The Keetch-Byram Drought Index and the Geographical Variability in Wildfire Size and Frequency in Eight Natural Areas of the United States

A continental-scale study of historic wildfire data within and across ecoregion provinces was conducted and geographical gradients in seasonal measures of wildfire size and frequency were observed. In the conterminous United States, western ecoregion provinces show north-south gradients in duration of season (short-to-long) and peak of season (early-to-late). Across the continent a gradient of unimodal to bimodal seasonal distributions of wildfire size and frequency was shown: western ecoregions have a single summer fire season and eastern regions have spring and late-summer fire seasons separated by an intervening dip in wildfire activity. From the ecoregion provinces with the highest wildfire frequency, average size, and area burned values, eight federal land units (four from the western and four from the eastern conterminous United States) were selected for a study of geographical variation in interactions between wildfire variables and the Keetch-Byram Drought Index (KBDI). Daily KBDI values for each location were provided by the North American Land Data Assimilation System (NLDAS). Confidence intervals around the mean for both days on which wildfires ignited and for days on which no new wildfires ignited were generated for each location using a bootstrap resampling method. A greater difference existed between nonire and fire-start KBDI values in the western locations, indicating a stronger association between KBDI and wildfire potential. At eastern locations, the difference between mean nonire and fire-start KBDI was lower than the minimum western mean difference for three of the four locations. The exception, the Great Dismal Swamp National Wildlife Refuge, showed the second highest difference between nonire and fire-start KBDI values of all eight federal land units. These results indicate that across the southeastern United States, the soil moisture (and, by extension, fuel moisture) cycle from field capacity (saturation) to drought (wilting point) and back to field capacity does not follow the regular seasonal pattern shown in the western states, and neither do geographical characteristics of wildfires