Natural Resources, School of


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An ASABE Meeting Presentation Paper Number: 20-070

Written for presentation at the 6th Decennial National Irrigation Symposium Sponsored by ASABE San Antonio, Texas November 30 – December 4, 2020


Estimation of evapotranspiration is under continual development and evolution, with significant developments and standardizations made during the past three decades for both reference ET (ETref) and for crop coefficients (Kc). These standardizations provide consistency and reproducibility in estimating ETref and a consistent basis for determining and expressing Kc curves, especially at the local scale. The application of the dual Kc procedure is growing, and has strong potential for improving accuracy of ET estimates as compared to the single Kc approach. This article describes current structures for estimating crop coefficients including the standardized FAO-56 dual Kc approach, with example applications. Emphasis is placed on estimation of parameters and special cases to be considered. Newer bases for establishing Kcb curves include thermal units and vegetation indices.