Weather scientists lend hand to safeguard Lake Mendocino water supply

The operators of Lake Mendocino have begun making flood releases for the first time in more than three years in anticipation of drenching storms expected to dump several inches of rain on Northern California starting this weekend.

The U.S. Army Corps of Engineers began making additional space in the reservoir early Wednesday, discharging more than five times the minimum outflow maintained in recent weeks in the Russian River.

The increased releases - though minimal by historic standards - mark a turning point for a region in which catastrophic drought has prompted water managers to hoard limited water behind Coyote Dam.

The Army Corps still plans to store more water in the lake than it is required to at this point in the year. But its priority and mission are flood protection of those downstream, said Mike Dillabough, chief of operations and readiness for the Army Corps’ San Francisco Division.

“The current weather prediction right now is that we’re going to have a heavy storm on Saturday,” he said.

He added that the current flood releases were short-term and would be restricted again beginning Friday until after the weekend rain, when the release schedule will be guided by the rate of incoming runoff and weather forecast.

Lake Mendocino on Thursday was at more than 100 percent of capacity for this time of year, holding more than 79,000-acre-feet, about 10,000-acre-feet above the winter-season threshold at which the federal agency is authorized to make flood releases. An acre foot equals 326,000 gallons, enough to flood a football field about a foot deep in water.

After March 1, that storage limit starts inching up by about 500 acre feet a day in recognition of the diminishing probability of severe winter storms, giving the Army Corps extra flexibility to hold additional water.

The flood releases this week followed a move by Sonoma County supervisors to formalize the county Water Agency’s leadership role in a groundbreaking, regional effort to refine extreme weather forecasting and develop strategies for more proactive reservoir management, aided by nearly $20 million in new state grant funding.

The project continues ongoing research into the moisture-laden storm systems known as atmospheric rivers and seeks to develop new tools that could help reservoir operators more efficiently safeguard water supplies while preserving flood capacity.

“Knowing when these (storms) are going to hit, and how they are going to hit and the type of precipitation, is critical,” Supervisor Shirlee Zane said during a review of the project Tuesday.

The Water Agency and its partners have been working for several years to better understand atmospheric rivers, which take shape as narrow storm bands that transport large amounts of moisture into regions like the Bay Area.

Unlike areas of California’s Central Valley, which are dependent on snow pack for water supply, much of Sonoma County’s drinking water is delivered to the region in the form of atmospheric rivers and held back in Lake Sonoma and Lake Mendocino, according to research meteorologist Marty Ralph, director of the Center for Western Weather and Water Extremes at UC San Diego’s Scripps Institution of Oceanography.

The density of atmospheric rivers in a given season, as well as their timing, determines whether there is drought, flooding or something in between, he said.

“We are situated in a land of high-frequency droughts and floods,” said Jay Jasperse, the Water Agency’s chief engineer. Atmospheric rivers “are primary drivers of floods and droughts.”

Lake Sonoma, the region’s largest reservoir, with more than three times the storage of Lake Mendocino, was at more than 96 percent of seasonal capacity on Thursday.

The new science push includes two main initiatives.

One, the Advanced Quantitative Precipitation Information Project, involves the placement of five specialized radar units around the North Bay, as well as other infrastructure, to target low-elevation atmospheric rivers and develop weather modeling focused on 12- and 48-hour lead times for flooding and other events, officials said.

It is being funded by state bonds approved by California voters a decade ago and awarded to the San Francisco Bay Area Integrated Regional Water Management Plan, which received a total $41.3 million in the round of funding announced in January.

Better predictive capability “has huge implications flood control, as well as water supply and sanitation issues” around the region, said Grant Davis, general manager of the Sonoma County Water Agency.

The second initiative, called the Forecast-Informed Reservoir Operations, aims to update the science on which water storage decisions are made.

It is considered a demonstration project that could be replicated elsewhere in California, where the Army Corps operates about half of the dams.

The work is informed by an incident in December 2012, when a series of storms nearly filled Lake Mendocino, prompting mandatory releases to make room for additional rainfall that never came and setting the stage for months in which it was feared the reservoir would run dry.

More than 25,000 acre feet of water was lost, Zane said Tuesday, “about $5 million of water supply in a time of horrible drought.”

Dillabough noted Thursday that those discharges occurred early in the season, when the forecast at the time called for “a wetter-than-average winter.”

“It was our last major flood release,” he said. “In fact, that was the only water flood release…. This is the highest the water has been since then.”

Dillabough also said that until the forecast research bears scientific proof, the Army Corps would continue its current management of Lake Mendocino.

Water Agency staffers and county officials, however, said they believe there is room to improve upon operational rules adopted in 1959 and based on 1950s science.

Ralph said he believed up to 20,000-acre-feet a year, on average, could be saved from going through the flood gates unnecessarily.

“We have an unbelievable sort of A-team on this technical topic,” Ralph told county supervisors, “and it is focused right here in Sonoma County on Lake Mendocino.”

You can reach Staff Writer Mary Callahan at 521-5249 or mary.callahan@pressdemocrat.com. On Twitter @MaryCallahanB.