Rapid soil moisture depletion during flash droughts accelerates wildfire spread in the United States

Abstract

Abstract Fire danger can escalate rapidly when vegetation dries over days to weeks. Flash droughts, characterised by rapid root-zone soil moisture depletion under high evaporative demand, may accelerate wildfire spread. Here, we quantify relationships between flash droughts and wildfire spread dynamics across land-cover types in the conterminous United States, using a model-derived root-zone soil-moisture flash-drought metric and contrast these with conventional drought conditions defined by persistently low soil moisture without rapid onset. We find that fires following flash droughts exhibit faster early spread than fires not associated with flash droughts, with model estimates indicating about 1.3–1.8-fold faster early burning across major flammable land covers. Grasslands and croplands show the strongest acceleration, followed by open shrublands, woody savannas, savannas and deciduous broadleaf forests, consistent with faster drying and greater fuel continuity in fine-fuel ecosystems. In contrast, conventional drought conditions produce weaker and less consistent changes in fire spread. Our results demonstrate that rapid soil-moisture drawdown—rather than background dryness—appears to provide a more direct pathway linking drought to accelerated wildfire spread. We recommend that monitoring declines in root-zone soil moisture in the days to weeks preceding ignition could improve early warning of fire risk in ecosystems prone to rapid drying.