Calibration‐Free Priestley‐Taylor Method for Reference Evapotranspiration Estimation

Abstract The Priestley‐Taylor (PT) method is widely used in hydrologic and crop models to calculate the atmospheric demand for water or reference evapotranspiration (ET o ). However, the default value of the PT coefficient cannot assure reliability under different climatic regions due to the inadequate consideration of the aerodynamic components. In addition, the spatial and temporal evolution of PT a at global scales and how meteorological parameters affect its change have not been elucidated to date. Here, we derived an analytical expression of PT a following the Penman‐Monteith method, i.e., , where S A and represent the available energy and wind speed impacts on ET o , respectively. The daily Princeton Global Forcing data set (0.25° × 0.25°, 1948–2016) was used to validate PT a . The calibration‐free PT a improved the simulation of aerodynamic components and, consequently, the accuracy of ET o estimation. The gridded global monthly PT a data set is fully open‐source and can be incorporated into hydrologic and crop models. We also demonstrated that PT a under equilibrium or quasi‐equilibrium evaporation conditions (1.26 ± 0.04) was very close to the default value. The simplified PT a , with relative humidity as the only input, still performed better than the original method. Results indicated that the radiative component was the driving factor of the global ET o changes from 1948 to 2016. The impacts of available energy and wind speed on global ET o variations tend to intensify in a warming climate. Our findings have important implications for understanding the roles of radiative and aerodynamic components in the global ET o changes.