LAUSR

Abstract Since 2002, the Tahoe Environmental Research Center has been studying the effects of climate change in the Tahoe Basin. We are using the output from four General Circulation Models downscaled by the method of Localized Constructed Analogues and provided by the Scripps Institution of Oceanography (SIO). The daily data were used by SIO to drive the Variable Infiltration Capacity (VIC) model and derive a suite of 24 hydrologic and climate variables at a 1/16° (ca. 6 km) grid scale, for two Representative Concentration Pathways (RCP 4.5 and 8.5). Here we focus on trends in the return levels of extreme climatic events, including annual maximum daily discharge of six basin streams, maximum and total annual kinetic energy (KE) of raindrops falling on snow-free ground, basin-wide climatic water deficit, and wind speed. To analyze time trends in historic and modeled future extreme values, we applied the program extRemes, based on the Generalized Extreme Value (GEV) distribution. Values of KE on snow-free ground were derived by statistically disaggregating daily rainfall to hourly, and using literature values to convert rainfall intensity (mm/hr) for days without snowpack to KE (Joules/m2/hr). We found strong upward trends in extreme values, with most of the effect due to loss of snowpack, but a significant effect of increasing rainfall for one model. To estimate future trends in maximum annual daily discharge of six streams, we adjusted the modeled future annual maximum runoff to the distribution of gage data, using regression of gage data on modeled historic runoff at equal return intervals. The GEV results for the six streams, averaged across the four models, indicate an increase in the 20-year flood of 65–117 percent. Climatic water deficit showed strong upward trends for three of the four models, with a maximum at mid-century for one model. Averaged across the basin and across the four models, average and maximum seasonal winds under RCP 8.5 are projected to decrease slightly in all seasons. The trends in averages and extreme values that we found will have important effects on vegetation, wildfire severity, flood hazards and the clarity of Lake Tahoe.