Functions to Predict Optimal Irrigation Programs

Davis, Calif., programming dates and depths of irrigations are based on comparison of the estimated sequence of maximum evapotranspiration rates (ET_max rates) throughout the season with the sequence of actual ET rates expected if no irrigation is given. The difference constitutes an expected sequence of ET deficits which carefully programmed irrigations may alleviate fully or in part. ET_max depend on the sequence of leaf canopy development and on evaporative conditions at the site being planned for. ET actual rates (unirrigated) depend on the sequence of root system development, available soil water in storage at planting time, and growing season rainfall. Time relationships of corn leaf canopy and root system development researched at Davis, Calif., are combined with irrigation planning site measurements of climate, soil, and soil water content to estimate the sequence of ET deficits. Measurement of actual irrigation operational efficiency completes the needed information.