Technical Note: Scheduling for Deficit IrrigationCrop Yield Predictor

Irrigators in many countries with dwindling water supplies face the prospect that they will not be able to fully irrigate their crops. In these cases, they still need to schedule their water applications to make the best economic use of available water. Major scheduling questions for deficit irrigation include: (1) will pre-season irrigation be beneficial; and (2) when should irrigation be started and stopped during the growing season. Traditional irrigation scheduling estimates crop evapotranspiration (ETc) to predict the amount and timing of irrigation events for the next several days. Usually these schedules assume that the water supply will provide irrigation to fully irrigate the crop to produce maximum crop yields. Irrigators practicing deficit irrigation need to predict irrigation schedules in advance of the growing season and make appropriate adjustments based on potential crop yields and economic returns. A computerized decision tool, the Crop Yield Predictor (CYP), has been developed to forecast yields from alternative irrigation schedules and designed for management decisions by irrigators, crop consultants, and extension personnel. Users of CYP determine soil water status before or during the cropping season and formulate potential schedules of irrigation dates and amounts. Soil water holding capacity and irrigation system water delivery capacity are constraints on the ability to supply water to the crop. CYP uses a daily soil water balance coupled with computations of effective evapotranspiration (ETe) to predict crop yields from regional yield-ET relationships. Multiple executions of CYP with alternative irrigation schedules lead to the schedules that project optimum net economic returns from the management scenarios. CYP is an example of adapting a crop simulation model into a tool for those who need to make irrigation management decisions.