How do environmental flows allocation and storage capacity changes impact the performance of multipurpose reservoirs?

Storage reservoirs are constructed and operated to overcome mismatch between supply of water and demands. Reservoir operation causes changes in downstream rivers. While operating a reservoir, environmental flows (Eflows) are released in the river to meet the requirements of aquatic ecosystems and mitigate impacts due to diminished flows. Numerous studies have estimated Eflows in different river basins, but trade-offs between sizing and operation of multipurpose reservoirs have not been adequately studied for seasonal rivers. This study has examined how the performance of reservoirs in seasonal basins is impacted as storage size and Eflows change. It is found that if the reservoir size is increased beyond a storage ratio (defined later) of about 0.7, additional irrigation benefits show small increase, but spills do not reduce significantly. The zone between storage ratios 0.2–0.8 is crucial in reservoir design. A framework has been presented to assess the trade-offs in reservoir sizing and operation to meet Eflows and other needs and arrive at acceptable solutions. Results show that the trade-offs depend on the relative magnitude of Eflows and other demands and on statistical properties of inflows and demands.