Advance in storm forecasting allows Lake Mendocino to hold more winter runoff
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.”