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Drought Watch: Is This the End of Our Dry Spell?

Jeffrey Mount December 12, 2014
OrovilleRiver

This is part of a continuing series on the impact of the drought.

This morning, the Bay Area and the Central Valley began to assess the mess created by Thursday’s big storm. And a big storm it was, with 50+ mph winds, torrential rains (more than 10 inches in some places, with blizzards in the Sierra), widespread flooding of streets, long power outages, and rivers and creeks that overtopped their banks.

This was an unusually powerful “atmospheric river” storm—California’s version of a hurricane—unmatched in intensity since January 2008. A deep low-pressure system came ashore in northern California and southern Oregon, generating violent winds throughout these regions. Spinning counterclockwise like a top, this system dragged warm, moist air into California from as far away as west of Hawaii—in satellite photos, this stream of moisture resembles a river, thus the name. All this activity led to prodigious amounts of rain in a very short period of time. And as cold air came in with the low pressure center, it produced abundant snow at higher elevations.

Beneath every headline about the intensity of the storm will be the question: Is the drought over? The answer: Not even close. It is important to remember that the state has been in severe rainfall deficit for three continuous years. One storm rarely busts a drought.

After the storm totals are added up, we will be at or slightly above average for this time of the year, and this is just the beginning of our wet season. At the same time, it has been so warm this season that total snowpack is likely to be below average, even with this big boost. The California Department of Water Resources has stated that we will need 150% of average precipitation this rainy season to recover from the drought. To have that kind of year, we need four or more additional atmospheric rivers (although not necessarily as strong as this one).

Indeed, history shows that as much as half of our annual rainfall is packed into just five to seven days of intense rainfall, highlighting the fine line between a wet year and dry one. And storms like this do little to address the problem of years of groundwater depletion. For that, we need many rainy days.

Still, despite the damage and inconvenience, this storm produced significant good. The state’s reservoirs are so depleted that there will be no trouble capturing this storm’s runoff. Small reservoirs, such as those serving Santa Cruz and other coastal communities hard hit by drought, will show dramatic improvements. The large reservoirs that rim the Central Valley—supplying water to 25 million people and more than seven million acres of farms—will get a good bump from this storm. Sometime this month, Folsom Reservoir, which does double duty by providing water and reducing Sacramento’s flood risk, is likely to encroach on the space it sets aside for flood control. (That Folsom Dam will be releasing water to protect against floods during a drought is testament to the difficulties the state faces in managing reservoirs for multiple, often conflicting needs).

This storm also was great for the environment. Native plants and animals are well-adapted to these kinds of events, and the physical changes that high flows bring to rivers and floodplains are needed to sustain habitat. One of the most important and stressed ecosystems in the state—the Sacramento–San Joaquin Delta—received a welcome flush of fresh water. During drought, the Delta becomes increasingly saline, harming habitat as well as the ability to export clean water to cities and farms. Bursts of fresh water reduce salinity and improve water quality for humans as well as the ecosystem (this is often lost on those who complain that this is water “wasted to the sea”).

In sum, this was an unusually powerful storm that caused damage, snarled traffic, and was a general nuisance for most northern Californians. Although it didn’t end the drought, this storm sure helped a lot.

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