Irrigation Scheduling for Deficit Irrigation

Many irrigators 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 preseason irrigation be beneficial; and (2) when should irrigation be started and stopped during the growing season. Traditional irrigation scheduling has been developed for irrigation systems that can supply the full irrigation requirements of the crop and produce maximum crop yields. Irrigation schedules are based on current crop water requirements and predict irrigation needs in the immediate future. 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 predict yields from alternative irrigation schedules. The user determines soil water status before or during the cropping season and formulates a potential schedule 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.