Flash Droughts and Wildfire Interactions: Influence of Detection Methods on Fire Risk and Speed Across U.S. Landscapes

Flash droughts are characterised by rapid soil-moisture depletion driven by elevated atmospheric evaporative demand from higher air temperatures, low humidity, stronger solar radiation and wind, especially when precipitation is limited. This heightened atmospheric evaporative demand enhances evapotranspiration, accelerates moisture depletion in the root zone and intensifies vegetation water stress. As plants dry and weaken, their flammability rises, creating a feedback loop that elevates wildfire risk during prolonged heat and drought conditions.

This study investigates the relationship between flash drought and wildfire dynamics using two commonly used methods of flash drought detection across diverse land-cover types in the continental United States. We show that the frequency and spatial patterns of flash drought and its relationship with wildfire is significantly influenced by the method used for flash drought detection. Flash drought events identified by the Standardized Evapotranspiration Stress Ratio (SESR) capture atmospheric evaporative stress, while Root Zone Soil Moisture (RZSM) reflects sustained soil drying that directly increases fuel flammability. Approximately 53% of fires occurred after flash droughts identified using SESR definition, whereas RZSM classified about 10%, with each producing different spatial footprints.

To quantify how flash drought alters fire evolution, we applied Kaplan–Meier survival analysis to time-to-burn, estimating the probability that pixels remain unburned as a function of time since ignition under flash drought and non-flash drought conditions, and used Cox proportional-hazards models to derive hazard ratios (HR), which measure the relative instantaneous burning rate under FD (HR > 1 indicating faster spread). Grasslands and croplands show the highest vulnerability to flash drought–related fires due to their fine, continuous fuels that rapidly dry and ignite, with stronger acceleration and earlier spread under RZSM identified flash droughts (HR ≈ 1.45 in grasslands, 1.33 in croplands, 1.84 in open shrublands; woody savannas ≈ 1.17), while SESR effects are small or near zero in several covers (HR ≈ 1.05 in croplands and grasslands; ≈ 0.99 in woody savannas).

We therefore recommend incorporating rapid soil-moisture drying dynamics into wildfire risk models and enhancing real-time monitoring to strengthen early warnings and fire management, especially in ecosystems prone to swift drying and ignition.