Lake Sonoma, Lake Mendocino get runoff boost from recent storms

Drought-weary North Bay residents are getting an early holiday present from Mother Nature as sporadic December storms have boosted the water level in the region’s largest reservoir and set the stage for bigger gains if rain continues to fall.

Lake Sonoma, the key source of water for 600,000 customers in Sonoma and Marin counties, hit 66.9 percent of water storage capacity Thursday, up two-tenths of a point since Dec. 1 - a small but significant increase that marked the reservoir’s first uptick since June 30, when it was 81.7 percent full.

Water managers call that an “inflection point,” and Jay Jasperse, the Sonoma County Water Agency’s chief engineer, said it is “good news,” with better news possibly on the way.

The storms that have dropped just over 5 inches of rain in the Santa Rosa basin since Oct. 1 have essentially saturated the near-surface soil, allowing water to run off into reservoirs, a trend that appears to be happening to California’s largest reservoirs, as well.

“Runoff hasn’t been huge,” Jasperse said, declaring he is “cautiously optimistic” that soggy soil will keep most of the upcoming rain on the surface and draining into reservoirs.

More rain is expected Friday, over the weekend and continuing next week, consulting meteorologist Jan Null said. “A wet pattern is setting up,” he said, with no downpours but “consistent rain through the end of the year.”

The Water Agency’s measurement for the Santa Rosa basin remains well below the 8.14-inch average for the rainy season that began Oct. 1. Rainfall in the Ukiah basin is at 6.5 inches compared with a 9.5-inch average for mid-December.

Last year brought a far more dramatic boost to Lake Sonoma from a heavy storm called an atmospheric river that pumped the reservoir up to 73 percent of capacity on Dec. 17, up from about 56 percent at the beginning of the month. By the end of December the reservoir west of Healdsburg was 78 percent full, but there was only one other major storm in the 2014-15 water year as California forged into a fourth year of drought.

Null said there are no atmospheric rivers in the offing, and Jasperse noted that the drought-busting potential from a strong El Niño is more likely to arrive in January and February.

But continued rain will keep wetting the soil, which is still drawing water to greater depths - down to about 40 inches - like a sponge, Jasperse said.

Assuming the precipitation continues, Jasperse said he expects a “sharper hydrological response” with more water reaching reservoirs. Once past the inflection point, he said, the response “jumps up pretty sharply.”

Lake Mendocino near Ukiah made a more modest uptick, rising to 39.56 percent of capacity on Tuesday, up from 39.51 percent on Dec. 1, but the shallow reservoir was back below that level Thursday.

The reservoir, which provides water to the Ukiah area and northern Sonoma County, registered its first increase last week after a steady decline since June 30.

California’s three largest reservoirs - Shasta, Trinity and Oroville - also rose in December due to rainfall and runoff. “The turnaround occurs each year about this time,” Department of Water Resources spokesman Doug Carlson said.

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