Exploring the trade-offs among hydropower benefits, environmental flow, and surface water temperature in a large reservoir under deep uncertainty

Abstract

The negative environmental impacts of large hydroelectric reservoirs have attracted wide attention. Reservoir operation alters seasonal flow patterns and reduces downstream flow during fish spawning seasons, which may lead to the loss of fish biodiversity. Meanwhile, reservoir impoundment reduces the flow velocity and raises the reservoir surface water temperature (SWT), potentially increasing the risk of algal blooms. While previous studies have designed various operation rules to balance the hydropower benefits and environmental flow or water temperature through multi-objective optimization approaches, a comprehensive analysis of trade-offs among the hydropower benefits, environmental flow, and SWT under an uncertain future remains lacking. This study contributes the many-objective robust decision-making (MORDM) framework to the multi-objective reservoir ecological operation for the first time. We quantify the trade-offs among maximizing the hydropower benefits, maximizing the reliability of environmental flow, and minimizing the number of days with high SWT; and evaluate the robustness of the operation rules under deep uncertainties from the climate conditions and electricity market. We apply MORDM to a large and deep reservoir, Nuozhadu, located in southwest China. Our results indicate that trade-offs exist between environmental flow and SWT, emphasizing the necessity to consider multiple environmental objectives in reservoir operation. Based on MORDM, we identify an operation scheme that can not only balance hydropower benefits and two environmental objectives, but also have robust performance against the uncertain future states. Scenario discovery analysis shows that air temperature is the most important uncertain factor that influences the robustness of operation rules. Our study provides a new perspective on reservoir operation planning to achieve balanced and sustainable developments in hydropower and environments under uncertain climate and electricity market conditions.