Re-assessing PET regularities at the global scale
Authors: Arheimer Berit , Pimentel Rafael , Crochemore Louise , Andersson Jafet , Pechlevanidis Ilias , Gustafsson David 
Published: February 2023

The importance of evapotranspiration (ET) for hydrology, agriculture, and meteorology has long been recognized. In fact, most of our current understanding of the physics of evaporation originated in early experiments during the past two centuries. Potential ET (PET) is a concept extendedly used for predicting ET and defined as the evaporation in case of an unlimited amount of water available. Different potential evapotranspiration (PET) formulas were developed for different purposes and are currently applied far beyond their origin. Accordingly, these formulas also result in different PET estimates due to their different assumptions and inputs requirements; hence, the regularities of the formulas should be re-assessed when applied for new scales or environmental conditions.In the current study, we experimented with three simplified PET models over the globe: Jensen-Haise, Hargreaves, and Priestly-Taylor. The World-Wide HYPE (WW-HYPE) global catchment hydrological model is applied as a virtual laboratory where we keep all other hydrological predictors constant except for PET to examine its influence on the model performance in term of streamflow and ET. 15 years of observations from 5,338 streamflow gauges and global evapotranspiration from Earth-observations (MOD16) were used as independent datasets. We tested model performance in a multi-process approach to select the best formula for catchments covering the global landmass. Catchment physiography and a classification in the Budyko space were used to explain differences in the model results.From comparing the results with land-cover, climate classification, water-energy limitations, we found that climate is the main driver behind the spatial patterns in model performance. We found a strong connection between the five main K&#246;ppen regions and the PET formulas, further supported by landcover analysis. The selection of a PET formula seems to be more critical in tropical regions close to the equator, where the differences in performance are above 50%. This is also where PET is highest. Hargreaves was the best PET formula in 50% of the catchments, most of them located in the Amazonas, central Europe, and Oceania. Jensen-Haise was better for catchments in northern latitudes (36%). Finally, Priestly-Taylor was the best formula for India and latitudes above 65&#8304; N. Hence, the PET formulas differed in their capacity to provide useful input to the water balance modelling, with complex formulas only giving improved predictions in temperate and polar regions; however, for the rest of the globe simpler formulas were better. We thus recommend applying different PET formulas based on climatic regions world-wide.&#160;References:Arheimer et al., 2020: Global catchment modelling using World-Wide HYPE (WWH), open data and stepwise parameter estimation, HESS 24, 535&#8211;559, https://doi.org/10.5194/hess-24-535-2020&#160; &#160;Pimentel et al., 2023: Which Evapotranspiration Formula to Use in Hydrological Modelling World-wide? WRR (in review)