Integrated UAV, USV, and satellite bathymetry reveals changes and causes in the core lakes of the Jiuzhaigou Nature Reserve

In recent years, the combined effects of seismic disturbances and extreme rainfall events have intensified water turbidity and sedimentation in several lakes within the Jiuzhaigou Nature Reserve located within the Jiuzhaigou watershed in Southwest China. In July 2024, turbidity events were observed in multiple core lakes, with noticeable impacts on local ecological functions and landscape values. To quantitatively characterize lake physical changes and explore their driving mechanisms, this study developed an integrated "air–space–ground" lake morphological measurement framework that combines unmanned aerial vehicle (UAV), unmanned surface vehicles (USV), and satellite remote sensing to enable high-precision acquisition of lake bathymetry, relationships between water level and surface area, and storage capacity. Additionally, time-series analyses of fractional vegetation cover (FVC) and the Revised Universal Soil Loss Equation (RUSLE) model were applied to characterize the patterns of change in vegetation cover and soil erosion intensity before and after seismic and extreme rainfall events. The results show that: (1) The physical morphology of the lakes has changed significantly compared with the field measurements conducted in 2004. The surface areas of the Arrow Bamboo Lake and the Rhinoceros Lake decreased by 15.2%~17.0%, and lake storage capacity decreased by 20%~33%. In Long Lake, the difference between the minimum and maximum daily water levels reached 13.21 m, and since 2021, the annual mean water level has exhibited a continuous decline at a rate of 1.46 m·a^-1. (2) Based on the results of the FVC time series analysis, the two earthquake events resulted in decreases in vegetation cover by 19.14% and 13.19%, and these decreases were accompanied by enhanced soil erosion under heavy rainfall conditions. This process facilitated the transport of large amounts of terrestrial material into the lakes, thereby contributing to water turbidity and sediment accumulation. RUSLE-based estimates further indicate that the two earthquakes increased the peak mean soil erosion modulus by 7.61 t· ha^-1· a^-1 and 4.10 t· ha^-1· a^-1, respectively, relative to pre-earthquake conditions. Based on these findings, it is recommended to implement slope vegetation restoration on the upstream areas of the core lakes, strengthen slope protection in accident-prone locations, and monitor water–sediment dynamics during heavy rainfall periods, to mitigate sedimentation risks and enhance the stability of the scenic area's ecosystem.