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A32 REGIONAL AQUIFER-SYSTEM ANALYSIS—CENTRAL VALLEY, CALIFORNIA others (1984) reported that hydraulic heads generally the 1960's; consequently, when heavy pumpage resumed declined 10 to 20 times as fast during the drought as during the 197647 drought, very little compaction oc- during the period of long-term drawdown and compaction curred.Pumpage was supplied by elastic storage(a small that ended in the late 1960's. In 1975, hydraulic heads quantity) rather than by inelastic storage, so heads were much higher than the lowest heads reached during declined rapidly. Following the drought, recovery to predrought water levels was rapid and compaction vir- 350 tually ceased. F A 1 AREAL EXIEWr ANn EmicTs aaa ! In the Central Valley, land has subsided largely in the San Joaquin Valley south of the Merced River. This 450 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 sao detailed contour maps and profiles showing the areal extent and magnitude of subsidence and hydrographs relating water levels and compaction. LU550 1,0 w More than one-half of the San Joaquin Valley,or about Z z 5,200 mit, has undergone land subsidence of more than 1 ui - ft (fig. 19). Three major areas of subsidence within the a soa 0-5 Q valley are the Los Banos-Kettleman City area (western Fresno County),the Tulare-Wasco area(Tulare County), N a and the Arvin-Maricopa area(Kern County)(Poland and Z 0 a Lofgren, 1984). Of these, the Los Banos-Kettleman area -I - ca underwent by far the largest volume of subsidence, a amounting to two-thirds of the subsidence observed in Co '0 the Central Valley up to 1980. This long trough-like area, cc extending for about 80 mi along the west margin of the LU B ! valley, contains three depressions all characterized by � 350 more than 20 ft of subsidence at their centers. The a maximum subsidence recorded in the United States(29.6 400ft) is within one of these depressions in western Fresno w County near the town of Mendota (fig. 23) (Ireland and others, 1984). 450 Subsidence began in the San Joaquin Valley in the y 1920's and increased slowly until World War II. Very {1, large increases in ground-water pumpage during the Sao t.n w 1940's and 1950's caused the volume of subsidence to increase dramatically. Pumpage increased further a through the mid-1960'.9 at an average withdrawal rate of 550 L, a 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). Nor- i 1960 1965 1970 1975 1980 1985 YEAR Ficu[tE 22.--Changes in hydraulic head and compaction at two FiGuitE 23.--Magnitude of subsidence at a site 10 mi southwest of wells in subsiding areas of San Joaquin Valley, 1960 to 1980 Mendota in the San Joaquin Valley. Joseph F. Poland, principal (modified from Ireland and others, 1981). A, Air;in-Maricopa subsidence researcher of the U.S. Geological Survey, alongside a area(well depth 1,480 ft). R, Los Banos-Kettleman City area power pole that shows approximate position of land surface in 1925, (well depth 1,3513 ft). 1955,and 1977- Land surface was lowered 29.6 ft from 1925 to 1977.