Joint thermal regulation by selective withdrawal in serial cascade reservoir systems effectively improves reservoir and downstream ecological health

Cascade reservoir construction has significantly altered water temperature, posing serious ecological problems. Selective withdrawal is an effective measure to regulate thermal regime, but its joint regulation capacity in serial cascade reservoir systems and the ecological impact remain inadequately demonstrated. This study employs a hydrodynamic-thermal model for the 4-level cascade reservoirs on the upper Yangtze River to elucidate the hydrothermal transport mechanisms during the stratified period, quantify the thermal responses, and infer the ecological impacts of serial cascade reservoirs under different selective withdrawal schemes. The results show that high-temperature inflows and high air temperature are the primary causes for stratification, and mainstream trajectory dominates the thermal stratification pattern. Shear instability in the reservoirs drives mixing and facilitates the downward movement of metalimnion to the reservoir bottom. Compared to the natural conditions, selective withdrawal prolongs stratification duration by up to 27 days, enhances stratification stability by up to 15.58 kJ/m², and elevates the metalimnion but accelerates its downward moving rate by up to 0.35 m/day. The maximum thermal regulation capacity of joint selective withdrawal in the serial cascade reservoirs, reflected by the raised outflow temperature of the terminal Xiangjiaba reservoir, is up to 1.3 °C. The effect of selective withdrawal mostly accumulates from the first- to the fourth-level of the reservoir, with a small portion lost due to reservoir heat storage. By raising withdrawal elevations, the risk of algal blooms is effectively reduced, and the dissolved oxygen concentration at the reservoir bottom is increased with the weakening and upwelling of the metalimnion. Additionally, the spawning window when the water temperature is favorable for the downstream fishes is also extended by up to 24 days. This study provides critical insights for the joint operation of cascade reservoirs in terms of protecting downstream aquatic ecosystems.