Correcting overestimated potential evaporation from the Penman-Monteith equation during water-limited conditions

Accurate estimation of potential evaporation is crucial for water resource management and trends in continental aridity. Potential evaporation is widely estimated using the Penman-Monteith (P-M) equation which has two main terms: the radiative forcing term, and the atmospheric dryness term, also called the aerodynamic component, which depends on the vapor pressure deficit (VPD). However, the aerodynamic component of the P-M equation overestimates potential evaporation in the presence of water limitation, exceeding the limits imposed by surface energy balance. This inconsistency arises because the high VPD in arid regions does not represent entirely the conditions of the idealized wet surface — an important underlying assumption for potential evaporation. Here we show that changes in VPD are mainly caused by changes in the diurnal temperature range (DTR).  For a given radiative forcing, soil water limitation amplifies the DTR through a reduction in the latent heat flux, this enhances the VPD, and therefore the aerodynamic component, yet without enhancing energy availability. We quantify this overestimation using FLUXNET observations and ERA-5 reanalysis data in combination with a thermodynamically-constrained surface energy balance approach. We find that soil water limitation amplifies DTR by up to 20 K, which increases VPD by up to 25 hPa. This additional VPD generates very high potential evaporation estimates in the aerodynamic component that exceed the available energy at the surface by over 100 W/m². When we remove the imprints of water limitation from DTR and VPD, the P-M equation leads to reduced potential evaporation rates approaching consistency with the surface energy balance. These results have significant implications for quantifying continental aridity and its changes with global warming.