Enhanced winter, spring, and summer hydroclimate variability across California from 1940 to 2019

California’s recent hydroclimatic pivots have caused billions of dollars in damages with the potential to become more extreme in the future, even if the mean total precipitation and streamflow do not change. In this study, we compile instrumental precipitation and streamflow records across California to quantify trends in hydroclimatic variability across spatial, temporal, and seasonal domains. We use the 10th and 90th percentiles of the distribution to examine whether any trends in variability are driven primarily by extreme low or high rainfall or streamflow events. Our analysis shows that there is a statistically significant positive trend in precipitation variability that is driven by long-term increases to the 90th percentile through most of California during winter (January–March), which has been steadily increasing since the mid-20th century. In contrast, significant positive streamflow trends are seen in winter, spring (April–June), and summer (July–September) and are driven by changes to both the 10th and 90th percentiles. Moreover, we found that there is a decreasing trend in fall (October–December) hydroclimatic variability, particularly over central California, that is related to declining values in the 90th percentile in both precipitation and streamflow. Our results suggest that California’s hydroclimate has been under a continually heightened state of volatility, leaving the state vulnerable to hydroclimatic extremes and elevated risk of associated compound events (floods, landslides, fires). Thus, water resources managers will need to incorporate this increased volatility into their planning for public and agricultural water supply, while being aware of the higher risks for wildfires and floods as well as the disproportionate and lasting impact on vulnerable ecosystem structures.