Modeling Multi-Reservoir Hydropower Systems in the Sierra Nevada with Environmental Requirements and Climate Warming

Hydropower systems and other river regulation often harm instream ecosystems, partly by altering the natural flow and temperature regimes that ecosystems have historically depended on. These effects are compounded at regional scales. As hydropower and ecosystems are increasingly valued globally due to growing values for clean energy and native species as well as and new threats from climate warming, it is important to understand how climate warming might affect these systems, to identify tradeoffs between different water uses for different climate conditions, and to identify promising water management solutions.;This research uses traditional simulation and optimization to explore these issues in California's upper west slope Sierra Nevada mountains. The Sierra Nevada provides most of the water for California's vast water supply system, supporting high-elevation hydropower generation, ecosystems, recreation, and some local municipal and agricultural water supply along the way. However, regional climate warming is expected to reduce snowmelt and shift runoff to earlier in the year, affecting all water uses. This dissertation begins by reviewing important literature related to the broader motivations of this study, including river regulation, freshwater conservation, and climate change. It then describes three substantial studies.;First, a weekly time step water resources management model spanning the Feather River watershed in the north to the Kern River watershed in the south is developed. The model, which uses the Water Evaluation And Planning System (WEAP), includes reservoirs, run-of-river hydropower, variable head hydropower, water supply demand, and instream flow requirements. The model is applied with a runoff dataset that considers regional air temperature increases of 0, 2, 4 and 6 °C to represent historical, near-term, mid-term and far-term (end-of-century) warming. Most major hydropower turbine flows are simulated well. Reservoir storage is also generally well simulated, mostly limited by the accuracy of inflow hydrology. System-wide hydropower generation is reduced by 9% with 6 °C warming. Most reductions in hydropower generation occur in the highly productive watersheds in the northern Sierra Nevada. The central Sierra Nevada sees less reduction in annual runoff and can adapt better to changes in runoff timing. Generation in southern watersheds is expected to decrease. System-wide, reservoirs adapt to capture earlier runoff, but mostly decrease in mean reservoir storage with warming due to decreasing annual runoff.;Second, a multi-reservoir optimization model is developed using linear programming that considers the minimum instream flows (MIFs) and weekly down ramp rates (DRRs) in the Upper Yuba River in the northern Sierra Nevada. Weekly DRR constraints are used to mimic spring snowmelt flows, which are particularly important for downstream ecosystems in the Sierra Nevada but are currently missing due to the influence of dams. Trade-offs between MIFs, DRRs and hydropower are explored with air temperature warming (+0, 2, 4 and 6 °C). Under base case operations, mean annual hydropower generation increases slightly with 2 °C warming and decreases slightly with 6 °C warming. With 6 °C warming, the most ecologically beneficial MIF and DRR reduce hydropower generation 5.5% compared to base case operations and a historical climate, which has important implications for re-licensing the hydropower project.;Finally, reservoir management for downstream temperatures is explored using a linear programming model to optimally release water from a reservoir using selective withdrawal. The objective function is to minimize deviations from desired downstream temperatures, which are specified to mimic the natural temperature regime in the river. One objective of this study was to develop a method that can be readily integrated into a basin-scale multi-reservoir optimization model using a network representation of system features. The second objective was to explore the potential use of reservoirs to maintain an ideal stream temperature regime to ameliorate the temperature effects of climate warming of air temperature. For proof-of-concept, the model is applied to Lake Spaulding in the Upper Yuba River. With selective withdrawal, the model hedges the release of cold water to decrease summer stream temperatures, but at a cost of warmer stream temperatures in the winter. Results also show that selective withdrawal can reduce, but not eliminate, the temperature effects of climate warming. The model can be extended to include other nearby reservoirs to optimally manage releases from multiple reservoirs for multiple downstream temperature targets in a highly interconnected system.;While the outcomes of these studies contribute to our understanding of reservoir management and hydropower at the intersection of energy, water management, ecosystems, and climate warming, there are many opportunities to improve this work. Promising options for improving and building on the collective utility of these studies are presented.