Flow regime alteration under progressive multi-reservoir development in the Upper Yangtze River basin

Study Region : The Upper Yangtze River basin, comprising the Jinsha, Min, Jialing, and Wu tributaries and the mainstream, contains 38 large reservoirs with a live storage capacity (LSC) of 80.5 billion m 3 . Study Focus : This study couples hydrological simulation with the Soil and Water Assessment Tool (SWAT) and reservoir-operation optimization to quantify flow regime alteration across three development stages: a natural baseline, a 21-reservoir configuration, and a 38-reservoir configuration. SWAT simulated daily natural flows for 1965–2024, and the Progressive Optimality Algorithm-Dynamic Programming Successive Approximation (POA-DPSA) optimized reservoir operations under a common objective and constraint framework. This counterfactual design isolates reservoir-operation effects from climatic variability and enables comparison among sub-basins with contrasting reservoir layouts and coordination structures. New Hydrological Insights for the Region : Reservoir operations reduced wet-to-dry flow ratios by up to 64.6% through seasonal redistribution. Under the fixed-parameter baseline, DOR (storage-to-annual-runoff ratio) above about 10% was associated with substantial flood attenuation, whereas low-flow augmentation emerged above about 5%. Across the three development stages examined in each basin, DOR and the coefficient of variation (CV) of daily flow were well described by linear fits (R ≥ 0.98), but fitted slopes differed among basins, indicating spatially variable sensitivity to DOR increases across development stages. A coordinated-versus-non-coordinated comparison showed basin-dependent coordination gains, strongest in the Jinsha cascade and weaker where mainstream multi-year reservoirs already provided substantial regulation.