Distinctive Impacts of Extreme Warm Season Precipitation and Climate Change on the Vulnerable Water Resources of the Southwestern United States

This dissertation explores the influence of extreme precipitation and the potential impacts of climate change on the vulnerable water resources of the southwestern United States. Specifically, it focuses on 1) the characteristics, origins, and impacts of historical extreme warm-season precipitation in the Lake Mead watershed, 2) improving existing bias correction techniques for projected future streamflows, and 3) investigating the vulnerability of California’s largest reservoir, Lake Shasta, to climate change under existing and adaptive operating protocols. Although the North American Monsoon (NAM) is the main driver of summertime climate variability in the American southwest, considerable knowledge gaps exist regarding its impact at the northern extent of the core region (northwestern Mexico, southern Arizona, and New Mexico). The first part of this dissertation catalogues historical extreme precipitation events in the Lake Mead watershed (located at the NAM’s northern boundary) and identifies unique synoptic drivers of extreme precipitation between the canonical NAM region and watersheds to the north. From here, the dissertation shifts its focus from the historical period to future climate projections. Motivated by a desire to connect bias correction techniques to the underlying dynamics within earth systems models, a novel statistical method is developed for projected streamflow wherein data are windowed based on hydrograph-relative time, rather than Julian day. This method yields improved preservation of original climate model data for both extreme and non-extreme events. Utilizing these bias corrected streamflow projections, and a simplified model of operations at California’s largest reservoir, Lake Shasta, developed by the author, coming threats to water supply and flood risk under existing operations and several forms of adaptive responses to climate change are analyzed. Compared to the historical period, we simulate 27% declines in carryover storage (storage on September 30th) at the end of the 21st century under a severe warming scenario if operations are left unchanged. Despite many simulated interventions favoring water supply over flood risk, historical levels of carryover storage were irretrievable at the end of the century under the warmer of the two warming scenarios examined in this study