The start of October marked the end of a roller coaster rainfall year in Marin County that included historic downpours and record dry periods.
A year ago, the county’s main reservoirs were 33% full and approaching critically low levels after being wracked by two years of historic drought. Residents were placed under mandatory water use restrictions. The Marin Municipal Water District was scrambling to build a $100 million pipeline across the Richmond-San Rafael Bridge to prevent running out of water as soon as mid-2022.
One year later, local reservoirs are now nearly 75% full — above average for this time of year — after being replenished by a series of unusual storms in October and December.
While last year’s rains brought some relief, California has been in a drought for eight of the last 11 years. The past three years have been the driest ever in California dating back to when records began in 1895, according to the State Water Board and weather station data collected by the Desert Research Institute.
“Let’s be clear. Things are going better this year than last year in most places in California because we got lucky,” said Jeffrey Mount, professor emeritus at the University of California, Davis, and senior fellow at the Public Policy Institute of California’s water center. “Instead of a repeat of 2021, we had the October record rains and then that very nice wet December and that was really enough to soften the blow.”
“If we had another 2021 this year we would be having a different conversation,” Mount said.
Now Marin water planners are faced with the decision of whether to potentially invest millions of dollars to obtain new water supply sources or to continue to rely on existing supplies and conservation.
“The record-breaking rainfall in October and December really provided us the time to consider what the next best move would be for the district in terms of augmenting water supply,” said Paul Sellier, water resources director for the Marin Municipal Water District.
Forecasts show California could experience an unusual third consecutive year of La Niña conditions, which is when Pacific Ocean waters are cooler along the equator. While the belief is that La Niña conditions mean a drier winter in northern California, the average rainfall in the Bay Area during the past 24 La Niña winters since 1950 has been above 90% of normal.
“What it really means for us in the central part of the state, it’s kind of a 50-50 chance of seeing wetter or drier conditions,” said Tony Williams, general manager of the North Marin Water District, which serves the Novato area and West Marin. “We’re always on that sort of dividing line.”
The Marin Municipal Water District, which serves 191,000 residents in central and southern Marin, has seven local reservoirs that are entirely reliant on rainfall. The reservoirs make up about 75% of the district’s total supply.
The district recorded about 51 inches of rain in the past “water year,” which runs from Oct. 1 through Sept. 30 of the following year. The average annual rainfall is about 52 inches.
Last year’s rainfall was nearly as much rain as the previous two water years combined. About 20.4 inches fell in the 2020-2021 water year — the district’s second-driest on record — and about 35 inches fell in the 2019-2020 water year.
The majority of the rain came from a series of “atmospheric rivers” in late October and December. Eighteen inches of rain fell in the Mount Tamalpais watershed in the last two weeks of October, making it the second-wettest October on the district’s records, behind the 26 inches that fell in October 1890.
One storm in October dumped nearly 11 inches of rain on Mount Tamalpais in a day, setting a district record for the most rainfall in a 24-hour period.
By the end of the year, the district’s reservoirs had refilled from near critically low levels of 32% to nearly 92%.
The North Marin Water District received 7.6 inches of rain in October, several times more than the 1.4 inches it receives for the month on average. The October rainfall was nearly as much rain as the district recorded for the entire 2020-2021 water year, which was its driest in records dating back to 1916.
But conditions shifted from January through March, the time of year that historically brings the majority of Marin’s rainfall.
The Marin Municipal Water District recorded only 2 inches of rain in January and February. The rainfall for this two-month period was the driest on record not only for the district but for California. Typically, the district receives about 20 inches of rain during that period, Sellier said.
The North Marin Water District recorded 19 inches of rain by the end of the water year, falling short of its average of 27 inches. The district’s only reservoir at Stafford Lake, which is typically only used during the summer, is now 46% full, which Williams said is better compared to last year at this time when it was at 28% of capacity.
Williams said the impacts of climate change are expected to affect the typical precipitation patterns in California.
“It’s going to come very sporadically and different from the patterns that we’re used to,” Williams said. “We’re starting to see that already, that the typical wet months of January and February are really not the wet months anymore.”
Both water agencies expect to have enough water stored to last through another dry winter. The Marin Municipal Water District expects to have reservoirs at nearly 70% of capacity by the start of December.
However, should this winter prove to be drier than normal and reservoirs only receive about 25% of their average runoff, the district could find itself in a similar position to where it was in the fall of 2020, according to district projections.
The district has about a two-year supply of stored water in its reservoirs compared to four-year supplies among other Bay Area water suppliers. The district’s storage capacity, as well as its reliance on rainfall and water imports from Sonoma Water, make it more vulnerable compared to other agencies that have a larger portfolio of water sources, Mount said.
“Nobody else in the Bay Area got that close to the word nobody likes to use: rationing,” Mount said.
The district is studying several new sources of supply, including an intertie to the State Water Project, desalinization, recycled water expansion, expanding local reservoir storage and conservation improvements, among others.
Williams said the North Marin Water District has the option to pump Russian River water into Stafford Lake this winter if it turns out to be dry in order to ensure the lake has water for the summer months. The district previously pumped what amounted to a quarter of the lake’s capacity in the winter of 2020-2021 in anticipation of a dry year. Had it not done so, the lake would have run dry by the end of the summer of 2021.
“We likely wouldn’t start that until the December time frame,” Williams said. “We have a little bit more time to see how the climate predictions are shaking out.”
Both Marin water agencies also rely heavily on Russian River water imports from Sonoma Water. The agency provides roughly 25% of the annual supply for the Marin Municipal Water District and 75% of the North Marin Water District’s.
In the summers of 2021 and 2022, Sonoma Water cut 20% of its water imports in response to low levels at its two main reservoirs, Lake Sonoma and Lake Mendocino. The 20% reduction is set to expire at the end of this month.
If Lake Sonoma drops below 100,000 acre-feet of storage before July 15, water imports could be reduced by 30%, though other options may be available, according to Sonoma Water engineer Don Seymour. The lake has 109,000 acre-feet, which is about 45% of capacity and well below the 200,000 acre-feet it normally holds around this time of year, Seymour said.
