Quantifying climate change impacts to City of Santa Barbara water supplies

Climate change affects water supply through changes in precipitation, temperature, and evapotranspiration. The City of Santa Barbara's water supply relies largely on water held in Lake Cachuma and Gibraltar Reservoir, located in the upper Santa Ynez River watershed. Quantifying climate change impacts to this watershed is critical to planning for future water supply. This project modeled the potential impacts of climate change on the Santa Ynez River watershed out to 2058 using the Soil and Water Assessment Tool. The model was calibrated using meteorological and streamflow data for the upper Santa Ynez River watershed. Future streamflow was then simulated for the high-emissions scenario (RCP 8.5) using five global climate models which produce a range of possible futures for California. The simulated streamflow was evaluated to estimate a range of future upper Santa Ynez River watershed inflows to Lake Cachuma under the different climate simulations. The results of the drought model simulation indicate: (1) a projected decrease of approximately 40% in average streamflow and (2) a decrease in the contribution of the upper Santa Ynez River watershed to Lake Cachuma by as much as a factor of 2 compared to the historical baseline. Compared to the historical baseline, the other four climate conditions that were simulated produced changes in average streamflow, from -20% to +20%, and upper Santa Ynez River watershed contributions to Lake Cachuma, from 0% to +20%. The results from the simulations will inform the City of Santa Barbara's water supply planning out to 2050.