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Southeastern Geographer Vol. 29, No. 1, May 1989, pp. 17-25 THE DROUGHT OF 1988: HISTORICAL RANK AND RECURRENCE INTERVAL Peter T. Soulé and Vernon Meentemeyer INTRODUCTION. The drought of 1988 received considerable press coverage from early spring until late summer. (1) By the end of June, over half the nation's counties (approximately 1,700) had been declared drought disaster areas by the U.S. Department of Agriculture. (2) The negative effects of this drought were far reaching. In addition to ruined crops and pastures, record low flows on streams and rivers disrupted the water-borne shipping industry and led to water restrictions for many municipal and industrial water users. The drought also was blamed for increased erosion oftopsoil, numerous forest fires, dried-out wetlands, a decrease in the ability of streams and rivers to filter and disperse pollutants , and other natural and human-related problems. Unfortunately, the geographic variation of this drought was treated in only the most general sense (e.g., the South, the Great Plains). The primary purpose of this paper is to put the drought of 1988 into historical perspective. It will address two questions: 1) what was the spatial extent and historical rank of the 1988 drought at its peak in June 1988, and 2) what was the areal pattern of its probability of recurrence (in years). Historical rank is often used in studies ofextreme streamflow, temperature, and precipitation. (3) Recent examples of drought analysis using statistical recurrence intervals include Carter's study of the 1980-81 hydrologie drought in Georgia and Karl and Young's study of the 1986 drought in the Southeastern United States. (4) At any given time in the United States, a place or region with drought of some intensity can be found. One can envision droughts from the perspective of spatial extent, intensity at a place, duration, or some mixture of these three measures. Even when using a numerical index to monitor drought conditions, delimiting precise beginning and ending times for drought events becomes a subjective process. For this study we chose to exclude the temporal aspects ofthe 1988 drought in order to focus on questions of spatial extent and severity. In the beginning of Mr. Soulé is a Ph.D. student in Geography and Dr. Meentemeyer is Professor of Geography at the University of Georgia in Athens, GA 30602. 18Southeastern Geographer July 1988, drought was extending to some new areas but was retreating in other areas. Consequently, we chose June to represent the "peak" of the 1988 drought. Because June represents a critical month for many agricultural activities in the United States, it provides a logical base for a historical comparison of drought conditions. DATA. June values of the Palmer Drought Severity Index (PDSI) from 1895 to 1988 were gathered for the 344 climatic divisions in the contiguous United States. (5) For the period 1895-1985, the data were extracted from the National Climatic Data Center's (NCDC) magnetic tape file TD9640. (6) For the period 1986-1988, the data were taken from National Weather Service (NWS) maps of values for the climatic divisions published in the Weekly Weather and Crop Bulletin. For each of these latter three years, the map with the latest June data was used. (7) The PDSI is probably the most widely accepted measure of drought severity in the United States. Palmer's definition of drought, a long period of moisture deficiency for the station or division under investigation , is based on deviations from climatic normals for each station or division. (8) This makes PDSI values comparable through time and space. The PDSI is described as a measure of general or "meteorological " drought conditions. (9) As such, it has a moderate rate of response to supply of water from precipitation and demand for moisture by évapotranspiration. PDSI values are calculated using temperature, precipitation , and various historical and current water budget components (e.g., runoff, soil-water recharge, actual and potential évapotranspiration ) as input. Index values of - 1.0 to - 1.9 are termed mild drought; -2.0 to -2.9, moderate drought; -3.0 to -3.9, severe drought; and values less than -4.0, extreme drought. Positive values indicate above normal supplies of...

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