Near-Term Global Warming Effect on Hetch Hetchy Inflow

Studies have shown that changes in winter air temperature have an effect on snow deposition and runoff patterns in the Sierra Nevada. Hetch Hetchy Reservoir, on the Tuolumne River at 3,806 feet, impounds 459 square miles of Yosemite National Park. Hetch Hetchy Water and Power (HHWP) uses a monthly runoff forecasting model that incorporates historical temperature, precipitation, and snowpack to predict monthly inflow. Given current conditions, the model estimates monthly runoff for the rest of the water year based on 48 years of historical climatology. An evaluation of an estimated 3 degrees F increase in temperature, assumed to occur by 2025, was performed. The historical daily maximum and minimum temperatures were increased by 3 degrees F, and the adjusted monthly runoff results were compared to the base run. On average, 7% of the annual runoff shifted from the April through June period to the November through March period. Less snowpack accumulated in the lower elevation zones due to the rise in the rain-snow line. The effect was proportionally larger for dry years than for wet years. Implications to water supply in the near term are minor, but long-term impacts that are coupled with increased demand are of concern.