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

Estimates of potential evaporation Ep are commonly employed in terrestrial water balance and net primary productivity models. This study compared a set of 11 Ep methods in a global-scale water balance model (WBM) applied to 3265 0.5° (lat. × long.) grid cells representing the conterminous US. The Ep methods ranged from simple temperature-driven equations to physically-based combination approaches and include reference surface Epr and surface cover-dependent Eps 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 Ep 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 Epr methods, the range of mean bias relative to observed runoff, and thus simulated actual evapotranspiration Es, varied from approximately −100 to +100 mm yr−1; Eps methods had a substantially smaller range, from about −50 to +50 mm yr−1. These results agree well with previous Ep intercomparison studies at the point scale. Some individual methods from both the Epr and Eps 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 Es across the US was significantly altered relative to that of Ep due to moisture-induced limits on soil drying. These limitations were most pronounced in hot, dry areas, where differences among Ep methods in excess of 700 mm yr−1 were reduced to differences of less than 200 mm yr−1 in Es and runoff. There was a correspondingly higher sensitivity of Es to the choice of Ep in more humid regions. These findings suggest that predictions made by macro-scale hydrology models like the WBM can be sensitive to the specific Ep method applied and that this sensitivity results in bias relative to measured components of the terrestrial water cycle. The adoption of particular Ep functions within such models should be conditioned upon the comparison of water budget calculations to suitable records of observed discharge.

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