A Growing Heavy Runoff Fingerprint of Climate Changes in the Sacramento San Joaquin Watershed

As climate warms in California mountain zones, shifts in runoff will occur, not only in its seasonal mean structure but also in the form of its daily runoff events. This transition will modify a historical hydroclimate where large precipitation events already have exceptionally strong influence. Although the intensification of heavy precipitation events has been studied extensively, climate change-driven shifts in heavy runoff events have received less attention. This investigation considers future changes in high-percentile daily runoff over complex topography, in response to future climate warming and occasionally heavier precipitation events. The study is based upon VIC hydrological simulations driven by 13 LOCA2-downscaled CMIP6 climate projections. The study domain covers 210 diverse lower- to higher-elevation catchments that drain into the Sacramento/San Joaquin Delta Watershed, in northern and central California, which provides many ecosystem services and supplies a major portion of the state’s water. Under a relatively high-emissions climate change scenario increase in daily runoff events that reach or exceed the 98th historical percentile are found, contributing nearly twice their cumulative volume relative to the 1952–2014 historical period. This increase occurs unevenly across the Delta Watershed, dominated by daily runoff events in medium-elevation mixed rain/snow catchments whose future occurrence increases by 179% of their historical mean. Highest-elevation catchments transition toward a winter/earlier spring runoff regime where 98th percentile occurrences decline to 62% of historical occurrences as warming causes snowpack to recede and historically familiar spring–summer snowmelt peaks fade. High-percentile runoff events are more likely during wet years and notably lacking during dry years. The results indicate a future with greater flood risk. Runoff days reaching or exceeding the historical 99.9th percentile (~2.7 year return period) occur more often across all elevations, likely in response to warm winter storms. These increases are especially impactful in medium and high elevations, where 99.9 percentile runoff days occur more than 240% more often than in the historical period and contribute more than 280% to annual runoff.