Assessing Terrain Effect on Evapotranspiration Using the SEBAL Model in Mountainous Regions

Evapotranspiration (ET) is a critical hydrothermal parameter for effective water resource management, particularly in land use. However, accurately estimating ET in mountainous regions is challenging due to terrain effects. In this study, we coupled a surface net radiation model accounting for topographic effect, with the Surface Energy Balance Algorithm for Land (SEBAL) to estimate daily ET and analyze the impact of terrain on ET. We compared ET estimated from SEBAL, using Landsat 8/9 datasets, with those from MOD16A2, PML-V2, and ETMonitor at Potatso National Park, China, from 2017 to 2022. The result showed that ET estimated by SEBAL exhibited more detailed spatial distributions influenced by terrain. ET estimated by SEBAL agreed well with the PML-V2, with an R², RMSE, and bias of 0.81, 1.02 mm/day, and 0.94 mm/day, respectively. Terrain effects on ET can be generalized as follows: spatial distributions of daily ET exhibit a bimodal pattern with increasing altitude, with ET peaks occurring at 2900–3200 and 3500–3800 m, respectively. Daily ET notably increases from north-to-northeast-to-east aspects, remains stable from east-to-south-to-southwest, and gradually decreases from southwest-to-west-to-northwest aspects. ET peaks were observed on slopes of 15°–25°, with significant increases followed by decreases as the slope increased, and the maximum difference was 0.69 mm/day on west slopes. In addition, ET peaks shifted to higher slopes on east-to-south slope compared to west-to-north slope, with a difference of less than 0.51 mm/day. This study presents a robust approach for accurately estimating ET and analyzing the effects of terrain on ET in mountainous areas.