Defining and Using Reference Evapotranspiration

Values of reference evapotranspiration $\left({\mathrm{ET}}_0\right)$ are used with crop coefficients $\left(\mathrm{KC}\right)$ for many aspects of irrigation and water resources planning and management. More than a score of methods are used for estimating ${\mathrm{ET}}_0$. Estimated values vary widely due to the lack of standardization of the reference. Evaporation (ET) measured by lysimeters of various grasses and/or alfalfas has been used as the standard for developing estimating equations. Due to variation in the references used, some international organizations now wish to promote the use of a single equation or method to avoid the confusion caused by the current diversity. Different versions of the Penman combination equation have been proposed. The Research Center for the European Community and the ASCE Committee on Irrigation Requirements have evaluated various equations for estimating ${\mathrm{ET}}_0$. Due to its simplicity and the accuracy of estimates, the 1985 Hargreaves et al. equation is recommended for general use. ${\mathrm{ET}}_0$ is used in irrigation planning, design, and scheduling and for other water adequacy studies.