The rain that fell on Marin in recent days has provided a small but welcome boost to local reservoirs and dampened the landscape after a record-breaking heat wave.
Rainfall in September is not unusual, but the amount of rain that fell over the weekend was much more than average for this time of year, according to Marin’s two largest water suppliers.
About 1.5 inches of rain fell in areas of the Mount Tamalpais watershed, which is more than three times the average amount the Marin Municipal Water District normally records from the start of July through this time of year.
The rain added a nominal 38 acre-feet of water to the district’s reservoirs, which can hold up to nearly 80,000 acre-feet. An acre-foot is about 326,000 gallons, or about half the volume of an Olympic-sized swimming pool.
The district’s seven reservoirs are at about 75% of capacity with 59,600 acre-feet, which is about 5% higher than the average storage the district has around this time of year. The district predicts it will have 55,000 to 60,000 acre-feet stored by the end of the year.
But the real benefit was to provide some much-needed moisture to the fire-prone watershed, said district official Paul Sellier, especially after last week’s heat wave that pushed temperatures to about 110 degrees in some areas.
“This rain has done us all a huge favor and dampened the watershed down and hopefully helped firefighters across northern California,” Sellier told the district board during a water supply update this week.
Chief Jason Weber of the Marin County Fire Department said the rain wasn’t enough to end the fire season, but it will provide a few weeks of relief before it dries out again, especially with higher temperatures forecast for October.
“We’re really looking for a series of storms,” Weber said. “This slows things down, and that’s good. But this moisture will dry out relatively quickly.”
The North Marin Water District recorded about three-fourths of an inch of rain at its Stafford Lake reservoir outside Novato. Rainfall records show the district normally receives about one four-hundredth of an inch on average for the entire month of September, according to Robert Clark, a district official.
Stafford Lake provides about 25% of the district’s water supply for the nearly 60,000 residents it serves in the greater Novato area. The lake had a small gain of about 3.4 million gallons of water, but Clark said that wasn’t expected to last long.
“That amount of rain is about equivalent to about one day of evaporation for this time of year for the lake elevation that it’s at,” Clark said. “We’d lose about 3.4 million gallons on average in a day this time of year.”
For the lake to receive more significant runoff, about 6 to 8 inches of rain over a short period is needed, Clark said. Stafford Lake is at about 52% of capacity, compared to its normal level of about 50% this time of year.