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Assessing temperature‐based PET equations under a changing climate in temperate, deciduous forests
Despite the availability of numerous approaches to estimate potential evapotranspiration (PET), temperature-based PET equations such as the Hamon equation and Thornthwaite equation are still used to predict changes in hydrology in a changing climate as temperature is one of the primary reported outputs from general circulation models. To isolate the actual dependence of PET on temperature, we analysed meteorological and energy balance measurements from five AmeriFlux deciduous forest sites in the eastern United States during periods with minimal soil moisture control on transpiration. For all five sites, when PET measurements with similar net radiation are grouped, temperature does not correlate to PET within each group. Conversely when PET measurements with similar temperature are grouped, net radiation strongly correlates to PET within each group. In terms of assessing standard PET models, when dormant and growing season PET are separated, we found that the Priestley–Taylor equation (a model primarily dependent on net radiation) consistently explained more of the variation in PET than temperature-based methods such as the Hamon equation (median R2 of 0·88 vs 0·66). We illustrate that the moderate ability of temperature-based equations to predict PET arises from the correlation between temperature and net radiation when the meteorological observations are averaged over at least several days. However, we suggest that because temperature is not the fundamental driver of PET and because the relationship between temperature and net radiation underlying temperature-based equations will shift with climate change, temperature-based equations in their current state will likely exaggerate PET in a changing climate. Copyright © 2010 John Wiley & Sons, Ltd.