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