Reservoir optimization operation considering regulating temperature stratification for a deep reservoir in early flood season

[Display omitted] •Reservoir optimization operation considering temperature stratification is conducted.•NSGA-II and MOPSO are combined with hydro-temperature model CE-QUAL-W2.•Releasing water late and large inflow rate can weaken the thermal stratification.•Thermal stability can be reduced to 14627.01 J/m2 (−5.32%) in the Pareto frontiers.•Large flow event should be optimized to regulate thermal regime for large reservoirs. Temperature stratification increases the thermal stability (TS) of the reservoir area, decreasing the vertical water exchange and going against the water environment improvement. It is affected by the reservoir operation scheme and should be considered with multiple objectives. Hence, considering the early flood season of Xiluodu Reservoir as a case, this study built a multiobjective optimization model of temperature stratification by coupling the hydro-temperature model CE-QUAL-W2. The non-dominated sorting genetic algorithm II and multiobjective particle swarm algorithm were used to solve the multiobjective optimization model, and the Pareto frontiers were evolved by reducing the peak rate of outflow (PRO), increasing total hydropower generation (THG), and decreasing TS. This paper shows that (1) the maximum-THG individual tends to release water late to increase the water level and hydraulic head, which is opposite for the minimum-TS individual. A temporally uniform outflow favors a decrease in PRO. (2) In the Pareto frontiers, THG can be increased from 47.42 × 108 to 52.15 × 108 kw·h (+9.97%), and TS can be reduced from 15448.27 to 14627.01 J/m2 (−5.32%). (3) When the inflow rate increases, the temperature stratification is further weakened, and the minimum TS is reduced from 14627.01 to 13233.54 J/m2. For large cascade reservoirs with low flood risk in the early or late flood season, their THG and thermal regime can be improved under a moderate outflow and should be considered for the administrations.