The Cosumnes River, before and after the atmospheric river storm of February 8-9. (Carson Jeffres)
Watching the TV news reporters out in their slickers, in front of suddenly raging rivers last weekend, it was tempting to open the taps and start celebrating the end of California’s Big Dry. But officials and forecasters were quick to point out that the festivities would be premature.
There are a few reasons for this. One is the sheer depth of California’s precipitation deficit. Over a three-day period, some places doubled their total seasonal rainfall totals–and still found themselves at just 30 or 40 percent of average.
Another was that this atmospheric river pouring in from the Pacific was a rifle, not a shotgun. It “kind of sat in one spot and hosed one part of the state,” explains Jeffrey Mount, a consulting geologist and co-founder of the Center for Watershed Sciences at University of California, Davis. “We’re in a statewide drought, so this did nothing to relieve anything basically south of Sacramento.”
But the third reason is more elusive. It has to do with how extremely dry conditions were when the storm arrived.
“It rained so hard that it saturated the ground right near the surface,” says Mount, who is a fluvial geomorphologist, which is to say an expert on how rivers change and respond to major weather events. “Once it’s saturated, that water starts running right off the surface. It’s not stored at all.”
Hence, rivers that were right in the path of the storm went from nearly bone-dry to running amok in a matter of hours. Carson Jeffres, a colleague of Mount’s at Davis, was up on the Cosumnes River, watching it swell. “The river went from dry to out in the floodplain in about 16 hours,” Jeffres wrote in an email.
The Russian River seen from Badger Park in Healdsburg, before and after the atmospheric river storm of February 8-9. (Before: Craig Miller. After: Jacob Katz)
But much of that water didn’t stick around long enough to recharge underground aquifers.
Mount says it has to do with “infiltration capacity” of the soils. “Think of your ability to absorb information,” he says. “You have an infiltration capacity. The soils are the same way. Lighter, more prolonged rains don’t tend to overwhelm the ground’s infiltration capacity. “It goes into soil and it actually starts moving downslope below the surface and discharges into the river, which is why you often will see rivers will rise very slowly in gentle storms and then go down very slowly, long after the storm has gone. That’s because the soils just below the surface are feeding water into the rivers.”
But when the water all comes at once, as it did in the early February deluge, “Up comes the river and down goes the river, very quickly,” says Mount.
In extremely dry conditions, Mount says the soil can turn “hydrophobic.” That doesn’t mean the dirt has rabies but it does mean it’s “afraid” of water, as he explains it. “The first rain doesn’t penetrate much. It literally bounces right off and runs off. The way water ran off during that storm, one would wonder if that was the case.”
Not that there weren’t winners from the storm. At one point, Mount says Folsom Lake, a lynchpin reservoir for the Sacramento region, was taking in 50,000 acre-feet per day from the suddenly engorged American River. “That’s enough for the City of Folsom to make it through a whole year,” he says. But similar flows on the Russian River, for instance, went straight out the ocean.