Planning for Deficit Irrigation

Abstract

Irrigators with limited water supplies that lead to deficit irrigation management need to make decisions about crop selection, water allocations to each crop, and irrigation schedules. Many of these decisions need to occur before the crop is planted and depend on yield-evapotranspiration (ET) and yield-irrigation relationships. The Kansas Water Budget (KSWB) predicts crop yields from inputs of daily weather parameters to calculate reference ET (ETr) and irrigation and precipitation events to calculate a soil water balance. Results from the KSWB were compared with data from a 4-year field study conducted in southwest Kansas. The field study had one irrigation treatment to meet full irrigation requirements and five deficit irrigation treatments. Average soil water contents from field data on discrete days during the growing season compared well with KSWB results, but field soil water contents varied among each irrigation treatment replication. Relative crop yields and crop ET (ETc) compared well with KSWB results for the fully irrigated treatment, but the KSWB results tended to underestimate field results as irrigation declined. These differences may be attributed to calibrations of the KSWB with historical data from conventional (tilled) management in contrast to the field study which was managed with no-till techniques. Field and KSWB yield-ET relationship results were almost identical. The KSWB can be a tool for deficit irrigation management decisions that need predictions of crop yields for planning crop rotations, allocations of irrigation to selected crops, and screening of anticipated irrigation schedules.