Drought: Marin reservoirs bolstered by rain surge

The Marin Municipal Water District logged nearly as much rain this past week as it did during all of last winter, making it the wettest October in more than 130 years.

The utility recorded more than 17 inches at its Lake Lagunitas reservoir between Oct. 18 and Monday, with 10.5 inches coming in on Sunday alone, according to district data. Typically, the district only sees an average of 2.8 inches of rain in October.

The last time the district saw close to this amount of rain in October was in 1890, when it received 26 inches.

“It’s really remarkable,” said district communications manager Emma Detwiler.

The rain this week is also about a third of the 52 inches of rainfall the district receives in an average year and just a few inches shy of the 20 inches during the entire 2020-2021 water year — its second driest year in 143 years of records.

As a result, water supplies in the district’s seven reservoirs in the Mount Tamalpais watershed increased from 32% to 43% of total capacity, with the total storage increasing from about 25,772 acre-feet on Oct. 18 to 34,535 acre-feet on Monday. The reservoirs, which serve 191,000 residents in central and southern Marin, provide 75% of the district’s water supply and rely entirely on rainfall runoff.

Local water officials say the rain is a welcome start to the upcoming rainy season, but that drought is not over. The district’s reservoirs are usually around 67% full by this time of year. Last year, reservoir levels were at 63% of capacity.

“Even if we had not had the severity of the drought we have had to date, we would still consider 34,500 acre-feet to be a very dry year,” said Cynthia Koehler, president of the district’s board of directors. “It certainly does provide some relief in terms of the direness of the situation, but it does not take us out of a drought situation.”

District staff also said the rains do not change the district’s projections of potentially depleting local reservoir supplies as soon as July should the district receive a similar amount of rain as it did last year.  And residents should not expect water use restrictions to ease up as a result of this storm.

But this week’s weather was a “best case scenario” to begin refilling reservoirs, Detwiler said. The rains last week soaked the drought-parched ground, allowing much of the deluge from Sunday’s storm to flow into local reservoirs. By Monday morning, Mount Tamalpais had received close to 27 inches of rain in seven days.

“Another event like this could get us back up to average storage,” Detwiler said.

As the county awaits more rainfall, the district is also continuing to plan for projects to bring in additional water supply, including an 8-mile pipeline across the Richmond-San Rafael Bridge that could bring in water purchased from the Sacramento Valley. The district board voted last week to invest $23.2 million to pre-purchasing piping, water tanks and pumping stations to have them ready by early next week should the pipeline project move forward.

A vote on a construction contract could take place in January if the coming months are dry.

“We are not letting up on our vigilance,” Koehler said.

The North Marin Water District recorded about 7.6 inches of rain near its Stafford Lake reservoir — its second-highest October rainfall count on record dating back to 1916, according to Drew McIntyre, district general manager. The district’s highest rainfall total for October was set in 1963 with 8.9 inches.

However, the rainfall didn’t generate significant runoff because the watershed near Stafford Lake typically requires 7 to 8 inches of rain before the ground is saturated. About 300 acre-feet of water, or about 5% of the lake’s capacity, was restored from the storm, McIntyre said. This week’s rainfall was also nearly as much rain as the district saw in all of the 2020-2021 water year of 8.6 inches.

“It set it up very well for what we hope will be some future storms coming down here,” McIntyre said.

The storm also helped prime the Sonoma Water agency to start receiving runoff for its two main reservoirs at Lake Sonoma and Lake Mendocino, McIntyre said. Sonoma Water provides 75% of North Marin Water District’s supply and about 25% of Marin Municipal Water District’s supply.

“The initial storm did not undo a two-year drought,” McIntyre said. “We’re still advocating that our customers need to help us save water during this drought.”