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Indices of annual diameter growth of trees were used to reconstruct drought in southern California back to A.D. 1700. A regional Palmer Drought Index served as predictand and tree-ring indices from eight sites as predictors in multiple linear regression analyses that yielded the prediction (reconstruction) equations. The regression explained 69 percent of the variance in Palmer Index in the period of calibration. The long-term reconstruction indicated that drought was rare in the first half of the current century relative to other discrete 50-year periods, and that based on evidence to date the last half of the 20th century may well turn out to be the most drought prone since A.D. 1700 in southern California.

(1975). A Long-Term History of Drought Occurrence in Western United States as Inferred from Tree Rings. Weatherwise: Vol. 28, No. 6, pp. 244-249.

Recently collected tree-ring data were used to reconstruct drought from 1700 to the present in four regionsflanking 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 regionallyaveraged annual precipitation from 1933 to 1977. Years of widespread severe drought clustered into droughtepochs lasting 5-10 years. A weighted mean of the four regional reconstructions pointed out the severity ofthe 1930's drought; the years 1934, 1936 and 1939 ranked among the driest 10 of 278 years. When droughtconditions were averaged over periods of three or more years, the 1930's drought was equaled or surpassedin severity by droughts in the 1750's, 1820's and 1860's. Spectral analysis of the 1700-1977 reconstructionindicated that precipitation averaged over the four regions had a penodicity of 16-19 years, but reconstructions for the individual regions deviated considerably fr...

Long-term streamflow series in the western United States were examined for evidence of secular changes related to climate. Streamflow series contained appreciable low-frequency variation related to the combined influence of temperature and precipitation. Evidence of nonstationarity was found in selected records for the Pacific Northwest and the Upper Colorado Basins: mean annual streamflow increased significantly (0.05 level) from the first to last half of the 1914–80 period in the Pacific Northwest, and decreased significantly over the same period in the Upper Colorado region. Correlation analyses and examination of drought years revealed a strong tendency for anomalies of opposite sign in the Pacific Northwest and the Southwest. Drought in the Upper Colorado Basin was statistically independent of drought in the Pacific Northwest. Under exceptional meteorological conditions (e.g., water-year 1976–77), however, low flows occurred over a vast area from the Northwest coast to the mountains of centr...

Previous tree-ring studies indicated that the total area affected by drought in the western United States has rhythmically expanded and contracted over the past 300 years, with a period near the 18.6-year lunar nodal and 22-year double-sunspot cycles. Recently collected tree-ring data from the U.S. Corn Belt for the years 1680 to 1980 were examined for evidence of either of these cycles on a regional scale. Spectral analysis indicated no periodicity in the eastern part of the Corn Belt, but a significant 18.33-year period in the western part. The period length changed from 17.60 to 20.95 years between the first 150 years and the last 151. High-resolution frequency analysis showed that the structure of the 18.33-year spectral peak was complex, with contributions from several frequencies near both the lunar nodal and double-sunspot periods. A t-test of difference of means in reconstructed annual precipitation weakly corroborated a previous finding of an association between drought area and the phase of the double-sunspot cycle. Both the high-resolution frequency analysis and the t-test results indicate that the periodic component of drought near 20 years is too weak and irregular to be of use in drought forecasting for the Corn Belt.

Families of Drought Area Indices (DAI) have been derived from tree-ring data for the entire U.S. west of the Mississippi River, for each year back to either 1700 or 1600 A.D., depending on the data base used. Each DAI is expressed in terms of the relative area in which the Palmer Drought Severity Index (PDSI) lies below a specified threshold value between −1 (mild drought) and −4 (extreme drought). Three families of DAI are considered in the analysis reported in this paper. Each DAI family is based on reconstructions from a selection of between 40 and 65 tree-ring sites ranging from Canada to Mexico and from the west coast to the Plains states. Variance spectrum analysis of the DAI series shows a concentration of variance at periods near 22 yr, at significance levels ranging from 5 to 0.1% (relative to a pink noise continuum). Band-pass filters tuned to periods near 22 yr are used in a form of harmonic dial analysis proposed by G. W. Brier to verify the extent of phase locking between the drought area variations and the Hale sunspot cycle since 1700 A.D. Phase locking is confirmed at significance levels of order 1 to 0.1% for all DAI families and all drought severity limits except extreme drought (PDSI < −4). A tendency is found for the amplitude of the 22 yr drought rhythm to vary systematically in parallel with the amplitude (envelope) of the Hale sunspot cycle, on the Gleissberg time scale of about 90 yr. This relationship is statistically significant between the 5 and 1% levels, and is independent of the phase locking found within the Hale cycle. The DAI series that extend back to 1600 A.D. reflect a well-defined 22 yr drought rhythm during the early stages of the Maunder Minimum of solar activity, but a very weak rhythm near the end of the Maunder Minimum. The average areal extent of drought was relatively low, and remained low for a prolonged period, during the Maunder Minimum. This analysis strongly supports earlier evidence of a 22 yr drought rhythm, or “cycle,” in the U.S. and suggests that the drought rhythm is in some manner controlled by long-term solar variability directly or indirectly related to solar magnetic effects. The solar control is best described as a modulation of terrestrial drought-inducing mechanisms, such that it alternately encourages and discourages the development of major continental droughts which are set up by evolutionary climatic processes unrelated to solar activity.