High-tech forecasting model scores scientific win at Lake Mendocino, showing promise for western reservoirs

Sandbars are spreading across rain-starved Lake Mendocino, the reservoir near Ukiah that is 35 feet lower than it was a year ago, a grim wintertime sight for the second major source of water for more than 655,000 people in Sonoma, Mendocino and Marin counties.

But the situation would be considerably worse without the payoff from a six-year, $50 million project applying high-tech weather forecasting to management of the reservoir behind Coyote Valley Dam built on the East Fork of the Russian River in 1958.

Thanks to the project, which replaces an inflexible dam operations manual with meteorological science unknown six decades ago, there is 20% more water — nearly 12,000 acre feet — now in Lake Mendocino as the North Bay teeters on the brink of drought.

“Imagine what would have been without this,” said Grant Davis, head of Sonoma Water, the agency that supplies water to 600,000 Sonoma and Marin county residents and another 55,000 people from Mendocino County’s Redwood Valley to Healdsburg.

A 141-page report released Thursday by Sonoma Water and seven partners provides “proof of concept” for Lake Mendocino’s Forecast Informed Reservoir Operations program, known as FIRO, Davis said.

The program’s success comes as climate change is “exacerbating the feast-or-famine nature of precipitation,” according to Sonoma Water, complicating work in its field and imperiling the supplies that go to tens of millions of residents and agricultural users across the water-scarce western United States.

Already, the model blending high-tech weather forecasting with new dam operations is under consideration at four other West Coast reservoirs, Sonoma Water said in a statement.

With forecast-based management, “we can store more water than we would’ve otherwise and we can do it safely,” Alan Haynes, hydrologist in charge at the California-Nevada River Forecast Center, a National Weather Service agency, said in the statement Thursday.

Future enhancement of FIRO — dubbed FIRO 2.0 — “will be important to further improving water supply and adapting to a changing climate,” the report said.

Applied to reservoir operations the past two years at Lake Mendocino, the smaller of Russian River basin’s two main reservoirs, FIRO uses technology — computers, satellites, wind- and rain-measuring radar, weather balloons and sensors on land and sea — to track atmospheric rivers, the megastorms that can carry as much water vapor as the Amazon River and typically provide nearly half of California’s precipitation.

With a Jekyll-and-Hyde capacity for dousing droughts and triggering floods, atmospheric rivers are at the heart of the North Coast’s increasingly variable climate, exacerbated by climate change, the report said.

The decades-old dam operations manual for Lake Mendocino capped wintertime water storage at 68,400 acre feet, or 22 billion gallons, with no regard for daily weather forecasting and far below the summertime allowance of 116,000 acre feet.

The idea was to assure sufficient capacity to control flooding from a deluge, and FIRO was borne from the bitter experience of December 2012, when an atmospheric river dumped 25,000 acre feet of water into Lake Mendocino.

Adhering to the manual, officials at the time dumped the water down the Russian River, but the storm’s end proved to be the start of California’s prolonged drought that dropped the lake in 2013 to 25,000 acre feet, about 3,000 acre feet below where it sits today.

“We said we’ve got to do better,” Davis said.

Work on FIRO began at a time when atmospheric rivers were known almost exclusively to scientists and not the public or government officials, he said.

Davis recalled a meeting that he and two other water experts had with California Sen. Dianne Feinstein about six years ago, explaining the harm caused by such megastorms in the Russian River watershed.

Knowing beforehand whether and when those storms will come, and the amount of moisture they could carry, gives water managers the option to retain far more of winter and early spring supply at Lake Mendocino — about 11,625 acre feet more than the dam manual allowed. (An acre foot is about the same amount of water that would cover a football field up to 1 foot deep and is generally enough water to sustain a family of four for a year.)

Accurate forecasting of atmospheric rivers allows time to lower the reservoir, if needed, before the rain arrives and retain water when no major storm is imminent.

Sonoma Water and the U.S. Army Corps of Engineers, which jointly operate Lake Mendocino, turned to a cadre of scientists at UC San Diego to devise the forecasting technology.

Their work passed its test in “two very different years,” Davis said, noting the 2019-20 rain year was wet, while the current year, which started in October, is the third driest in 127 years.

Marty Ralph, director of the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography, which developed the FIRO program, said in a statement it “demonstrated the ability to apply science to save water, which is essential to California given how common droughts are in the region.”

Sonoma County Board of Supervisors Chair Lynda Hopkins in a statement called FIRO a water management breakthrough that “demonstrates significant regional benefits for people, the environment and the economy.”

An assessment funded in part by the U.S. Bureau of Reclamation estimated FIRO would yield annual benefits of $9.4 million to agricultural, municipal and industrial water supplies. Recreation and wildlife habitat stand to gain as well, including $5.7 million in annual benefits to fisheries, based on the cost of raising Coyote Valley Dam as an alternative to providing suitable conditions for federally protected fish in the upper Russian River.

Josh Fuller, a biologist with the National Marine Fisheries Service in Santa Rosa, hailed the multiagency collaboration on FIRO, calling it a “solution in a watershed known for drought, flooding and endangered species.”

The project’s $50 million cost was funded by federal and state grants and a $5 million investment by Sonoma Water, Davis said.

FIRO’s success at Lake Mendocino has “implications throughout the West” in other settings where “atmospheric rivers are dominant,” Sonoma Water’s statement said.

The program is under consideration at Prado Reservoir in Southern California, New Bullards Bar Reservoir in Yuba County and at Lake Oroville in Butte County, the second largest reservoir in the state, as well as the Howard Hanson Dam near Seattle.

Meanwhile, the first atmospheric river of the winter had a minimal impact on North Bay water supplies and left ranchers, water managers and most residents yearning for more.

Lake Mendocino was left at 41% of targeted capacity, with Lake Sonoma northwest of Healdsburg, the region’s major water source, at 64% of capacity and 30 feet lower than a year ago.

About a mile of muddy sediment lies between the boat ramp at Lake Mendocino’s north end and the water line.

Last week’s sustained 72-hour storm, which dropped 2 to 3 inches of rain over the region, was rated a “weak” atmospheric river in the Russian River area, Ralph said.

The storm from Tuesday night through Friday brought 2.17 inches of rain to the Charles M. Schulz-Sonoma County Airport, boosting the total for the rain year starting Oct. 1 from 31% to 41% of normal through Jan. 26, the National Weather Service said.

It left Santa Rosa with just under 9 inches of rain Tuesday, far below the 21-inch average for the period about halfway through winter.

Cloverdale at the county’s north end gained 4.13 inches, while Petaluma at the dry southern end of the county gained 1.74 inches.

Venado, in the hills 10 miles west of Healdsburg heralded as the county’s soggiest spot, gained 8.04 inches during the same period. Known for 100-inch rain years, Venado was at 20.8 inches since Oct. 1, the weather service said.

“It definitely helped,” said Joe Pozzi, a Valley Ford cattle and sheep rancher. The rain will help get grass growing on county farmlands as the days grow longer, making livestock ranchers “better off than we were a week ago,” he said.

But the rain still hasn’t sufficiently saturated the soil to yield runoff that would replenish farm ponds and natural springs, Pozzi said.

“We’re getting to the point where we’re depending on that one big storm,” he said.

A warming and drying trend that began Thursday is expected to continue through the weekend in the Bay Area, with daytime highs in the 60s, the weather service said.

Accuweather, a private company that does long-range weather forecasting, sees no rain in Santa Rosa until the last day of February and less than 2 inches in March.

You can reach Staff Writer Guy Kovner at 707-521-5457 or guy.kovner@pressdemocrat.com. On Twitter @guykovner.