Future implications of enhanced hydroclimate variability and reduced snowpack on California's water resources

The Sierra Nevada snowpack, which supplies sixty percent of California’s consumptive water use, is under threat due to anthropogenic climate change. While previous studies have examined the impacts of climate change on mountain snowpack in the Sierra Nevada and across the Western US, few have quantified the risks to monthly irrigation water resources posed by shifting hydroclimate patterns and declining snowmelt runoff. Because they use coarse-resolution models, existing global-scale studies lack regional specificity, while existing regional studies rely on statistical or dynamical ‘downscaling’ of coarse-resolution global models. We use a new simulation of the variable resolution Community Earth System Model 2, which provides high spatiotemporal resolution estimates (14 km horizontal grid spacing, daily-to-hourly outputs) of California’s historical and future hydroclimate. We leverage the US Geological Survey’s recent irrigation water use reanalysis to evaluate basin-scale irrigation water consumption across the Sacramento, San Joaquin, and Tulare basins. Our study provides a comprehensive assessment of the water cycle, examining shifts in precipitation regimes, snowpack dynamics, and the timing and magnitude of runoff under warming scenarios of +1.5 °C, +2.0 °C, and +3.0 °C, based on the 1985–2005 reference period. Additionally, we evaluated the potential of rainfall- and snowmelt-derived runoff to meet monthly basin-scale irrigation water consumption and quantified the resulting water gaps under both modeled historical conditions and the +3°C climate scenario. The Sierra Nevada region is projected to shift from a snow-dominated to a rain-dominated hydrology as the climate warms. In the +3 °C warming scenario, the fraction of precipitation that falls as snow decreases from 51% to 24% in the Northern Sierra Nevada and from approximately 64% to 40% in both the Central and Southern Sierra Nevada. This results in a decline and earlier peak in snow water equivalent, as well as an earlier onset and shorter duration of snowmelt runoff. We find that changes in runoff timing and magnitude under a +3.0 °C scenario will amplify water gaps during the summer months and introduce a new water gap as early as May in the Tulare basin. Under this scenario, the Tulare basin is projected to exhibit the largest yearly water gap (5.8 km³), followed by the San Joaquin basin (4 km³), and the Sacramento basin (3.1 km³). Our findings highlight the vulnerability of California’s agricultural water security to warming-driven shifts in hydroclimate patterns and snowpack loss.