Enhanced Hydroclimatic Variability Across California

Abstract

Most of the annual precipitation over California occurs in a short window between November and March and is highly dependent on the frequency of winter storms. Thus, the amount of precipitation can vary strongly from year to year, as demonstrated in the most recent decade (2012-2022), when year-to-year swings between high and low winter precipitation extremes resulted in drought, wildfires, and floods throughout California that caused billions of dollars in damage. Despite this year-to-year variability, mean precipitation and streamflow over the state do not show any long-term trends over the observational records or future projections. However, some studies suggest that hydroclimatic variability will increase into the end of the 21st century with changes to both the magnitude and frequency of dry and wet extremes. In this dissertation, I explore this topic in-depth using instrumental records and records derived from tree-ring-based hydroclimate reconstructions across California. In the first study, which is detailed in Appendix A, I used instrumental monthly and seasonal precipitation and streamflow records at various spatial scales. I show that hydroclimatic variability across the state shows an increasing trend starting in the 1950s that is primarily caused by intensified wet extremes. In the second study, detailed in Appendix B, I examine this 20th century trend in a longer-term context using tree-ring-based hydroclimate reconstructions throughout California. I find that the 20th century increases in precipitation and streamflow variability across the state, driven by increasingly wetter extremes, are unprecedented over the past 600 years. I also show that there is a comparable period of enhanced variability in the 16th century that is, however, driven by increasingly dry extremes. Finally, in the third study, detailed in Appendix C, I assess the influence of the 20th century increase in variability on the monthly storage in the largest man-made reservoirs located in California. I find that statewide and regional water supply has not changed over the past 45 years, but that trends are strongly dependent on record length, which does not always match management or climate variability time scales. The results of these three studies are consistent with climate model simulations that suggest an increasingly volatile future for California’s hydroclimate, especially as wet extremes intensify further. This work highlights the intrinsic nature of the California hydroclimate to meander in the extremes, and the future challenges that await the state into the future as warming temperatures continue to intensify the hydrologic cycle.