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

This study examines how flash droughts influence wildfire spread across the conterminous United States. Using a model-derived root-zone soil moisture (RZSM) flash drought metric, the authors quantified relationships between flash droughts and wildfire spread dynamics across different land-cover types and compared these with conventional drought conditions defined by persistently low soil moisture without rapid onset. The analysis used ERA5 soil moisture data, MODIS burned-area and land-cover datasets, and wildfire clusters from 2001–2024 to evaluate spatial co-occurrence, spread rates and fire behaviour.

The study finds 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. In contrast, conventional drought conditions produce weaker and less consistent changes in fire spread. The authors conclude that rapid soil-moisture drawdown, rather than background dryness, provides a more direct pathway linking drought to accelerated wildfire spread. They recommend monitoring declines in root-zone soil moisture in the days to weeks preceding ignition and incorporating indicators of rapid soil-moisture decline into fire danger systems to improve early warning and preparedness.