Similarity and difference of potential evapotranspiration and reference crop evapotranspiration – a review

Potential evapotranspiration (ETp) and reference crop evapotranspiration (ETo) differ in their developments, concepts, equations and application fields, however, many researchers have mixed the utilization of the two terms. Thus, it is necessary to clarify the terms to guide their proper usage. The aim of this study is to provide a comprehensive review of the concepts, developments, equations and applications of ETp and ETo. The review shows clearly that the concepts and developments of ETp and ETo have long histories. The original concept of ETp was proposed by Thornthwaite, and that core idea with various improvements has been used up to now. The development of ETo, although initially confused with that of ETp, was formally defined as a standard method. The ETp equations were classified as mass-transfer-based, temperature-based, radiation-based and combination types. While the ETo equations were classified as being temperature-based, radiation-based, combination and pan-evaporation-based. Both terms are connected to evaporation and evapotranspiration, and although there have been ambiguities, ETp has been applied mostly in hydrology, meteorology and climatology, while ETo has been applied mostly in agronomy, agriculture, irrigation and ecology. ETp has been used to estimate drought indices, and ETo has been used in the single and dual crop coefficient methods and the Shuttleworth-Wallace method for estimating actual field evapotranspiration. This review serves to clarify the origins, definitions, and uses of ETp and ETo. It addresses common ambiguities between the terms in order to aid the proper utilization of the terms.