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SFPUC hydrologic modeling finds runoff declines in most climate scenarios; supply impacts to be modeled
Summary
SFPUC staff told commissioners that a new hydrologic sensitivity analysis of Hetch Hetchy runoff shows declines in most scenarios, earlier runoff timing and substantial snow‑storage losses later in the century; operations modeling to translate runoff changes into water‑supply impacts is scheduled for summer 2012.
SAN FRANCISCO — The San Francisco Public Utilities Commission heard on the record that a newly calibrated hydrologic model for the Hetch Hetchy watershed projects lower runoff in nearly all examined climate scenarios and a shift in runoff timing that could reduce late‑spring and summer flows.
David Behar, Climate Program director for the Water Enterprise, summarized a sensitivity analysis based on six climate scenarios and said: "Runoff declines in all of the scenarios that we tested but 1." He added that snow storage is expected to fall significantly later in the century and that combined temperature increases with precipitation decreases produce the largest reductions in runoff.
Behar said the study was a preliminary, watershed‑level analysis and was not intended to deliver direct water‑supply projections. "The next steps in this analysis where we run these stream flow numbers through our HHLSM operations model ... will tell us what the potential water supply impact will be," he told the commission, and staff have scheduled that operations modeling for completion in summer 2012. The comprehensive climate‑change assessment that will follow is expected to take roughly 18 months once it begins.
Commissioners and staff stressed the analysis' limitations and the value of a staged approach. The report tested low, moderate and high temperature increases, and a range of precipitation outcomes; some global climate models used show precipitation declines of about 15% toward the end of the century under the drier scenarios considered.
Public commenters and environmental stakeholders praised the modeling and urged the commission to continue no‑regrets investments — work staff described as improving system resilience regardless of which scenario occurs. The commission did not adopt new policy at the meeting; it directed staff to continue the modeling and to return with the HHLSM results and subsequent recommendations.
