Measured Soil Moisture is a Better Predictor of Large Growing‐Season Wildfires than the Keetch–Byram Drought Index

Abstract

Core Ideas Fraction of available water capacity (FAW) was determined from measured soil moisture. Wildfire relationships were compared for FAW and Keetch–Byram drought index (KBDI). Growing‐season wildfire danger was more accurately represented by FAW than KBDI. Neither FAW nor KBDI alone accurately represented dormant‐season wildfire danger. We recommend replacing KBDI with FAW in growing‐season wildfire danger assessments In situ soil moisture measurements have the potential to improve wildfire danger assessments, which often rely on the Keetch–Byram Drought Index (KBDI) as a soil moisture surrogate. However, the relative merits of measured soil moisture and KBDI as indicators of wildfire danger are unknown. Therefore, our objectives were to (i) identify relationships between drought indices (KBDI or fraction of available water capacity, FAW) and wildfire size for 34,939 growing and dormant‐season wildfires, (ii) compare relationships between each drought index and wildfire probability for 501 large (≥ 405 ha) growing‐season and dormant‐season wildfires, and (iii) quantify relationships between KBDI and FAW for each season in Oklahoma, the United States. Neither KBDI nor FAW accurately represented dormant‐season wildfire danger, with wildfires ≥ 121 ha occurring across nearly the entire range of each index. During the growing season, however, we found that a smaller percentage of wildfires ≥ 121 ha occurred under extreme levels of KBDI than under equivalent levels of FAW (66% vs. 81%), and a logistic regression model based on FAW correctly classified more growing‐season days with large wildfires than the KBDI model (84% vs. 79%). Furthermore, while FAW represented soil moisture in near real‐time, KBDI responded slower to soil drying and recharge, so FAW provided about 10 d earlier warning of extreme wildfire potential for the 10 largest growing‐season wildfires in our study. We therefore recommend replacing KBDI with FAW in growing‐season wildfire danger assessments in Oklahoma and regions with similar climate (temperate, subhumid to semiarid) and vegetation types (primarily herbaceous).