Evaporation Modeling: Potential

Abstract Potential evaporation ( E p ) has had a relatively broad range of definition over the past century. It is defined here as the quantity of water evaporated per unit area per unit time from an idealized extensive free‐water surface under existing atmospheric conditions. Three primary, common means to estimate E p have been used during the past century. These are (i) pan evaporation measurement, (ii) an estimate of potential evapotranspiration based on weather data, and (iii) a reference evapotranspiration. Of these three, reference evapotranspiration ( ET ref ) has the more practical application. “Potential evapotranspiration” ( ET p ) had widespread usage from the 1940s through the 1970s, when the term was used to represent a maximum evaporative index from which to derive estimates of actual ET from vegetation. However, there are several major, contrasting definitions for ET p and several challenges associated with its usage. One of the primary definitions used for ET p is the rate of evaporation and transpiration from a saturated (free‐water) vegetated surface so that the evaporation process occurs at the potential level. Challenges in sustaining a saturated surface and in measuring weather data that are coincident with such a surface make this definition for ET p theoretically attractive, but practically difficult. Standardized parameterizations of the Penman–Monteith equation are described for calculating ET for grass and alfalfa references. The reference evapotranspiration, despite some shortcomings, can be a consistent and reproducible index for a weather‐based potential evaporation.