Estimating the Contribution of a Perched Water Table to the Seasonal Evapotranspiration of Cotton1

Abstract Water supplied to a growing crop by capillary rise from a shallow water table can be an important resource. Several thousand hectares in the western San Joaquin Valley in California have a perched water table created by irrigation and a slowly permeable subsurface zone. When irrigations are made according to a schedule that is optimum for soils without a shallow water table, cotton (Gossypium hirsutum L.) frequently shows plantgrowth that is characteristic of excessive irrigation. Field studies were conducted to evaluate the contribution of a perched water table to the evapotranspiration (ET) demand of cotton and to develop an irrigation schedule that uses the resource effectively. Two independent procedures were used to estimate the water table contribution (WT) through capillary rise. A water budget method used a previously developed yield‐ET function to estimate ET from observed production levels. Soil moisture depletion (SMD), stored irrigation water (IW), and deep percolation (PW) were determined from neutron thermalization measurements on volumetric soil water content. WT was determined by rearranging the relation ET = SMD + IW − PW + WT. A second approach, associated the chloride in the water table (17.4 meq/l) with increased chloride concentrations in the soil above the water table. The equivalent water depth necessary to effect this change was calculated. Both procedures estimated WT to be near 36 cm for the growing season. A high standard deviation (18.7 cm) was caused, however, by considerable spatial variability for the chloride tracer method. A revised irrigation schedule for cotton was developed that uses water, of acceptable quality, derived from a water table as a beneficial raource.