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