Assessing the Effect of Climate Change in Two Snow-Dominated Mountainous Basins in the California Sierra Nevada Using CMIP5 data and the Soil and Water Assessment Tool (SWAT)

Abstract:

American River and Upper Merced basins are located on the snow-dominated area in the California’s Sierra Nevada mountain range. American River basin, located north of the Sierra Nevada, is 4,781.2 km² and snowmelt runoff from the basin feeds the Folsom Lake reservoir which is primarily used to generate hydroelectric power as part of the Central-Valley project. To the south of the mountain range, 5,252.8 km² Upper Merced basin is located and streamflow from the watershed is mainly used for irrigation. Because of the importance of the two basins to the state water resources, we have studied the future projected change in streamflow of the basins. The Soil and Water Assessment Tool (SWAT) model was calibrated (R² and NSE values of 0.93 and 0.91 at American River basin and 0.86 and 0.85 at Upper Merced River basin, respectively) to simulate the hydrologic response of the basins. Monthly precipitation and temperature data from the Centre National de RecherchésMeteorologiques (CNRM) and NOAA’s Geophysical Fluid Dynamics Laboratory (GFDL) outputs were collected from World Climate Research Programme’s (WCRP) downscaled and bias corrected CMIP5 dataset. The watersheds were studied using RCPs 4.5 and 8.5 future projections for 50 year period (2015-2065). Compared to the baseline (1965-1999), peak discharge in Upper Merced basin is projected to increase by 7% to 57% at the USGS stream gauge site located upstream in the basin. At the basin outlet, peak discharge in July decreases by 15% for RCP 4.5 scenariobut increases by 30% for RCP 8.5 scenarios. Winter streamflow increases for both RCPs 4.5 and 8.5 scenarios by 50-125%. At the American River basin,there is no change in projected summer streamflow but 30-68% increase is projected in winter streamflow at the basin outlet. Overall, the high elevated mountain zones are projected to have small change compared to the lower elevated downstream zones of the basins. Based on the selected GCM outputs from the CMIP5 dataset, winter precipitation and temperature will increase in the watersheds which will cause significant change in temporal distribution of the regions’ water resource.