Ground-water withdrawal from the Central Valley aquifer system varies seasonally. The highest demand is generally during the peak growing season in spring and summer, which are the driest seasons of the year. Demand for ground water is greatest in the semiarid San Joaquin Valley where natural recharge is least. Withdrawal rates increase significantly during dry years (fig. 89).
Ground water accounted for only a small part of the water withdrawn for irrigation before 1900. Streams and distribution canals supplied most of the demand. However, the need to con-tinue irrigation in dry years when surface-water supplies are undependable, as well as the expansion of agriculture into areas distant from surface-water sources, prompted increased ground-water development. By the 1960's, ground-water withdrawals from the Central Valley aquifer system averaged 11.5 million acre-feet per year, which was one-half or more of the water withdrawn from all sources (fig. 90) and was about 20 percent of the total irrigation withdrawals for the entire United States. During that same period, withdrawals for domestic and industrial uses accounted for about 5 percent of all ground-water withdrawals in the Central Valley. Historically, the largest withdrawal of ground water--15 million acre-feet--was during 1977, a drought year.
Increased importation of surface water for irrigation since 1977, as well as generally wetter weather through 1985, resulted in decreased ground-water withdrawals. During 1985, ground water accounted for only about 37 percent of the total withdrawals in the Central Valley (fig. 91); total ground-water withdrawals were 10.1 million acre-feet (fig. 92). Of that amount, agricultural withdrawal accounted for 8.8 million acre-feet, all of which was used for irrigation (fig. 92A). This amount was about 11.5 percent of all ground water withdrawn in the United States for all purposes during 1985. The remaining 1.3 million acre-feet was used for public supply and industrial purposes, and by domestic and commercial users. Almost one-half of the water was withdrawn from the Tulare Basin (fig. 92B).
Although gains have been achieved by importing surface water from areas of surplus to areas of deficit, projected water needs in the San Joaquin Valley may require temporary withdrawal of ground water in excess of recharge in the future. The California Department of Water Resources has estimated that by 2010 demand for water in the Sacramento Valley, the San Joaquin Valley (exclusive of the Tulare Basin), and the Tulare Basin would be 7, 8, and 9 percent greater, respectively, than 1980 demands. The Sacramento Valley is expected to have sufficient supplies to meet agricultural demand until at least 2010. However, without increased surface-water imports, the San Joaquin Valley (exclusive of the Tulare Basin) and the Tulare Basin might require withdrawals of 150,000 and 2,400,000 acre-feet per year, respectively, in excess of recharge. Those estimates probably underestimate additional increased demand that would result from sustained dry weather. Occasional large withdrawals from an aquifer are a viable solution to the problem of reduced surface-water supplies in dry periods, provided the aquifer is replenished during wet years. However, continual withdrawal of ground water in excess of recharge can increase the cost of pumping, reduce water availability, and, in certain hydrogeologic settings, can cause land subsidence.