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GROUND WATER 1N THE.CENTRAL VALLEY,CALIFORNIA A17 chapter D(Williamson and others, 1989). The simulation the valley's center is about 49 inlyr, exceeding precipi- utilized the U.S. Geological Survey's three-dimensional tation rates. The occurrence of upward direction of finite-difference model (Trescott, 1975; Trescott and hydraulic gradients in the central part of the valley is Laren, 1976). The model was modified to include a shown by the large area of flowing wells that were procedure first described by Meyer and Carr(1979)that documented prior to 1900 (Hall, 1889; Mendenhall and simulates the effects of land subsidence due to inelastic others, 1916). In the southern San Joaquin Valley, compaction of clays (Prudic and Williamson, 1986). The ground water was discharged to Tulare Lake and as resulting model considers the valley deposits as one evapotranspiration in the area surrounding it(shown by aquifer system characterized by variations in vertical the closed depression in fig. ll). Water discharging to leakage properties. The leakage depends not only on the stream channels flowed into the Sacramento and San vertical permeability of the sediments, but also on the Joaquin Rivers, then into the Delta and westward into density of wells constructed with long perforated sections San Francisco Bay. or multiple screens, because such wells provide vertical The regional hydraulic gradients in the aquifer system hydraulic connection within the aquifer system. were steeper in the Sacramento Valley than in the San Four aquifer layers were specified within the model:an Joaquin Valley for the following reasons:(1)The outlet at upper layer representing the shallow water-table zone, the confluence of the Sacramento and San Joaquin Rivers two middle layers representing the lower pumped zone, is closer to the northern end of the Central Valley, (2) and a basal layer representing the continental deposits recharge rates were higher in the Sacramento Valley, below the deepest wells in the valley (fig. 9). The model and (3) average permeabilities are lower in the Sacra- simulated recharge from precipitation, streams, and mento Valley. irrigation returns and simulated discharge to streams, The ground-water flow system has been greatly al- evapotranspiration, and wells. Emphasis was placed on tered by large-scale ground-water development and very simulation of the period from 1961 to 1977 because of large diversions and redistribution of surface water availability of data and because this period was repre- through the Central Valley. Heavy pumpage from wells, sentative of long-term climatic conditions including wet averaging 11.5 million acre-ft annually during the 1960'( years and dry years. The discussion of regional ground- and 1970's, combined with increased recharge due to water flow presented here draws heavily on the results of irrigation returns from redistributed surface water, simulation by Williamson and others (1989). caused changes in ground-water levels throughout most The natural pattern of ground-water movement and of the Central Valley. Examples of long-term ground- the rates of recharge and discharge have been signifi- water-level changes in some wells caused by the water cantly altered by water development. Prior to develop- development are shown in figure 12. ment, ground water generally moved from recharge The configuration of the water table in 1976 (fig. 13) areas in the higher ground surrounding the Central shows the effects of heavy pumpage from wells. Ground Valley toward topographically low areas in the center of water now flows primarily toward cones of depression at the valley. The general pattern of lateral flow in the f pumping centers rather than toward the preexisting valley before development is shown by the water-table natural discharge area• along the Sacramento and San contour map in figure 11. Note that ground water flowed Joaquin Rivers and around Tulare Lake,but there is still largely toward the Sacramento or San Joaquin Rivers a large component of flow toward the Delta area. The except in the southern San Joaquin Valley, where flow largest ground-water-level declines have occurred in the was toward Tulare Lake. western and southern parts of the San Joaquin Valley. Recharge was supplied primarily by streams entering Declines are much less in the Sacramento Valley, but a the valley from the Sierra Nevada and 1,2amath Moun- major pumping depression has formed just north of the tains and, to a lesser extent, directly from precipitation. Delta. Recharge from irrigation returns has caused The streamflow (mostly snow meltwater) was largest ground-water levels to rise above their predevelopment from January to June. Recharge via stream channels took levels in parts of northwestern San Joaquin Valley and place mostly in their upper reaches shortly after entering parts of central Sacramento Valley. (See further discus- the valley. Downward hydraulic gradients undoubtedly sion of water-level declines in the section "Effects of were present in these recharge areas, but they are not Ground-Water Withdrawal on the Central Valley Aquifer well documented because of the smmity of data from System.") older deep wells. The combination of increased recharge to the water Prior to irrigation development, most ground water table and increased pumping from the lower zone has was discharged as evapotranspiration in the central caused a reversal in the direction of the hydraulic trough of the valley, and to a lesser extent, it was gradient(from upward to downward)in the center of the discharged to streams. Potential evapotranspiration in Central Valley (Williamson and others, 1989). Large