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GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A21 withdrawals of ground water from the west side of the the amount of regional flow, it has also decreased the San Joaquin Valley have profoundly affected hydraulic amount of water in storage and caused the land surface to gradients and flow patterns there. The heads in the lower subside over a large area. These effects are discussed in zone (originally above land surface) are now below sea the section"Effects of Ground-Water Withdrawal on the level and the direction of lateral ground-water flow has Central Valley Aquifer System." been reversed(Bull and Miller, 1975). Water in the lower zone previously flowed toward the center of the valley GROUND WATER IN STORAGE and discharged in the slough near the San Joaquin River (fig. 14). However,by the 1960's,flow was mostly toward The quantity of water in storage in the aquifers of the the pumping center on the west side of the valley. Central Valley has been estimated by several investiga- The total flow through the aquifer system has in- tors. All such estimates are based on use of average creased from about 2 million acre-ft/yr prior to develop- values of specific yield for different lithologies and an ment to nearly 12 million acre-ft/yr after development arbitrary thickness of the aquifer system. Earlier inves- (Williamson and others, 1989). The increased pumpage is tigators restricted their estimates to the shallow part of supplied largely by increased recharge, mostly from the aquifer system. Olmsted and Davis (1961) estimated imported surface water or recirculated pumped water. I that there were 28 million acre-ft of water in storage in Increased pumpage of ground water has not only changed the upper 200 ft of sediments in the Sacramento Valley. SOUTHWEST NORTHEAST FEET � 2,000 e m a � 5 0 y iti tl U5 1,000 Potentiometric surface, rz lower zone,1906 I I Sea �~ ��—— _� ——— — f� ._ — .- .- ✓ levelUppeBasg r zone Corcoran Clay Member th=Tulare For'ation f 1,000 Lower zone of trg' A VERTICAL EXAGGERATION X 20 2,000 SOUTHWEST NORTHEAST d FEET 2,000 K o � tl � 1,000 Potentiometric surface, n lower zone,1966 co I I � Sea Upper zone . { level 7Lower X—Corcoran Clay Member of the Tulare Formation1,000 � Basg of fresh Hatet .� B VERTICAL EXAGGERATION X 20 2,000- 0 5 10 MILES 0 5 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.