On the Estimation of Potential Evaporation Under Wet and Dry Conditions

• Published in: Water resources research 2022-04, Vol.58 (4), p.n/a
• Main Authors: Tu, Zhuoyi, Yang, Yuting
• Format: Article
• Language: English
• Subjects: Agriculture; Atmospheric forcing; Estimates; Evaporation; Evaporation rate; Hydrology; Modelling; Moisture availability; Moisture effects; Net radiation; Ocean surface; Potential evaporation; Radiation; Radiation balance; radiation‐surface temperature ‐evaporation coupling; Real variables; Surface temperature; Temperature dependence; the maximum evaporation; Water supply
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2021WR031486

Potential evaporation (EP) is an important concept that has been extensively used in hydrology, climate, agriculture and many other relevant fields. However, EP estimates using conventional approaches generally do not conform with the underlying idea of EP, 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 EP using a recently developed ocean surface evaporation model (i.e., the maximum evaporation model) that explicitly acknowledges the inter‐dependence between evaporation, surface temperature (Ts) and radiation such that is able to recover radiation and Ts 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 EP. The mean annual (1979–2019) global land EP from the maximum evaporation model (EP_max) is 1,272 mm yr−1, which is 11.2% higher than that estimated using the widely adopted Priestley‐Taylor model (EP_PT). The difference between EP_max and EP_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 (Rn) when restoring the dry surfaces to hypothetical wet surfaces, despite a lower Ts obtained under hypothetical wet conditions in the maximum evaporation model. Key Points We estimate potential evaporation (EP) using a maximum evaporation model that recovers radiation and temperature to a hypothetical wet condition Mean global land EP from the maximum evaporation model is 1,272 mm yr−1, which is 11.2% higher than the Priestley‐Taylor estimate The higher EP from the maximum evaporation model is mainly caused by higher net radiation over hypothetical wet surfaces