Before development, the net circulation through the aquifer system was approximately 2 million acre-feet per year (fig. 88A). Of an average annual 12.4 million acre-feet of precipitation that fell on the valley floor, 10.9 million acre-feet was lost to evaporation because of the arid conditions that characterize the valley; thus, only 1.5 million acre-feet of precipita-tion entered the aquifer system as recharge. Water that moved from surface-water bodies to the aquifer system provided the remaining 500,000 acre-feet per year of recharge. The recharge was balanced by discharge from the aquifer system to rivers (300,000 acre-feet) and evapotranspiration (1.7 million acre-feet).
Development added two components to the water budget--withdrawals and return flow from irrigation--and increased the volume of water that flowed through the ground-water system approximately sixfold from 1961 through 1977. Ground-water withdrawals for irrigation, municipal supply, and industrial use totaled about 11.5 million acre-feet annually. Seepage from irrigation returned about 9 million acre-feet to the ground-water system (fig. 88B). During the period 1961 through 1977, the rate of ground-water withdrawals from the aquifer system was greater than the net recharge from all sources. Withdrawals in excess of recharge resulted in a loss of water from storage in the aquifer of 800,000 acre-feet per year. In the case of the Central Valley aquifer system, some of the loss from storage is permanent because some of the water was removed from beds of fine-grained materials, which, when drained, become compacted and cannot store water again. Compaction of fine-grained materials led to land subsidence in the Central Valley. By the late 1970's, however, sufficient surface-water supplies were imported by aqueducts to reduce substantially the volume of ground water that was withdrawn. Although some additional surface water has been imported since 1977 and ground-water withdrawals have slightly decreased, the water budget shown in figure 88B is representative of current (1995) conditions.