Designing Instream Flows to Satisfy Fish and Human Water Needs

Abstract

The relicensing of nonfederal hydropower projects and the mandated reevaluation of federal water projects require policy makers to balance the human use of water with instream releases for environmental values. To meet the need for planning level tools for instream flow determination, we develop a flexible multiobjective optimization model. The model considers both the size and frequency of water supply shortages and the habitat available for fish species as the fish progress through life stages. We use a habitat capacity metric to combine expected mortality, the fraction of a life stage in a particular month, and the areal habitat needs per individual fish. The model incorporates human water supply concerns such as monthly variations in human water demand, water-year types, and flood control restrictions. We apply this monthly optimization model to a west-slope Sierra Nevada stream used for municipal and agricultural supply and for supporting an anadromous fish population. Results identified a range of alternative solutions that involve trade-offs between water shortage levels and fish population capacity.