Evapotranspiration estimates under deficient water supplies

Effective use of water supplies via irrigation requires good methods for determining crop water needs. To implement irrigation scheduling programs requires an accurate estimate of water use by the crop. This study was undertaken to compare different forms of the evapotranspiration (ET) equations that include Priestley-Taylor and Penman-Monteith for reference ET. To estimate actual ET, the Priestley-Taylor with an adjusted coefficient for available soil water and the Penman-Monteith with a variable surface resistance were compared to water use for grain sorghum, cotton, and grass forage at three locations: Davis, Calif.; Lubbock, Tex.; and Logan, Utah. Both models provided acceptable results; however, the Penman-Monteith model with daily meteorological data input provided more consistent results over the growing season. The Priestley-Taylor overestimated actual ET when crops were limited in soil water because the adjustment for available soil water was not sensitive to soil-water depletion. Irrigation scheduling using the Penman-Monteith model requires daily meteorological data, an estimate of the available soil water depletion, and a measurement of crop leaf area. This method would be useful for irrigation scheduling programs.