Rethinking environmental flow management strategies in reservoir operations from an integrated water quality perspective

Abstract

River damming causes appreciable nutrient retention in reservoirs, increasing the risk of eutrophication in reservoir water. Environmental flow (e-flow) management strategies have been extensively studied to direct reservoir release to meet water quantity demands of downstream ecosystems. However, such strategies also impact reservoir nutrient transport and transformation as well as stored and discharged water quality, which has rarely been considered in existing e-flow management. This study combines a reservoir operation optimization model and a coupled hydrodynamic-eutrophication-sediment model to evaluate e-flow management strategies from an integrated water quality perspective. Multiple nutrients, water, and sediment processes are accounted for as well as the quality of both stored and discharged water. Five common e-flow strategies that focus on various ecosystem protection targets were used to generate operation scenarios. Subsequently, the coupled hydrodynamic-eutrophication-sediment model was used to simulate nutrient cycling processes and water quality under each scenario. Modeling results show that e-flow management strategies cause complex effects on reservoir nitrogen, phosphorus, and algal dynamics. In general, reservoir water quality was the worst in the scenario with largest e-flow release, implying that tradeoffs exist between water quantity and quality of reservoir release. Lower e-flow release resulted in reservoir storage level increases and promoted sediment nutrient release, but the effect was offset by the increase in the nutrient dilution capacity of reservoir water. Additionally, the comparison between inflow and discharged water quality showed that the reservoir contributed to reducing river nitrogen loads under low e-flow releases while increasing river phosphorus loads in all scenarios. The findings support e-flow management decision-making to account for both water quantity and quality targets as well as to coordinate reservoir services and ecological protection.