California has been in drought for some time now, visible in California data for the Palmer Drought Severity Index, or PDSI. It’s a measure of, well, drought severity. On the PDSI scale negative numbers mean drought (i.e. unusually dry conditions) while positive numbers mean unusually wet conditions, so it’s the most extreme negative PDSI values that mark the most extreme drought. Anyway, here are 12-month moving averages of the data through April


Much of the cause of the drought is reduced precipitation for many years now. But similar reductions have been felt before, without leading to such severe drought. What’s different this time is that in addition to reduced precipitation, California has been feeling the heat.

Here are 12-month moving averages of temperature in California:


Not only have the drought years been exceptionally hot, temperature has recently soared to new heights.

Conservation measures are already in place, with extreme water rationing not far behind. One of the problems is that even during “normal” years, there’s precious little rainfall in most parts of California during most of the year. Winter is the “wet season,” so no relief is in sight — at least in terms of precipitation — for most of the rest of this year.

Worse yet, the usual sources of water for Californians, which get them through the most-of-the-year dry season, are also in bad shape. One of (perhaps the) most important sources is snowmelt, from the accumulated snowpack in the Sierra Nevada mountains. But the precipitation deficit hasn’t just hit rainfall, it’s also notable in snowfall, so this year the snowpack has reached record lows.

That situation is made worse by high temperature, which not only increases evaporation (and therefore drought), it also melts the snowpack earlier and faster. One measure of its overall effect is the total snowpack on April 1st of the year. This year it sank to a new low, a mere 8% of its normal value.


It’s even more obvious on a plot which may itself be more relevant, a log-transformed plot:


Yes, the drought in California is the worst we’ve seen.

Yet another major source of water for California is the nation’s largest fresh-water reservoir, Lake Mead at the southern tip of Nevada. It’s a crucial source, not just for California, but for Nevada, Arizona, other western states, and even Mexico. Unfortunately Lake Mead recently sank to record low levels, meaning that yet another water reservoir for California (and many other areas) is perilously low.

California is one of the most productive agricultural areas in the nation. But the drought has put a huge dent in their agriculture already. If the water situation doesn’t improve things will be even worse, and honestly, there’s not much hope for improvement soon. It underscores the fact that threats to the water supply can also threaten the food supply.

There’s also enhanced risk from wildfire. Numerous studies have shown that the increase in wildfire in the western U.S. is not just some consequence of fire management, as deniers like to claim (if they can’t deny a problem they’re sure to find something else to blame it on). It’s because of climate change. Western states are expecting a very bad wildfire season this year, and for Washington state they’re already off to a horrific start.

I hope the situation improves soon. Even if it does, the recent drought should be taken as a warning that our tampering with the climate has consequences. We all need to prepare for those consequences, in full knowledge that as bad as things can get today (and California is in bad shape), future instances won’t just be a repeat of a dire situation. They’ll be worse.

Unfortunately, if the California drought situation does improve soon, the natural human tendency will be to forget about the danger — “out of sight, out of mind.” The best we can hope for is that California will soon recover and that the last few years’ troubles will be a “word to the wise.”

It’s not that the word isn’t clear enough. Usually, it’s just the fact that we’re not wise enough.

24 responses to “CA H2O

  1. Thank you as usual.

    What could be the consequences of the coming “El Nino” event for California?

  2. I have read in places that the Palmer Drought Severity Index (PSDI) is seen by some geophysicists as not being a good indicator of drought. Clearly the other aspects of the West’s situation indicate a severe drought, irrespective of what PSDI says. But I never found a concise explanation of what it exactly is about PSDI that’s considered wanting. Anyone know?

  3. Western Ridge is forming again, leading to very hot temperatures in Pacific NW this weekend. We used to think summer started on July 4–now it seems to be beginning of June.

  4. Re question of hypergeometric: Here is a good reference:

    Click to access 20th%20century%20drought-Heim-02.pdf

    On page 1157: “The index was specifically designed to treat the drought problem in semiarid and dry subhumid climates where local precipitation is the sole or primary source of moisture (Doesken et al. 1991). Palmer himself cautioned that extrapolation beyond these conditions may lead to unrealistic results (Palmer 1965; Guttman 1991). During the last 30 years, several scientists have evaluated the model as applied under different climate regimes and have expressed concerns with some of the model’s assumptions. These concerns fall into two broad categories: the use of water balance models in general, and Palmer’s model in particular.” Not being a meteorologist, I would not dare to try to critique this statement or the technical discussion that follows it. The entire paper reveals a century of attempts to quantify drought conditions. I found the section at the end on the drought monitor measures educational also. However, none of this calls into question Tamino’s presentation, in my opinion.

    • Yes, I think that the question is very relevant to the IPCC’s attempts to grapple with drought. The question of best metric seems to be in flux, as of AR5:

      “There are very few direct measurements of drought related variables,
      such as soil moisture (Robock et al., 2000), so drought proxies (e.g.,
      PDSI, SPI, SPEI; Box 2.4) and hydrological drought proxies (e.g., Vidal
      et al., 2010; Dai, 2011b) are often used to assess drought. The chosen
      proxy (e.g., precipitation, evapotranspiration, soil moisture or streamflow)
      and time scale can strongly affect the ranking of drought events
      (Sheffield et al., 2009; Vidal et al., 2010). Analyses of these indirect
      indices come with substantial uncertainties. For example, PDSI may not
      be comparable across climate zones. A self-calibrating (sc-) PDSI can
      replace the fixed empirical constants in PDSI with values representative
      of the local climate (Wells et al., 2004). Furthermore, for studies
      using simulated soil moisture, the type of potential evapotranspiration
      model used can lead to significant differences in the estimation of the
      regions affected and the areal extent of drought (Sheffield et al., 2012),
      but the overall effect of a more physically realistic parameterisation is
      debated (van der Schrier et al., 2013).

      “Because drought is a complex variable and can at best be incompletely
      represented by commonly used drought indices, discrepancies in the
      interpretation of changes can result. For example, Sheffield and Wood
      (2008) found decreasing trends in the duration, intensity and severity
      of drought globally. Conversely, Dai (2011a,b) found a general global
      increase in drought, although with substantial regional variation and
      individual events dominating trend signatures in some regions (e.g.,
      the 1970s prolonged Sahel drought and the 1930s drought in the USA
      and Canadian Prairies). Studies subsequent to these continue to provide
      somewhat different conclusions on trends in global droughts and/
      or dryness since the middle of the 20th century (Sheffield et al., 2012;
      Dai, 2013; Donat et al., 2013c; van der Schrier et al., 2013).”

      Click to access WG1AR5_Chapter02_FINAL.pdf

    • hypergeometric,
      Nkemdirim (2015) in the Encyclopedia of Atmospheric Sciences may be worth a read for the pros & cons of PDSI. This link should point to the section of interest.

  5. Gah! Sorry for the horrible formatting there.

  6. Danger of warm El Nino rains will be reduced snowpack and increased need for reservoir storage – exacerbated by rains taking down burned hillsides. I’m glad I don’t live there any more, not only for this summer but for this coming winter and then next spring and summer. Far from over yet.



  7. California groundwater supply also dropping.

    “Worst hit, according to NASA, are the Sacramento River and San Joaquin River basins, where water has been pumped out to support agriculture in the Central Valley and elsewhere. Since 2011, the amount of water removed from these river basins each year added up to 4 trillion gallons. That’s “an amount far greater than California’s 38 million residents use in cities and homes annually,” NASA noted.”

    • Horatio Algeranon

      Big Ag in CA is stealing from future generations to grow exceedingly water-intensive crops like almonds (Ten Percent of California’s Water goes to Almond farming)

      The state asks ordinary folks to conserve while almond growers and other profit seekers continue to suck it all up.

      This same game plan plays out all over the west. When I lived in Tucson, officials asked people to conserve water (and even fined people for using too much water to wash their car) while golf courses and other water intensive developments sprung up all over the place and used water out the yin-yang.

      • Horatio Algeranon

        Computer chip manufacturing in Silicon valley also sucks up huge amounts of water.

      • Not many chip fabs left in CA; most moved out and offshore decades ago but there are a number in AZ, NM, OR & TX

  8. There are time series for PDSI, temperature and snowpack. But because it is a drought, is there also a time series for precipitation?

  9. Looking at that first graph, it appears that since 1980, the variation (both wet and dry) is larger than before 1980. Is that significant or just reading tea-leaves?

    • Andrew Dodds

      Yes, I noticed that as well..

      Don’t know if it is statistically relevant (anyone know a stats expert who can check?) although a general principle of global warming seems to be greater swings as well as overall warming.

  10. In Australia, we built desalination plants because of major droughts. In the eastern states, the droughts have gone (mostly) and the desal plants sit idle. But in Perth, where we have a long term rainfall decline, we would be in huge trouble if we hadn’t built not only the original desal plant, but another bigger one. Are they building desal plants in California?
    Incidentally, or idiot Prime Minister, Tony Abbott, recently complained about the wind turbine on Rottnest Island, Perth’s holiday island. The wind turbine powers the reverse osmosis plant that gives Rottnest its fresh water. No doubt Tony would have preferred a diesel engine powering the plant.

  11. Tamino :: not relevant to this thread but there’s another correlation exercise with AMO etc being put forward at

  12. and touted by Curry at

    questions: why the 13-year smoothing? and is “adjusting the amplitude of the oscillatory mode to maximize the correlation between the temperature time series and the combined function” kind of a fix?

    • David Sanger,
      I see no problem with 13-year smoothing. Surely the central problem is that this is but puerile curve-fitting, something that Judy Curry is rather partial to. The major crime is the silly attribution. For instance, why is “…the monotonic mode (mostly due to increasing atmospheric CO2)” seen as being linear? Why are the wobbles solely due to the AMO/PDO? You could also consider the thesis to be an exercise in cherry-picking. Why is the attributed AGW so tiny in the US while (I assume this because in the following the wobbles are not big or entirely absent) AGW remains so large in Russia or in Brazil or in India or in Australia or even in Canada? It is exceeding odd. Perhaps the reason is because AGW is so un-American that its effect is duly diminished across the 48 continental states.

    • Horatio Algeranon

      “Lucky Thirteen”

      Lucky thirteen
      To smooth your data
      Makes the
      Correlation greata

      No matter what it takes, Judith Curry will be touting PDO as the driver of global temperature until her last breath

      PDO = Pretty Darned Ornery

  13. Tamino,

    I’ve left it to let this thread move on, as the UK situation is not as dire as in California. But, for what it’s worth, there’s a plot of UK annual average rainfall anomalies on this blog post.