SOIL-WATER AND ET ESTIMATES FOR A WIDE-RANGE OF RAIN-FED AND IRRIGATED CONDITIONS

The reference crop procedure, which is commonly used for estimating evapotranspiration (ET) of irrigated crops, was tested over a wide range of soil water for rainfed and irrigated winter wheat. The procedure, when using the Penman-Monteith equation combined with published FAO crop coefficients, provided close estimates of ET for irrigated winter wheat when adjustments were made for soil water depletion using the natural log algorithm developed in Southern Idaho. However, when the same procedure was applied to rainfed winter wheat with much lower soil water, there were considerable differences between estimated and measured ET. A daily soil water balance was performed to test various soil water depletion models for estimating ET and soil water throughout the summer-growing season. Soil water depletion adjustment algorithms which have relatively low slopes as the soil water depletion approaches the wilting point provided the best results under high soil water depletion conditions while algorithms having large negative slopes near the wilting point performed poorly.