Abstract
Abstract. Potential evaporation (Ep) is a crucial variable for hydrological forecasting and drought monitoring. However, multiple interpretations of Ep exist, which reflect a diverse range of methods to calculate it. A comparison of the performance of these methods against field observations in different global ecosystems is urgently needed. In this study, potential evaporation was defined as the rate of terrestrial evaporation (or evapotranspiration) that the actual ecosystem would attain if it were to evaporate at maximal rate for the given atmospheric conditions. We use eddy-covariance measurements from the FLUXNET2015 database, covering 11 different biomes, to parameterise and inter-compare the most widely used Ep methods and to uncover their relative performance. For each of the 107 sites, we isolate days for which ecosystems can be considered unstressed, based on both an energy balance and a soil water content approach. Evaporation measurements during these days are used as reference to calibrate and validate the different methods to estimate Ep. Our results indicate that a simple radiation-driven method, calibrated per biome, consistently performs best against in situ measurements (mean correlation of 0.93; unbiased RMSE of 0.56 mm day−1; and bias of −0.02 mm day−1). A Priestley and Taylor method, calibrated per biome, performed just slightly worse, yet substantially and consistently better than more complex Penman-based, Penman–Monteith-based or temperature-driven approaches. We show that the poor performance of Penman–Monteith-based approaches largely relates to the fact that the unstressed stomatal conductance cannot be assumed to be constant in time at the ecosystem scale. On the contrary, the biome-specific parameters required by simpler radiation-driven methods are relatively constant in time and per biome type. This makes these methods a robust way to estimate Ep and a suitable tool to investigate the impact of water use and demand, drought severity and biome productivity.
Highlights
Since its introduction 70 years ago by Thornthwaite (1948), the concept of potential evaporation (Ep), defined as the amount of water which would evaporate from a surface unconstrained by water availability, has been widely used in multiple fields
We show that the poor performance of Penman– Monteith-based approaches largely relates to the fact that the unstressed stomatal conductance cannot be assumed to be constant in time at the ecosystem scale
We focus here on the parameter estimates of the unstressed record based on the energy balance criterion (Sect. 2.4)
Summary
Since its introduction 70 years ago by Thornthwaite (1948), the concept of potential evaporation (Ep), defined as the amount of water which would evaporate from a surface unconstrained by water availability, has been widely used in multiple fields. It has been incorporated in hydrological models dedicated to estimate runoff There has been an increasing awareness of the impact of the underlying assumptions and caveats in traditional Ep formulations (Weiß and Menzel, 2008; Kingston et al, 2009; Sheffield et al, 2012; Seiller and Anctil, 2016; Bai et al, 2016; Milly and Dunne, 2016; Guo et al, 2017). Ep has been typically defined as the evaporation which would occur in given meteorological conditions if water was not limited, either (i) over open water (Shuttleworth, 1993); (ii) over a reference crop, usu-
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