UC Merced

Headwaters watersheds and forests of the Sierra Nevada are crucial in ensuring water security, supplying over 70% of clean water to the California water system. How- ever, severe drought-induced tree mortality and wildfires fueled by climate change, lower snowpack, and overgrown forests are threatening the health and resilience of this critical landscape and the water supplies originating in these areas. The seasonal divergence between precipitation and forest water use, typical of the Mediterranean climate, highlights the importance of snowpack and subsurface water storage in sus- taining vegetation during the growing season. During warmer years, a loss of snow- pack increases the asynchrony between the timing of water input to the root zone and the peak forest water use, intensifying forest water stress and tree mortality. A potential aid can come from forest restoration practices. A reduction in canopy interception and transpiration following forest treatments can lead to an increase in available water for the remaining trees and runoff. However, the impact of forest management on water balance can be highly variable due to differences in climate, topography, location, and vegetation. In this dissertation, we used statistical and processed-based models to explore the effect of climate stressors and forest treat- ments on forests and water resources. Firstly, we used generalized additive models to infer and quantify how snowpack loss affects forest water stress in Sierra Nevada. Next, we used SWAT+ to investigate how decisions regarding location, intensity, and scale of forest treatments can affect the water balance across the water-energy gradi- ent in a large watershed such as the upper Kings River Basin. Lastly, we evaluated the variability of the hydrological response to warming and forest treatments. The findings of this dissertation suggest that the failure of a reduced snowpack to mitigate water deficit exacerbates forest water stress and tree mortality, providing insights into the importance of the Sierra Nevada snowpack under a warming climate. The hy- drological response to forest treatments is driven by water and energy limitations, as well as climate variability. With warming, although forest restoration may not mit- igate warming impacts completely, it can contribute to reducing streamflow losses, the severity of forest water stress, and tree mortality.