| Summary: | Study region: Santa Ynez River Basin, Santa Barbara County, California. Study focus: Lake Cachuma, a reservoir on the Santa Ynez River, provides water for over 280,000 residents and agricultural lands of Santa Barbara County, California. This area experiences high inter-annual precipitation variability, which we hypothesize is driven by the presence or absence of a few large precipitation events each year. We use daily precipitation observations from 1965 to 2017 to identify extreme precipitation events, defined as those exceeding the 90th percentile. We examine the role of these events, their associated synoptic patterns, and the El Niño Southern Oscillation (ENSO) in driving inter-annual precipitation variability in this basin. New hydrological insights for the region: On average, a wet year features three or more extreme events, a normal year 1–2 events, and a dry year 0–1 events. We identify four distinct synoptic-scale weather patterns associated with extreme events and find that 74% of events are associated with atmospheric rivers. El Niño years tend to have a greater number of extreme events, though this relationship is not dependable. The reliance on just a few extreme precipitation events and diversity among these events highlights the challenges of seasonal prediction and resource management in this area. This novel approach to defining variability on a watershed scale can support ecological, geological, and hydrological studies as well as regional water resource management. Keywords: Lake Cachuma, Southern California, Precipitation extremes, Precipitation variability, Drought, Water resources
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