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Dam operators are planning to store nearly 4 billion extra gallons of water this winter in Lake Mendocino, the reservoir near Ukiah that plays a critical role in providing water for residents, ranchers and fish along the upper Russian River and to communities in Sonoma and Marin counties.

Retaining that much more water — enough for about 97,000 people for a year — comes about as a four-year and $10 million program, proven in computer models but not in practice, gets its first field test.

The program, blending high-tech weather forecasting with novel computer programming, is intended to pinpoint the arrival of rain-rich atmospheric rivers that have been both a drought-busting blessing and a flood-causing curse to the Russian River region.

It evolved from a searing lesson water managers got six years ago, when they released more than a third of the reservoir’s allowed capacity in anticipation of storms that never arrived. Then the state’s prolonged drought set in.

Under the new program, called Forecast Informed Reservoir Operation, or FIRO, the Army Corps of Engineers, which operates the reservoir, will hold onto the extra water as long as no atmospheric river is imminent.

Should a drenching storm loom, that water will be released, enabling Lake Mendocino to capture the new runoff and control flooding, the mission it was built to serve 60 years ago.

The goal is to head into summer with as much water in Lake Mendocino as possible. Army Corps officials, the phalanx of scientists who developed FIRO at a branch of UC San Diego and Sonoma Water, are confident it will work.

“We want to capture more of what Mother Nature gives us,” said Jay Jasperse, chief engineer at Sonoma Water, the county agency that supplies water to 600,000 Sonoma and Marin residents. That must be done without compromising public safety from floods, he said.

To the Army Corps, the allowance for extra water is a “deviation” from the water control manual published in 1959, a year after Coyote Valley Dam was built on the East Fork of the Russian River, creating the reservoir.

The manual imposed a rule that Lake Mendocino could not hold more than 68,000 acre-feet of water from Nov. 1 through the end of February, preserving capacity to accommodate an additional 42,000 acre-feet. An acre-foot is 326,000 gallons, enough to supply a family of four for a year.

In December 2012, when an atmospheric river dumped an unwanted 25,000 acre-feet of water into the reservoir, officials played it by the book, dumping the water down the river while no storms were on the horizon.

Unfortunately, the end of that storm marked the beginning of a prolonged drought that dropped Lake Mendocino to a mere 25,000 acre-feet roughly a year later.

Jasperse called that experience the “catalyzing event” that ultimately created the FIRO program.

Nick Malasavage, chief of operations and readiness at the corps office in San Francisco, said the program’s trial run, approved by top brass early last month, “allows us the flexibility to leverage some of the newer technology in weather forecasting.”

Computer modeling backs him up, but Malasavage acknowledged FIRO is “a machine that has yet to be road tested. We still haven’t gotten behind the wheel.”

Marty Ralph, director of the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography, heads the cadre of about three dozen scientists — meteorologists, oceanographers, climatologists, engineers, economists and more — that did the program’s technological heavy lifting.

Computers, satellites, wind- and rain-measuring radar, weather balloons and sensors on land and sea give FIRO a forecasting capacity that didn’t exist when the Lake Mendocino water manual was written in the 1950s, he said.

There’s far more than scientific achievement at stake, however.

Janet Pauli, a Potter Valley rancher, said ranchers and residents on the river’s upper reach hope to avoid the hardships imposed by the low water in 2013.

An economic analysis pegged the total value of Mendocino County agriculture based on Russian River water, including grapes and other crops, wine, jobs and taxes, at $743 million a year, she said.

More than 55,000 people are served by water districts and municipal utilities from Redwood Valley to Healdsburg and nearly 43,000 acres of cropland depend on water from the river’s upper reach, according to a Sonoma Water report.

The famed vineyards of Alexander Valley north of Healdsburg are also river-dependent.

Tito Sasaki, a Sonoma Valley grape grower who recently chaired the Sonoma County Farm Bureau’s Water Committee, said FIRO will “prevent wasteful discharge of water from Lake Mendocino” and enable the area to cope with lengthy dry spells.

“We cannot afford to waste a drop of water just to follow an outdated flood precaution rule,” he said.

Water from Lake Mendocino plays a critical role in Sonoma Water’s mandate to maintain minimum flows on the upper river to sustain imperiled salmon and steelhead trout runs, Jasperse said.

Releases from Lake Sonoma, the region’s largest reservoir, travel down Dry Creek and reach the river at Healdsburg. Pumps near Forestville draw water from riverside gravel beds for delivery to 600,000 Marin and Sonoma county residents.

Ultimately, the new forecasting program is an effort to track atmospheric rivers, long bands of moisture moving in from the Pacific Ocean that deliver about half of the Russian River region’s precipitation and make or break a rain season.

Research shows they have caused most of the floods on the river since 1948, while also breaking more than half of the droughts since 1950.

Coyote Valley Dam was authorized by Congress in the wake of a catastrophic 1937 flood on the Russian River. The 136-foot earthen dam, completed two decades later, was the region’s first major flood control facility.

You can reach Staff Writer Guy Kovner at 707-521-5457.

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