Potential evaporation functions compared on US watersheds: Possible implications for global-scale water balance and terrestrial ecosystem modeling

Abstract

Estimates of potential evaporation E p are commonly employed in terrestrial water balance and net primary productivity models. This study compared a set of 11 E p methods in a global-scale water balance model (WBM) applied to 3265 0.5° (lat. × long.) grid cells representing the conterminous US. The E p methods ranged from simple temperature-driven equations to physically-based combination approaches and include reference surface E pr and surface cover-dependent E ps algorithms. Cover-dependent parameters were assigned a priori based on grid cell vegetation. The WBM applies mean monthly climatic drivers and other biophysical inputs to compute water budgets on individual grid cells using a quasi-daily time step. For each E p method water budgets were computed and compared against mean monthly and annual streamflow from 679 gauged watersheds, assumed to be representative of the grid cells in which they reside. Procedures were developed for excluding watersheds for which this assumption was questionable, and 330 of the original 1009 watersheds were removed from further analysis. Among E pr methods, the range of mean bias relative to observed runoff, and thus simulated actual evapotranspiration E s, varied from approximately −100 to +100 mm yr −1; E ps methods had a substantially smaller range, from about −50 to +50 mm yr −1. These results agree well with previous E p intercomparison studies at the point scale. Some individual methods from both the E pr and E ps groups yielded relatively small overall bias when compared with observed discharge data, suggesting the utility of simple as well as physically-based evaporation functions in continental- and global-scale applications. For any individual method, the spatial distribution of E s across the US was significantly altered relative to that of E p due to moisture-induced limits on soil drying. These limitations were most pronounced in hot, dry areas, where differences among E p methods in excess of 700 mm yr −1 were reduced to differences of less than 200 mm yr −1 in E s and runoff. There was a correspondingly higher sensitivity of E s to the choice of E p in more humid regions. These findings suggest that predictions made by macro-scale hydrology models like the WBM can be sensitive to the specific E p method applied and that this sensitivity results in bias relative to measured components of the terrestrial water cycle. The adoption of particular E p functions within such models should be conditioned upon the comparison of water budget calculations to suitable records of observed discharge.