The Sierra Club has sued the San Francisco Public Utilities Commission, alleging the agency overstated future water demand in a plan that helps guide the region’s water decisions and state planning. The lawsuit, filed last week in San Francisco Superior Court, seeks revisions to the agency’s Urban Water Management Plan, which was completed last year. The Sierra Club says the plan relies on inflated assumptions about population growth and household water use, and that the agency set aside lower demand estimates from its financial department in favor of higher projections without adequately explaining why. The plan forecasts that water demand from San Francisco residents will rise 17% by 2050, while demand from the agency’s wholesale customers is expected to increase 14%. Together, the projected need is listed at 222 gallons per day. The Sierra Club contends the forecasts overstate demand by about one-third. Mark Shahinian, a member of the Sierra Club’s San Francisco Bay Area Chapter executive committee, said the projections could support new infrastructure and increased water storage and withdrawals. Higher capital costs could, in turn, contribute to rate increases, the lawsuit argues. The agency draws much of its water from the Tuolumne River in Yosemite National Park and supplies San Francisco as well as more than two dozen communities in San Mateo, Santa Clara and Alameda counties. Environmental groups have long argued that water stored at the city’s Hetch Hetchy Reservoir comes at a cost to salmon and the surrounding landscape. During peak runoff season, the Tuolumne’s average flow is about 21% of its natural level, according to the figures cited in the dispute; the river is also affected by withdrawals from other users. The SFPUC did not immediately respond to a request for comment on the lawsuit. The agency has generally defended its water planning as necessary to serve the Bay Area’s residents and economy. The legal challenge arrives as California debates new limits on river diversions through a proposed update to the Bay Delta Plan. The state-mandated water plans are overseen by the California Department of Water Resources. The lawsuit argues that alleged flaws in San Francisco’s plan violate the Urban Water Management Planning Act and should require the agency to revise it.