Thermal response of deep monomictic reservoir under different selective withdrawal types

Selective withdrawal is an effective measure to mitigate the adverse effects caused by reservoir construction. The main types of selective withdrawal include multi-level withdrawal and internal weir withdrawal, each with distinct characteristics. It is urgent to elucidate the thermal response differences between these two types of selective withdrawal to improve scheduling accuracy. Taking the Xiluodu Reservoir (XLDR) as a representative of monomictic reservoirs in this study, a hydrodynamic-thermal numerical model is employed to investigate the thermal response of large deep monomictic reservoirs under different selective withdrawal types and schemes. Additionally, we also explore the regulatory mechanisms of selective withdrawal on water temperature and its applicability under different hydrological conditions. The results indicated an asynchrony in the variations of the vertical temperature gradient (VTG) and the Schmidt stability index (SSI). SSI is identified as the optimal index for comprehensively reflecting the characteristics of water temperature stratification. The multi-level withdrawal generates a strong velocity zone and a strong temperature gradient zone near the intake, enhancing the overall stratification strength and surface water temperature and increasing the thermocline thickness but weakening its strength. In contrast, internal weir withdrawal accelerates the downward movement of the thermocline and strengthens it but reduces the thermocline thickness. As the withdrawal depth increases, the surface water temperature gradually increases, the thermocline position moves downward, and the duration of strong stratification extends. Different selective withdrawal schemes should be matched with different water demands, based on water quality health and ecological temperature requirements, we provided selective withdrawal strategy recommendations for large monomictic reservoirs, considering the thermal response characteristics of each withdrawal scheme, including changes in temperature structure and outflow temperature characteristics. The findings of this study provide a theoretical basis and technical support for reservoir optimal management and the design of selective withdrawal engineering.