Spatial Pattern of Temporal Trends in Moisture Conditions in the Southeastern United States

Abstract

Palmer's Drought Severity Index (PDSI) is used to represent moisture conditions in the southeastern United States. Temporal trends in annual PDSI for 53 climate divisions in the region are examined for the period 1895 to 1988. Spearman's rank correlation reveals that 19 climate divisions have increasing trends in annual PDSI at the 0.05 significance level, changing towards a wetter condition during the study period. There is no division with a significant decreasing trend. Such temporal changes also display a welldefined spatial pattern. All but one of the divisions with significant increasing trends are located in the southwestern part of the region, centered at Alabama. Climate divisions in Alabama have experienced the most consistent change towards a wetter condition. The increasing rates in the annual PDSI during the study period range from 0.021/year to 0.037/year. Further analysis indicates that the increases in annual PDSI are mostly the result of increasing annual precipitation. Introduction Recent climatic fluctuations have attracted a great attention. Many studies focus on the temporal variation of climatic variables, such as temperature and precipitation. In the southeastern United States, temporal variations in temperature have been discussed by several workers. Although there is a general warming trend corresponding to the increase of CO2 in the atmosphere since the turn of the century, the temperature in the southeast has been characterized by a cooling trend since the 1930s with more colder winters lately (Barnett 1983, McCabe 1988, Skeeter 1985, Suckling 1986 and 1988). Studies on precipitation are fewer than that on temperature. There is probably a slight increase in precipitation in the southeastern United States. For example, Plummer (1983) found an increase in statewide precipitation for Georgia. Howarth (1989) also found an increase in statewide precipitation for Kentucky. However, changes in precipitation are often highly localized Geografiska Annaler · 75 A (1993) · 1-2 and few studies have been able to generalize a regional pattern. Recently, several studies are focused on droughts in the southeastern United States, probably prompted by the severe droughts occurred in the 1980s (Eder et al. 1987, Henry and LeBoutillier 1990, Soul6 and Meentemeyer 1989). Severe droughts have been recorded in the 1930s, 1950s, and especially the 1980s. 1981 was a very dry year for Georgia and South Carolina (Carter 1983). In 1986, the southeast was hit by one of the worst droughts in the century (Karl and Young 1987). This drought was ranked as unprecedented for the past 277 years in the southern Appalachian Mountains (Cook et al. 1988). The southeast was affected again in 1988 by a drought covering a large portion of the United States (Soule and Meentemeyer 1989). 1990 saw another dry year for Georgia and Florida (LeComte 1991). These severe droughts, particularly when occurring in consecutive years, are great threats to agriculture in the region, causing crop failures and lower income to farmers. Severe droughts also cause water supply shortage in urban areas where surface water is the main source, such as Atlanta. Furthermore, frequent wild fires are often associated with droughts (Izlar 1984). After 3 years of consecutive drought years in south Florida, the Everglades were on fire in early 1990. The fire continued for more than 2 months and burned more than 20,000 acres (LeComte 1991). A wild fire broke out later in 1990 in the Okefenokee Swamp National Wildlife Refuge in south Georgia and swept over more than 5000 acres in just two days. The refuge spent $6.6 million to fight the fire (Leavenworth 1991). Palmer's Drought Severity Index (PDSI) has been widely used in regional climatic studies because it summarizes the effect of changing temperature and precipitation. Although it was originally designed to represent drought conditions, its val-