A novel approach for estimating evapotranspiration by considering topographic effects in radiation over mountainous terrain

Abstract

Mountains are one of the hotspots of climate change, and their complex morphology makes the monitoring of water and energy fluxes extremely challenging. Evapotranspiration (ET) is a crucial component of the water cycle and energy budget and its accurate estimation is essential for water resources management and ecosystem protection in mountains. It has been reported that the topography substantially controls the distribution of ET in mountainous terrain. However, most of the existing models neglect the impact of topography, leading to considerable errors and deviations for ET simulation. To address this issue, this study proposed a terrain-extended ET model (TEEB) based on the principle of energy balance, which can be used for ET estimation over complex terrains. Given the substantial impact of topography on net shortwave radiation (NSR), the mountain radiative transfer scheme was employed for NSR estimation, the net radiation model considering topographic effects was then constructed. Soil heat flux and sensible heat flux were then estimated from net radiation. The proposed TEEB model was tested using data from seven eddy covariance (EC) flux towers and a multidimensional comparison was made with the most widely used Surface Energy Balance Algorithm for Land (SEBAL) model. Regarding the results, the simulation of the TEEB model had a high consistency with EC measurements, with a root-mean-square-error of 0.713 mm/d, and was significantly superior to the SEBAL model. Moreover, the spatial pattern of estimated ET with the TEEB model exhibited distinct topographic characteristics, such as the ET on shady slopes being much lower than on sunlit slopes. Meanwhile, topographic analyses revealed that ET estimates on shady slopes would be reduced by 46 % with a proper consideration of topographic effects. The TEEB model can improve the estimation accuracy of ET in mountains, and provide a useful reference for maintaining ecological balance and optimizing water resources management.