San Francisco's plan to take more water from the Tuolumne River -- especially in dry years -- faces a challenge from the Modesto and Turlock irrigation districts.
MID and TID leaders suspect faulty assumptions may have led the San Francisco Public Utilities Commission to overstate how much river water would be available to divert during a drought year.
"That's probably our biggest concern," the MID's Walt Ward said Wednesday, "the assumption that 27,000 acre-feet can be diverted from the river (even) during dry years."
That is about 24 million gallons a day.
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MID officials are confused, saying that there is no extra water flowing in the Tuolumne, especially in a drought year.
Diverting more water by 2030 -- San Francisco already diverts 225 million gallons a day from the Tuolumne -- is a key part of a $4.3 billion San Francisco Public Utilities Commission plan to refurbish and rebuild its Hetch Hetchy water conveyance system.
San Francisco's plan includes adding new pipeline segments and rebuilding others to help the water system better withstand earthquakes.
TID spokeswoman Jennifer Stone said both irrigation districts "strongly support" San Francisco's efforts to replace aging pipelines and other water infrastructure, as well as proposed earthquake retrofits.
But Ward, the MID's assistant general manager of water operations, said a lot more information is needed about San Francisco's diversion plan.
Ward said the MID and TID have been seeking San Francisco's computer modeling data for months without success.
He said that request was renewed Monday in formal written comments on San Francisco's improvement plan the districts sent to the San Francisco Planning Department.
SFPUC spokesman Tony Winnicker said Wednesday the in-formation requested would be shared with both districts as soon as possible.
Winnicker said an "unfortunate miscommunication" between San Francisco and the districts apparently led to the computer modeling data not being made available sooner.
While San Francisco may have a legal right to divert more water from the upper Tuolumne, the TID's Robert Nees said Wednesday, federal law requires the SFPUC to exhaust other potential water sources first.
"We think," said Nees, the TID's assistant general manager of water resources and regulatory affairs, "that they need to give more consideration to alternatives."
His point was underscored in the written comments.
"San Francisco's water sources of choice should be water supplies close to San Francisco and alternative resources," the districts said, "not additional diversions from the Tuolumne River."
The districts go on to say that San Francisco's draft program environmental impact report "does not give adequate attention to local water supplies nor to alternatives such as desalinization, advanced treatment processes, water conservation or reclamation (water recycling).
"San Francisco should turn to these sources first to meet its own and the future needs of its wholesale water customers."
Winnicker said the districts' concerns would be addressed in writing, and there will be another chance to comment in May 2008, when the final report is made public.
Nearly 2.5 million people in San Francisco, San Mateo, Santa Clara and Alameda counties depend on Tuolumne water. It's moved from the mountains in Yosemite National Park to the Bay Area though a network of pipes and tunnels.
The federal Raker Act, approved by Congress in the early 20th century, granted San Francisco the right to take some Tuolumne water.
But the law also preserved rights held by the TID and MID to draw water from the river.
If, at the end of the day, San Francisco is able to prove that taking more Tuolumne water is its only viable alternative, Nees and Ward said, it would be better to make the diversion from the lower Tuolumne.
Diverting water near the Tuolumne's confluence with the San Joaquin River, they said, would ensure the Tuolumne has enough water flow- ing through it to sustain salmon and other aquatic life.
Bee staff writer Michael G. Mooney can be reached at email@example.com or 578-2384.