National estimation of regulated water storage of reservoirs in China

The need for man-made reservoirs has increased with the growing population and changing climate. One of the most intensive human-induced alterations of the hydrological cycle is the regulated water storage in reservoirs, however, its quantification in large-scale reservoirs in China is inadequate. The lack of such information limits the rational management and utilization of water resources in reservoirs. To address this issue, we first mapped the dense time series of high-resolution (10 m) water inundation areas based on Sentinel-2 image scenes from 2017 to 2022 and then composed the annual surface water frequency (WF) maps of Chinese reservoirs. We calculated the water area of each reservoir at different WFs and estimated the corresponding water storage using our previously developed machine learning model. The water storage difference between the maximum water area (WF ≥ 5 %) and year-long (WF ≥ 75 %) water area was assumed as the regulated water storage of reservoirs. Results show that the multiyear-averaged water storage of all Chinese reservoirs at the maximum water area is estimated at 784.60 km³ from 2017 to 2022, and the water storage at the year-long water area is approximately 318.86 km³. The average annual regulated water storage of the reservoirs is estimated at 465.74 km³, accounting for 43.72 % of the total designed water storage of reservoirs in China. Among the basins, the Yangtze River Basin and Pearl River Basin have the highest average annual regulated water storage of 152.52 and 84.94 km³, respectively. The basins with larger irrigation areas and higher populations tend to have higher reservoir regulated water storage. The normalized water storage (NS) shows high spatial variations across reservoirs in different basins, with most basins in the south having higher NS than those in the north.