Abstract: The water surface evaporation loss in river reaches caused by hydropower station construction has become a concern. Existing studies mostly focus on changes in pan evaporation within reservoir areas, lacking quantitative calculation and comparative analysis of evaporation-induced water resource losses. Targeting the quantitative analysis of water surface evaporation in the reservoir reach before and after the commissioning of A Hydropower Station on the Jinsha River, this study utilized Sentinel-2 and Landsat 8 satellite images as data sources to design a water area extraction method based on feature field screening and image reconstruction. Combined with local area fitting of single-scene images, the time series of water area in the reach from 2018 to 2023 was expanded from 198 to 294 points, achieving high-temporal-frequency monitoring of water area changes. The PenPan model, with a fitted wind speed function, was employed to interpolate and extend the pan evaporation time series. Water surface evaporation rates were then calculated by combining this with reconstructed conversion coefficients based on variations in the water surface saturation deficit. By integrating the two, the evaporative water resource loss was estimated and analyzed. The results indicated that: ① After the operation of the hydropower station, the water area in the reach expanded significantly, while the daily average water surface evaporation rate increased by 25.5% compared to pre-impoundment period; the superposition of both led to a 5.26 times increase in daily evaporative water loss. ② The reservoir formed by the hydropower station affected the regional microclimate, causing a "simultaneous reduction" in both the radiation and aerodynamic terms of evaporation from December to June, which reduced pan evaporation; conversely, a significant increase in the aerodynamic term from July to November increased pan evaporation. ③ The superposition of pan evaporation and the changes in wind speed and surface saturation deficit induced by the reservoir′s thermal environment effect caused the water surface evaporation rate in the reach to decrease by a maximum of 55.17% from April to June compared to pre-reservoir levels, while it increased substantially in the remaining months, with a maximum increase of 90.31%. The findings provide a reference for revealing the changes in water surface evaporation in reaches influenced by reservoir impoundment and offer data support for the refined water resource management of hydropower stations.