Optimal pumping strategies for managing shallow, poorquality groundwater, western San Joaquin Valley, California

Abstract Continued agricultural productivity in the western San Joaquin Valley, California, is threatened by the presence of shallow, poor-quality groundwater that can cause soil salinization. We evaluate the management alternative of using groundwater pumping to control the altitude of the water table and provide irrigation water requirements. A transient, three-dimensional, groundwater flow model was linked with nonlinear optimization to simulate management alternatives for the groundwater flow system. Optimal pumping strategies have been determined that substantially reduce the area subject to a shallow water table and bare-soil evaporation (that is, areas with a water table within 2.1 m of land surface) and the rate of drainflow to on-farm drainage systems. Optimal pumping strategies are constrained by the existing distribution of wells between the semiconfined and confined zones of the aquifer, by the distribution of sediment types (and associated hydraulic conductivities) in the western valley, and by the historical distribution of pumping throughout the western valley. INTRODUCTION Continued agricultural productivity in the western San Joaquin Valley, California, is threatened by the presence of shallow, poor-quality groundwater that can cause soil salinization. The long history of irrigation in the western valley has contributed to a significant rise in the altitude of the water table. Infiltrating irrigation-return water also has caused a redistribution of soluble forms of naturally occurring salts in the soils of the valley, including constituents toxic to waterfowl such as selenium, into the underlying ground water (Gilliom et al., 1989). Historically, subsurface tile drains have been used to control the altitude of the water table and to manage subsurface water quality. However, the presence of selenium-bearing agricultural drainwater has resulted in the closure of contributing regional-collector drainage systems (Fig. 1). In the