Spatiotemporal Change in Evapotranspiration across the Indus River Basin Detected by Combining GRACE/GRACE-FO and Swarm Observations

Evapotranspiration (ET) is an important approach for enabling water and energy exchange between the atmosphere and the land, and it has a very close relationship with terrestrial water resources and the ecological environment. Therefore, it is of great scientific to accurately quantify the spatiotemporal change in ET and its impact factors to understand the terrestrial water change pattern, maintaining water resource security and protecting the ecological environment. Our goal is to study the spatiotemporal characteristics of ET in the Indus River basin (IRB) and their driving factors. In our study, we first integrated the multi-source satellite gravimetry observations using the generalized three-cornered hat and least square methods to obtain the high-precision and continuous spatiotemporal evolution features of ET in the IRB from 2003 to 2021. Finally, we combined nine hydrometeorological and land cover type data to analyze the factors influencing ET. The results indicate that the algorithm used in our study can improve the ET accuracy by 40%. During the study period, ET shows a significant increasing trend (0.64 ± 0.73 mm/a), and the increasing rate presents spatial distribution characteristics of high variability in the northern areas and low variability in the southern areas of the study region. ET has a close relationship with precipitation, specific humidity, total canopy water storage, surface temperature and wind speed (with a correlation coefficients greater than 0.53 and variable importance of projection greater than 0.84). Among these factors, precipitation, specific humidity and surface temperature have significant correlations with ET (correlation coefficients greater than 0.85 and variable importance of projection greater than 1.42). And wind speed has a more significant positive effect on ET in the densely vegetated regions. The impacts of climate change on ET are significantly greater than those of land cover types, especially for similar land cover types. Ice and snow are significantly different to other land cover types. In this region, ET is only significantly correlated with precipitation, specific humidity and snow water equivalent (variable importance of projection greater than 0.81), and the impacts of precipitation and specific humidity on ET have been significantly weakened, while that of snow water equivalent is significantly enhanced. Our results contribute to furthering the understanding of the terrestrial water cycle in subtropical regions.