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GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A21 withdrawals of ground water from the west side of the San Joaquin Valley have profoundly affected hydraulic gradients and flow patterns there. The heads in the lower zone (originally above land surface) are now below sea level and the direction of lateral ground-water flow has been reversed (Bull and Miller, 1975). Water in the lower zone previously flowed toward the center of the valley and discharged in the slough near the San Joaquin River (fig. 14). However, by the 1960's, flow was mostly toward the pumping center on the west side of the valley. The total flow through the aquifer system has in- creased from about 2 million acre-ft/yr prior to develop- ment to nearly 12 million acre-ft/yr after development (Williamson and others, 1989). The increased pumpage is supplied largely by increased recharge, mostly from imported surface water or recirculated pumped water. Increased pumpage of ground water has not only changed the amount of regional flow, it has also decreased the amount of water in storage and caused the land surface to subside over a large area. These effects are discussed in the section "Effects of Ground-Water Withdrawal on the Central Valley Aquifer System." GROUND WATER IN STORAGE The quantity of water in storage in the aquifers of the Central Valley has been estimated by several investiga- tors. All such estimates are based on use of average values of specific yield for different lithologies and an arbitrary thickness of the aquifer system. Earlier inves- tigators restricted their estimates to the shallow part of the aquifer system. Olmsted and Davis (1961) estimated that there were 28 million acre-ft of water in storage in the upper 200 ft of sediments in the Sacramento Valley. SOUTHWEST NORTHEAST Potentiometric surface lower zone, 1906 Corcoran Clay Member of the Tulare Formation VERTICAL EXAGGERATION X 20 2,000 SOUTHWEST FEET 2,000 "I 1,000 - NORTHEAST Corcoran Clay Member of the Tulare Formation 1,000 - 2,000 10 KILOMETERS FIGURE 14. Predevelopment and current ground-water flow conditions (shown by arrows) in the San Joaquin Valley (from Bull and Miller, 1975, fig. 20). A, About 1900. B, 1966.