ALTERNATIVE TO AGRICULTURAL DRAINS IN CALIFORNIA SAN-JOAQUIN VALLEY - RESULTS OF A REGIONAL-SCALE HYDROGEOLOGIC APPROACH

The occurrence of selenium in agricultural drainage water derived from the central part of the western San Joaquin Valley has focused concern on alternatives to agricultural drains for managing shallow, poor-quality groundwater. A transient, three-dimensional simulation model was developed to evaluate the response of the water table to alternatives that affect recharge to or discharge from the groundwater flow system. The modeled area is 551 mi(2) (1 mi(2) = 2.59 km(2)) and includes both the semiconfined and confined zones above and below the Corcoran Clay Member of the Tulare Formation of Pleistocene age. The simulation model was calibrated using hydrologic data from 1972 to 1988, and was extended to the year 2040 to forecast for various management alternatives, including maintenance of present practices, land retirement, reduced recharge, increased groundwater pumping, and combinations of these alternatives. Maintenance of present practices results in a worsening of the situation: the total area subject to bare-soil evaporation increases from 224 mi(2) in 1990 to 344 mi(2) in 2040, and drain flow increases from 25,000 ac ft/yr (1 ac ft = 1234 m(3)) to 28,000 ac ft/yr. Although land retirement results in elimination of bare-soil evaporation and drain flow in the areas retired, it has little to no effect in adjacent areas. In contrast, regional-scale changes in recharge and pumping are effective for regional management. The area subject to bare-soil evaporation can be reduced to 78 mi(2), and drain flow to 8000 ac ft/yr if (1) recharge is reduced by 15% (26,009 ac ft/yr) in areas that currently use surface and groundwater (362 mi(2)); (2) recharge is reduced by 40% (28,000 ac ft/yr) in areas that currently use only surface water (137 mi(2)); and (3) pumping rates are uniformly incremented by 0.5 ft/yr (160,000 ac ft/yr) in both areas. If these water budget changes were to be implemented in the study area, and in adjacent areas with similiar hydrogeologic characteristics, then approximately 400,000 ac ft/yr of surface water would be made available. Thus a shift in the hydrologic budget in the central part of the western San Joaquin Valley improves the prospects for sustaining agriculture in the area, and could provide substantial water resources for other uses.