A robust Sentinel-2-based high-frequency optical sensing framework for understanding asymmetric turbidity responses...

Water level fluctuation zones (WLFZ) in large deep reservoirs are critical interfaces mediating sediment dynamics, yet the mechanistic link between hydrological regulation rates and nearshore turbidity dynamics remains poorly elucidated. To address this gap, we developed a semi-quantitative optical proxy framework by integrating high-frequency Sentinel-2 observations (2021-2025) with in-situ water level records (2021-2023), specifically designed to overcome the low signal-to-noise ratio challenge in ultra-oligotrophic clear waters. We then applied this framework to investigate the spatiotemporal heterogeneity of nearshore turbidity and its dynamic response to water level regulation in the ultra-oligotrophic Danjiangkou Reservoir, Central China. Results show that green band reflectance (ρ_green) is a robust optical proxy (P value <0.05) that effectively captures turbidity dynamics in scenarios where traditional inversion algorithms are ineffective. Spatially, a persistent lateral turbidity gradient was detected, with the shoreline zone consistently having higher reflectance than the pelagic center, especially during dry and flood seasons. Notably, we identified a distinct asymmetric response of nearshore turbidity to hydrological regulation: active drawdown phases (∆H < -0.1 m/month) trigger pronounced sediment resuspension, forming turbidity plumes largely restricted to the proximal 600 m zone; conversely, impoundment phases are dominated by dilution effects that suppress nearshore turbidity. These findings challenge the existing static view of WLFZ management and indicate that controlling drawdown rates and establishing spatial buffer zones are essential strategies for protecting the water quality of deep reservoirs serving as strategic drinking water sources.