Estimating land evapotranspiration from potential evapotranspiration constrained by soil water at daily scale

Accurate simulation of evapotranspiration is of substantial importance to hydrology, ecology, agriculture, and water resources management. Evapotranspiration is equal to the fraction of potential evapotranspiration (PET) constrained by soil water. PET can be calculated from meteorological observations with a wide global distribution and high density. However, it is necessary to determine how to accurately simulate daily evapotranspiration through PET. We have developed a non-linear function for simulating evapotranspiration through PET constrained by soil water at daily scale. The evaluation results show that the accuracy of the evapotranspiration simulation using the non-linear function was higher than that of linear relations and complementary relationship (CR) methods. In the temperature-based PET equations, the Hargreaves-Samani equation was the closest to the Penman-Monteith calculation values. The simulation accuracy of the CR methods obviously improved after parameter calibration. The accuracy has a large variability at the global scale. Daily evapotranspiration can be simulated with PET data in some regions with a high accuracy (Nash and Sutcliffe efficiency coefficient > 0.60), including most regions of Eurasia, eastern and southern North America, and northern South America. However, other regions showed a poor performance (Nash and Sutcliffe efficiency coefficient < 0.20), including western North America, the Mediterranean region, and the eastern and western coastal regions of Australia. Our results indicate that the accurate simulation of daily evapotranspiration can be achieved based on meteorological data in most regions of the world. Owing to the wide distribution of global meteorological observations, the accurate simulation of the daily evapotranspiration method proposed in this study can be applied in other regions across the globe.