Historical climate records for the Great Plains provide only a brief snapshot of the drought climatology for the region. For most portions of the region, climatic records cover the period since about 1900, and at only a few locations. To learn more about the occurrence and patterns of drought before that time, tree-ring data can be used to reconstruct the drought history of the region. These data provide insights into past climates extending back many centuries. Figure 1 is adapted from the work of Weakly (1965) for western Nebraska. His work was based largely on tree rings from Red Cedar for a 748-year period, from 1210 to 1958. The results of his study showed the occurrence of 21 drought periods of 5 years or more duration. The most remarkable of these drought periods was from 1276 to 1313, a period of 38 years. However, the average duration of the droughts was 12.8 years. A similar study was conducted in northern and southern Texas by Stahle and Cleaveland (1988) for the period 1698 to 1980. This analysis revealed numerous drought events during this nearly 300-year study period. The most severe individual drought years before 1900 for north Texas were 1772, 1790, 1805, 1855, 1872, and 1887. The years 1917, 1925, 1939, and 1956 were the driest years of the twentieth century. For south Texas, individual drought years before 1900 were 1790, 1805, 1855, 1857, and 1887, and the driest years of the twentieth century occurred in 1917, 1925, 1956, 1967, and 1971. Other tree-ring studies in the region confirm the recurrent nature of single and multiyear droughts in the region (Stockton and Meko, 1975, 1983).
Figure 2 provides a historical perspective of the percent area of the Great Plains in severe to extreme drought, according to the Palmer Drought Severity Index (PDSI) (Palmer, 1965) from 1895 to 1995. The PDSI is a meteorological drought index that integrates many variables in a water balance accounting procedure. This index is calculated routinely for each of the climate divisions in the United States. PDSI values commonly range from + 4.0 (extreme wetness) to — 4.0 (extreme drought), although values above and below these levels are often computed. For example, during August 1977, PDSI values reached — 7.0 in parts of the upper Midwest. For the Great Plains region, Figure 2 illustrates three interesting characteristics. First, the percent area in drought is highly variable from year to year. The peak drought year was 1934 in which 95% of the region was experiencing severe to extreme drought; other severe drought years were 1936 and 1956, with 90 and 80%, respec-
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