Optimization of Selective Withdrawal Strategy in a Warm Monomictic Reservoir Based on Thermal Stratification

Selective withdrawal from reservoirs is an effective strategy for balancing economic goals and environmental demands. Multilevel intake operation (MIO) is one of the main options for improving the thermal regime of deep reservoirs. In this study, a combination of measured data and a two-dimensional hydrodynamic model (CE-QUAL-W2) was utilized to analyze the impact of MIO processes on the thermal stability of a reservoir and the improvement effect of the withdrawal water temperature (WWT). The purpose of this study was to identify an optimal selective withdrawal strategy to meet the water temperature needs of fish downstream during their spawning and breeding activities. The results showed that the MIOs raised the withdrawal water intake position (8.3–11.2 m) for the WWT. Meanwhile, MIOs resulted in statistically significant changes in the water temperature structure of the reservoir (p < 0.01). The stratification stability of the water column weakened (6.8 %-34.5 %), and there were interannual differences. The WWT increased by 0.0 °C∼1.9 °C every ten days, and the time to reach the optimum temperature (18 °C) for fish spawning was 7 to 17 days earlier. There was a statistically significant positive correlation between the WWT improvement and the stability index (SI) (R = 0.7817, p < 0.01). The threshold values of the SI (above the water intake) for average WWT improvements of 0.5 and 1.0 °C were 90.3 and 221.9 kg/m2, respectively, and the recommended operating period was from early-April to May. Under MIOs, the weakening stratification and the increasing WWT during the warming period reduced risks to the water quality and ecological health in the reservoir and downstream rivers. This study provides a helpful reference for the implementation of selective withdrawal operations and the optimization of water quality management strategies in similar large, deep reservoirs.