Expanding the geography of evapotranspiration: An improved method to quantify land-to-air water fluxes in tropical and subtropical regions

The development of new reference evapotranspiration (ETo) methods hold significant promise for improving our quantitative understanding of climatic impacts on water loss from the land to the atmosphere. To address the challenge of estimating ETo in tropical and subtropical regions where direct measurements are scarce we tested a new method based on geographical patterns of extraterrestrial radiation (Ra) and atmospheric water potential (Ψ(air)). Our approach consisted of generating daily estimates of ETo across several climate zones in Brazil–as a model system–which we compared with standard ETo(PM) (Penman-Monteith) estimates. In contrast with ETo(PM), the simplified method (ETo(MJS)) relies solely on Ψ(air) calculated from widely available air temperature ((o)C) and relative humidity (%) data, which combined with Ra data resulted in reliable estimates of equivalent evaporation (E(e)) and ETo. We used regression analyses of Ψ(air) vs ETo(PM) and E(e) vs ETo(PM) to calibrate the ETo(MJS(Ψair)) and ETo(MJS) estimates from 2004 to 2014 and between seasons and climatic zone. Finally, we evaluated the performance of the new method based on the coefficient of determination (R(2)) and correlation (R), index of agreement “d”, mean absolute error (MAE) and mean reason (MR). This evaluation confirmed the suitability of the ETo(MJS) method for application in tropical and subtropical regions, where the climatic information needed for the standard ETo(PM) calculation is absent.