Managing water resources in semiarid watersheds is challenging because of limited water supply and uncertain impacts of future climate conditions on groundwater resources. This paper examines the impact of future climate changes on the urban Yucaipa Valley watershed in southern California using an integrated hydrological model referred to herein as the Yucaipa model. Coupled Groundwater and Surface-water FLOW (GSFLOW) modeling software was used to simulate the nonlinear relationships between climate trends and precipitation partitioning into evapotranspiration (ET), runoff, and subsurface storage. Four global climate models (also known as general circulation models or GCMs), each with two greenhouse-gas (GHG) scenarios: Representative Concentration Pathway 4.5 (RCP45) (low emissions) and RCP85 (high emissions) are used to project varying future climate conditions. GCMs include the Canadian Earth System Model (CanESM2), Centre National de Recherches Météorologiques Climate Model version 5 (CNRM-CM5), Hadley Centre Global Environment Model version 2 – Earth System (HadGEM2-ES), and Model for Interdisciplinary Research on Climate version 5 (MIROC5) models. RCP85 scenarios tend to be wetter and warmer than RCP45, and in some cases the increased precipitation offsets increased loss to ET. The Yucaipa model's simulated hydrological conditions using climate projections predict decreased groundwater in storage in most scenarios due to increased natural ET, vegetation consumptive use, and streamflow out of the watershed. Only scenarios with substantial increases in annual precipitation were able to maintain groundwater in storage approximately the same as at the end of the historical period. The study also highlights increased future aridity despite increased precipitation, and larger precipitation events, which increase the risk of urban floods and decrease stream leakage.
