Drought History….

Yes, I’m still working on the model stuff. And yes, I’m busy trying to teach myself R. In the mean time, I felt like sharing this with you: The Drought History of the US:

% Area of the US Moderately to Extremely Dry (red) and Severely to Extremely Dry (Orange).
Just so that there is a hypothesis test here: Did anyone hypothesize that AGW would cause more droughts in the US? If so, I think this is enough to invalidate that idea.

But I doubt anyone familiar with the actual data or science would make such a silly claim. So it was probably Al Gore.

Sources:

http://www.ncdc.noaa.gov/oa/climate/research/2009/jul/uspctarea-wetdry-svr.txt

http://www.ncdc.noaa.gov/oa/climate/research/2009/apr/uspctarea-wetdry-mod.txt

And yes we are getting wetter. Bad? I don’t think so. But people are welcome to spurn rain if they wish.

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3 Responses to “Drought History….”

  1. Tony Hansen Says:

    Andrew,
    What are their definitions of moderate and extreme?
    I am at a bit of a loss to define what is or is not a drought. When gov depts start talking 1 in 20 year events it seldom seems to fit the on ground stuff, eg a 50% deficit in 24 months gets a badge but not a 70% deficit in 20 months or a 45% deficit in 36 months. Your thoughts?
    Cheers

  2. timetochooseagain Says:

    Tony- NOAA comments extensively on how drought is measured and the difficulties:

    http://www.wrh.noaa.gov/fgz/science/drought.php?wfo=fgz

    ” A drought is defined as “a period of abnormally dry weather sufficiently prolonged for the lack of water to cause serious hydrologic imbalance in the affected area.” -Glossary of Meteorology (1959).

    In easier to understand terms, a drought is a period of unusually persistant dry weather that persists long enough to cause serious problems such as crop damage and/or water supply shortages. The severity of the drought depends upon the degree of moisture deficiency, the duration, and the size of the affected area.

    There are actually four different ways that drought can be defined.

    Meteorological-a measure of departure of precipitation from normal. Due to climatic differences, what might be considered a drought in one location of the country may not be a drought in another location.

    Agricultural-refers to a situation where the amount of moisture in the soil no longer meets the needs of a particular crop.

    Hydrological-occurs when surface and subsurface water supplies are below normal.

    Socioeconomic-refers to the situation that occurs when physical water shortages begin to affect people.

    So what counts as a drought is highly dependent on how it is defined. That is of course rather problematic. My understand is that the source I use for these graphs:

    http://www.ncdc.noaa.gov/sotc/?report=drought&year=2009&month=9&submitted=Get+Report

    Use the Palmer Drought Index, which, according to wiki is:

    “The Palmer Drought Index is based on a supply-and-demand model of soil moisture. Supply is comparatively straightforward to calculate, but demand is more complicated as it depends on many factors – not just temperature and the amount of moisture in the soil but hard-to-calibrate factors including evapotranspiration and recharge rates. Palmer tried to overcome these difficulties by developing an algorithm that approximated them based on the most readily available data — precipitation and temperature.

    The index has proven most effective in determining long-term drought — a matter of several months — and not as good with conditions over a matter of weeks. It uses a 0 as normal, and drought is shown in terms of minus numbers; for example, minus 2 is moderate drought, minus 3 is severe drought, and minus 4 is extreme drought. Palmer’s algorithm also is used to describe wet spells, using corresponding positive numbers. Palmer also developed a formula for standardizing drought calculations for each individual location based on the variability of precipitation and temperature at that location. The Palmer index can therefore be applied to any site for which sufficient precipitation and temperature data is available.”

    http://en.wikipedia.org/wiki/Palmer_Drought_Index

    I hope that helps.

  3. Tony Hansen Says:

    Thanks Andrew – much appreciated.

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