Climate change impacts on hydrological services in southern California

Water availability is one of the most critical issues facing southern California. Consequently, the role and management of intact watersheds on public lands that supply water are paramount. We undertake the first regional study of climate impacts on hydrological services (runoff, recharge, and climatic water deficit) across the four national forests of southern California-the Angeles, Los Padres, Cleveland and San Bernardino. We assess the exposure, sensitivity, and vulnerability of water resources by comparing current conditions (1981-2010) to mid-century (2040-2069) and end-of-century (2070-2099) using three general circulation models (GCMs) under RCP8.5. Half of the study area is projected to exceed 2015's drought conditions in 10%-30% of the years between now and end-of-century under the moderate GCM(CCSM4), and one-third of the area is projected to exceed 2015 in 50% of the years under the hotter, drier projection (MIROC-ESM). Under a moderate projection, mean runoff increased by 1.2 x by the end-of-century for three of the national forests, while mean recharge decreased by 0.9 x across all forests. Projected end-of-century climatic water deficit increased on average 1.1 x across the four forests. We assessed the vulnerability of watersheds by comparing the projected mean change between current and future climates with the current inter-annual variability using three categories of vulnerability. Under the moderate projection, one-third of the 385 watersheds were moderately vulnerable to changes in runoff and recharge (+/-0.2 to +/-1 x the standard deviation of current inter-annual variability) and similar to 12 watersheds were highly vulnerable, suggesting an era of new hydrological conditions by the end-of-century. Half of the Forest Service's priority management watersheds had moderate or high vulnerability for runoff and recharge. Spatial data on hydrological services and their vulnerability can directly assist in climate-smart planning, allowing tradeoffs to be assessed between proposed management actions and their effect on hydrological services.