The reference ET (ETref) concept was introduced by irrigation engineers and researchers to avoid the confusions that existed in the definition of potential ET. By adopting a reference crop (grass or alfalfa), it became easier and more practical to select consistent crop coefficients and to make reliable actual crop ET estimates in new areas. Introduction of the reference ET concept also helped to enhance the transferability of the crop coefficients from one location to another. Two reference crops have been used to represent the reference ET: grass and alfalfa.
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