Low water levels at San Luis Reservoir in February, 2014. (Josh Cassidy/KQED)
Low water levels at San Luis Reservoir in February, 2014. (Josh Cassidy/KQED)

Vegas has nothing on Davis.

As talk turns to whether California’s drought will stretch into a fourth year, two co-founders of the Center for Watershed Sciences at U.C. Davis decided to handicap it. Their conclusion: don’t bet on wet.

Jay Lund, who specializes in the engineering side of water and geologist Jeff Mount, now a senior fellow at the Public Policy Institute of California, looked at more than a hundred years of precipitation records and drought patterns in the Sacramento Valley, and calculate that the chances of another winter with below-average precipitation to be nearly three in four. Lund and Mount figure there’s about a one-in-four chance of a “critically dry” year, using the five-category nomenclature of state water managers. “There’s a good chance that if you’re in a dry year this year, you’ll be in a dry year next year,” says Lund. They write on the center’s California Water Blog:

“In all, there’s a 71 percent chance that next [water] year will be Below Normal or drier and only a 29 percent chance of experiencing an Above Normal or Wet year.

Based on 106 years of record, only 13 percent of years have been Critically Dry. But the odds facing California for next year aren’t as good. In the Sacramento Valley — the state’s largest source of water supply — there’s a 29 percent chance that the 2014-15 water year will also be Critically Dry, and a 64 percent chance that it will be Dry or Critically Dry — not favorable conditions for water management.”

Given the changing climate, I asked Lund if history is a reliable gauge anymore.

“Well if history is no gauge, then maybe it’s a 50-50 chance,” says Lund. “But I think history is probably a better gauge than we’d like to think.”

(Center for Watershed Sciences, U. C. Davis)
(Center for Watershed Sciences, U. C. Davis)

Clearly as things stand, anything on the dry side would be bad news. “Even if we have a wet year next year, we’re not out of the drought,” warns Mount, a specialist in river hydrology. “We’ve drawn down so much on our groundwater, we have such low reserves within our reservoirs, and our soils are very, very dry.” The two say that the likelihood of a longer drought isn’t merely statistical. For instance, after an extended dry spell, soils tend to soak up more of the rain that does fall, and that reduces runoff into reservoirs. The current 2014 “water year” (ending September 30) is on track to be the third driest on record.

Lund and Mount agree that betting on El Niño to bring rain and snow is a chump’s game. In fact, says Mount, he and Lund were motivated to make their calculations in part because of all the “media froth” over the prospects for a rainmaking El Niño condition in the Pacific.

“You could actually hear the response of people saying, ‘We’re going to have an El Niño next year, so everything’s going to be fine.”

Not necessarily, says Mount, noting that Central and Northern California lie “at the swing point,” where effects from the legendary oscillation are less predictable than in far Southern California or the Pacific Northwest. As for the Las Vegas metaphor, “Only a fool would make book on the weather,” cautions Mount. “Our predictive capacity for year-to-year weather is very, very low.”


A Fourth Drought Year for California: What Are the Odds? 16 June,2014Craig Miller

  • Robert Pyke

    Pure statistics do not tell the whole story unless you are
    studying something that is truly random.
    But weather patterns are not entirely random. See for instance http://www.fixcawater.com/climate-variability.html
    for a history of climate variability (that is heavy on geologic input) and at
    least partial explanation for this variability.

    As pointed out by Lynn Ingram in “The West Without Water”,
    El Nino’s tend not to cause wet winters in California unless the Pacific
    Decadal Oscillation (PDO) is in its positive phase. Also the last long dry spell in California,
    in 1976-77, coincided with the PDO flipping from negative to positive. In the long period of positive PDO that
    followed, there were two very wet El Nino events, in 1982-83 and 1997-98. However, for most of this century the PDO has
    been negative and a really big question has been weather or not it would flip
    again, coincident with the long dry spell.
    See http://jisao.washington.edu/pdo/PDO.latest for the answer, which is, yes, it has. Therefore, throw out the statistics and get
    out the sand bags!

    Robert Pyke Ph.D., G.E.

    • It’s an interesting point, Robert. The original Lund-Mount post (linked
      from this one) talks about ENSO’s weak correlation to Northern
      California precipitation in general but does not address the possible compounding (or maybe I should say, “confounding”) affects of the PDO. I think it’s a topic that deserves more attention.

  • CharliePeters

    Shell ethanol affect the water?

    Dr. Stan’s California water & fuel supply opinion


    • CharliePeters

      BP GMO fuel affect the beef?


Craig Miller

Craig is KQED’s science editor, specializing in weather, climate, water & energy issues, with a little seismology thrown in just to shake things up. Prior to his current position, he launched and led the station’s award-winning multimedia project, Climate Watch. Craig is also an accomplished writer/producer of television documentaries, with a focus on natural resource issues.

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