DROUGHT RECURRENCE IN THE GREAT PLAINS AS RECONSTRUCTED FROM LONG-TERM TREE-RING RECORDS.

Recently collected tree-ring data were used to reconstruct drought from 1700 to the present in four regions flanking the Great Plains. Regions were centered in Iowa, Oklahoma, eastern Montana and eastern Wyoming. Reconstructions derived by multiple linear regression explained from 44 to 56% of the variance in regionally averaged annual precipitation from 1933 to 1977. Years of widespread severe drought clustered into drought epochs lasting 5-10 years. A weighted mean of the four regional reconstructions pointed out the severity of the 1930's drought; the years 1934, 1936 and 1939 ranked among the driest 10 of 278 years. When drought conditions were averated over periods of three or more years, the 1930's drought was equaled or surpassed in severity by droughts in the 1750's, 1820's and 1860's. Spectral analysis of the 1700-1977 reconstruction indicated that precipitation averaged over the four regions had a periodicity of 16-19 years, but reconstructions for the individual regions deviated considerably from this result. The Iowa region was dominated by a 22-year periodicity, the Oklahoma region by a 17-23 year periodicity, and the other two regions by a relatively strong 60-year periodicity. Separate analysis of 88-year subperiods of reconstruction indicated that evidence for a 22-year periodicity was strongest in the most recent period (1890-1977), weaker for 1802-89 and lacking entirely from 1714 to 1801.