Discharge Response to Cascade Reservoir Operations and Optimized Freshwater Replenishment Strategies for Salinity Suppression in the Yangtze River Estuary

To mitigate water supply security risks induced by saltwater intrusion in the Yangtze Estuary during dry seasons, this study investigates the characteristics of low-flow runoff variation at the Datong Station and develops a one-dimensional hydrodynamic model for the reach from Yichang to Jiangyin. The model is used to simulate the discharge response at Datong under various augmentation release schemes from the upstream reservoir group, dominated by the Three Gorges Dam (TGD). Using the extreme drought event of 2022 as a validation case, we propose optimized freshwater replenishment strategies for salinity suppression. Key findings include the following: (1) The seasonal water-storage operation (i.e., storing wet-season water for dry-season use) significantly increases dry-season discharge at Datong and substantially reduces the frequency of extreme low-flow events. (2) The response time of the maximum influenced discharge depends primarily on the duration of augmentation; as the TGD release duration increases from 1 to 30 days, the peak influenced discharge at Datong appears between 12 and 33.5 days after the start of replenishment. (3) The TGD augmentation release exhibits a linear positive correlation with the maximum influenced discharge at Datong; when augmentation lasts more than 30 days, the replenishment efficiency stabilizes at 80–82.5%. Considering the long-distance runoff propagation time along the mainstream, a steady-discharge mode is recommended for salinity suppression. In emergency conditions, replenishment should be initiated at least 4 days in advance; if augmentation is to be maintained for 10 days, then reservoir regulation should commence 18–20 days before the anticipated saltwater intrusion event. Practical application during the 2022 drought confirms that coordinated augmentation from the upstream reservoir group effectively elevates discharge at Datong and pushes the saltwater front downstream, substantially extending the water intake window for estuarine water sources. The derived runoff response characteristics and optimized dry-season regulation schemes can improve water supply reliability for key water sources in the Yangtze Estuary, ensure water security for Shanghai and surrounding areas, and provide theoretical and technical support for joint salinity-regulation control and estuarine water security management.