U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2009–3057

July 2009 Printed on recycled paper

California’s Central Valley Groundwater Study: 

A Powerful New Tool to Assess Water Resources in California’s Central Valley 

The U.S. Geological Survey 

(USGS) has released results from a study 

on the largest water reservoir in the 

State of California, the Central Valley 

groundwater system. The findings show 

continued loss of stored groundwater in 

the southern part of the valley (see figure 

below). Since about 1960, groundwater 

has been depleted by almost 60 million 

acre-feet, which is, on average, enough to 

supply every resident of California with 

water for 8 years. 

In order to complete the study, the 

USGS developed an extensive, detailed 

three-dimensional (3D) computer model 

of the hydrologic system of the Central 

Valley (Faunt, 2009). The Central Valley 

Hydrologic Model (CVHM) simultaneously accounts for changing water supply 

Variable

to Dry

Variable

to Dry

Variable

to Wet Dry Wet Dry Wet

WATER YEAR

CHANGE IN GROUNDWATER STORAGE, 

IN MILLIONS OF ACRE-FEET

0

-20

-60

-40

-80

20

1962 1966 1970 1974 1978 1982 1986 1990 1994 1998 2002

Entire Central Valley

By region

Sacramento Valley

Delta and Eastside Streams

San Joaquin Basin

Tulare Basin

Groundwater Loss

and demand across the landscape, and 

simulates surface water and groundwater 

flow across the entire Central Valley. 

This new hydrologic modeling 

tool can be used by water managers to 

understand how water moves through the 

aquifer system, predict water-supply 

scenarios, and address issues related to 

water competition in California and the 

Central Valley including:

• Conjunctive water use (interdependent 

use of surface water and groundwater);

• Conservation of agricultural land;

• Land-use change, including environmental concerns and urbanization, and 

its effects on water resources; and 

• Effects of climate change.

Central

Valley California

Sacramento Valley

San Joaquin 

Basin

Tulare Basin

Delta and 

Eastside Streams

The USGS Groundwater Resources Program 

funded this study, one of 30 regional aquifer 

studies the USGS is conducting to assess the 

Nation’s groundwater availability. Intense 

competition for groundwater resources 

in California was an important factor in 

choosing the Central Valley as one of the 

first studies undertaken and completed. 

Competition for water resources is growing throughout California, particularly in the Central Valley. Since 

1980, the Central Valley’s population has nearly doubled to 3.8 million people. It is expected to increase to 

6 million by 2020. Statewide population growth, anticipated reductions in Colorado River water deliveries, 

drought, and the ecological crisis in the Sacramento–San Joaquin Delta have created an intense demand for 

water. Tools and information can be used to help manage the Central Valley aquifer system, an important 

State and national resource.

Central Valley Facts

•	 California’s Central Valley

covers about 20,000 square

miles, and is one of the most

productive agricultural regions

in the world.

•	 More than 250 different crops

are grown in the Central Valley,

with an estimated value of $17

billion per year. 

•	 Approximately one-sixth of the

Nation’s irrigated land is in the

Central Valley.

•	 About one-fifth of the Nation’s

groundwater pumpage is from

the Central Valley aquifer

system.
Detail of model grid Pacific Ocean

San

Francisco

Central

Valley

C A L I F O R N I A

A

A' Land surface

Layer 10

Layer 9

Layer 8

Layer 7 Layer 6

Layer 3

Layer 2 Layer 1

Corcoran Clay 

(layers 4,5)

Cross section 

Not in model

A A'

0 57 DISTANCE ALONG SECTION, IN MILES

ALTITUDE, IN FEET -2,500

Sea 

Level

1,000

To create enough detail 

to be practical for watermanagement decisions, 

the aquifer was divided 

spatially into 20,000 

model cells of 1 square 

mile each, and vertically 

into ten layers ranging in 

thickness from 50 to 

750 feet. 

The Central Valley Hydrologic Model 

The CVHM is built on knowledge from USGS and other Federal, State, and local studies. The CVHM 

was constrained by comparing simulated and historically observed groundwater levels, streamflows, and 

subsidence. The detail and breadth of this hydrologic modeling tool provides a better understanding of valleywide hydrologic processes. This tool also provides realistic boundary conditions so that finely gridded models 

of smaller areas can be easily developed.

Model Features

•	 A new tool for simulating the use and movement of water on the landscape, called “Farm Process”, combined with the

USGS MODFLOW groundwater/surface-water model to form a hydrologic model

•	 A comprehensive geospatial database developed using a Geographic Information System (GIS)

•	 A texture model to characterize the hydraulic properties of the valley-fill materials

Farm Process

The complex hydrologic system of the Central 

Valley is simulated by using the USGS hydrologic 

model, MODFLOW. Combined with the Farm Process, 

MODFLOW provides a tool to simulate natural and 

human-induced water movement and consumption over 

the landscape. Groundwater and surface-water flow 

are fully coupled. The model accounts for supply-constrained and demand-driven conjunctive use of water in 

agricultural, urban, and natural settings. In the Central 

Valley, the tool simulates unmetered historical pumpage 

and the delivery of surface water for 21 water-balance 

regions for water years 1962 to 2003. 
0 to 10

>10 to 20

>20 to 30

>30 to 40

>40 to 50

>50 to 60

>60 to 70

>70 to 80

>80 to 90

>90 to 100

Approximate extent

of Tulare Lake bed

Percent coarsegrained valley fill

Corcoran

Clay

Line of cross section

(shown on opposite page)

( > is greater than)

not in 

model

not in 

model

Information from 8,500 drillers’ 

logs helped scientists create 

a 3D model of the geologic 

characteristics of the

aquifer system. 

Centralized

Geospatial

Database

Soils Topography Remote Sensing

Hydrology Chemistry Human Influences

Geology Vegetation & Land Use

Climate Geophysics

Soil Surveys (STATSGO)

Soil-Properties Data

Digital-Elevation Models

Topographic Maps

Satellite Imagery

MoDIS

Geologic Maps

Geologic Cross Sections

Drillers Logs

Land-Use/Land-Cover Maps

Agricultural Data

Crop Data

Precipitation Data

Temperature Data

Humidity

Evapotranspiration

Water-Well Information

Aquifer Tests

Aquifer Properites

Aquifer Chemistry

Spring Chemistry

Surface-Water Chemistry

Surface-Water Delivery

Surface-Water Diversions

Environmental Requirements

Surface-Water Data

Gravity

Seismic/Earthquake

Magnetics/Resistivity

Many different types of data are 

managed within the Central Valley 

Hydrologic Model’s extensive 

Geospatial database. 

Well locations

Geospatial Database

A comprehensive geospatial database was developed using a Geographic 

Information System (GIS). The GIS was 

used to compile, manage, store, and analyze the large quantities of data needed 

to run the model, and to understand and 

visualize the data output. 

Because the 3D groundwater flow 

model of the heterogeneous Central 

Valley aquifer system includes complex 

surface-water management processes, 

the GIS is extremely useful for recognizing and understanding spatial relations 

within and among data sets. 

Texture Model

A geologic texture model was developed to characterize 

the coarseness of valley-fill materials that make up the aquifer 

system. Lithologic data from approximately 8,500 drillers’ 

logs of boreholes ranging in depth from 12 to 3,000 feet below 

land surface, along with other information, were compiled 

and analyzed. The resulting geologic model was then used 

to estimate hydraulic properties for every cell in the model. 

The texture model is key to understanding how water moves 

through the Central Valley groundwater system. 
0 50 Miles

0 50 Kilometers

Land Use 2000 Sacramento Valley Hydrology

San Joaquin Valley Hydrology Area of land subsidence

Results

Study results show 

that human activities 

such as surface-water 

deliveries and associated 

groundwater pumping 

have a dramatic influence on the present-day 

Central Valley hydrology. 

These human activities have increased flow 

through the groundwater 

system by roughly a 

factor of six compared to 

the pre-development period. 

Regional differences throughout the hydrologic system 

can be seen in variations in 

water availability and land use, 

and in water-delivery systems. 

These human activities are often 

in response to spatial and temporal 

climate variability. 

The San Joaquin Valley, which includes the San Joaquin and 

Tulare Basins, has experienced large changes in groundwater 

storage. In the early 1960s, groundwater pumping caused 

water levels to decline to historic lows on the west side of the 

San Joaquin Valley, which resulted in large amounts of surface 

subsidence. In the late 1960s, the surface-water delivery system 

began to route water from the wetter Sacramento Valley and Delta 

regions to the drier, more heavily pumped San Joaquin Valley. The 

surface-water delivery system was fully functional by the early 

1970s, resulting in groundwater-level recovery in the northern and 

western parts of the San Joaquin Valley. Overall, the Tulare Basin 

portion of the San Joaquin Valley, the hottest and driest part of the 

Central Valley, is still showing declines in groundwater levels and 

accompanying depletion of groundwater storage.

The Central Valley includes urban, 

native, and agricultural land uses. 

Because both surface-water and 

groundwater needs vary by 

location and land use, the land 

use affects groundwater 

availability.

The surface-water delivery system developed for the Central Valley 

redistributes a significant portion of the water from north to south. The 

abundance of surface water in the Sacramento Valley results in less 

pumping, so the Sacramento Valley and the Delta generally have 

experienced relatively mimimal groundwater-storage depletion.

For more information contact:

Claudia Faunt, Ph.D.

Hydrologist

ccfaunt@usgs.gov

U.S. Geological Survey

California Water Science Center

4165 Spruance Road, Suite 200

San Diego, CA 92101

Reference:

Faunt, C.C. ed., 2009, Groundwater Availability of 

the Central Valley Aquifer: U.S. Geological Survey 

Professional Paper 1766, 225 p. Available at 

http://pubs.usgs.gov/pp/1766/

—Claudia C. Faunt, Randall T. Hanson, 

Kenneth Belitz, and Laurel Rogers

Fraction of total area 

by land-use type

Land-use type

Water

Urban

Native vegetation

Idle/fallow

Truck, nursery, 

 and berry crops

Citrus and subtropical

Field crops

Vineyards

Pasture

Grain and hay

Deciduous fruits 

 and nuts

Rice

Cotton