DEVELOPING THE RESOURCE POTENTIAL OF A SHALLOW WATER TABLE

Observations and estimates indicate that shallow water table encroachment affects increasingly large areas of agriculturally productive land in the central and western San Joaquin Valley. With current management techniques, the present drain and disposal facilities are inadequate to effectively handle the water volume that moves through soils and becomes a part of the shallow water system. In the presence of active plant roots, soil water is depleted in upper profile zones with the establishment of a potential gradient sufficient to effect upward capillary water movement in the presence of a shallow water table. A three-year study was done to evaluate the resource potential of shallowperched water tables as a resource to meet crop evapotranspiration (ET) requirements. Using water-budget and chloride-tracer techniques independently to measure shallow water table contributions to crop ET revealed that as much as 50 to 60 percent of crop ET could be met by the shallow water table. The amount of water contributed by the shallow water table was strongly conditioned by water table salinity and depth to the shallow water. Study results show the need for a total management system approach for effective utilization of the resource potential of shallow water tables in contrast to specific entities considered in isolation.