Impacts of Reservoir Operation on Hydrodynamic and Thermal Conditions in a Large Deep Reservoir and the Potential Ecological Impacts

Many large-deep reservoirs have been built, are under construction, or are planned in southwestern China, to meet increasingly high energy demands. The construction of dams can significantly alter the natural hydrodynamic and thermal conditions of rivers, thereby, threatening aquatic ecosystems, e.g., fish and algae communities. However, the impacts of large-deep reservoir operations on the hydrodynamic and thermal conditions and their potential influence on aquatic ecosystems have seldom been addressed. In this study, a quasi-3D hydrodynamic model is developed to describe the hydrodynamic and thermal conditions of a large deep reservoir, Nuozhadu, the largest reservoir along the Lancang River. The water temperature and flow velocity after the impoundment of the reservoir are examined. To explore how reservoir operation changes the hydrodynamic and thermal conditions, five groups of reservoir operational scenarios based on different water levels and discharge rates are conducted. Our results indicate that a lower water level combined with a higher discharge rate can increase the flow velocity and decrease the surface water temperature in the reservoir, which may contribute to diminishing the growth of Cyanobacteria. Meanwhile low water levels can also increase the outflow water temperature to protect the downstream fish. Further, the LowLevel and MaxPower schemes were designed to provide ecologically favorable hydrodynamic conditions, and to maximize power generation, respectively. The LowLevel scheme was found to reduce the duration of high surface water temperature (>25°C) by 18 days annually and reduce the outflow temperature by 0.8°C in the fish spawning period, at the expense of a 10% reduction in power generation. This study contributes to a better knowledge of the linkage among hydrodynamic, thermal conditions, and ecological impacts due to reservoir operation, and provides a novel perspective regarding the ecological safety of a large deep reservoir operation.