Were bigger Oklahoma wildfires linked to drier soil?

Details

Soil moisture data are becoming more available for wildfire danger assessment, but research on how soil moisture relates to wildfires is scarce. Researchers set out to identify relationships between soil moisture and wildfire size in Oklahoma during the growing season (May–October) and dormant season (November–April). They hypothesized that soil moisture influences fire size when vegetation is growing but is less important when most vegetation is dead or dormant.

For 38,419 wildfires from 2000–2012, the researchers determined soil moisture as a fraction of available water capacity (FAW), along with commonly measured weather variables. They grouped fires into five size classes: <4.05, ≥4.05 and <40.5, ≥40.5 and <121, ≥121 and <405, and ≥405 hectares. They used the Kruskal–Wallis test with multiple comparisons to identify differences in each variable between fire size classes and seasons.

Large fires occurred at lower FAW than small fires in both seasons (P < 0.001). Growing-season fires of ≥405 hectares occurred over a narrow FAW range of 0.05–0.46, whereas dormant-season fires of that size occurred across the entire FAW range of 0.05–1.05. Among growing-season fires of ≥121 hectares, 91% occurred at FAW <0.5 and 77% at FAW <0.2.

The researchers concluded that the occurrence of large growing-season fires exclusively under low soil-moisture conditions highlights the need to develop methods for using soil moisture data in wildfire danger assessments.