New ‘rainfall year’ begins 30 inches short

The numbers are in: The official total for the 2019-2020 “rainfall year” in the city of Sonoma was 14.46 inches, a full 30 inches less than the previous year, which totaled 44.46.

The precipitation total is not a calendar year, but runs from Oct. 1 to Sept. 30 of the following year. The rainfall year, also known as the “water year,” thus spans the months of most precipitation (usually October through April) instead of “splitting” them according to the annual calendar.

Since 2000, when the U.S. Drought Monitor was initiated, the longest duration of drought in California lasted 376 weeks - seven years - beginning in late 2011, and ending on March 5, 2019 – just last year, when we had the 44-inch total.

But if last year’s totals were a drought-buster, this year hasn’t been. Average precipitation in Sonoma is around 30 inches, double what we’ve had. Much of California is in a moderate state of drought (32 percent) and another 32 percent is in severe drought, including Sonoma County and north to the Oregon border and beyond – where drought conditions are qualified as extreme.

While what some are calling “significant rainfall” is possible late next week – a rain event around Oct. 10 that may arrive as a result of a Pacific hurricane—firefighters aren’t popping the champagne quite yet. An inch of rain is necessary to quench this year’s raging fire season, and there’s only an outside chance of anything like that.

A dramatic drop-off in rainfall once the calendar year began – with no rainfall at all in February, June or July, and precious little otherwise – gave rise to concerns over regional water supply, and shortage-sensitized water planners took preemptive measures.

So in June, the Sonoma County Water Agency, also known as Sonoma Water, filed a Temporary Urgency Change Petition (TUCP) with the state Water Resources Control Board to reduce in-stream flows this summer, meaning less release from Lake Mendocino and Lake Sonoma into regional streams.

“In fact this has been the third driest water year on record in the Ukiah region since records started being kept 107 years ago. In the Santa Rosa area maybe the fifth or sixth,” said Donald Seymour, principal engineer in Sonoma Water’s Resources and Energy Planning Sector.

The rainfall in Ukiah is significant because that region feeds into Lake Mendocino via the Potter Valley Project, diverting water from the Eel River watershed into Lake Mendocino, and thus the Russian River watershed – which provides water for most of Sonoma County and northern Marin.

Keeping a reliable if lean water flow into Lake Mendocino and Lake Sonoma enables Sonoma Water to deliver water to its customer water districts – including the City of Sonoma and the Valley of the Moon Water District.

But despite the low rainfall, the reduced water releases and consumers’ improved water use habits have put Lake Mendocino right at the target level that the energy planning section hoped for. “In our minds, our goal was to have Lake Mendocino at 40,000 acre feet,” said Seymour; an acre foot is about 326,000 gallons. “Now it’s about 39,900 acre feet – so from that perspective we feel it’s been successful.”

Customer behavior has a lot to do with keeping water delivery adequate. “In mid ’90s, the per capita on average was 150-160 gallons per day; 25 years later, we’ve driven it down to a little under 100 gallons per day,” said Seymour.

That level is about 60 percent of the reservoir’s capacity. Lake Sonoma is running below 75 percent.

“Folks in Sonoma County have been very aggressive in improving water use efficiency, not just in responding to drought but in doing this day-to-day,” Seymour said.

With the arrival of the Shady and Glass fires, the messaging from local water departments has shifted to the same type of limitations on water use as found during the depths of the drought years, in the interest of conserving water for wildlife response: use less, irrigate less, postpone washing vehicles and other outdoor water uses, and avoid waste.

Email Christian at christian.kallen@sonomanews.com.