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A32 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA others (1984) reported that hydraulic heads generally declined 10 to 20 times as fast during the drought as during the period of long-term drawdown and compaction that ended in the late 1960's. In 1975, hydraulic heads were much higher than the lowest heads reached during 350 400 450 500 111111 550 w Q O_i LU CD DC 111 $ O 600 0.5 O 300 350 400 450 500 550 1.0 UJai u. 0.5 1960 1965 1970 1975 YEAR 1980 1985 FIGURE 22. Changes in hydraulic head and compaction at two wells in subsiding areas of San Joaquin Valley, 1960 to 1980 (modified from Ireland and others, 1984). A, Arvin-Maricopa area (well depth 1,480 ft). B, Los Banos-Kettleman City area (well depth 1,358 ft). the 1960's; consequently, when heavy pumpage resumed during the 1976-77 drought, very little compaction oc- curred. Pumpage was supplied by elastic storage (a small quantity) rather than by inelastic storage, so heads declined rapidly. Following the drought, recovery to predrought water levels was rapid and compaction vir- tually ceased. AREAL EXTENT AND EFFECTS In the Central Valley, land has subsided largely in the San Joaquin Valley south of the Merced River. This subsidence was extensively documented by Poland and others (1975) and by Ireland and others (1984), and it is briefly summarized here. These two reports present detailed contour maps and profiles showing the areal extent and magnitude of subsidence and hydrographs relating water levels and compaction. More than one-half of the San Joaquin Valley, or about 5,200 mi2 , has undergone land subsidence of more than 1 ft (fig. 19). Three major areas of subsidence within the valley are the Los Banos-Kettleman City area (western Fresno County), the Tulare-Wasco area (Tulare County), and the Arvin-Maricopa area (Kern County) (Poland and Lofgren, 1984). Of these, the Los Banos-Kettleman area underwent by far the largest volume of subsidence, amounting to two-thirds of the subsidence observed in the Central Valley up to 1980. This long trough-like area, extending for about 80 mi along the west margin of the valley, contains three depressions all characterized by more than 20 ft of subsidence at their centers. The maximum subsidence recorded in the United States (29.6 ft) is within one of these depressions in western Fresno County near the town of Mendota (fig. 23) (Ireland and others, 1984). Subsidence began in the San Joaquin Valley in the 1920's and increased slowly until World War II. Very large increases in ground-water pumpage during the 1940's and 1950's caused the volume of subsidence to increase dramatically. Pumpage increased further through the mid-1960's at an average withdrawal rate of nearly 12 million acre-ft/yr and subsidence increased accordingly. As of 1970, the total volume of subsidence was 15.6 million acre-ft/yr (Poland and others, 1975). FIGURE 23. Magnitude of subsidence at a site 10 mi southwest of Mendota in the San Joaquin Valley. Joseph F. Poland, principal subsidence researcher of the U.S. Geological Survey, alongside a power pole that shows approximate position of land surface in 1925, 1955, and 1977. Land surface was lowered 29.6 ft from 1925 to 1977.