yr for average years; 

and a pump-back recovery of stored water of 5,000 

acre-ft/yr for dry years;
126 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

(4) Increased artificial recharge by the UWCD at 

El Rio and Saticoy based on the projected increased 

capacity of the Freeman Diversion (Steve Bachman, 

United Water Conservation District, written commun., 

1996). With the addition of the Rose pit near Saticoy, 

the projected artificial recharge ranges from 0 to 

127,900 acre-ft/yr. The spectral approach used 

estimates ranging from 6,000 to 92,000 acre-ft/yr;

(5) Reduced average pumpage from the loweraquifer system by the city of Port Hueneme, the 

Channel Islands Beach Community Services District, 

and the U.S. Navy base at Port Hueneme for a 

combined reduction of as much as 1,000 acre-ft/yr in 

lieu of new deliveries of imported water from the State 

water project starting in January 1997;

(6) Reduced pumpage by the PVCWD in lieu of 

5,000 acre-ft/yr of new surface-water deliveries from 

the city of Thousand Oaks Hill Canyon wastewatertreatment plant starting in January 1998.

Four simulations, referred to as “base-cases 1–

4,” were used to project future ground-water 

conditions. Base-case 1 represents the adjusted 

1984–93 mean annual pumpage for the six proposed 

water-supply projects listed above for the 24-year 

period (1994–2017). Two additional base-case 

scenarios were simulated to address the existing 

FGMA ordinance 5.5 (Fox Canyon Groundwater 

Management Agency, 1997) of a rolling cut back in 

pumpage (base-case 2) and the step cut-back reduction 

of pumping which began in the early to middle 1990s 

(base-case 3). These two cut-back simulations are 

based on average pumpage throughout the entire Santa 

Clara–Calleguas Basin for 1984–89. These two basecase projections used the 24-year period of projection 

and the same historical period of recharge, streamflow, 

and climate-related demand conditions. Base-case 4 is 

the simulation of future ground-water flow for the 

extended 44-year period; this simulation is based on 

the spectral estimate of precipitation and uses the same 

adjusted mean pumpage for 1984–93 that was used for 

base-case 1.

The mean historical pumpage for 1984–93 for 

the six hypothetical projects yielded a mean adjusted 

pumpage of about 241,000 acre-ft/yr for base-case 1 

for the 24-year period and 240,000 acre-ft/yr for 

base-case 4 for the 44-year period (table 6). This 

represents about 59 percent (141,000 acre-ft/yr) of the 

total pumpage in the FGMA area and 41 percent 

(100,000 acre-ft/yr) in non-FGMA areas for 

base-case 1 (table 6). The total adjusted mean pumpage 

for base-case 1 for the FGMA area was about 

6,000 acre-ft/yr less than the total mean pumpage for 

the FGMA area for the 1984–93 period.

Simulation of the rolling cut-back (base-case 2) 

scenario shows the potential effect of the FGMA 

Ordinance 5.5 (Fox Canyon Groundwater Management 

Agency, 1997) and represents the 25-percent total cut 

back in pumpage as a 5-percent rolling cut back every 

5 years through the 2010. This is equivalent to a 

5-percent cut back in average pumpage for 1994, a 

10-percent cut back in average pumpage for the years 

1995–99, a 15-percent cut back in average pumpage for 

the years 2000–2004, a 20-percent cut back in average 

pumpage for the years 2005–2009, and a 25-percent 

cut back in average pumpage for the years 2010–2017. 

Total average pumpage with climatic variation in 

demand was about 229,000 acre-ft/yr, of which 

56 percent is for the FGMA area of the basin and 

44 percent is for the non-FGMA areas of the basin 

(table 6). The total adjusted average pumpage for 

base-case 2 is about 18,000 acre-ft/yr less than that for 

the 1984–93 period for the entire modeled area. Most 

of the reduction was in the FGMA area and represents 

an average 12-percent reduction in pumpage in the 

FGMA area for the 24-year period.

The simulation of the step cut-back (base-case 3) 

scenario represents the potential effect of continuing 

the apparent reduction in pumping that occurred in the 

mid-1990s. The reduction is based on the estimated 

total pumpage of about 100,000 acre-ft for 1996 for the 

FGMA area, which represents a 37-percent cut back 

from the average pumpage for 1984–89. This 

37-percent reduction was applied uniformly to all 

pumpage within the FGMA boundaries for the entire 

projection period; it was not applied to pumpage in the 

Piru, Fillmore, Santa Paula, and Mound subareas or in 

the eastern part of the Santa Rosa Valley subarea, areas 

that are outside the FGMA area. The projected climatic 

variations increased overall demands on pumpage and 

added an average additional 13,600 acre-ft/yr to the 

reduced pumpage rate in the FGMA area. Total mean 

pumpage for base-case 3 is about 213,000 acre-ft/yr, of 

which about 53 percent is for the FGMA area of the 

basin and 47 percent is for non-FGMA area (table 6). 

Total mean pumpage is about 34,000 acre-ft/yr less 

than that for the 1984–93 period for the entire modeled 

area. 
Analysis of Ground-Water Flow 127

Summary of Projected Ground-Water Conditions

Differences in ground-water levels and changes 

in ground-water storage between 1994 and 2017, the 

end of the projected period, are shown in figure 27 for 

the four base-case simulations. The water-level-change 

maps (fig. 27A–D) indicate a continued decline in the 

Oxnard Plain subarea, the Santa Clara River Valley 

subareas, and the East and West Las Posas Valley 

subareas for base-case 1; the declines are as much as 

67 ft in the upper aquifer system (fig. 27A). The rolling 

cut-back (base-case 2) and the step cut-back (basecase 3) projections progressively show decreased 

declines and increased recoveries (fig. 27B,C). Total 

ground-water storage change ranges from a withdrawal 

from storage of about 65,200 acre-ft (2,700 acre-ft/yr) 

for base-case 1 to a return of water to storage of about 

168,100 acre-ft (7,000 acre-ft/yr) for base-case 3 

(table 6). The large withdrawals of water from storage 

in the Oxnard Plain subareas were coincident with the 

withdrawals in the Oxnard Plain Forebay and the 

Northeast Oxnard Plain subareas. The changes in 

storage during the projection period were as much as 

60,000 acre-ft during dry-year periods in the Oxnard 

Plain Forebay and Fillmore subareas (fig. 27E). In the 

step cut-back and rolling cut-back simulations, the 

storage changes were reduced for the Oxnard Plain 

Forebay but were comparable for the Piru, Fillmore, 

and Santa Paula subareas (fig. 27A,B,C,E). The cut 

backs did not affect the magnitude of pumpage in these 

Santa Clara River Valley subareas because they were 

outside the FGMA area (fig. 26). The step cut-back 

projection (base-case 3) resulted in the largest 

reduction of coastal landward flow (seawater intrusion) 

in the lower-aquifer system and the largest increase of 

coastal seaward flow in the upper-aquifer system 

because the pumpage reductions were applied for the 

entire projection period (fig. 27E). This is illustrated by 

the hydrographs of supply well 1N/22W-3F4 for the 

city of Oxnard which show that the simulated water 

levels for the step cut-back projection (base-case 3) are 

always higher than those for the rolling cut-back 

projection (base-case 2) (fig. 27F). The higher 

hydraulic head near the coast results in less coastal 

landward flow (seawater intrusion).

A comparison of model results between the 

spectral projection for base-case 4 and the historical 

hydrology projection for base-case 1 indicates 

differences in the amount of water-level declines, 

changes in storage, cumulative coastal landward flow 

(seawater intrusion), and the timing of wet and dry 

periods. The spectral projection for base-case 4 

indicates that water-level declines and losses in storage 

were comparable to those of base-case 1 at the end of 

the 44-year spectral projection period 1994–2037 

(fig. 27A, D, and E). However, the major cycles of 

water-level declines and storage losses were opposite 

in phase during 2017 and occurred earlier in the 

projection period of base-case 4 (fig. 27A,D). For 

example, the difference in water levels in supply well 

1N/22W-3F4 for the city of Oxnard was as much as 

80 ft between base-cases 1 and 4 during periods when 

the projections were out of phase (fig. 27F). 

Projections of base-case 4 show significantly more 

coastal landward flow (seawater intrusion) by 2017 

(fig. 27D) than was projected in base-case 1 (fig. 27A). 

The comparison of base-cases 1 and 4 shows the 

importance of the range of possible wet and dry periods 

and the sequence of events that may affect the state of 

the system and the management of the water resources.

Recharge

The historical inflow conditions for the base-case 

1–3 projections are similar and consist of recharge of 

about 179,000 acre-ft/yr, of which about 63,500 acreft/yr was artificial recharge from the UWCD spreading 

grounds in Piru subarea and in the Oxnard Plain 

Forebay (table 6). Recharge of diverted streamflow 

from the spreading grounds was about 9,000 acre-ft/yr 

more than the average historical recharge for 1984–93; 

the projected increase in recharge was due to the 

increased capacity of the Freeman Diversion. This 

increase in recharge, however, did not stop water-level 

declines throughout most of the Santa Clara River 

Valley and the Oxnard Plain subareas (fig. 27A).

The simulation of the proposed CMWD ASR 

project (base-case 3) in the East Las Posas Valley 

subarea for the injection of 3,750 acre-ft/yr added 

about 90,000 acre-ft of net imported water to groundwater storage in the lower-aquifer system (table 6)

during 1994–2017. An additional 25,000 acre-ft of 

injected water was pumped back during dry years or 

years with average precipitation. Water-level rises 

relative to 1993 simulated conditions were more than 

30 ft in most of the lower-aquifer system for all basecase projections (fig. 27A–D). Water-level rises in the 

lower-aquifer system reduced downward vertical 

leakage, which contributed to water-level rises in the 

upper-aquifer system for all the base-case projections.
128 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Figure 27. Simulated differences in ground-water levels from 1993 to 2017 for proposed water-supply projects in the existing management plan for the 

Santa Clara–Calleguas ground-water basin, Ventura County, California. A, Historical reported pumpage averaged over the period 1984–1993 and estimated 

or measured historical recharge, streamflow, and diversion data (base-case 1). 

Pacific Ocean

Pacific Ocean

118°45'

34°

22'

30"

R22W R18W

119°

0

0

10 MILES

10 KILOMETERS

T

2

N

T

4

N

34°

07'

30"

119°15'

118°45'

34°

22'

30"

119°

T

2

N

T

4

N

34°

07'

30"

119°15'

0

0

10 MILES

10 KILOMETERS

4

3

20

19

18

17

7

5

1

34

33

32

31

8

2

30

28

27 26

13

12

11

10

9

6

29

16

14

25

24

23

22

21

15

Calleguas Creek

Santa Clara River

Lake

Piru

Calleguas Creek

Santa Clara River

Lake

Piru

100

76 4,972

1,116

1,061

2,723

1,5

1,925 19

168

573

532

791

1,930

3,065

3,525

2,847

1,139

1,630

450

622

14,813

16,798

21

106

A Base-case 1

EXPLANATION

River and selected streams

Hydrologic Unit boundary Subarea number –

See figure 17B for subarea names

Model-grid boundary of

flow region and subareas

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -66 to 67

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -258 to 84

-66 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 15

15 < ∆h ≤ 67

-258 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 30

30 < ∆h ≤ 84

R22W R18W

949

10

22

Upper-Aquifer

System

Lower-Aquifer

System

Upper-aquifer system

Lower-aquifer system

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium

Shallow alluvium and unconsolidated deposits

Consolidated and unconsolidated deposits

Rise

Decline

Rise

Decline

Mean simulated ground-water

underflow and coastal flow –

in acre-feet per year
Analysis of Ground-Water Flow 129

Figure 27—Continued. B, Rolling cut back in pumpage and estimated or measured historical recharge, streamflow, and diversion data (base-case 2)

Pacific Ocean

Pacific Ocean

118°45'

34°

22'

30"

R22W R18W

119°

0

0

10 MILES

10 KILOMETERS

T

2

N

T

4

N

34°

07'

30"

119°15'

118°45'

34°

22'

30"

119°

T

2

N

T

4

N

34°

07'

30"

119°15'

0

0

10 MILES

10 KILOMETERS

4

3

20

19

18

17

7

5

1

34

33

32

31

8

2

30

28

27 26

13

12

11

10

9

6

29

16

14

25

24

23

22

21

15

Calleguas Creek

Santa Clara River

Lake

Piru

Calleguas Creek

Santa Clara River

Lake

Piru

2,721

1,062

5,037

358

633

429

27

1,184

1,238

2,193

178

672

16,779

2,928 2,736

939

15,479

3,315

1,729

1,373

429

409

152

17

B

EXPLANATION

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -77 to 62

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -272 to 74

-77 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 15

15 < ∆h ≤ 62

-272 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 30

30 < ∆h ≤ 74

R22W R18W

River and selected streams

Hydrologic Unit boundary

Model-grid boundary of

flow region and subareas

Base-case 2

Upper-Aquifer

System

Lower-Aquifer

System

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium

Shallow alluvium and unconsolidated deposits

Consolidated and unconsolidated deposits

Rise

Decline

Rise

Decline

Subarea number –

See figure 17B for subarea names

949

10

22 Upper-aquifer system

Lower-aquifer system

Mean simulated ground-water

underflow and coastal flow –

in acre-feet per year
130 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Figure 27—Continued. C, Step cut-back reduction in pumpage and estimated or measured historical recharge, streamflow, and diversions data (base-case 3).

Pacific Ocean

Pacific Ocean

118°45'

34°

22'

30"

R22W R18W

119°

0

0

10 MILES

10 KILOMETERS

T

2

N

T

4

N

34°

07'

30"

119°15'

118°45'

34°

22'

30"

119°

T

2

N

T

4

N

34°

07'

30"

119°15'

0

0

10 MILES

10 KILOMETERS

4

3

20

19

18

17

7

5

1

34

33

32

31

8

2

30

28

27 26

13

12

11

10

9

6

29

16

14

25

24

23

22

21

15

Calleguas Creek

Santa Clara River

Lake

Piru

Calleguas Creek

Santa Clara River

Lake

Piru

28

496

1,648

188503

754

357

1,057

1,159

485

4,618

322

329

259

13

1,309

3,946

2,184

3,227

15,471

16,271

3,038

4,835

1,301

C

EXPLANATION

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -94 to 55

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -283 to 65

-94 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 15

15 < ∆h ≤ 55

-283 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 30

30 < ∆h ≤ 65

R22W R18W

River and selected streams

Hydrologic Unit boundary

Model-grid boundary of

flow region and subareas

Base-case 3

Upper-Aquifer

System

Lower-Aquifer

System

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium

Shallow alluvium and unconsolidated deposits

Consolidated and unconsolidated deposits

Rise

Decline

Rise

Decline

Subarea number –

See figure 17B for subarea names

949

10

22 Upper-aquifer system

Lower-aquifer system

Mean simulated ground-water

underflow and coastal flow –

in acre-feet per year
Analysis of Ground-Water Flow 131

Figure 27—Continued. D, Historical reported pumpage averaged over the period 1984–1993 and spectral-based estimates of recharge, streamflow, and 

diversions (base-case 4).

Pacific Ocean

Pacific Ocean

118°45'

34°

22'

30"

R22W R18W

119°

0

0

10 MILES

10 KILOMETERS

T

2

N

T

4

N

34°

07'

30"

119°15'

118°45'

34°

22'

30"

119°

T

2

N

T

4

N

34°

07'

30"

119°15'

0

0

10 MILES

10 KILOMETERS

4

3

20

19

18

17

7

5

1

34

33

32

31

8

2

30

28

27 26

13

12

11

10

9

6

29

16

14

25

24

23

22

21

15

Calleguas Creek

Santa Clara River

Lake

Piru

Calleguas Creek

Santa Clara River

Lake

Piru

741

231

1,441 16,429

3,636

475

1,428

74

703

14,381

2,548

3,251

142

125

102

2,024

22

2,804

1,549

877

558 42

1,023

4,960

D

EXPLANATION

Base-case 4 Difference in water-level altitude (∆h)

from 1993, in feet

Total range -66 to 67

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -258 to 84

-66 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 15

15 < ∆h ≤ 67

-258 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 30

30 < ∆h ≤ 84

R22W R18W

River and selected streams

Hydrologic Unit boundary

Model-grid boundary of

flow region and subareas

Upper-Aquifer

System

Lower-Aquifer

System

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium

Shallow alluvium and unconsolidated deposits

Consolidated and unconsolidated deposits

Rise

Decline

Rise

Decline

Subarea number –

See figure 17B for subarea names

949

10

22 Upper-aquifer system

Lower-aquifer system

Mean simulated ground-water

underflow and coastal flow –

in acre-feet per year
132 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Figure 27—Continued. E, Cumulative changes in ground-water storage and ground-water flow for selected subareas during 1993–2037.

17 31

3 4

7 8

1 2

5 6

25 13

15

21

BASE-CASE 4

1 2

17 31

3 4

7 8

5 6

25 13

BASE-CASE 1

15

21

1 2

17 31

3 4

7 8

5 6

25 13

15

21

BASE-CASE 3

1 2

17 31

3 4

7 8

5 6

25 13

15

21

BASE-CASE 2

Subarea

number –

See figure 17B

for subarea

names

10

22

EXPLANATION

1990200020102020

1990200020102020

1990200020102020

199020002010202020302040

YEAR

50

100

0

-50

-100

50

100

0

-50

-100

0

50

-50

-100

-150

CUMULATIVE FLOW TO OCEAN, IN THOUSANDS OF ACRE FEET

E

0

50

-50

-100

-150

0

50

-50

-100

-150

0

50

-50

-100

-150

0

50

-50

-100

-150

0

50

-50

-100

-150

0

50

-50

-100

-150

0

50

-50

-100

-150

CUMULATIVE STORAGE, IN THOUSANDS OF ACRE FEET

Upper-aquifer

system

Lower-aquifer

system

Upper-aquifer

system

Lower-aquifer

system
Analysis of Ground-Water Flow 133

Figure 27.—Continued. F, Water-level altitudes for the city of Oxnard public-supply well (1N/22W-3F4). Upper hydrograph generated from historical and 

spectral simulation data; lower hydrograph generated from spectral simulation data.

Measured water-level altitude

Simulated future water-level elevation for base-case 1

Simulated future water-level elevation for base-case 2

Simulated future water-level elevation for base-case 3

Simulated future water-level elevation for base-case 4

1890 1915 1940 1965

YEAR

1990 2015 2040

Oxnard Water Supply Well (1N/22W-3F4)

(Total depth = 232 feet,

Screened depth = 141-232 feet below land surface)

Oxnard Water Supply Well (1N/22W-3F4)

(Total depth = 232 feet,

Screened depth = 141-232 feet below land surface)

Measured water-level altitude

Flow model historical water-level altitude

Flow model historical water-level altitude

Flow model future (1984-93 maximum average pumpage)

Flow model future (1992-93 average pumpage)

Flow model future (1989-90 average pumpage)

Flow model future (1984-93 average pumpage)

-75

-25

Sea

level

25

75

Maximum historical water-level

decline with subsidence

in future conditions

below this water level

Equivalent

freshwater

head in Oxnard

aquifer at the

submarine outcrop -

seawater intrusion

occurs below this level

1890 1915 1940 1965 1990 2015 2040

WATER-LEVEL ALTITUDE, IN FEET ABOVE SEA LEVEL

Land surface

at well

Land surface

at well

-75

-25

Sea

level

25

75

Maximum historical water-level

decline with subsidence

in future conditions

below this water level

Equivalent

freshwater

head in Oxnard

aquifer at the

submarine outcrop -

seawater intrusion

occurs below this level

End of

2017

End of

2017

F
134 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

The mean streamflow recharge rates were similar 

for the base-case 1–3 projections, ranging from about 

59,000 acre-ft/yr for continued historical demand 

(base-case 1) to about 50,000 acre-ft/yr (a rate reduced 

by about 11,000 acre-ft/yr) for the step cut back of 

FGMA pumpage (base-case 3). Mean streamflow 

recharge for base-case 1 was about 3,000 acre-ft/yr less 

than that for the historical period 1984–93. Although 

the simulated water-level declines were reduced in the 

western part of the Santa Paula subarea for the rolling 

(base-case 2) and step cut-back projections (base-case 

3) (fig. 27A,B,C), the streamflow recharge was similar 

to base-case 1 because most of the recharge occurred in 

the Piru and Fillmore subareas and the eastern part of 

the Santa Paula subarea (fig. 27A,B,C). The mean 

recharge for the spectral 44-year projection (base-case 

4) was about 80,800 acre-ft/yr, which is about 19,000 

acre-ft/yr higher than the historical projections (table 

6). This difference was largely due to the projection of 

a severe and prolonged drought spanning 1999 through 

2006. The projected drought caused water levels to 

decline below streambeds resulting in greater 

streamflow recharge. The larger streamflow recharge 

may also have been due to the use of regression 

estimates (Appendix 4) of future streamflow which do 

not completely capture the extremes of streamflow. 

Changes in streamflow recharge had little effect on 

ground-water discharge. ET was simulated at about 

1,000 acre-ft/yr and was similar for the three 

historically based projections, but was about twice this 

rate for the spectral projection. 

Coastal Flow

The simulation of coastal flow yielded one of the 

largest differences among the base-case scenarios. All 

the base-case simulations indicated some coastal 

landward flow (seawater intrusion) into the upperaquifer system during dry years but a cumulative 

coastal seaward flow along the coast, and coastal 

landward flow (seawater intrusion) into the loweraquifer system for the entire period (table 6). The 

projection of historical average pumpage for the six 

hypothetical ground-water/surface-water projects 

(base-case 1) resulted in about 95,300 acre-ft of coastal 

seaward flow from the upper-aquifer system and about 

114,500 acre-ft of coastal landward flow (seawater 

intrusion) to the lower-aquifer system. This was almost 

10 times the total coastal seaward flow simulated for 

the historical period (1984–93), even though the 

simulation period was only 2.4 times longer. The 

largest mean coastal seaward flow in the upper-aquifer 

system occurred in the Northwest and South Oxnard 

Plain subareas, and the largest coastal landward flow 

occurred in the lower-aquifer system in the South 

Oxnard Plain subarea (fig. 27). The reductions in 

pumpage increased the total simulated coastal seaward 

flow by about 43,000 acre-ft for base-case 2 and about 

131,000 acre-ft for base-case 3 while reducing coastal 

landward flow (seawater intrusion) in the lower-aquifer 

system only about 32,400 acre-ft for base-case 2 and 

about 63,400 acre-ft for base-case 3 relative to the 

projection of historical average pumpage with selected 

projects (table 6).
Analysis of Ground-Water Flow 135

Flow Between Subareas and Aquifer Systems

The mean horizontal ground-water underflow to 

and from subareas surrounding the Oxnard Plain 

subareas are shown in figure 27, and the total mean 

downward flow between aquifer systems is given in 

table 6. More than 20,000 acre-ft/yr of underflow 

entered the Northwest and South subareas from the 

inland subareas of the Oxnard Plain for base-case 1 

(fig. 27A). Even larger subregional underflows were 

indicated for the cut-back projections for base-cases 2 

and 3 (fig. 27B,C). Changes in horizontal flow of 

ground water as underflow to the Oxnard Plain 

subareas were directly proportional to the reductions in 

pumpage in the FGMA area, ranging from about 9,850 

acre-ft/yr for base-case 1 to about 10,720 acre-ft/yr for 

base-case 3 (table 6). The largest components of 

underflow were from the Santa Clara River Valley and 

the Pleasant Valley subareas. These flow rates are small 

relative to coastal landward flow (seawater intrusion), 

water derived from storage, and downward flow 

between aquifers, but are important locally near 

subarea boundaries (fig. 27, table 6). The mean rate of 

underflow for the base-case 1 projection was about half 

the rate simulated for 1984–93 (table 6). Ground water 

was flowing from the Oxnard Plain subareas and 

adjacent inland subareas toward the Pleasant Valley 

subareas in both aquifer systems during the 1984–93 

period (fig. 25). Yet, the base-case projections 

simulated flow of water toward Pleasant Valley in the 

upper-aquifer system, flow from the Northeast Oxnard 

Plain subarea in the lower-aquifer system, and a 

reversal of flow toward the South Oxnard Plain 

subarea. The rate of underflow from the Santa Clara 

River Valley subareas was similar to that for base-case 

1, for the rolling cut-back (base-case 2), and was 

almost half that for the step cut-back (base-case 3). The 

direction of mean underflow from the South Pleasant 

Valley subarea was reversed for both cut-back 

projections (fig. 27B,C, table 6). About 1,800 and 

1,400 acre-ft/yr of underflow left the Oxnard Plain 

subarea to the South Pleasant Valley subarea for basecase 2 and for three projections, respectively (fig. 

27B,C, table 6). Similarly, the direction of net 

underflow was reversed in the upper-aquifer system 

toward the West Las Posas Valley subarea. The net 

inflow to the Oxnard Plain subareas for the historical 

period of pumpage was a net flow of about 900 acreft/yr (fig. 25A), and the net mean inflow toward the 

West Las Posas Valley subarea for base-cases 1–3 was 

less than 500 acre-ft/yr (fig. 27 A–C, table 6). The 

spectral-based projection (base-case 4) was similar to 

the base-case 1 projection, that is, there was a large 

underflow component from the Santa Clara River 

Valley subareas but a net mean flow toward the 

Pleasant Valley and Las Posas Valley subareas 

(fig. 27D, table 6). 

Mean downward flow between aquifer systems 

in the Oxnard Plain subareas changed directly with 

changes in potential pumpage in the FGMA area, but it 

varied only between about 17,400 acre-ft/yr and 

20,900 acre-ft/yr for the four base-case projections 

(table 6). Net water-level declines reversed to waterlevel recoveries throughout most of the subareas in the 

FGMA areas, as well as the adjacent Mound and Santa 

Paula subareas, for the cut-back projections (base-cases 

2 and 3) (figs. 25, 26, 27B,C).

Land-Subsidence

The water derived from aquifer-system 

compaction also was reduced and was proportional to 

the cut backs in pumpage and related water-level 

recoveries. The total amount of water derived from 

storage owing to the compaction of fine-grained 

deposits was about 36,400 acre-ft (1,500 acre-ft/yr) for 

base-case 1; the amount was reduced to about 10,000 

acre-ft (420 acre-ft/yr) for base-case 2 and was 

reversed to about 8,000 acre-ft (330 acre-ft/yr) 

returning to storage in the fine-grained deposits for 

base-case 3 (table 6). For the spectral analyses, the total 

amount of water from compaction was about 47,250 

acre-ft for the entire 44-year period. The larger amount 

simulated for the spectral analysis relative to base-case 

1 is due to the prolonged drought estimated by the 

spectral precipitation method. The simulated 

subsidence, which was driven by this extended 

drought, resulted in potential subsidence of about 1 ft 

throughout most of the Northeast Oxnard Plain 

subarea, the northeastern part of the South Oxnard 

Plain subarea, and the West and East Las Posas Valley 

subareas.
136 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

The base-case projections generally produced 

water-level recoveries or modest water-level declines 

that generally were less than previous maximum 

declines (figs. 25 and 27). However, as much as an 

additional 1 ft of subsidence was simulated in the 

Northeast Oxnard Plain subarea, the northern part of 

the South Oxnard Plain subarea, the West Las Posas 

Valley subarea, and the western part of the East Las 

Posas Valley subarea during the early dry-year period 

for base-case 1. Simulated subsidence for the rolling 

cut-back (base-case 2) projection was reduced to a 

smaller areal extent and generally from about 0.5 ft 

(base-case 1) to 0.1 ft throughout most of the South and 

Northeast Oxnard Plain subareas. Simulated 

subsidence was further reduced for the step cut-back in 

pumpage for the FGMA areas for base-case 3. 

However, about 1 ft of subsidence persisted in the basecase 3 simulation in the northeastern part of the Oxnard 

Plain subareas and the South Pleasant Valley subarea 

and in the East Las Posas Valley and North Pleasant 

Valley subareas. The extended drought simulated in the 

early part of the 44-year projection of base-case 4 

produced water-level declines in most of the Oxnard 

Plain Forebay subarea (fig. 27D) and, to a lesser extent, 

in the remainder of the Oxnard Plain subareas and the 

inland subareas in the FGMA areas (fig. 26), which 

resulted in additional subsidence.

Projected Future Ground-Water Flow for Alternative WaterSupply Projects

The analysis of future ground-water flow for 

alternative water-supply projects was simulated for the 

same 24-year period used for the analysis of the 

proposed projects for the existing management plan. 

The simulations included well-by-well average groundwater pumpage for the 1984–93 period, irrigation 

return flow estimated using the 1969 land-use 

distribution, and climatically varying recharge, 

streamflow, and pumpage. Each projection of future 

ground-water flow that includes potential alternative 

projects also includes the proposed projects described 

in the previous section. 

Each of these potential future projects was 

simulated individually, but they include the base-case 1 

set of projects and assumptions. These seven 

alternative water-supply projects were proposed to help 

manage the effects of increasing demand and variable 

supply on seawater intrusion, subsidence, increased 

withdrawal from storage, and vertical and lateral flow 

between subareas and aquifer systems. 

The model cells used to simulate the alternative 

water-supply projects (referred to as potential cases 

1–7) are shown in figures 26 A,B. The simulated 

differences in ground-water levels and the cumulative 

changes in ground-water storage, coastal flow, and 

mean ground-water underflow in and out of the Oxnard 

Plain are shown in figure 28. In general, reductions in 

water derived from subsidence in the alternative watersupply projects were proportional to the cut backs in 

pumpage and related water-level recoveries. The 

potential-case projections resulted in water-level 

recoveries or modest water-level declines that generally 

were less than historical maximum declines (fig. 27). 

However, an increase in subsidence was simulated in 

the FGMA areas and in the Fillmore subarea during the 

early dry-year period for all seven alternative watersupply projects (potential cases 1–7). Selected details 

for each alternative water-supply project (potential 

case) are presented below.
Analysis of Ground-Water Flow 137

Potential Case 1—Seawater Barrier and Increased Pumpage 

in the Oxnard Plain Forebay

For potential case 1, pumpage by the city of 

Oxnard was reduced by 4,000 acre-ft/yr. The reduced 

pumpage was supplanted with CMWD deliveries, and 

a seawater-intrusion barrier project was implemented 

by injecting 20,000 acre-ft/yr of imported water and 

reclaimed sewage into the upper-aquifer system along 

the South Oxnard Plain subarea from Port Hueneme to 

just south of the wastewater treatment plant. Ground 

water that had been historically pumped from the 

lower-aquifer system from the El Rio-OH wells was 

pumped from the upper-aquifer system in the Oxnard 

Plain Forebay. This offset the injection of effluent and 

imported water and reduced the pumpage stress on the 

lower-aquifer system. These projects collectively 

started in the year 2000.

Results of potential case 1 show that the 

simulated seawater-barrier injection stopped coastal 

landward flow (seawater intrusion) in the upper-aquifer 

system but did not reduce the coastal landward flow 

(seawater intrusion) in the lower-aquifer system. The 

rates of coastal landward flow (seawater intrusion) in 

the lower-aquifer system were comparable to those 

simulated for base-case 1 (figs. 27A and 28A; table 6). 

Injecting water into the upper-aquifer system to form a 

seawater-intrusion barrier for the South Oxnard Plain 

subarea south of the Hueneme submarine canyon 

(fig. 1) produced water-level rises as great as 30 ft 

(fig. 28A) that resulted in heads as much as 20 ft above 

sea level (add water-level changes from fig. 28A to 

water-level elevations from fig. 25A). For this case, 

more water was pumped from storage in the Oxnard 

Plain Forebay without increasing coastal landward flow 

(seawater intrusion) in the upper-aquifer system. 

However, this additional pumpage produced a small 

amount of additional subsidence. A 24-percent increase 

in net underflow from the Santa Clara River Valley 

subareas to the Oxnard Plain subarea was simulated for 

this case with the increase of 20,000 acre-ft/yr of 

pumpage in the Oxnard Plain Forebay at the OH wells 

(figs. 26B and 28A). In addition, the pumpage reduced 

the net ground-water underflow away from the Oxnard 

Plain Forebay to the Northeast and Northwest Oxnard 

Plain model subareas by about 11,000 acre-ft/yr in the 

upper-aquifer system compared with the net underflow 

in base-case 1 (figs. 27A and 28B). As in the base-case 

1 projection, as much as an additional foot of 

subsidence was simulated in the northeast Oxnard 

Plain subarea, the northern part of the South Oxnard 

Plain subarea, the West Las Posas Valley subarea, and 

the western part of the East Las Posas Valley subarea 

during the early dry-year period for potential case 1. 

Subsidence of a few tenths of a foot was further 

extended across the Oxnard Plain Forebay subarea 

owing to the additional 20,000 acre-ft/yr of pump-back 

pumpage, and the extent of subsidence in the South 

Oxnard Plain subarea along the coast was reduced 

owing to the 20,000 acre-ft/yr injection project in the 

upper-aquifer system.

Potential Case 2—Artificial Recharge in Happy Camp Canyon

 For potential case 2, additional recharge of 

15,000 acre-ft/yr was added as surface-spreading to the 

upper-aquifer system at the mouth of Happy Camp 

Canyon along the northeast border of the East Las 

Posas Valley subarea beginning in 2000. The projected 

additional recharge contributed about 204,000 acre-ft 

of water going into storage but resulted in simulated 

water levels being significantly above land surface (not 

feasible) in the upper-aquifer system in the East Las 

Posas Valley subarea (figs. 27A and 28B, table 6). 

Although this case resulted in simulated water levels 

that were above land surface in the East Las Posas 

Valley subarea, essentially no changes were simulated 

in the hydrologic conditions in the Oxnard Plain, 

Pleasant Valley, or Santa Clara River Valley subareas. 

The simulated water-level rise above land surface at the 

mouth of Happy Camp Canyon may, in part, be due to 

the hydraulic properties and layering used in the 

model. 
138 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Figure 28. Simulated differences in ground-water levels from 1993 to 2017 for alternative water-supply projects using the base-case 1 set of projects 

and assumptions in the Santa Clara–Calleguas ground-water basin, Ventura County, California. A, Seawater intrusion barrier project in the southern Oxnard 

Plain subregion and equal pump-back from the Oxnard Forebay in the upper-aquifer system (potential case 1).

Pacific Ocean

Pacific Ocean

118°45'

34°

22'

30"

R22W R18W

119°

0

0

10 MILES

10 KILOMETERS

T

2

N

T

4

N

34°

07'

30"

119°15'

118°45'

34°

22'

30"

119°

T

2

N

T

4

N

34°

07'

30"

119°15'

0

0

10 MILES

10 KILOMETERS

4

3

20

19

18

17

7

5

1

34

33

32

31

8

2

30

28

27 26

13

12

11

10

9

6

29

16

14

25

24

23

22

21

15

Calleguas Creek

Santa Clara River

Lake

Piru

Calleguas Creek

Santa Clara River

Lake

Piru

136

2,017

2,726

157

152

1,72

1,073

1

444

543

732 4,798

1,572

748

565 639

2,274

4,144

3,652

9,274

1,066

910

11,665

1,012

18

A

EXPLANATION

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -67 to 71

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -258 to 84

-67 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 15

15 < ∆h ≤ 71

-258 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 30

30 < ∆h ≤ 84

R22W R18W

River and selected streams

Hydrologic Unit boundary

Model-grid boundary of

flow region and subareas

Potentialcase 1

Upper-Aquifer

System

Lower-Aquifer

System

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium

Shallow alluvium and unconsolidated deposits

Consolidated and unconsolidated deposits

Rise

Decline

Rise

Decline

Subarea number –

See figure 17B for subarea names

949

10

22 Upper-aquifer system

Lower-aquifer system

Mean simulated ground-water

underflow and coastal flow –

in acre-feet per year
Analysis of Ground-Water Flow 139

Figure 28—Continued. B, Additional artificial recharge added at mouth of Happy Camp Canyon, East Los Posas subarea (potential case 2).

Pacific Ocean

Pacific Ocean

118°45'

34°

22'

30"

R22W R18W

119°

0

0

10 MILES

10 KILOMETERS

T

2

N

T

4

N

34°

07'

30"

119°15'

118°45'

34°

22'

30"

119°

T

2

N

T

4

N

34°

07'

30"

119°15'

0

0

10 MILES

10 KILOMETERS

4

3

20

19

18

17

7

5

1

34

33

32

31

8

2

30

28

27 26

13

12

11

10

9

6

29

16

14

25

24

23

22

21

15

Calleguas Creek

Santa Clara River

Lake

Piru

Calleguas Creek

Santa Clara River

Lake

Piru

1,630

622

450

3,065

791

1,930

14,813

2,847

3,525

1,139

16,798

100

1,925

2,723

168

1,061 1,519

1,116

76

573

531

4,972

106

21

EXPLANATION

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -4,489 to 67

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -259 to 84

-4,489 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 15

15 < ∆h ≤ 67

-259 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 30

30 < ∆h ≤ 84

R22W R18W

River and selected streams

Hydrologic Unit boundary

Model-grid boundary of

flow region and subareas

B Potentialcase 2

Upper-Aquifer

System

Lower-Aquifer

System

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium

Shallow alluvium and unconsolidated deposits

Consolidated and unconsolidated deposits

Rise

Decline

Rise

Decline

Subarea number –

See figure 17B for subarea names

949

10

22 Upper-aquifer system

Lower-aquifer system

Mean simulated ground-water

underflow and coastal flow –

in acre-feet per year
140 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Figure 28—Continued. C, Cessation of pumpage in the southern Oxnard Plain subregion (potential case 3).

Pacific Ocean

Pacific Ocean

118°45'

34°

22'

30"

R22W R18W

119°

0

0

10 MILES

10 KILOMETERS

T

2

N

T

4

N

34°

07'

30"

119°15'

118°45'

34°

22'

30"

119°

T

2

N

T

4

N

34°

07'

30"

119°15'

0

0

10 MILES

10 KILOMETERS

4

3

20

19

18

17

7

5

1

34

33

32

31

8

2

30

28

27 26

13

12

11

10

9

6

29

16

14

25

24

23

22

21

15

Calleguas Creek

Santa Clara River

Lake

Piru

Calleguas Creek

Santa Clara River

Lake

Piru

28

496

1,648

188503

754

357

1,057

1,159

485

4,618

322

329

259

13

1,309

3,946

2,184

3,227

15,471

16,271

3,038

4,835

1,301

C

EXPLANATION

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -94 to 55

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -283 to 65

-94 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 15

15 < ∆h ≤ 55

-283 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 30

30 < ∆h ≤ 65

R22W R18W

River and selected streams

Hydrologic Unit boundary

Model-grid boundary of

flow region and subareas

Base-case 3

Upper-Aquifer

System

Lower-Aquifer

System

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium

Shallow alluvium and unconsolidated deposits

Consolidated and unconsolidated deposits

Rise

Decline

Rise

Decline

Subarea number –

See figure 17B for subarea names

949

10

22 Upper-aquifer system

Lower-aquifer system

Mean simulated ground-water

underflow and coastal flow –

in acre-feet per year
Analysis of Ground-Water Flow 141

Figure 28—Continued. D, Shifting pumpage from the Pumping-Trough Pipeline (PTP) wells from the lower- to upper-aquifer system (potential case 4).

Pacific Ocean

Pacific Ocean

118°45'

34°

22'

30"

R22W R18W

119°

0

0

10 MILES

10 KILOMETERS

T

2

N

T

4

N

34°

07'

30"

119°15'

118°45'

34°

22'

30"

119°

T

2

N

T

4

N

34°

07'

30"

119°15'

0

0

10 MILES

10 KILOMETERS

4

3

20

19

18

17

7

5

1

34

33

32

31

8

2

30

28

27 26

13

12

11

10

9

6

29

16

14

25

24

23

22

21

15

Calleguas Creek

Santa Clara River

Lake

Piru

Calleguas Creek

Santa Clara River

Lake

Piru

102

1,759

168

1,193

1,374

623 543

570

1,187

4,705

2,594

20

1,486

558

489

14,756

777

1,768

92

1,007

3,080

2,480

2,920

17,198

EXPLANATION

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -67 to 67

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -258 to 84

-67 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 15

15 < ∆h ≤ 67

-258 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 30

30 < ∆h ≤ 84

R22W R18W

River and selected streams

Hydrologic Unit boundary

Model-grid boundary of

flow region and subareas

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium

Shallow alluvium and unconsolidated deposits

Consolidated and unconsolidated deposits

D Potentialcase 4

Upper-Aquifer

System

Lower-Aquifer

System

Rise

Decline

Rise

Decline

Subarea number –

See figure 17B for subarea names

949

10

22 Upper-aquifer system

Lower-aquifer system

Mean simulated ground-water

underflow and coastal flow –

in acre-feet per year
142 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Figure 28—Continued. E, Shifting pumpage from the lower- to upper-aquifer system in the northeastern Oxnard Plain subarea (potential case 5).

Pacific Ocean

Pacific Ocean

118°45'

34°

22'

30"

R22W R18W

119°

0

0

10 MILES

10 KILOMETERS

T

2

N

T

4

N

34°

07'

30"

119°15'

118°45'

34°

22'

30"

119°

T

2

N

T

4

N

34°

07'

30"

119°15'

0

0

10 MILES

10 KILOMETERS

4

3

20

19

18

17

7

5

1

34

33

32

31

8

2

30

28

27 26

13

12

11

10

9

6

29

16

14

25

24

23

22

21

15

Calleguas Creek

Santa Clara River

Lake

Piru

Calleguas Creek

Santa Clara River

Lake

Piru

1,031

381

690

14,318

3,166

736

1,359

59

730

1,636

1,519

19,558

166

110

1,204

17

2,261

429

2,524

1,790

2,773

1,860

557

795

EXPLANATION

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -69 to 67

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -258 to 84

-69 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 15

15 < ∆h ≤ 67

-258 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 30

30 < ∆h ≤ 84

R22W R18W

River and selected streams

Hydrologic Unit boundary

Model-grid boundary of

flow region and subareas

E Potentialcase 5

Upper-Aquifer

System

Lower-Aquifer

System

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium

Shallow alluvium and unconsolidated deposits

Consolidated and unconsolidated deposits

Rise

Decline

Rise

Decline

Subarea number –

See figure 17B for subarea names

949

10

22 Upper-aquifer system

Lower-aquifer system

Mean simulated ground-water

underflow and coastal flow –

in acre-feet per year
Analysis of Ground-Water Flow 143

Figure 28—Continued. F, Shifting pumpage from the lower- to upper-aquifer system in the southern Oxnard Plain subarea (potential case 6).

Pacific Ocean

Pacific Ocean

118°45'

34°

22'

30"

R22W R18W

119°

0

0

10 MILES

10 KILOMETERS

T

2

N

T

4

N

34°

07'

30"

119°15'

118°45'

34°

22'

30"

119°

T

2

N

T

4

N

34°

07'

30"

119°15'

0

0

10 MILES

10 KILOMETERS

4

3

20

19

18

17

7

5

1

34

33

32

31

8

2

30

28

27 26

13

12

11

10

9

6

29

16

14

25

24

23

22

21

15

Calleguas Creek

Santa Clara River

Lake

Piru

Calleguas Creek

Santa Clara River

Lake

Piru

5,113

2,790

14,931

3,076

37

1,640

776

17,062

1,418

45

482

237

1,138

4,737

169

565

103

1,635

14

1,428

1,655

693

1,173

1,104

EXPLANATION

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -69 to 67

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -258 to 84

-69 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 15

15 < ∆h ≤ 67

-258 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 30

30 < ∆h ≤ 84

R22W R18W

River and selected streams

Hydrologic Unit boundary

Model-grid boundary of

flow region and subareas

F Potentialcase 6

Upper-Aquifer

System

Lower-Aquifer

System

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium

Shallow alluvium and unconsolidated deposits

Consolidated and unconsolidated deposits

Rise

Decline

Rise

Decline

Subarea number –

See figure 17B for subarea names

949

10

22 Upper-aquifer system

Lower-aquifer system

Mean simulated ground-water

underflow and coastal flow –

in acre-feet per year
144 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Figure 28—Continued. G, Shifting pumpage from the lower- to upper-aquifer system in the Pleasant Valley subarea (potential case 7).

Pacific Ocean

Pacific Ocean

118°45'

34°

22'

30"

R22W R18W

119°

0

0

10 MILES

10 KILOMETERS

T

2

N

T

4

N

34°

07'

30"

119°15'

118°45'

34°

22'

30"

119°

T

2

N

T

4

N

34°

07'

30"

119°15'

0

0

10 MILES

10 KILOMETERS

4

3

20

19

18

17

7

5

1

34

33

32

31

8

2

30

28

27 26

13

12

11

10

9

6

29

16

14

25

24

23

22

21

15

Calleguas Creek

Santa Clara River

Lake

Piru

Calleguas Creek

Santa Clara River

Lake

Piru

1,310

501

3,681

797

1,931

109

1,113

3,279

2,601

14,684

3,032

17,067 4,520

1,234

1,546

1,849

397

98

1,632

18

2,290

1,092

169

611

EXPLANATION

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -40 to 152

Difference in water-level altitude (∆h)

from 1993, in feet

Total range -262 to 81

-40 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 15

15 < ∆h ≤ 152

-262 ≤ ∆h ≤ -30

-30 < ∆h ≤ 0

0 < ∆h ≤ 5

5 < ∆h ≤ 30

30 < ∆h ≤ 81

R22W R18W

River and selected streams

Hydrologic Unit boundary

Model-grid boundary of

flow region and subareas

G Potentialcase 7

Upper-Aquifer

System

Lower-Aquifer

System

Santa Clara-Calleguas ground-water basin

Outside Santa Clara-Calleguas ground-water basin

Shallow alluvium

Shallow alluvium and unconsolidated deposits

Consolidated and unconsolidated deposits

Rise

Decline

Rise

Decline

Subarea number –

See figure 17B for subarea names

949

10

22 Upper-aquifer system

Lower-aquifer system

Mean simulated ground-water

underflow and coastal flow –

in acre-feet per year
Analysis of Ground-Water Flow 145

Figure 28—Continued. H, Cumulative changes in ground-water storage and ground-water flow for selected subareas, 1993–2017. 

1 2

17 31

3 4

5 6

25 13

POTENTIAL-CASE 1

15

21

7 8

1 2

17 31

7 8

5 6

25 13

15

21

POTENTIAL-CASE 3

1 2

17 31

3 4

7 8

5 6

25 13

15

21

POTENTIAL-CASE 2

1990200020102020

1990200020102020

1990200020102020

YEAR

YEAR

50

100

0

-50

-100

50

100

0

-50

-100

0

50

-50

-100

-150

CUMULATIVE FLOW TO OCEAN, IN THOUSANDS OF ACRE FEET

50

100

0

-50

-100

50

100

0

-50

-100

CUMULATIVE FLOW TO OCEAN, IN THOUSANDS OF ACRE FEET

H

0

50

-50

-100

-150

0

50

-50

-100

-150

0

50

-50

-100

-150

0

50

-50

-100

-150

0

50

-50

-100

-150

0

50

-50

-100

-150

CUMULATIVE STORAGE, IN THOUSANDS OF ACRE FEET

0

50

-50

-100

-150

0

50

-50

-100

-150

0

50

-50

-100

-150

0

50

-50

-100

-150

0

50

-50

-100

-150

0

50

-50

-100

-150

0

50

-50

-100

-150

CUMULATIVE STORAGE, IN THOUSANDS OF ACRE FEET

0

50

-50

-100

-150

-200

100

150

50

0

-50

-100

Subarea

number –

See figure 17B

for subarea

names

10

22

EXPLANATION

Upper-aquifer

system

Lower-aquifer

system

Upper-aquifer

system

Lower-aquifer

system

3 4
146 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Figure 28—Continued. H, Cumulative changes in ground-water storage and ground-water flow for selected subareas, 1993–2017. 

1 2

17 31

3 4

7 8

5 6

25 13

POTENTIAL-CASE 4

15

21

1 2

17 31

3 4

7 8

5 6

25 13

15

21

POTENTIAL-CASE 5

1 2

17 31

3 4

7 8

5 6

25 13

15

21

POTENTIAL-CASE 6

1 2

17 31

3 4

7 8

5 6

25 13

15

21

POTENTIAL-CASE 7

1990200020102020

1990200020102020

19902000201020201990200020102020

YEAR

50

100

0

-50

-100

50

100

0

-50

-100

0

50

-50

-100

-150

CUMULATIVE FLOW TO OCEAN, IN THOUSANDS OF ACRE FEET

H

0

50

-50

-100

-150

0

50

-50

-100

-150

0

50

-50

-100

-150

0

50

-50

-100

-150

0

50

-50

-100

-150

0

50

-50

-100

-150

0

50

-50

-100

-150

CUMULATIVE STORAGE, IN THOUSANDS OF ACRE FEET

Subarea

number –

See figure 17B

for subarea

names

10

22

EXPLANATION

Upper-aquifer

system

Lower-aquifer

system

Upper-aquifer

system

Lower-aquifer

system
Analysis of Ground-Water Flow 147

Potential Case 3—Eliminate Agricultural Pumpage in the 

South Oxnard Plain Subarea

For potential case 3, the pumping of ground 

water was stopped in the South Oxnard Plain subarea 

in lieu of additional pipeline deliveries of diverted 

streamflow or imported water beginning in 1998. This 

case shows increased recovery in the upper- and loweraquifer systems throughout the Oxnard Plain and 

Pleasant Valley subareas relative to base-case 1. This 

reduction in pumpage increased the coastal seaward 

flow in the upper-aquifer system and reduced the 

coastal landward flow (seawater intrusion) in the loweraquifer system. Stopping pumpage primarily in the 

lower-aquifer system in the South Oxnard Plain 

subarea had the largest effect on reducing coastal 

landward flow (seawater intrusion) of all the potential 

cases evaluated. Coastal landward flow (seawater 

intrusion) in the lower-aquifer system was reduced by 

48 percent, yet coastal seaward flow in the upperaquifer system was increased by 85 percent compared 

with base-case 1 (figs. 27A and 28C; table 6). The 

largest net underflow to the South Pleasant Valley 

subarea was simulated with cessation of pumpage in 

the South Oxnard Plain subarea (fig. 28C, table 6). 

Similarly, the cessation of pumpage in the South 

Oxnard Plain subarea resulted in ground-water 

underflow to the Northeast Oxnard Plain from the 

South Oxnard Plain subareas—a reversal in underflow 

relative to the base-case 1 (figs. 27A and 28C). For 

cessation of pumpage in the South Oxnard Plain, 

simulated subsidence was not completely eliminated 

but was reduced to a few tenths of a foot along the 

northern boundary with the northeastern part of the 

Oxnard Plain subareas. This potential case also resulted 

in an additional 0.1 ft of subsidence over much of the 

Oxnard Plain Forebay subarea and the adjacent Santa 

Paula subarea relative to base-case 1. Cessation of 

pumpage in the South Oxnard Plain also reduced the 

extent and magnitude of subsidence in the Northeast 

Oxnard Plain subarea. 

Potential Case 4—Shift Pumpage to Upper-Aquifer System in 

PTP Wells

For potential case 4, pumpage from the 

Pumping-Trough Pipeline (PTP) wells was shifted 

from the lower-aquifer system to the upper-aquifer 

system beginning in 1998. This change produced 

water-level declines over a larger areal extent in the 

upper-aquifer system in the Oxnard Plain Forebay 

subarea (fig. 28D) relative to base-case 1 (fig. 27A), as 

well as a small reduction in coastal seaward flow in the 

upper-aquifer system and a small reduction of coastal 

landward flow in the lower-aquifer system compared 

with base-case 1 (fig. 28D). The shifting of PTP-well 

pumpage to the upper-aquifer system also resulted in 

increased underflow from the lower-aquifer system in 

the Northeast Oxnard Plain to the South Oxnard Plain 

and the South Pleasant Valley subareas by 900 acreft/yr compared to net underflow in base-case 1 

(figs. 27A and 28D). The shifting of PTP-well 

pumpage to the upper-aquifer system reduced the 

extent and magnitude of subsidence in the Northeast 

Oxnard Plain subarea but had little to no effect 

elsewhere. 

Potential Case 5—Shift Pumpage to Upper-Aquifer System in 

the Northeast Oxnard Plain

For potential case 5, pumpage throughout the 

Northeast Oxnard Plain subarea was shifted from the 

lower-aquifer system to the upper-aquifer system 

beginning in the year 1998. The change for this 

simulation is similar to the change in potential case 4. 

The increase in pumpage in the upper-aquifer system 

produced increased water-level declines in the 

Northeast Oxnard Plain subarea, reduced underflow 

from the Northeast Oxnard Plain subarea to adjacent 

subareas in the upper-aquifer system, and reduced 

coastal seaward flow in the upper-aquifer system 

relative to base-case 1 (figs. 27A, 28E). The reduced 

pumpage in the lower-aquifer system resulted in 

reduced coastal landward flow in the lower-aquifer 

system (fig. 28E). The net coastal seaward flow was 

decreased by about 1,090 acre-ft/yr in the upperaquifer system and the coastal landward flow (seawater 

intrusion) was decreased by about 1,180 acre-ft/yr 

(figs. 27A, 28E). The shifting of pumpage to the upperaquifer system in the Northeast Oxnard Plain subarea 

also reduced the extent and magnitude of subsidence 

throughout the Oxnard Plain subareas but did extend 

some potential subsidence of less than 0.1 ft into the 

Northwest Oxnard Plain subarea.
148 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Potential Case 6—Shift Pumpage to the Upper-Aquifer 

System in the South Oxnard Plain Subarea

For potential case 6, pumpage throughout the 

South Oxnard Plain subarea was shifted from the 

lower-aquifer system to the upper-aquifer system 

beginning in the year 1998. The shift in pumpage 

produced coastal landward flow (seawater intrusion) in 

the upper-aquifer system and reduced coastal landward 

flow (seawater intrusion) into the lower-aquifer system 

in the South Oxnard Plain subarea by about half 

(fig. 28F) relative to base-case 1 (fig. 27A). Relative to 

the base-case 1 projection, shifting pumpage from the 

lower- to the upper-aquifer system in the South Oxnard 

Plain subarea resulted in the second largest reduction 

(33 percent) of total coastal landward flow (seawater 

intrusion) of all the potential cases evaluated. In 

addition, shifting pumpage to the upper-aquifer system 

in the South Oxnard Plain subarea resulted in reduction 

in coastal seaward flow in the upper-aquifer system, an 

increase in underflow from the Northeast Oxnard Plain 

subarea to the South Oxnard Plain subarea, and in a 

reversal of underflow from the South Oxnard Plain to 

the South Pleasant Valley subarea in the lower-aquifer 

system (figs. 27A and 28F). The net coastal seaward 

flow was decreased by about 1,750 acre-ft/yr in the 

upper-aquifer system, and the net coastal landward 

flow (seawater intrusion) was decreased by about 

1,590 acre-ft/yr (figs. 27A, 28F) in the lower-aquifer 

system relative to base-case 1. The shifting of pumpage 

from the lower- to the upper-aquifer system in the 

South Oxnard Plain subarea yielded the largest 

combined effect on coastal flow with a reduction of 

coastal landward flow in the lower-aquifer system and 

coastal seaward flow from the upper-aquifer system. 

Similarly, shifting pumpage in the South Oxnard Plain 

subarea to the upper-aquifer system reduced the 

magnitude of potential additional subsidence 

throughout the Northeast and South Oxnard Plain 

subareas. 

Potential Case 7—Shift Pumpage to Upper-Aquifer System, 

Pleasant Valley

For potential case 7, pumpage throughout the 

Pleasant Valley subareas was shifted from the loweraquifer system to the upper-aquifer system beginning 

in the year 1998. This simulation produced coastal 

seaward flow in the upper-aquifer system similar to that 

in base-case 1 and a small decrease of coastal landward 

flow (seawater intrusion) in the lower-aquifer system 

compared with that for base-case 1 (fig. 28G). Shifting 

pumpage to the upper-aquifer system in the Pleasant 

Valley subareas resulted in more flow from the upperaquifer system in the Northeast Oxnard Plain subarea 

and a reversal of flow in the lower-aquifer system 

toward the Oxnard Plain subareas from the South 

Pleasant Valley subarea (figs. 27A and 28G). Shifting 

pumpage to the upper-aquifer system in the Pleasant 

Valley subareas reduced potential subsidence in the 

North Pleasant Valley subarea and resulted in reduced 

subsidence in the Oxnard Plain subareas—a result 

similar to that caused by shifting pumpage to the 

upper-aquifer system in the Northeast and South 

Oxnard Plain subareas.

SUMMARY AND CONCLUSIONS

Ground water from the regional alluvial-aquifer 

system is the main source of water in the Santa Clara–

Calleguas Basin in southern California. A steady 

increase in the demand for water in the basin since the 

late 1800s has resulted in streamflow depletion, 

ground-water overdraft, seawater intrusion, interaquifer flow, land subsidence, and ground-water 

contamination. Construction of reservoirs and 

discharge of shallow ground water and treated sewage 

effluent have contributed to regulated flow and 

modification of river systems in the basin, changing 

flows in the Santa Clara River and the Calleguas Creek 

and in some tributaries to predominantly perennial or 

intermittent flow. The use of ground water and surface 

water also is affected by wet and dry climatic periods 

that control the quantity and distribution of streamflow 

and recharge. These periods, which have persisted 

since the late 1600s, are estimated to have had periods 

of about 22, 5.3, and 2.2–2.9 years during the past 100 

years. Dry to wet cycle precipitation increases by a 

factor of 1.8 for winters and by 1.6 for springs.
Summary and Conclusions 149

The Santa Clara–Calleguas Basin in Ventura 

County, California, is composed of northeast-trending 

anticlinal mountains and synclinal valleys in the 

Transverse Ranges physiographic province. The 

onshore part of the alluvial basin is about 32 mi long 

and includes about 310 mi2 bounded by rugged 

topography. An additional 193 mi2 of the ground-water 

basin is an extensive sloping offshore plain truncated 

by steeply dipping submarine cliffs and dissected by 

several submarine canyons. The two largest submarine 

canyons dissect the offshore plain west of Port 

Hueneme and Point Mugu. The Santa Clara River and 

the Calleguas Creek and their tributaries drain the basin 

to the Pacific Ocean.

Growth and increasing water use in the Santa 

Clara–Calleguas Basin have continued over the last 

century, and because of the proximity to the Los 

Angeles metropolitan area, they may continue to 

transform the basin from an agriculture-based economy 

to an urban and industrial economy. Agricultural land 

use increased less than 5 percent between 1969 and 

1980, and population in Ventura County increased 28 

percent between 1980 and 1992. Agricultural water use 

increased to a historical high during the 1950s owing in 

part to the introduction of truck crops and refrigerated 

railroad transportation. Estimated pumpage ranged 

from 34,800 acre-ft for the drought years of the 1920s 

to a maximum pumpage of 301,400 acre-ft/yr during 

the 1990 drought year.

The Santa Clara–Calleguas Basin consists of 

multiple aquifers grouped into upper- and loweraquifer systems. The upper-aquifer system includes the 

Shallow, Oxnard, and Mugu aquifers. The loweraquifer system includes the Hueneme, Fox Canyon, 

and Grimes Canyon aquifers. Layers of the aquifer 

systems include basal coarse-grained sediments 

overlying regional unconformities; these coarsegrained layers are the major source of ground-water 

production and pathways for seawater intrusion. The 

aquifer systems are surrounded and underlain by 

consolidated bedrock that forms a relatively 

impermeable boundary to ground-water flow. 

Numerous faults act as barriers and boundaries to 

ground-water flow. The aquifer systems crop out 

offshore along the edge of the submarine shelf and 

within the coastal submarine canyons. Submarine 

canyons have dissected these regional aquifers, 

providing a hydraulic connection to the ocean through 

the submarine outcrops of the aquifer systems. 

Analysis of hydrological, geophysical, and 

geochemical data and simulation results indicates that 

the upper-aquifer system receives most of the natural 

and artificial recharge, and thus is more dynamic than 

the lower-aquifer system. Owing to development, many 

changes have occurred in the regional flow system: 

streamflow has changed from predominantly 

floodflows to a combination of regulated flows and 

floodflows; large quantities of diverted streamflow and 

treated sewage effluent are used for artificial recharge; 

streamflow infiltration has increased due to pumpage of 

ground water; ground water that flowed toward the 

ocean now flows toward the major pumping centers in 

the northeastern part of the Oxnard Plain, in Pleasant 

Valley, and in the western part of the East Las Posas 

Valley; aquitard compaction has resulted in land 

subsidence in the southern Oxnard Plain; and vertical 

flow occurs as leakage between aquifer systems and 

intraborehole flow within water-supply wells.

A numerical ground-water flow model of the 

Santa Clara–Calleguas Basin was developed as part of 

the USGS RASA Program. The flow model was 

developed to better define the geohydrologic 

framework of the regional ground-water flow system 

and to analyze problems affecting water resources of a 

typical coastal aquifer system. Development of the 

model included compilation of geographic, geologic, 

and hydrologic data and estimation of hydraulic 

properties and flows. The transient-state model was 

calibrated to historical surface-water and ground-water 

flows for 1891–1993.

Sources of water to the regional ground-water 

flow system are natural and artificial recharge, coastal 

landward flow from the ocean (seawater intrusion), 

storage in the coarse-grained beds, and water from 

compaction of fine-grained beds (aquitards). Inflows 

used in the regional flow model simulation include 

streamflows routed through the major rivers and 

tributaries; infiltration of mountain-front runoff and 

infiltration of precipitation on bedrock outcrops and on 

valley floors; and artificial ground-water recharge of 

diverted streamflow, irrigation return flow, and treated 

sewage effluent. 
150 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Most natural recharge occurs through infiltration 

(losses) of streamflow within the major rivers and 

tributaries and the numerous arroyos that drain the 

mountain fronts of the basin. Most streamflow loss 

occurs during wet-year periods when flows are the 

greatest, although the percentage of streamflow loss is 

larger during dry-year periods (37 percent during dryand 22 percent during wet-year periods). Total 

simulated natural recharge was about 114,100 acreft/yr for 1984–93: 27,800 acre-ft/yr of mountain-front 

and bedrock recharge, 24,100 acre-ft/yr of valley-floor 

recharge, and 62,200 acre-ft/yr of net streamflow 

recharge. 

Artificial recharge (spreading of diverted 

streamflow, irrigation return, and sewage effluent) is a 

major source of ground-water replenishment to the 

Santa Clara–Calleguas ground-water basin. Streamflow 

has been diverted to spreading grounds since 1929, and 

treated-sewage effluent has been discharged to stream 

channels since 1930. During 1984–93, the estimated 

average artificial recharge at spreading grounds was 

about 54,400 acre-ft/yr, which is about 13 percent less 

than simulated streamflow recharge (62,200 acre-ft/yr). 

Estimated recharge from irrigation return flows on the 

valley floors and treated sewage effluent for 1984–93 

averaged about 51,000 acre-ft/yr and 9,000 acre-ft/yr, 

respectively.

 Surface-water outflows from the Santa Clara–

Calleguas Basin are streamflow discharged to the 

Pacific Ocean and to streamflow diversions used for 

agriculture and artificial ground-water recharge. The 

streamflows consist of floodflows, regulated surfacewater flows, such as releases from Lake Piru and 

discharge of treated sewage-effluent, and intermittent 

baseflow from rejected ground water. 

Ground-water discharge from the Santa ClaraCalleguas ground-water basin is pumpage, coastal 

seaward flow to the Pacific Ocean, and 

evapotranspiration along the flood plains of the major 

rivers and tributaries. Under predevelopment 

conditions, the largest discharge from the ground-water 

system was outflow as coastal seaward flow and 

evapotranspiration. Pumpage of ground water from 

thousands of water-supply wells has diminished these 

outflows and was the largest outflow from the groundwater flow system for the simulation period 1891–93. 

The distribution of pumpage for 1984–93 indicates that 

most of the pumpage occurs in the Oxnard Plain 

subareas (37 percent) and in the upper Santa Clara 

River Valley subareas (37 percent). 

The total simulated pumpage for 1984–93 

averaged about 247,000 acre-ft/yr, 146,000 acre-ft/yr 

from the Fox Canyon Groundwater Management 

agency (FGMA) subareas and 101,000 acre-ft/yr from 

the non-FGMA subareas. This large demand for 

ground water exceeded the natural and artificial supply 

of surface water and ground water for parts of the two 

aquifer systems and resulted in an overdraft of the 

potable water supply. Of the total 1984–93 pumpage, 

46 percent was contributed by natural recharge, 22 

percent was contributed by artificial recharge from 

diverted streamflow, 20 percent was contributed by 

irrigation return flow, and 4 percent was contributed 

from sewage-effluent infiltration, 6 percent was 

contributed by storage depletion, and 2 percent was 

contributed by coastal landward flow (seawater 

intrusion).

Ground-water pumping has resulted in large 

water-level declines in the Las Posas Valley and the 

Pleasant Valley subbasins. A monotonic water-level 

decline occurred in the Las Posas Valley subbasins 

from agricultural pumping. In the Las Posas Valley and 

South Pleasant Valley subbasins, water-level declines 

of 50 to 100 ft have occurred in the upper-aquifer 

system, and declines of about 25 to 300 ft or more have 

occurred in the lower-aquifer system since the early 

1900s.

The combination of variable demand from 

ground-water pumpage and variable supply, which 

changes in response to climatic cycles, has resulted in 

large cycles of decline and recovery in ground-water 

levels in the upper- and lower-aquifer systems. The 

largest seasonal and decadal changes in ground-water 

levels occur in the Oxnard Plain Forebay subarea 

owing to artificial recharge and pumping, and in the 

South Oxnard Plain and Pleasant Valley subareas 

owing to agricultural pumping.

The simulated direction of ground-water 

underflow in the Oxnard Plain is from the artificialrecharge areas in the Oxnard Plain Forebay subarea 

toward pumping centers in the Northwest and 

Northeast Oxnard Plain subareas. The mean simulated 

underflow to the Oxnard Plain subareas from the Santa 

Paula, West Las Posas Valley, and South Pleasant 

Valley subareas for 1984–93 was about 5,77; 500; and 

5,720 acre-ft/yr, respectively. 
Summary and Conclusions 151

Pumpage from both aquifer systems has resulted 

in large simulated water-level differences between 

aquifer systems during dry-year periods that range 

from 20 to 30 ft near the Hueneme submarine canyon, 

50 to 90 ft near Mugu submarine canyon in the Oxnard 

Plain, 10 to 25 ft in the Santa Clara subareas, and 30 to 

more than 100 ft in the Las Posas Valley subareas. As a 

result, inter-aquifer flow occurs as leakage. The 

simulated vertical downward flow from the upper to the 

lower-aquifer system averaged about 22,700 acre-ft/yr 

for the Oxnard Plain subareas for 1984–93.

Seawater intrusion was first suspected in 1931 

when water levels were below sea level in a large part 

of the Oxnard Plain. The simulation of regional 

ground-water flow indicated that coastal landward flow 

(seawater intrusion) began in 1927 and continued to the 

end of the period of simulation in 1993. During wet 

climatic periods or periods of reduced demand for 

ground-water pumpage, the simulated direction of 

coastal flow is reversed in the upper-aquifer system 

from landward to seaward. During the 1984–93 

simulation period, the total net coastal seaward flow 

was 9,500 acre-ft in the upper-aquifer system, which is 

less than the 16,000 acre-ft/yr coastal seaward flow 

simulated for predevelopment conditions. During the 

same simulation period, total coastal landward flow 

was 64,200 acre-ft in the lower-aquifer system. This 

simulated coastal landward flow was supported by 

increased chloride concentrations and increased EM 

conductivities in many of the coastal monitoring wells. 

Water-level declines induced land subsidence 

that was first measured in 1939. The model indicates 

that land subsidence began prior to the 1940s, with 

most of the decline occurring after the drought of the 

late 1920s and during the agricultural expansion of the 

1950s and 1960s. From 1939 through 1993, water-level 

declines contributed to 2.7 ft of measured land 

subsidence in the southern part of the Oxnard Plain. 

For this same period, the model simulated a total 3 ft of 

land subsidence in the South Oxnard Plain subarea, and 

as much as 5 ft in the Las Posas Valley subareas. Model 

results indicate that subsidence occurred primarily in 

the upper-aquifer system prior to 1959, but in the 

lower-aquifer system between 1959–93 owing to an 

increase in pumpage from the lower-aquifer system.

The calibrated ground-water flow model was 

used to assess future ground-water conditions based on 

proposed water-supply projects in the existing 

management plan for the Santa Clara–Calleguas 

ground water basin and seven alternative water-supply 

projects. Two different approaches were used to 

estimate future recharge, streamflow, and climaterelated water-demand conditions for input to these 

model simulations: (1) a 24-year projection (1994–

2017) using historical estimates of recharge and 

measured streamflow, and (2) a 44-year projection 

(1994–2037) using spectral estimates of future 

precipitation. The model simulations were used to 

assess the effects of increased recharge, reduced 

pumpage, and shifted pumpage (from lower- to upperaquifer system) on ground-water storage depletion and 

related coastal landward flow (seawater intrusion) and 

land subsidence.

The model simulations of the proposed watersupply projects in the existing management plan 

assume average pumpage from 1984–93 with historical 

inflows (base-case 1) and with spectral estimates of 

inflows (base-case 4), a rolling cut back in pumpage 

(base-case 2), and a step cut back in pumpage (basecase 3). All the simulations of the proposed watersupply projects reduced pumpage in the FGMA areas 

which resulted in a reduction but not an elimination of 

storage depletion and related coastal landward flow 

(seawater intrusion) and subsidence, a reduction in 

streamflow recharge, and an increase in coastal 

seaward flow and underflow to adjacent subareas from 

the Oxnard Plain. However, the immediate reduction in 

pumpage represented by the step cut-back projection 

showed the largest reduction in coastal landward flow 

(seawater intrusion) and land subsidence. A 

comparison of simulations of future ground-water 

conditions, based on historical inflows (base case 1) 

and a spectral estimate of inflows (base case 4), shows 

increased coastal landward flow (seawater intrusion), 

storage depletion, and increased land subsidence for 

base-case 4 due to a drought projected earlier in the 

spectral estimate of inflows than in the historical 

inflows. The spectral estimate probably provides a 

smoother and more realistic transition between 

historical and future climatic conditions.
152 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

Simulations of alternative water-supply projects 

indicated some differences in hydrologic responses 

relative to the simulations of the proposed water-supply 

projects in the existing management plan. Stopping 

pumpage primarily in the lower-aquifer system in the 

South Oxnard Plain subarea had the largest effect on 

reducing coastal landward flow (seawater intrusion) of 

all the potential cases evaluated. The shifting of 

pumpage from the lower- to the upper-aquifer system 

in the South Oxnard Plain subarea yielded the largest 

combined effect on coastal flow with a reduction of 

coastal landward flow in the lower-aquifer system and 

coastal seaward flow from the upper-aquifer system. A 

seawater-barrier injection projection stopped coastal 

landward flow (seawater intrusion) into the upperaquifer system but also resulted in large quantities of 

coastal seaward flow. The recharge of water in Happy 

Camp Canyon resulted in water-level rises that were 

above land surface (not feasible) in the East Las Posas 

Valley subarea and did not result in significant changes 

in hydrologic conditions in other parts of the basin.

Water-resource management alternatives may 

require implementation of feasible demand-side 

pumpage strategies that do not create adverse effects, 

such as seawater intrusion and land subsidence, during 

the driest parts of the dry climate cycles. Management 

practices should consider the natural climatic cycles 

that are dominant factors in the supply and demand 

aspects of the hydrologic budget and hydrologic cycle.

Management of the regional-aquifer system may 

require the implementation of feasible supply-side 

recharge projects that do not create adverse effects 

during the wettest parts of the wet climate cycles; such 

effects include the potential for liquefaction or 

contaminant mobilization from water levels that could 

approach the land surface. Near-surface ground-water 

levels currently controlled by ground-water pumpage 

along Arroyo Simi in Simi Valley could occur in areas, 

such as South Las Posas Valley subarea and the Oxnard 

Plain Forebay, where additional recharge projects are 

planned. Contaminant mobilization of organic and 

inorganic constituents from agricultural and treated 

sewage effluent can occur when unsaturated sediments 

become saturated or semiperched systems are 

hydraulically reconnected to the upper-aquifer system 

by rising water levels. Evaluation of future 

management projects may require simulating multiple 

projects as opposed to individual water-supply projects 

as was done for this study. Optimization modeling may 

be used to better evaluate the effects of multiple watersupply projects, allocate the final distribution of 

resources among the final set of supply and demand 

components, and delineate the limits of feasibility of 

any combination of water-supply projects and waterresource management policies.

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