During the past water year, San Francisco received nearly 84 percent of its normal annual rainfall. With California likely entering its fourth year of drought, near-average levels are good news.
Friday marks the end of the current water year, a 12-month period that began last October. 1. According to the National Weather Service, the water year totals about 19 inches.
Despite the ongoing drought, it wasn’t one of the driest water years on record for the city. In fact, it didn’t make the top 70.
By comparison, in 2021, downtown San Francisco will receive only about 40% of the precipitation in a normal water year.
Instead, San Francisco is close to reaching its average annual rainfall. But what’s a little fuzzy in the numbers is that the vast majority of rain occurs within two months: October and December 2021. Rainfall in 2022 is less than 3 inches.
“Overall, the rain is good,” said Roger Gass, a meteorologist with the National Weather Service’s Bay Area office. “But sometimes the timing of the precipitation and the snow can tell a different story.”
A similar pattern emerged in other parts of the Bay Area. At Santa Rosa’s long-running airport site, rainfall this year is about 27 inches, 80 percent of normal. But only 5.7 inches have fallen since Jan. 1.
San Jose has seen 58 percent of its normal rainfall, or 7.8 inches, in the past water year. By 2022, only about 1.2 inches of that will fall.
This calendar year has been dry across the state. According to the latest data from NOAA’s National Center for Environmental Information, the first eight months from January to August were the driest on record for California.
“It’s really been a roller coaster water year,” said John Abazoglu, a climate scientist at the University of California, Merced.
The water year does not start in January. Instead, it is defined as October to September. A big reason is because of the snow.
Kurt Richter points to one of his rice fields in Colusa. He normally planted 5,000 acres, but drought limited him to 1,300 acres.
Brontë Wittpenn, photographer / The Chronicle“Snowdrifts are a very important part of our water resources and storage across the state,” Gass said. This is why the timing of precipitation is so important. Unfortunately, this has not worked out in recent water years.
“We were excited in December, and then it went away pretty quickly,” Abatzaglou said.
Wet water years are enough to lift the state out of drought conditions.
“A very wet water year, often interrupted by a slew of atmospheric rivers, can be a drought nemesis,” Abatzoglou said.
For example, the drought from 2012 to 2016 ended with a strong rainy season in 2017. Precipitation is sufficient to replenish the reservoir.
But there are other ways to look at droughts.
“If you’re concerned about the impact of some ecosystems on chronic drought, individual years may or may not be doing that,” Abatzoglou said.
Cumulative drought years add up. If La Niña predictions are successful, California will enter its fourth straight year of drought.
The September 2019-August 2022 period was the state’s driest 36 months on record, ending August, according to the latest NOAA data.
The timing of the fall in precipitation also has a major impact on snowpack, which is critical to California’s water supply.
“We could even hit 130 percent of the average,” Gass said. “If it comes at the wrong time, it doesn’t necessarily get us out of a prolonged drought.”
Jack Lee (he/he) is a staff writer for the San Francisco Chronicle. Email: jack.lee@sfchronicle.com