Soil moisture models unlock better fire spread forecasts
Fire managers rely on fuel moisture and wildfire danger models to assess conditions for prescribed burning and prioritize resources during busy wildfire seasons. These models have historically focused on the moisture content of surface fuels, like sticks and logs, but have ignored the role of soil moisture and its effects on living fuels, such as shrubs and trees. As it turns out, soil moisture is a critical driver of fire spread and can be used to produce operational maps of fire spread potential.
Two recently published studies shed light on the role that soil moisture plays in regulating fire spread potential in the U.S. Northern Rocky Mountains and Pacific Northwest. Using thermal imagery from satellites, researchers reconstructed daily fire progression maps for several hundred recent wildfires and then extracted the weather, vegetation and fuel moisture conditions for the day and location of each burning day. They also included soil moisture and live vegetation stress as potential indicators of daily fire spread rates. From more than 50 different weather, fuel and dryness indicators, soil moisture and vegetation stress were the strongest overall predictors of fire spread and significantly improved the accuracy of models when they were included.
Researchers developed simple versions of these models and integrated them with daily weather and soil moisture forecast data from the National Weather Service to provide daily fire spread forecast maps for the Northwest United States. Fire managers have used these maps for the past 3 fire seasons and found that they are useful, consistently representing conditions on the ground and often coinciding with emerging wildfires.
These studies highlight the critical role that water content of the soil plays in regulating fire spread in this region of the United States and provide a way to incorporate soil moisture models into operational fire danger assessments.
Daily fire spread potential forecasts for the Sawlog Fire. Upper plots show probability of fire spread greater than 70 acres; lower plots show predicted acres of new growth. Black dot on each map shows the approximate fire start location. Photo on the right shows smoke from the Sawlog Fire on May 3rd, 2025.