Adopting Meteorological Approaches in Irrigation Scheduling for High Rainfall Areas

The irrigation scheduling program developed by Jensen, et al. and used by the Bureau of Reclamation, irrigation districts, and several private consultants has been widely accepted by the irrigators subscribing to the scheduling service. The program, summarized in the previous paper, forecasts the next date of irrigation by maintaining a water budget and estimating the number of days until the soil water depletion approaches an optimum value. The depletion or evapotranspiration rate used for estimating the next irrigation is a 6-day average occurring at the time of forecasting. The irrigation-scheduling program accounts for the precipitation that occurs before the date of forecast but assumes no additional rainfall before the date of irrigation. Most of the areas in which the irrigation-scheduling program has been used are located in the arid and semiarid Western United States. In these areas, limited rainfall has little effect on the predicted date of irrigation. Also, 6-day average evapotranspiration is more uniform from week to week than in sub-humid regions. Most of the variability in consecutive estimates of the date of irrigation is caused by the differences between the estimated evapotranspiration and that actually occurring in the forecast period. Adapting the irrigation-scheduling program to sub-humid and humid regions may require a more stable forecast of evapotranspiration and the inclusion of rainfall probability. This paper describes procedures for including the precipitation probability in the program for scheduling irrigations, and the effects of using long-time average evapotranspiration rates for the forecasts