When can we prioritise environmental flow release without affecting hydropower and water demand satisfaction?

Large multi-purpose reservoirs are required to support socioeconomic development in many parts of the world. However, impoundment of streamflow also negatively impacts instream aquatic ecology. Simultaneously, there are often conflicts between different objectives of multi-purpose reservoirs, such as hydropower generation and water storage for sustaining demands using dry seasons. Thus, releasing environmental flows, while ideal, is often accorded secondary priority unless there are legislative norms in place to facilitate their release. Here, we quantify the compromises between environmental flow maintenance, hydropower generation, and water demand satisfaction across five major multi-purpose reservoirs in India. We investigate these compromises for two type of release priority orders, which are reflected in two versions of a multi-objective decision problem (a) one that prioritizes environmental flow releases over demand satisfaction (PF_MEF) and another that does not (PF_nMEF). Pareto approximate strategies for reservoir operation are identified for each formulation using the Borg multi-objective evolutionary algorithm considering multiple objectives related to annual demand deficits (minimize), hydropower production (maximize), reliability of maintaining environmental flows downstream (maximize), and reliability of non-exceedance of high flows downstream (maximize). In each case, radial basis functions (RBFs) are used to develop flexible release rules as a function of reservoir storage. The resultant Pareto approximate strategies are analyzed to understand when environmental flow prioritization over other releases is practical. In all five projects, hydropower generation exhibits trade-offs with both demand satisfaction and environmental flow reliability. However, the slope and the nature of tradeoffs are governed by other factors such as the storage inflow ratio and the demand-to-inflow ratio at annual time scales. We aim to provide a generalized guideline as to – when prioritizing environmental flow does not impact other objectives of the multi-purpose reservoir project.