Soil Moisture Affects Growing-Season Wildfire Size in the Southern Great Plains

The increasing availability of soil moisture data presents an opportunity for its use in wildfire danger assessments, but research regarding the influence of soil moisture on wildfires is scarce. Our objective was to identify relationships between soil moisture and wildfire size for Oklahoma wildfires during the growing (May-October) and dormant seasons (November-April). We hypothesized that soil moisture influences wildfire size when vegetation is growing but is less important when most vegetation is dead or dormant. Soil moisture, as fraction of available water capacity (FAW), and commonly measured weather variables were determined for 38,419 wildfires from 2000–2012. Wildfires were grouped by size class (<4.05, ≥4.05 and <40.5, ≥40.5 and <121, ≥121 and <405, and ≥405 ha), and the Kruskal–Wallis test with multiple comparisons was used to identify differences in each variable between wildfire size classes and seasons. Large fires occurred at lower FAW than small fires during both seasons (P < 0.001), but growing-season wildfires ≥405 ha occurred over a narrow range of FAW (0.05–0.46) whereas dormant-season fires of this size occurred across the entire range of FAW (0.05–1.05). For growing-season fires ≥ 121 ha, 91% occurred at FAW < 0.5 and 77% occurred at FAW < 0.2. Our finding that large growing-season wildfires occurred exclusively under conditions of low soil moisture highlights the need to develop methods to use soil moisture data in wildfire danger assessments.