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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] Sediment size Sand ........................ Clayey sand .............. Sand-silt-clay ............ PllflVPV <5llt Silty sand ................. Sandy silt ................. Silt .......................... Silty clay .................. Clay ........................ Number of samples _ ... . , ,, Specific yield 1 used to determine .-.,,, .. (percent) specific yield and porosity 126 28 95 107 137 49 79 86 0 19-35 27 10-28 16 2-20 12 <l-7 3.5 <1-15 7.5 1-12 7.5 1-7 3 <l-8 4 No pure clays Porosity 1 (percent) 31-65 40 28-52 37 31-56 37 32-61 42 25-41 34 34-37 36 34-56 43 35-52 43 analyzed Average hydraulic conductivity2 (ft/d) ,. , Hori- Vertical , zontal 11.5 14 .02 .02 .0001 .21 .16 .02 .13 .0002 .0001 .002 1 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). laboratory 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 confined in the sense that the vertical permeabilities of sediments are much lower than the horizontal permeabil- ities. In the confined aquifers, water released by com- pression of fine-grained lenses, rather than that released from dewatering of pore space, may be the major source of water release from storage (Jacob, 1940). Therefore, it is necessary to define and measure another storage parameter, specific storage (Ss ). This parameter is the volume of water released from or taken into storage per unit volume of aquifer material per unit change in head (Lohman and others, 1972, p. 13). Below the zone of water-table fluctuation, only Ss and the thickness of the aquifer are used to calculate water in storage. However, when effective (grain-to-grain) stress is increased, some of the fine-grained lenses undergo reorientation and deformation. Therefore, Ss has two significant values. If water released from storage is due to the expansion of water and compressibility of the aquifer in response to a decrease in hydraulic head, the specific storage is elastic. Conversely, if a decrease in hydraulic head causes deformation and reorientation of sediments in fine- grained lenses, the specific storage is inelastic. The coefficients of elastic and inelastic specific storage de- rived from field tests and computer simulation are shown in table 2. Note that values of inelastic specific storage are much larger than those of elastic specific storage. For their simulation of regional ground-water flow, William- son and others (1989) used the average specific-storage values in table 2. HYDRAULIC CONDUCTIVITY The term "hydraulic conductivity" (K) allows relative comparison of the transmission properties of different aquifers or parts thereof. The hydraulic conductivity of a saturated porous medium (aquifer material) is the volume of water that the material will transmit in a unit of time through a cross section of unit area, under a hydraulic gradient of unit change in head through a unit length of flow (Lohman and others, 1972, p. 6). Average horizontal hydraulic conductivity (Kh ) in the valley ranged from 14 ft/d for sand to 0.002 ft/d for silty clay (table 1) as determined from laboratory tests of core