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GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A15 TABLE 2.—Specific-storage values (Sd for aquifers in the Central Valley Specific storage coefficient Source of data (per foot) Remarks Fine Coarse grained grained Elastic Poland — 1.4 x 10-6 Assumed 1,000 ft of (1961, p. 1353) aquifer thickness of which 700 ft is coarse and 300 ft is fine. Estimate is for coarse material only. Riley and — 0.7 to 1 x 10--6 Detailed leaky-aqui- McClelland fer analysis of (1972, p. 77) pumping tests near Fresno, San Joa- quin Valley. Helm 4.5 x 10- — Average of several (1978, p. 193)) model runs. Inelastic Poland 2 x 10-4 — For 300 ft of clay (1961, p. 1353) with inelastic stor- age coefficient of 5 x 10-2. Helm 3 x 10-4 — Average for seven (1978, p. 193) sites;range, 1.4x10-' to 6.7 x 10-4. samples. However, the average Kh of the entire Central that are present in the aquifer system. Different view- Valley aquifer system is estimated to be 6 ft/d based on points on the role of the fine-grained lenses have resulted calibration of a regional ground-water flow model in two concepts of the aquifer system, as follows: (Williamson and others, 1989). This value is somewhat less than the average value for sand but probably reflects 1. Until recently, most investigators considered the the lateral discontinuity of sand beds and more accurately Sacramento Valley as containing one unconfined represents the conductivity that controls ground-water aquifer (Bloyd, 1978) and the San Joaquin Valley as flow on a regional scale. The average hydraulic conduc- containing two aquifers separated by a regional tivity of the Sacramento Valley is about one-half the confining unit. The San Joaquin sequence was de- average for the San Joaquin Valley, probably because of scribed in descending order by Poland and Lofgren more fine-grained volcanic-derived sediments in the (1984) as a semiconfined aquifer (upper water-bear- Sacramento Valley (Williamson and others, 1989). ing zone), a regional confining unit (Corcoran Clay Member of the Tulare Formation), and a confined aquifer (lower water-bearing zone). GROUND-WATER FLOW SYSTEM 2. More recently, Williamson and others (1989) pro- posed the concept of a single heterogeneous aquifer REGIONAL FLOW system for the Central Valley. This concept is that "the entire thickness of continental deposits is one Regional ground-water flow in the Central Valley is aquifer system that has varying vertical leakance and strongly influenced by the numerous clay and silt lenses confinement depending upon the properties of fine-