Reservoir operating rules to sustain environmental flows in regulated rivers

Sustainable water resource developments require reservoir operations that provide environmental flows (e-flows) to support the downstream riverine ecosystem by minimizing the degree of hydrologic alteration under given water supply constraints. The effective management of e-flows below dams was explored by combining three e-flow management strategies for normal,'' wet, and dry year situations, delivering a range of e-flow regimes for different reservoir storage levels and different natural flow (reservoir inflow) conditions. We combined these into a single combined e-flow strategy and coupled this with reservoir operating rule curves to form a reservoir operating approach that optimized e-flow provision under given water supply constraints. We also explored constraints imposed by the dam's valve release capacity. To investigate the capability of the approach to minimize the degree of hydrologic alteration downstream from a reservoir, we presented a case study of the Tanghe Reservoir in China's Tang river basin. The approach triggered a preferred e-flow strategy when the reservoir water level was high to better maintain the ecological functions within downstream riverine ecosystems, a basic e-flow strategy when the water level was low to reduce the water supply risk, and an acceptable strategy under intermediate conditions. The approach effectively balanced human and ecosystem needs and demonstrated the minimum levels of flow regime alteration achievable within the regulated river.