Fifty More Inches of Rain Needed to Fill Cachuma

January rains have brought welcome relief to South Coast farms and gardens, but not to Lake Cachuma, the reservoir that normally provides most of the region’s water supply.

After five years of severe drought, the stream runoff into Cachuma this month to date totals about a year’s supply for 15,000 people, out of a South Coast population of 207,000. Currently at only nine percent of capacity, Cachuma places dead last among the 23 reservoirs on the state’s daily water storage list. Because of the recent extreme wet weather in Northern California, most reservoirs are showing higher-than-average levels for this time of year.

In addition to runoff, there is imported state aqueduct water flowing into Cachuma. But the minimal rain runoff into the lake means the South Coast remains stuck, for now, in the dark brown blob on national weather maps that stands for “exceptional drought.” The classification also includes much of Ventura County and parts of Kern and Los Angeles counties, covering about two percent of California. Just two weeks ago, 18 percent of the state was in exceptional drought.

“The few times it’s rained here, we’ve gotten a little stream flow, but not enough,” said Jayme Laber, a hydrologist with the National Weather Service in Oxnard who determines the local drought classifications. “The Santa Ynez watershed is so dry, it’s going to take quite a bit of rainfall to get close to saturation. We’re still in a significant drought here.”

Three incoming storms starting Wednesday night and ending Monday are each forecast to bring between one and three inches of rain to the local mountains and foothills, Laber said – not enough to fill Cachuma, but likely enough to trigger substantial flow into the lake.

The county’s rainfall gauge at the Gibraltar Dam on the Santa Ynez River, a barometer of wet or dry conditions in the steep and rugged mountains upstream of Cachuma, is registering nine inches of rain for the season to date, or 90 percent of average for this time of year. But the watershed likely needs another 40 to 50 inches more rain to generate enough runoff to fill and spill Cachuma, said Tom Fayram, deputy director of Santa Barbara County water resources. The lake last spilled in March 2011.

“We’re setting up nicely to get there,” Fayram said. “The heart of our winter is January.”

Significant runoff depends both on rainfall volume and intensity, and on the intervals between storms, said Shawn Johnson, county senior hydrologist. The same number of annual inches may or may not produce a spill, depending on whether it is spread out over months, allowing the ground to dry out between rains, or it arrives in back-to-back storms.

“In our winter season, a big storm or two typically can make the difference between a dry year and a wet year,” Johnson said.

The Bradbury Dam that created Lake Cachuma was built in 1953. County records show that the lake spilled 14 times in the 28 years from 1959 to 1987, but only eight times in the 28 years from 1988 to 2016.

Yet historically, Cachuma has made some dramatic comebacks. In February of 1991, the lake was at 15 percent capacity after five years of drought. Then the “March Miracle” storms of 1991 dumped 23 inches of rain in the watershed at Gibraltar. But the lake did not spill until 1993, when Gibraltar got 49 inches.

After two dry years from 2002 to 2004, county records show, Cachuma had dropped to 35 percent of capacity. But it filled and spilled after storms drenched the watershed with 17 inches in late December, 2004, followed by 21 inches in January, 2005, and 16 inches in February – 54 inches of rain overall.

“If we had a repeat of 2004 – 2005, that would fill the lake,” Fayram said.