The primary source of California water supply is the forested headwaters of the Sierra Nevada, which are undergoing shifts in vegetation because of prior fire exclusion policies and the recent drought. Forest conditions include vegetation overgrowth, extensive tree mortality, and increased risk of high-severity wildfire. These ecosystem impacts are changing the amount of precipitation transpired by vegetation or routed to streams and reservoirs, but it is still poorly understood how reservoir management for objectives like flood control and hydropower is affected. This research investigates the question: How does fire and vegetation thinning change the frontier of achievable reservoir management objectives, and does vegetation thinning affect projected climate impacts on reservoir management? We used the Regional Hydro-Ecologic Simulation System to simulate daily inflows to French Meadows Reservoir in the American River basin, featuring a mixed-conifer watershed that receives a mix of rain and snow. Vegetation scenarios were simulated with historical and projected climate data (1950-2100) to generate reservoir inflows. A multi-objective optimizer then adjusted reservoir operation rules given management constraints of reservoir capacity, maximum release curves, flood pools, and environmental flow restrictions. The optimizer creates trade-off frontiers between four reservoir management objectives: hydropower production, water supply, ecological flows, and flood risk.
Results/Conclusions
Guided by established forest management constraints, forest thinning was simulated over 47% of the catchment area. Thinning resulted in higher reservoir inflows during the wet winter season from reduced vegetation water demand. Climate warming projected towards the end of the century showed more precipitation events occurring as rain and increased rates of snowpack melt, with peak streamflow occurring 2-4 weeks earlier. Vegetation thinning had minimal impact on changes in streamflow timing. Vegetation impacts from wildfire were estimated using historical fire events in adjacent watersheds. Forest conditions after wildfire resulted in patches of high tree mortality, contributing to increases in the frequency of extreme winter inflows. Projected climate impacts of more rain and quicker snowmelt also resulted in the highest inflow events after wildfire. The extreme flow events from wildfire and climate warming required additional space for reservoir flood pools to maintain prescribed flood control reliability. These results indicate the potential of landscape-scale forest vegetation change to impact reservoir water supply, hydropower production, and environmental flows. Incorporating shifts in forest vegetation and climate into reservoir operations can improve management of both source watersheds and connected downstream reservoir systems, when reservoir operation rules are re-optimized to consider these anticipated inflow changes.