On the estimation of potential evaporation under wet and dry conditions

Potential evaporation (E_P) is an important concept that has been extensively used in hydrology, climate, agriculture and many other relevant fields. However, E_P estimates using conventional approaches generally do not conform with the underlying idea of E_P, since meteorological forcing variables observed under real conditions are not necessarily equivalent to those over a hypothetical surface with an unlimited water supply. Here, we estimate E_P using a recently developed ocean surface evaporation model (i.e., the maximum evaporation model) that explicitly acknowledges the inter-dependence between evaporation, surface temperature (T_s) and radiation such that is able to recover radiation and T_s to a hypothetical wet surface. We first test the maximum evaporation model over land by validating its evaporation estimates with evaporation observations under unstressed conditions at 86 flux sites and found an overall good performance. We then apply the maximum evaporation model to the entire terrestrial surfaces under both wet and dry conditions to estimate E_P. The mean annual (1979-2019) global land E_P from the maximum evaporation model (E_{P_max}) is 1272 mm yr^-1, which is 11.2% higher than that estimated using the widely adopted Priestley-Taylor model (E_{P_PT}). The difference between E_{P_max} and E_{P_PT} is negligible in humid regions or under wet conditions but becomes increasingly larger as the surface moisture availability decreases. This difference is primarily caused by increased net radiation (R_n) when restoring the dry surfaces to hypothetical wet surfaces, despite a lower T_s obtained under hypothetical wet conditions in the maximum evaporation model.