Napa’s main reservoirs see some refill from storms

Recent rainfall in California has started refilling reservoirs, and that includes Lake Hennessey, the city of Napa’s primary water source.

The reservoir holds up to 31,000 acre-feet of water — one acre-foot of water is equivalent to about 325,851 gallons. As of Jan. 4, Lake Hennessey was at 78.6% of its capacity, with 24,395 acre-feet, according to city data.

Water levels last year dropped to a low of 23,350 acre-feet, or about 75% capacity.

Joy Eldredge, the city’s deputy utilities director, said the recharge so far hasn’t been especially significant, but the storms have helped saturate the watershed. That means subsequent rain will take the form of runoff that will create a more significant refill.

“This is pretty typical for us,” she said. “It’s kind of a big footprint, so it takes a while for the ground to really become wet first, then we start to see some substantial runoff.”

Lake Hennessey’s water levels were higher at the same time last year, with the reservoir reaching 83% capacity on Jan. 4, 2023. But at that time the city was in the midst of a series of atmospheric rivers, and reservoir water levels shot above 100% storage capacity Jan. 9, causing water to flow down the reservoir spillway.

As a result of those storms filling reservoirs across the state and bringing heavy snow, California’s drought emergency — and Napa County’s own state of emergency — was lifted and Napa’s jurisdictions ended strict water-use restrictions that were in effect since 2021. Lake Hennessey remained at or over capacity until May 21, 2023.

Eldredge said the city is primed for more recharge so long as rainy conditions continue.

“As long as we have a couple consistent storms coming in and it continues to stay rainy even if they’re not big storms, we’ll continue to see recharge,” she said. “As long as we don’t see three weeks of straight sunshine and things dry back out, then it’s almost like we have to start over with saturated ground.”

Along with Lake Hennessey, the city of Napa also depends on water supplies from the State Water Project — the city has an annual allocation of up to 21,900 acre-feet from the state — and uses the local Milliken Reservoir, which stores up to 1,390 acre-feet of water, as a seasonal source.

The city’s been given a 10% allocation from the State Water Project so far this year, Eldredge said, and the city has carryover water from last year. Given that the city received a 100% allocation last year, and the city is allowed to carry 50% of that over to this year, the city is already starting the year with 10,500 acre-feet.

“The only thing that could change that is, operationally, the place where that water is technically stored is San Luis Reservoir, and so as long as San Luis doesn’t spill that carryover water is available,” Eldredge said.

The Milliken Reservoir started to spill this week, she added, through holes bored into the dam in 2008, 16 feet below the top, for seismic reasons.

“So that will be full for us as we start the year, and we’ll keep looking forward to a little more recharge in Hennessey,” Eldredge said. “Ideally we get Hennessey back up to a spill situation. We like that for water quality reasons, as well.”

You can reach Staff Writer Edward Booth at 707-521-5281 or edward.booth@pressdemocrat.com.