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It's been worse than we've ever seen: the long view on droughts

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Willard Drake Johnson
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USGS Photographic Library, http://libraryphoto.cr.usgs.gov/

When explorer Stephen Long led his expedition across the western Great Plains in 1819-1820, it was during a period of widespread drought.  With only a single reference point in time, he concluded the area “is almost wholly unfit for cultivation, and of course uninhabitable by a people depending upon agriculture for their subsistence”.  He also marked the region on his maps as the “Great American Desert”, a label used by other map makers for decades to come.

Today we can take a much longer view of drought patterns on the High Plains thanks to paleoclimatology research of the sort conducted by Anthony Layzell and Catherine Evans of the Kansas Geological Survey.  Actual temperature and precipitation data have been recorded in the region since 1895 but their research is able to extend the historical view of droughts all the way back to 850 AD by measuring tree rings.

This technique precisely measures the width of the annual growth rings in living trees and preserved wood to reconstruct climatic patterns. Such Wide rings in highly drought-sensitive trees typically indicate a long growing season with adequate moisture, and very narrow rings usually signify drought conditions.  

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Credit R. K. Adams; courtesy of Laboratory of Tree-Ring Research, University of Arizona
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Cross section showing tree rings of a Douglas-fir.

The exact calendar year a tree ring was formed can be determined by matching the patterns in tree-ring characteristics among several trees in a region with overlapping growth periods.  Through this technique called crossdating, a continuous chronology of climatic trends for a region can be constructed. Such a chronology dating back to 837 has been developed for western Kansas.

To measure the relative severity of droughts across this long time period, the researchers use the Palmer Drought Severity Index (PDSI).  PDSI values range from -4 (extremely dry) to 4 (extremely wet), although the range is unlimited. 

In western Kansas there is a 35% chance of a drought as severe as the Dust Bowl in any decade, a 70% chance within a 20-year span, and a 100% chance over the estimated 40-year working lifetime of a western Kansas farmer.

Layzell and Evans note in their writings that, “Although a PDSI value of -4 or less (even more extreme) is daunting, a persistent drought averaging moderate (-2) to severe (-3) PDSI values over many years may actually cause more damage than a more severe but shorter episode.” 

For example, the PDSI value for 2002 in southwest Kansas was -7.1, compared to -5.0 for the peak year of the Dust Bowl, yet the situation was not as dire in 2002 because it was bounded by years with positive PDSI values. The negative effects of one extremely dry year can be overcome relatively quickly when preceded or followed by wetter years, but several years of nearly uninterrupted drought can lead to serious long-lasting socioeconomic and environmental problems.

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Credit Anthony Layzell; Kansas Geological Survey
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Annual PDSI reconstructions showing drought severity in southwest Kansas. Dashed lines indicate 1934 (black) and 1956 (red) PDSI values.

Droughts of unusually long duration are commonly referred to as "megadroughts," generally considered to last 20 or more years, with individual years of normal or even above-average precipitation during the period.  Such droughts appear to have been prevalent in Kansas between 850 AD and 1500 AD.   This spans the time frame of the Medieval Warm Period (MWP), a time of significant climatic variability that lasted from about 900 to 1300 in many parts of the world.  These recurring droughts likely afflicted Native American populations of the region, particularly those with permanent settlements and agricultural societies.

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Credit Anthony Layzell; Kansas Geological Survey
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Smoothed PDSI reconstructions showing drought durations for southwestern Kansas. Light gray bars indicate episodes of similar duration to the 1930s and 1950s droughts and dark gray bars represent episodes of greater duration. Annual PDSI values have been smoothed to filter out anomalous high and low values over a 10-year range (blue) and a 50-year range (red).

Though not as dramatic as other natural disasters like tornadoes and wild fires, the costs and long term consequences of droughts are far more significant.  Understanding the full range of potential severity and duration of droughts over long periods of time, not just recent memory, is essential to adequately planning for droughts.  Many water conservation and management practices are commonly designed to handle the "drought of record".  For Kansas, that is the drought of 1952 to 1957.  However, Layzell and Evans caution that planning for a worst-case scenario of only a five-year duration, “does not prepare the state for multi-decade megadroughts that modern-day agricultural and water systems - dependent on the state's limited groundwater and surface-water resources - may not have the resilience to withstand”.

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Further explanation and details of Anthony Layzell’s and Catherine Evans’ research and findings can be found at the Kansas Geological Survey website.  Layzell is interviewed on their work in a High Plains Journal article by Doug Rich.

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Credit Map by John M. Atwood. Published by J.H. Colton, New York. Courtesy of Library of Congress, Geography and Map Division.
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Detail from a 1849 map with the label "Great American Desert" sweeping down the High Plains.