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| On a statewide basis, May 2004 ranked as the 57th driest (54th wettest) May in the 110-year record. However, considerable variation occurred across the state. The month was wet in eastern Nebraska, with the Northeast division ranking in the top tenth (wet) percentile, while it was dry in the west, with the Nebraska panhandle ranking in the bottom tenth (dry) percentile. This spatial variability is also reflected in the May 2004 Palmer Z Index and the long-term Palmer Drought Index. |
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Paleoclimatic Perspective
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Western Nebraska has been experiencing drought conditions since the summer of 2000. In July 2002, the Palmer Drought Severity Index (PDSI) for the Panhandle (climate division 1) reached the most severe value of the instrumental record. Extensive reliable instrumental records go back only about 100 years, but tree rings are a form of paleoclimatic data that can reliably extend the climate record back several more centuries.
Ponderosa pine tree growth in western Nebraska is related to soil moisture conditions in the spring. Because of this relationship, variations in tree-ring widths can be used as a proxy for May drought as measured by the PDSI. A comparison between the instrumental record of Nebraska Division 1 (the Panhandle of western Nebraska) PDSI (red line) and a tree-ring chronology from a site called Canyon Road (blue line) for 1900-1997 shows a correspondence between low growth and drought particularly after about 1920. The match between the two series for the wet years is not as good, so this record is best used as a proxy for dry conditions (small graph - large graph). Both the tree-ring reconstruction and the Panhandle PDSI time series have been converted to standardized z-scores using their respective means and standard deviations over the 1931-1990 PDSI calibration period. |
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The tree-ring record can be used to place 20th and 21st century droughts into a long term context. In the graph to the right above, annual tree-ring reconstructed values are plotted against the May PDSI from each year. The Canyon Road chronology extends from 1489 to 1997 (blue line), while the instrumental PDSI record (red line) extends from 1900 to 2004. The 2000-2004 drought has the most severe PDSI values in the instrumental record, followed by the 1930s drought which is the most persistent drought of the last 100 years. The tree-ring record suggests that many single years exceed the severity of these years. The ring for 1934 was the smallest ring (driest spring) of the 20th century, but it was only the 25th smallest ring in the 509-year record. The smallest growth year in this longer time span is 1824, indicating severe drought conditions. Other very dry years were 1496, 1685, and 1579. Even when considering the 1930s and the recent droughts, the 20th and 21st centuries, in the context of the full tree-ring record, appear to be a period of moderate drought extremes. Several periods of time in past centuries are notable for a high occurrence of extreme drought years, including periods from about 1848-1880 and 1625-1685.
Reference: More information on the Canyon Road tree-ring chronology can be found in this publication:
Additional Resources: The Canyon Road tree-ring chronology is available from the International Tree-Ring Data Bank: Additional paleoclimatic information can be found at the NOAA Paleoclimatology Program web site: |
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