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A22 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA This estimate was based on dividing the sediments into four lithologic groups (channel deposits, alluvial plain and fan deposits, dissected alluvial deposits, and valley de- posits) and assigning storage characteristics to each group. Davis and others (1959) estimated that there were 121 million acre-ft in the upper 200 ft of the aquifer system in the San Joaquin Valley. As part of this RASA study, Williamson and others (1989) estimated that there were about 830 million acre-ft of freshwater in the upper 1,000 ft of the continental deposits in the Central Valley (as of 1961). This estimate was derived from a study of several thousand well logs, in which values of specific yield were assigned to depth intervals according to texture as mapped by Page (1986). Average values of specific yield for designated aquifer layers and subareas were then computed and are given in table 7 of Williamson and others (1989). The thickness of the aquifer system was taken as the difference between the 1961 water table and the lesser of either the depth to base of freshwater, or depth to the base of continental deposits, or 1,000-foot depth. The product of the specific yields and thicknesses, so derived, provided values of ground water in storage as follows: Area Sacramento Valley . Delta ................... San Joaquin Valley Tulare Basin.......... Average specific yield 0.07 0.08 0.10 0.10 Volume of ground water in storage (million acre-ft) 170 130 160 370 Central Valley 0.09 830 As discussed in the section "Effects of Ground Water Withdrawal on the Central Valley Aquifer System," ground water in storage was depleted at an average rate of 800,000 acre-ft annually during the 1960's and 1970's. Thus, the total ground water in storage as of 1986 was probably about 810 million acre-ft. GROUND-WATER DEVELOPMENT HISTORY OF DEVELOPMENT Ground-water development began in the Central Val- ley about 1880. However, development of surface water, primarily for irrigation, had been underway for the previous 100 years. By 1900, an extensive system of canals had been built to supply surface water to the southern San Joaquin Valley, and ground water was providing only a very small part of the irrigation water. After 1900, the construction of wells and rate of ground-water withdrawal increased slowly. By 1913, total well pumpage for the Central Valley was estimated to be 360,000 acre-ft annually. During the 1940's and 1950's, the pumpage of ground water for irrigation increased sharply. During the 1960's and 1970's, ground- water pumpage averaged about 11.5 million acre-ft/yr and was providing about 50 percent of the water used for irrigation. This withdrawal rate represented about 20 percent of the total yearly ground-water pumpage in the United States during that time. Pumpage for domestic and industrial use rose slightly during the 1960's and 1970's, but by 1977, it constituted only 5 percent of the total ground-water withdrawal. A summary of ground- water pumpage in the Central Valley from 1961 to 1977 was provided by Diamond and Williamson (1983). In the late 1960's, increased importation of surface water in some areas caused ground-water pumpage to decline and many wells to be unused. However, a drought in 1976 and 1977 decreased the availability of imported surface water, and ground-water pumpage increased sharply, reaching a maximum of 15 million acre-ft in 1977. Thus, in recent years, annual ground-water pumpage has fluctuated depending upon the availability of imported surface water. The areal distribution of the relatively light ground-water pumpage during the wet year of 1975 is compared with the heavy pumpage during the drought year of 1977 in figure 15. During the early 1980's, ground-water pumpage de- creased slightly from the 11.5 million acre-ft annual rate of the 1960's and 1970's, and the 1980's pumpage is about equal to the estimated recharge. Pumping in the Central Valley is seasonal, and most of the water is withdrawn during the spring-summer growing season. The autumn- winter period is usually a period of water-level recovery. Historically, the highest withdrawal rates have been in the drier areas the south-central part of the San Joa- quin Valley. DEPTH AND YIELD OF WELLS Most of the (approximately) 100,000 high-capacity wells in the Central Valley are used for either irrigation or public water supply. Yields in excess of 1,000 gal/min are generally required and can be obtained nearly every- where. The depth at which such yields can be obtained, however, varies depending on the local geology. Poor- quality water at shallow depths in some areas requires deep wells. Well depths in the Sacramento Valley are generally less than those in the San Joaquin Valley, and they range from an average depth of 120 ft in the highly permeable areas to nearly 500 ft in the less permeable areas. An analysis of performance tests on 2,783 wells reported by Olmsted and Davis (1961) indicated that most of those wells yielded 250 to 1,700 gal/min. For these wells,