Sonoma County's effort to preserve the dwindling water in Lake Mendocino could involve a major revision of how the reservoir is managed.

The Sonoma County Water Agency is asking the State Water Resources Control Board for permission to cut flow from the lake below 75 cubic feet per second, the normal minimum for a dry year.

But instead of simply asking the state to allow a lower flow, as it has done in the past, the agency is seeking to break with the half-century-old way that drought conditions are analyzed in order to set flow rates.

Since at least the 1960s, the state has used flow rates on the nearby Eel River to determine whether a year is "normal," "dry" or "critical" in the headwaters of the Russian River. The designation determines how much water the agency is obligated to release downriver to protect agriculture, fish spawning grounds, recreational uses and drinking water for cities such as Healdsburg, Cloverdale and Ukiah.

In a petition delivered to the state Thursday, the agency asks for the determination of the flow rate to be based instead directly on the water level in Lake Mendocino, which is today only around 30-percent full with no prospect of significant rain in the forecast over the next several weeks.

"You really are beginning to focus the attention on the storage at Lake Mendocino, where it belongs," agency General Manager Grant Davis said. "That's the good news, that we are focusing in on the area of vulnerability."

Should the state grant the request, it would change the management of the dam for only as long as 180 days. If the system works out well, the agency plans to request that the system be made permanent.

The change may seem arcane and technical, but it has important implications for the future water supply on the upper Russian River, agency officials say. The conditions in the Eel River have become an increasingly unreliable guide to conditions in the Russian River over the years, and basing management decisions on real conditions in Lake Mendocino would give the agency much greater control over its own water resources.

The Water Resources Control Board did not comment on the details of the plan by Thursday afternoon, but in general it is open to considering long-term changes in the way the dam is managed, provided the agency can make a case for its idea, said Amanda Montgomery, manager of permitting and licensing for the board.

It appears the board originally linked the management of Lake Mendocino to conditions on the Eel River because of a century-old tunnel that links Lake Pillsbury, a reservoir on the Eel, with the upper reaches of the Russian River. PG&E sends water down that tunnel to run a power turbine and the water winds up in the Russian River.

At one time, that was a significant source of water for Lake Mendocino, but a 2004 change in PG&E's license cut that back dramatically. Just this week, PG&E secured federal permission to slash the flow even further, from 35 cubic feet per second to just 5, a move designed to conserve water in Lake Pillsbury, which is in even worse condition than Lake Mendocino after two extremely dry years.

But even as Lake Pillsbury faded as a source of water for Lake Mendocino, the decisions on how to manage the dam remained linked to conditions on the upper Eel River, an indicator that is "is no longer so reliable," agency Assistant General Manager Pam Jeane said.

In addition to seeking state permission to cut flows from Lake Mendocino, the Water Agency is going to launch an unusual wintertime drive to urge consumers to save water. The larger reservoir at Lake Sonoma is still about 70 percent full, but the agency is worried that another year of drought could put pressure on that water source, too.

The agency will advertise "an aggressive water reduction, to make sure you're checking your leaks, making sure you're doing everything you can to use water wisely," Davis said. "And it's a year-round thing; I just don't see a day when you can say this is seasonal."

While it won't help ease the current drought, the agency is looking at longer-term ways to conserve water as well, such as reconfiguring its system of detention ponds to help recharge ground water, injecting excess water into aquifers during rainy periods to preserve supplies deep underground, and marketing treated wastewater to farmers and other users to slow down the rate at which they irrigate using fresh well water.

The agency also is updating its planning models for the upper Russian River, taking a look at who is using the water, long-term growth plans of communities that rely on the river, and the emerging models for what effect climate change will have on rainfall patterns. That report is due to the state by the end of 2014.

"We're entering into an era of increasing uncertainty," Davis said. "We have to plan for extreme droughts and floods over multiple years, and be able to manage our water supply under those conditions."

You can reach Staff Writer Sean Scully at 521-5313 <QA0>or sean.scully@pressdemocrat.com. On Twitter @BeerCountry.