Lake Sonoma, Mendocino reservoirs ‘healthy’ at start of 2024

Sonoma County is starting out the new year much differently than it did 2023 considering the amount of water it has stored.

As of Tuesday, water levels in Lake Sonoma and Lake Mendocino, the region’s main reservoirs, were at 92% and 77%, respectively, according to Sonoma Water. That’s higher than most previous years.

Lake Sonoma is currently storing 234,742 acre-feet. Its storage capacity — not its overall capacity — is 381,000 acre-feet. And Lake Mendocino has 61,913 acre-feet with a storage capacity — not overall capacity — of 122,400 acre-feet.

Though considered good, the storage levels are not record breaking for the time of year, said Brad Sherwood, assistant general manager of the Sonoma County water agency. The levels lag behind those recorded in 2017.

Still, the numbers signify an optimism that was largely absent at the start of 2023 — that is, until a string of atmospheric rivers brought a deluge and contributed to ending the yearslong drought.

Water was released from the two reservoirs last year due to excess storage for the first time in about four years. The two facilities ended August 2023 with the highest combined storage level since 1985, the first full year after Lake Sonoma was constructed.

The solid footing this year is also due to the management of the reservoirs, Sherwood said. Which helped to preserve more of the water after the winter storms.

“It’s essentially not releasing water when you don’t have to,” he said about the management, adding that the techniques hinge on accurate forecasting. “You’re saving that water in the reservoir while still maintaining flood protection and water supply.”

The Forecast Informed Reservoir Operations helped to retain about 5,000 acre-feet of water from Oct.1, 2022, to Sept. 30, 2023 in Lake Mendocino. At Lake Sonoma, some of the same tactics preserved about 19,000 acre-feet of water.

In addition to the reservoirs, recycled water generated and stored by the city of Santa Rosa is at just about average levels for the year, said Mike Prinz, deputy director of Regional Water Reuse Operations.

Water that goes through the city’s treatment plant is mainly used for irrigation, industry and agriculture — not drinking water. But it makes up a good slice of the water supply pie.

As of Tuesday, the city has about 600 million gallons of recycled water stored.

“I would say everything is hunky dory,” Prinz said. “Everything is just sort of normal.”

The picture is a bit less clear as to how much groundwater is being stored.

Sonoma County’s three groundwater sustainability agencies are still developing their data sets and are unaware of how storage levels this year compare to others, Sonoma Water spokesperson Andrea Rodriguez said.

However, between 2019 and 2022, the ground water stored in the Santa Rosa Plain subbasin declined by about 9,900 acre-feet, according to the 2022 groundwater annual report.

“The bottom line is that both reservoirs are healthy for this time of year,” Sherwood said.

For the reservoirs to stay in this good position, he said, a good amount of rainfall is still needed. The treatment system is also weather-dependent and produces more recycled water during wetter winters.

There have been a few storms since the start of the water year, Oct. 1, 2023, that have brought rainfall totals near the 30-year average, or what is considered “normal.”

A total of 11.56 inches of rain has fallen at the Charles M. Schulz-Sonoma County Airport in Santa Rosa since Oct. 1, just under 0.7 inches from normal. The bulk of that, over 8.2 inches, fell from Dec. 16 through Monday.

Another storm midweek is expected to bring another inch or so of rain to the North Bay valleys and about 2 inches to the coastal mountains, said Dylan Flynn, a meteorologist with the National Weather Service’s Monterey office.

Fortunately, the long-term forecast from the Climate Predication Center is pointing to rain totals “leaning above” normal for the next three months.

“For the rest of the winter, from January, February and into March, California and the Bay Area specifically is more likely to have an above normal year than a below normal year,” Flynn said.

While Sherwood said it is too early to tell what the rest of the winter and water year will like, he is hoping for the best.

“All we can be is optimistic that the storm door will remain open,” he said.

You can reach Staff Writer Madison Smalstig at madison.smalstig@pressdemocrat.com. On X, formerly Twitter, @madi.smals.