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A14 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA TABLE 1.-Laboratory values of selected hydraulic and physical properties of unconsolidated sediment in the Central Valley [ft/d,feet per day;<,less than;—,no data] Average hydraulic conductivity2 de Sediment size used to determine Number samples Specific yield' Porosity' (ft/d) specific yield and porosity (percent) (percent) Vertical Hori- zontal Sand ........................ 126 19-35 31-6527 40 11.5 14 Clayey sand .............. 28 10-28 28-52 _ _ 16 37 Sand-silt-clay95 2-20 31-56 .02 .02 ............ 12 37 Clayey silt ................ 107 <1-7 32-613.5 42<1-15 25-41 0001 — Silty sand ................. 137 7,5 34 21 .16 Sandy silt ................. 49 7,5 36 1-12 34-37 02 .13 Silt .......................... 79 1-7 34-56 ,00023 43 Silty clay .................. 86 <1-8 35-52 4 43 0001 ,002 Clay ........................ 0 No pure clays analyzed — — 'Range of values above line;mean value below line.Specific yield and porosity values were compiled from Stearns and others(1930), Piper and others(1939),Johnson(1967),and Johnson and others(1968). 2Laboratory determinations of hydraulic conductivity values were obtained from the U.S.Bureau of Reclamation,Sacramento, California,and Johnson and others(1968). below the upper few hundred feet should be considered grained lenses, the specific storage is inelastic. The confined in the sense that the vertical permeabilities of coefficients of elastic and inelastic specific storage de- sediments are much lower than the horizontal permeabil- rived from field tests and computer simulation are shown ities. In the confined aquifers, water released by com- in table 2. Note that values of inelastic specific storage pression of fine-grained lenses, rather than that released are much larger than those of elastic specific storage. For from dewatering of pore space, may be the major source their simulation of regional ground-water flow, William- of water release from storage(Jacob, 1940). Therefore,it son and others (1989) used the average specific-storage is necessary to define and measure another storage values in table 2. parameter, specific storage (Ss). This parameter is the volume of water released from or taken into storage per HYDRAULIC CONDUCTIVI'T'Y unit volume of aquifer material per unit change in head (Lohman and others, 1972, p. 13). Below the zone of The term "hydraulic conductivity" (K) allows relative water-table fluctuation, only SS and the thickness of the comparison of the transmission properties of different aquifer are used to calculate water in storage. However, aquifers or parts thereof. The hydraulic conductivity of a when effective (grain-to-grain) stress is increased, some saturated porous medium(aquifer material)is the volume of the fine-grained lenses undergo reorientation and of water that the material will transmit in a unit of time deformation. Therefore, SS has two significant values. If through a cross section of unit area, under a hydraulic water released from storage is due to the expansion of gradient of unit change in head through a unit length of water and compressibility of the aquifer in response to a flow (Lohman and others, 1972, p. 6). decrease in hydraulic head, the specific storage is elastic. Average horizontal hydraulic conductivity (Kh) in the Conversely, if a decrease in hydraulic head causes valley ranged from 14 ft/d for sand to 0.002 ft/d for silty deformation and reorientation of sediments in fine- clay(table 1)as determined from laboratory tests of core