Water-Energy Sector Vulnerability to Climate Warming in the Sierra Nevada: A Method to Consider Whether Dams Mitigate Climate Change Effects on Stream Temperatures

In this paper, we provide a method for examining water quality objectives below dams at the mesoscale using a multi-model approach. Our method is a pilot application intended to leverage climate-forced hydrologies into reservoir inflows, combined with in-reservoir thermal dynamics, reservoir release operations, and downstream temperature changes driven by atmospheric conditions. Habitats for coldwater fish species, such as trout and salmon, have been reduced by water development, dam-building, and land use changes that alter thermal regimes of rivers. Climate change is an additional threat. Because regulated flows in California’s Sierra Nevada are complex and highly valued for downstream beneficial uses such as hydropower and water deliveries to municipal and agricultural uses, a multi-model method can potentially capture the complexities of changing flow regime and water quality. Changing hydroclimatic conditions will likely have associated changes to water temperature that will have cascading ecological impacts downstream (although we assume reservoir operations remain unchanged through time). We model hypothetical reservoirs of different sizes, elevations, and latitudes to examine the potential to manage stream temperatures for downstream coldwater habitat for 4 time periods representing a progressively farther outlook for climate change, and 2 climate models to illustrate climate model uncertainty. Climate-driven Variable Infiltration Capacity hydrologic data was input into a regional equilibrium stream temperature model. The Water Evaluation and Planning model was used to estimate reservoir outflow and develop generic operating rules for each reservoir and Water Quality for River-Reservoir Systems software was used to estimate water temperature within each reservoir. All models are 1-dimensional and operate on a weekly timestep. Our approach is useful as a proof of concept study, although we believe that stream temperature results are too cool and do not represent the current or future thermal regime. Air temperatures used as input data for this study are 3-7°C cooler than Daymet interpolated climate data, and NCARPCM1 air temperatures for the end of the 21st century are cooler than Daymet historical data. We recommend quantifying the accuracy of historical climate data in mountain regions and modifying reservoi