Testing a maximum evaporation theory over saturated land: implications for potential evaporation estimation

State-of-the-art evaporation models usually assume net radiation (R-n) and surface temperature (T-s; or near-surface air temperature) to be independent forcings on evaporation. However, R-n depends directly on T-s via outgoing longwave radiation, and this creates a physical coupling between R-n and T-s that extends to evaporation. In this study, we test a maximum evaporation theory originally developed for the global ocean over saturated land surfaces, which explicitly acknowledges the interactions between radiation, T-s and evaporation Similar to the ocean surface, we find that a maximum evaporation (LEmax) emerges over saturated land that represents a generic trade-off between a lower R-n and a higher evaporation fraction as T-s increases. Compared with flux site observations at the daily scale, we show that LEmax corresponds well to observed evaporation under non-water-limited conditions and that the T-s value at which LEmax occurs also corresponds with the observed T-s. Our results suggest that saturated land surfaces behave essentially the same as ocean surfaces at timescales longer than a day and further imply that the maximum evaporation concept is a natural attribute of saturated land surfaces, which can be the basis of a new approach to estimating evaporation.