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GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA TABLE 2. Specific-storage values (SJ for aquifers in the Central Valley Specific-storage coefficient (per foot) A15 Source of data Remarks Fine grained Coarse grained Elastic Poland (1961, p. B53) 1.4x10- Riley and McClelland (1972, p. 77) Helm (1978, p. 193)) 0.7 to lx 10- 4.5x10- Assumed 1,000 ft of aquifer thickness of which 700 ft is coarse and 300 ft is fine. Estimate is for coarse material only. Detailed leaky-aqui- fer analysis of pumping tests near Fresno, San Joa- quin Valley. Average of several model runs. Inelastic Poland (1961, p. B53) Helm (1978, p. 193) 2x10- 3x10- For 300 ft of clay with inelastic stor- age coefficient of 5xlO-2 . Average for seven sites; range, 1.4xl(T*to 6.7 xKT4 . samples. However, the average Kh of the entire Central Valley aquifer system is estimated to be 6 ft/d based on calibration of a regional ground-water flow model (Williamson and others, 1989). This value is somewhat less than the average value for sand but probably reflects the lateral discontinuity of sand beds and more accurately represents the conductivity that controls ground-water flow on a regional scale. The average hydraulic conduc- tivity of the Sacramento Valley is about one-half the average for the San Joaquin Valley, probably because of more fine-grained volcanic-derived sediments in the Sacramento Valley (Williamson and others, 1989). GROUND-WATER FLOW SYSTEM REGIONAL FLOW Regional ground-water flow in the Central Valley is strongly influenced by the numerous clay and silt lenses that are present in the aquifer system. Different view- points on the role of the fine-grained lenses have resulted in two concepts of the aquifer system, as follows: 1. Until recently, most investigators considered the Sacramento Valley as containing one unconfined aquifer (Bloyd, 1978) and the San Joaquin Valley as containing two aquifers separated by a regional confining unit. The San Joaquin sequence was de- scribed in descending order by Poland and Lofgren (1984) as a semiconfined aquifer (upper water-bear- ing zone), a regional confining unit (Corcoran Clay Member of the Tulare Formation), and a confined aquifer (lower water-bearing zone). 2. More recently, Williamson and others (1989) pro- posed the concept of a single heterogeneous aquifer system for the Central Valley. This concept is that "the entire thickness of continental deposits is one aquifer system that has varying vertical leakance and confinement depending upon the properties of fine-