Projected Changes in Precipitation, Temperature, and Drought across California’s Hydrologic Regions

This study investigates potential changes in future precipitation, temperature, and drought across 10 hydrologic regions in California. The latest climate model projections on these variables through 2099 representing the current state of the climate science are applied for this purpose. Changes are explored in terms of differences from a historical baseline as well as the changing rate (trend slope). Results indicate that warming is expected throughout the 21st Century across all regions in all temperature projections. RCP 4.5 and RCP 8.5 warm about the same amount by mid-21st Century, but thereafter the higher rate of warming produces greater increases in temperature under RCP 8.5. There is no such consensus in precipitation, with projections ranging from -25 percent to +50 percent different from the historical baseline. There is no statistically significant increasing or decreasing trend in historical precipitation and in a majority of the projections of precipitation. On average, projected precipitation changes are small compared to the natural variability observed in historical precipitation. Compared to wet regions, dry regions are projected to have higher increases in temperature and more severe droughts. Droughts are increasingly extreme in the late 21st Century, especially in RCP 8.5 simulations. The study also shows that the coolest North Lahontan region tends to have the highest increases in both minimum and maximum temperature. The region is also projected to experience increases in wet season precipitation. For the driest region, the Colorado River region, all projections consistently show higher increasing trends in temperature and drought risk compared to their historical counterparts. Overall, these findings are meaningful from both scientific and practical perspectives. From a scientific perspective, these findings provide useful information that can be utilized to improve the current flood and water supply forecasting models or develop new predictive models. From a practical perspective, these findings can help decision-makers in making better-informed adaptive strategies for different regions to address adverse impacts posed by those potential changes.