Earlier this year, the SFPUC completed
a report on the potential changes (increase or decrease) in Tuolumne River inflows to Hetch Hetchy Reservoir with anticipated changes in climate. The study used modeled temperature and precipitation data from a range of previously modeled climate change scenarios. While the likelihood of any particular climate future was not assessed, and the report doesn't seek to address potential water supply impacts of climate change, the simulated 2040, 2070, and 2100 hydrologic conditions show a progressively altered snow and runoff regime in the watershed. Snow accumulation is reduced and snow melts earlier in the spring. Fall and early winter runoff increases while late spring and summer runoff decreases, and these changes become more significant at the later time periods. Total runoff is projected to decrease under the climate change scenarios evaluated, in some cases marginally and others significantly. The consequences of these potential changes for UTREP efforts is unknown at this time, but will be considered in our ongoing instream flow work.
