Remote sensing-based estimation of monthly CO2 flux in Three Gorges Reservoir drawdown areas.

The drawdown areas of the Three Gorges Reservoir (TGR) have emerged as significant greenhouse gas (GHG) hotspots, yet their contribution remains poorly quantified due to the difficulty of monitoring large, dynamic, and frequently cloud-covered landscapes. To address these challenges, we developed a remote sensing-based framework to generate the first high-resolution (30 m, monthly) estimates of CO2 fluxes from the TGR drawdown area over 2015-2023. By integrating all-weather Sentinel-1 SAR imagery with digital elevation models (DEM) and hydrological data, we successfully mapped the dynamic boundary between submerged and exposed lands, achieving an overall accuracy of 92.3 % (Kappa >0.8). Our results show that the drawdown area, covering only 21.7 % of the reservoir surface, contributes 392.2 ± 19.5 Gg of CO2 annually-accounting for 44.4 % of total emissions from the reservoir surface. Critically, we identify the seasonally exposed terrestrial phase as the overwhelming driver, responsible for 82.4 % of the drawdown area's total flux. A strong inverse correlation (r = -0.91) between CO2 emissions and reservoir water levels demonstrates that the anthropogenic operational regime for flood control and power generation-not natural seasonality-is the definitive control on these emissions. These findings establish that drawdown area emissions are a critical and previously under-accounted component of hydropower's carbon footprint. This work provides robust remote sensing-driven insights and actionable recommendations for sustainable reservoir operations and informed carbon accounting frameworks. We propose that adaptive reservoir management may mitigate these emissions.