Marin water reservoirs spilling as summer nears

The Marin Municipal Water District basins have been full since late January, while Stafford Lake in Novato has been full since early February.

A rainy winter and early spring have replenished reservoirs in Marin County, but water suppliers remain cautious heading into the dry months.

Despite wet weather recently, April signals the waning of state’s rainy season, providing a clear picture of how much water the region has to work with until the rain returns in the fall.

Since October, Mount Tamalpais has received 41.47 inches of rain, according to Nicole Sarment, a meteorologist with the National Weather Service. Sarment said the number does not account for the recent snowfall reported on the peak on Thursday.

Marin Municipal Water District reservoirs, which serve most of the county, were at 100% capacity on March 31; the average storage capacity for that date is 91.49%. The district’s reservoirs have been full and spilling since Jan. 22, according to spokesperson Adriane Mertens.

In Novato, the North Marin Water District’s sole reservoir, Stafford Lake, has been full and spilling since Feb. 4, according to Tony Williams, general manager for the district.

Water managers in Marin look to the National Oceanic and Atmospheric Administration for seasonal outlooks on precipitation and temperature.

The current El Niño, a climate pattern that influences temperatures and precipitation globally, is one of the strongest on record, according to the federal agency. But that might be about to change. Experts at NOAA are predicting the probability of La Niña — and its drier conditions — beginning to build in June and August.

“It’s too soon to predict the forecast for later summer weather conditions, but we know to expect dry conditions based on what we’ve seen the last several years,” Mertens said. “Fortunately, thanks to above-average rainfall and continued lowered consumption from customers, we have adequate water supply to provide for customers for the next year.”

About 75% of the Marin Municipal Water District supply relies on rainfall on the Mount Tamalpais watershed, which fills the district’s seven reservoirs. Mertens said that from October to March of this year, Lake Lagunitas received 57.2 inches of rain — a total that is in the top 25 percentile of the 146 years of the district’s rainfall data.

“It’s great to see that we’re just about at the total rainfall that we’d expect for a full water year,” Lucy Croy, water quality manager at the district, said in an update on the water supply at a meeting on March 5.

Croy said the water year began in October with reservoirs around 10,000 or 15,000 acre-feet below capacity. An acre-foot is about 326,000 gallons of water.

Croy said to fill to capacity — 80,000 acre-feet — with 50 inches as of March, seems impressive, but that the timing of the rain helped.

“If you can have these storms one after another, this is the kind of impact we do see,” Croy said. “So we’ll see how many more storms we have over the next few months, but right now we’re sitting well in our local storage.”

Mertens said the average total rainfall for the six-month period is 45.9 inches, which is below recent years. In 2017, Lake Lagunitas received 88.7 inches, in 2019 it got 64.7, and last year 71.2 inches of rainfall were recorded.

In drier years, Lake Lagunitas has not fared as well: In 2020 it got 30.6 inches of rainfall, and in 2021, only 19 inches. However, the average cumulative rainfall for April and May is 5.3 inches.

“So there’s a chance we could see more rain through May,” Mertens said.

The North Marin Water District has tracked about 32 inches of rain from July 2023 to April 4. The prior year tracked about 10 more inches of rainfall for the same time period.

The Marin Municipal Water District still buys about a quarter of its supply from the Sonoma County Water Agency, whose water comes from the Russian River.

Comparatively, the North Marin Water District reservoir can only supply on average around 20% of the demand in the Novato service area. Williams said a better metric for predicting water supply through the summer is the Sonoma County Water Agency’s reservoirs at Lake Sonoma and Lake Mendocino. Both are full.

“We rely on imported water from Sonoma Water on a regular basis and have been since the 1960s, so purchasing imported water every year is a given,” Williams said.

Mertens said water usage typically goes up in the summer because of swimming pools, spas and outdoor irrigation. However, Williams said the district has not seen a similar increase.

“We are seeing a flat trend in water use for the last three years likely due to the dry conditions in 2021 and 2022 and resulting emergency conservation ordinances mandated by the state,” he said. “We are assuming continued flat growth for the coming year in light of the past trends and the second year of good rainfall and late season rains.”