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A26 REGIONAL AQUIFERSYSTEM ANALYSIS—CENTRAL VALLEY,CALIFORNIA tion of fine-grained materials within the aquifer system. 1 100 ft except locally in the southern part of the San Thus, the vertical permeability of the Corcoran Clay Joaquin Valley. In a few areas, increased recharge from Member and the many clay beds in the section may have irrigation returns has caused the water table to rise as been reduced by 1.5 to 6 times (Williamson and others, 1989). Changes in the aquifer system's vertical flow were Recharge from precipitation investigated with the finite-difference flow model as (t.5) described in chapter D (Williamson and others, 1989). Evapotranspiration Simulations suggest that the average vertical leakance 1171 increased by about an order of magnitude from the predevelopment era to the 197Ws. This increase is due to Surface-water podies the large number of wells that contain lou intervals of Stream gain "g g CENTRAL VALLEY perforated casing [fig. 17]. In some areas of the valley, -AQUIFER SYS TEM, the simulated increase in vertical leakage was more than Stream ioss three orders of magnitude. Such localized, very large increases are most logically explained by vertical move- I ,A ment of water through many wells. Simulations intended to determine possible decreases in vertical leakance due to inelastic compaction of clays were inconclusive because Recharge from precipitation of the dominant effect of vertical flow through unpumped (t.sl wells. Irrigation return Includes ground-water Rumpage and surface water sources Calculations presented in chapter D indicate that if iii.$) tg;o1 large-diameter wells perforated over a long interval are y evenly distributed, the vertical leakance of one well is r stream gain Surface-water bodies about the same as that of the fine-grained beds in about CENTRAL VALLEY • ia�31 7 mis AQUIFER.SYSTEM � - of the aquifer system. Therefore, in areas with • . _ many wells, the vertical flow is significantly altered by Stream loss well density. Decrease in ground-water storage CHANGES 1N AQUIFER STORAGE B (0.8) Ground-water levels have been significantly altered by FICLIBE 16.—Change in regionak ground-water flow system due to development in the Central Valley(fig. 12). For the most development (all values in millions of achefeet per year). A, Prede- part, long-term declines of the water table are less than velopmenL B.Average rates during 1960's and 1970's. Sandr7and ----Clays==-