Restoring Hetch Hetchy: The Cherry Solution
Report Notes and Modeling Methodology
- The Cherry Intertie is evaluated by comparative simulations of the TREWSSIM (Tuolumne River Equivalent Water Supply Simulation) model, which was originally developed by the Environmental Defense Fund for Paradise Regained: Solutions for Restoring Yosemite’s Hetch Hetchy Valley (2004). TREWSSIM is based on publicly available data and documents provided by San Francisco, including:
- Water Supply System Modeling Report, Hetch Hetchy / Local Simulation Model (SFPUC, 2007)
- Hetch Hetchy / Local Simulation Model (HHSM-LSM) output – see example with with system demand of 249 TAF. (Note the SFPUC uses various units for water volumes, including millions of gallons per day, millions of gallons and acre-feet. The TREWSSIM model primarily uses only thousands of acre-feet (TAF) to measure water volumes.)
- The simulations in Restoring Hetch Hetchy: The Cherry Solution assume an annual demand for 220 TAF for the San Francisco Regional Water System – the average of actual deliveries between 2012 and 2022 (a 19% drop from the previous decade). See SAN FRANCISCO PUBLIC UTILITIES COMMISSION
ALTERNATIVE WATER SUPPLY PLAN FEBRUARY 2024. Total use in 2024-2025 was even lower – 209 TAF. - Current instream flow requirements below Don Pedro Reservoir are expected to change after years of study, debate and legal action. The California State Water Board has recommended substantial increases to protect and enhance fisheries, a proposal supported by many conservation and fishing groups. Other state agencies have sided with San Francisco and the Turlock and Modesto Irrigation Districts who have proposed a “Voluntary Agreement“, that would provide minimal increases in flow as well as habitat and other non-flow measures designed to help the fishery. Restoring Hetch Hetchy: The Cherry Solution assumes these Voluntary Agreement flows will be implemented – requiring an average additional flow of 28 TAF in the downstream reaches of the Tuolumne River. Under the terms of the fourth agreement between the Districts and San Francisco, the SFRWS would be responsible for 51.7% of the increase, or 15 TAF per year.
- Restoring Hetch Hetchy: The Cherry Solution proposes an intertie between Cherry Reservoir and San Francisco’s Mountain Tunnel as well modifying the Canyon Tunnel so the Tuolumne River can be diverted when its natural flow is sufficient large. TREWWSIM, like most water system models, operates on a monthly basis and uses monthly hydrology data from 1922 through 1994. Because daily flow data is not available for this time period, TREWSSIM employs regression analysis, based on daily data for the years 1971-2010 (provided ot the Federal Energy Regulatory Commission), to estimate what volume of daily flows could be diverted into the Canyon and Mountain Tunnels. The regression analysis, including the parameters used to determine what fraction of monthly natural flows at Hetch Hetchy could be diverted into the Canyon and Mountain Tunnels, is provided in Monthly Diversion Regressions for Hetch Hetchy Daily Natural Flows.
- Summary Data comparing the Current Operations Scenario and the Cherry Solution Scenario is posted online here.
Inquiries can be sent to restoration@hetchhetchy.org.