How have the drawdown zones of large reservoirs changed over the past two decades in China?

This study focuses on the Reservoir Drawdown Zones (RDZs) of 727 large reservoirs in China. The research investigates the spatiotemporal dynamics of RDZs from 2000 to 2021, utilizing remote sensing data from the Global Surface Water Dataset (GSWD). It examines trends in RDZ area changes, their spatial distribution across major river basins, and the key drivers, including reservoir operations, climate variability, and human activity. According to the findings, RDZ zones constitute around 35 % of a major reservoir's maximum water surface area. Although there are significant fluctuations over time, the total area of RDZs shows an increasing trend on a temporal scale. Prior to 2015, there were some variations in the growth of RDZ areas, which were likely caused by external factors such as operating plans, early reservoir commissioning, and climate change. The capacity of reservoir operations to adjust to climate change was enhanced after 2015 due to the construction and operation of additional large reservoirs, as well as improvements in scheduling and management. As a result, RDZ regions continued to expand at a rate significantly faster than before 2015. Geographically, RDZs are primarily distributed in eastern, central, and southern China, particularly in the Songhua and Liaohe River Basins, the Yangtze River Basin, and the Pearl River Basin. From a spatial perspective, local population density, economic growth, and water resource management techniques, in addition to geographic and climatic factors, may influence the distribution patterns of RDZs across reservoirs of varying sizes. In RDZ regions, more than one-fourth of the reservoirs exhibit notable interannual changes, where climate or human activity may be the primary driving factor. This challenges the conventional wisdom that RDZ regions typically exhibit consistent interannual patterns. • Spatiotemporal changes of drawdown zones in China's large reservoirs are explored. • The Danjiangkou Reservoir hosts the largest drawdown zone in China. • Over 1/4 reservoirs’ drawdown zone experience drastic inter-annual changes in area. • Drawdown zone accounts for roughly one-third of its reservoir area on average.