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A26 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA <br />tion of fine-grained materials within the aquifer system. <br />Thus, the vertical permeability of the Corcoran Clay <br />Member and the many clay beds in the section may have <br />been reduced by 1.5 to 6 times (Williamson and others, <br />1989). <br />Changes in the aquifer system's vertical flow were <br />investigated with the finite-difference flow model as <br />described in chapter D (Williamson and others, 1989). <br />Simulations suggest that the average vertical leakance <br />increased by about an order of magnitude from the <br />predevelopment era to the 1970's. This increase is due to <br />the large number of wells that contain long intervals of <br />perforated casing (fig. 17). In some areas of the valley, <br />the simulated increase in vertical leakage was more than <br />three orders of magnitude. Such localized, very large <br />increases are most logically explained by vertical move- <br />ment of water through many wells. Simulations intended <br />to determine possible decreases in vertical leakance due <br />to inelastic compaction of clays were inconclusive because <br />of the dominant effect of vertical flow through unpumped <br />wells. <br />Calculations presented in chapter D indicate that if <br />large-diameter wells perforated over a long interval are <br />evenly distributed, the vertical leakance of one well is <br />about the same as that of the fine-grained beds in about <br />7 mi2 of the aquifer system. Therefore, in areas with <br />many wells, the vertical flow is significantly altered by <br />well density. <br />CHANGES IN AQUIFER STORAGE <br />Ground-water levels have been significantly altered by <br />development in the Central Valley (fig. 12). For the most <br />part, long-term declines of the water table are less than <br />100 ft except locally in the southern part of the San <br />Joaquin Valley. In a few areas, increased recharge from <br />irrigation returns has caused the water table to rise as <br />Recharge from precipitation <br />(1.5) <br />\ <br />Evapotranspiration <br />(1.7) <br />.. CENTRAL VALLEY:.. <br />'AQUIFER SYSTEM- <br />c Stream gain v <br />(0-3) ^ <br />Stream loss <br />(0-5) <br />Surface-water bodies <br />Recharge from precipitation <br />(1.5) <br />Irrigation return includes ground-water <br />Pumpage <br />(11.5) <br />and surface-water sources <br />(9.0) <br />Stream gain <br />(0-3). CENTRAL VALLEY a <br />AQUIFER SYSTEM' <br />Surface-water bodies <br />Stream loss <br />(0.5) <br />B <br />Decrease in ground-water storage <br />(0.8) <br />FIGURE 16. Change in regional ground-water flow system due to <br />development (all values in millions of acre-feet per year). A, Prede- <br />velopment. B, Average rates during 1960's and 1970's. <br />Sand <br />Sand <br />' '-.'. .Sand.. <br />' '.' -. :::.':-'. <br />-.'-.' <br /> -.- .-- '-' <br />US Compacted clayjgggl <br />B <br />Sand ': " "- <br />Well with long perforated <br />interval - not pumping <br />FIGURE 17. Change in effective vertical conductance of Central Valley aquifer system due to development; size of arrows represents relative <br />magnitude of flow. A, Predevelopment effective vertical conductance. B, Current effective vertical conductance.