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GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A27 much as 40 ft. Head declines in the lower pumped zone in the Sacramento and northern San Joaquin Valleys gen- erally are small less than 80 ft (fig. 18). However, in the heavily pumped western and southern parts of the San Joaquin Valley, heads have declined from 100 to 400 ft since development began. Since the late 1960's, the increased availability of imported surface water in these areas and the accompanying decrease in ground-water pumpage has stopped the long-term decline and allowed some recovery of ground-water levels. Year-to-year changes in ground-water levels have reflected the avail- ability of surface water. During wet or average years, more imported surface water is available for irrigation; as a result, well pumpage decreases and ground-water levels rise. During drought years, such as 1976 and 1977, less surface water is available, wells are more heavily pumped, and ground-water levels decline. When heads have declined sufficiently in the lower pumped zone for inelastic compaction of clay beds to occur, the rate of water-level decline slows. This slower rate results because the effective storage coefficient is significantly increased (Williamson and others, 1989). The result of the decline in gruund-water levels from the start of development until 1977 has been the loss of an estimated 60 million acre-ft of aquifer storage. This depletion of storage is made up of three components. 1. L^ng-term lowering of the water table that results from dewatering of the shallow sediments 40 mil- lion acre-ft. 2. Inelastic compaction (permanent reduction of pore space) 17 million acre-ft. 3. Elastic storage (compression of sediments and ex- pansion of water) 3 million acre-ft. The changes in storage were calculated with the computer model as described in chapter D (Williamson and others, 1989). The decrease in storage due to dewatering is the product of specific yield, water-table decline, and area of the shallow aquifer dewatered. Similarly, the change in elastic storage is the product of elastic specific storage, thickness of confined aquifer, head decline, and area of aquifer affected. The loss of storage from inelastic compaction of clay beds causes a permanent loss of pore space that in turn is balanced by an equivalent volume of land subsidence (see next section). Extensive leveling by the National Geo- detic Survey over many years has established the extent of land subsidence in the Central Valley (Poland and others, 1975; Ireland and others, 1984). The 17 million acre-ft of storage loss attributed to inelastic compaction of fine-grained sediments is simply the volume of land subsidence derived from these surveys. During the 1960's and 1970's, the annual decrease in ground-water storage was about 800,000 acre-ft, repre- senting about 7 percent of annual pumpage (fig. 16). The long-term decrease in aquifer storage of 60 million acre-ft, although very large, represents only a small part of the more than 800 million acre-ft of freshwater stored in the upper 1,000 ft of sediments in the Central Valley. Nevertheless, the lowering of water levels in the upper and lower zones caused a significant increase in pumping lifts and thus a significant increase in the cost of pumping ground water. During the early 1980's, ground-water pumpage decreased, ground-water levels rose in many areas, and there was virtually no further decrease in ground-water storage. LAND SUBSIDENCE The largest volume of land subsidence in the world caused by human activities is in the Central Valley. The area affected by subsidence includes much of the south- ern part of the San Joaquin Valley and smaller areas in the Sacramento Valley and the Delta (fig. 19). By far, the largest volume of land subsidence is caused by ground- water pumpage and the resulting compaction of clay in the San Joaquin Valley. However, other processes have contributed to land subsidence locally as described by Poland and Evenson (1966) and Poland and Lofgren (1984). Briefly, the five processes that cause subsidence are: 1. Compaction of fine-grained sediments in the aquifer system resulting from head declines due to heavy ground-water pumpage. 2. Compaction of sediments in petroleum reservoir rocks caused by oil and gas extraction. 3. Hydrocompaction compaction of moisture-deficient sediments following the first application of water. 4. Compaction of peat soils following land drainage. 5. Tectonic subsidence. Compaction of peat soils and subsequent land subsid- ence has occurred in an area of about 450 mi 2 in the Delta area formed at the confluence of the San Joaquin and Sacramento Rivers (Poland and Evenson, 1966). Islands in the Delta area that were originally at or slightly above sea level are now 10 to 20 ft below sea level. This type of subsidence results from oxidation and compaction of peat soils following drainage of marshlands for agriculture. This area was drained in the middle and late 1800's, which resulted in subsidence that is continuing today. Increased pumping is required to maintain a lowered water table for cultivation. Hydrocompaction refers to the compaction of moisture- deficient deposits above the water table following the