Ground-Water Flow in the Central Valley, California

The agricultural productivity of the Central Valley is dependent on the availability of water from irrigation. About 7.3 million acres of cropland in the Central Valley receives about 22 million acre-feet of irrigation water annually. One half of this irrigation water is supplied by ground-water, which amounts to about 20 percent of the Nation’s ground-water pumpage. Ground water is important as a stable supply of irrigation water because of the high variability of surface-water supplies in the Central Valley. This large ground-water development during the past 100 years has had major impacts on the aquifer system, such as decline in water levels, land subsidence, depletion of the aquifer storage, and increase in recharge. The flow conditions before and during development were simulated on a regional scale using a three-dimensional finite-difference flow model.

The Central Valley is a large (20,000-square-mile) structural trough filled with poorly permeable marine sediments that are overlain by coarser continental sediments. In general, previous investigators have conceptualized the northern one-third of the valley the Sacramento Valley as a water-table aquifer and the southern two-thirds the San Joaquin Valley as a two-aquifer system separated by a regional confining clay layer. A somewhat different concept of the aquifer system was suggested during this study by analyses of water-level measurements, texture of sediments interpreted from electric logs, and flow model simulations. Vertical hydraulic head differences are found through out much of the Central Valley. Early in development, flowing wells and marshes were found throughout most of the central part of the valley. More than 50 percent of the thickness of the continental sediments is composed of fine-grained lenticular deposits that are discontinuous but are distributed throughout the stratigraphic section in the entire Central Valley.

The concept presented in this report considers the entire thickness of the continental deposits as one aquifer system which has varying vertical leakance that depends on several factors, including amount of fine-grained sediments. The average horizontal hydraulic conductivity is about 6 feet per day, and the average thickness of the continental deposits is about 2,400 feet.

Irrigation use, which averaged 22 million acre-feet of water per year during 1961-77, increased evapotranspiration about 9 million acre-feet per year over its predevelopment value. This is a large figure compared with the average annual surface-water inflow to the Central Valley of 31.7 million acre-feet per year. Precipitation on the valley floor is mostly lost to evapotranspiration. The overall post-development recharge and discharge of the aquifer system was about 6 times greater than the predevelopment estimated values. The increases of pumpage associated with development mostly in the San Joaquin Valley have caused water-level declines that exceed 400 feet in places and have resulted in the largest known volume of land subsidence due to fluid withdrawal in the world.

Water in aquifer storage declined about 60 million acre-feet from predevelopment to 1977; 40 million acre-feet were derived from the water-table zone, 17 million acre-feet from compaction of sediments, and 3 million acre-feet from elastic storage.

During 1961-77, ground water withdrawn from aquifer storage averaged about 800,000 acre-feet per year. The flow model constructed during this study was calibrated principally in accordance with the hydrologic data observed during 1961-75 because little predevelopment data were available for analysis. An explicit algorithm to simulate land subsidence was developed and calibrated. The simulated land subsidence was within 6 percent of the estimated observed volume; however, the time lag associated with this type of subsidence was not adequately simulated. Simulated water level changes averaged 2.6 and 12 feet higher than observed water level changes for the water table and the lower pumped zones, respectively, and the standard deviation of the simulated changes minus the observed change was 22 and 27 feet, respectively.

The flow model was tested for the period of 1976-77 drought with good results. The simulations indicated that vertical leakance greatly increased from the predevelopment values as a result of water flowing through some of the more than 100,000 irrigation well casings that are open to different aquifer layers.