The Encroachment Factor (EF) or Encroachment Probability is a raster-based input used to represent the relationship between wildland vegetation and developed areas in the Wildland-Urban Interface (WUI). It helps the Urban Conflagration (UC) model identify locations where fire behavior may be influenced by the proximity of vegetation to structures, infrastructure, or other built-environment features.
EF is provided as a geospatial raster layer aligned to the simulation domain and processed with the other input rasters used by the fire model. Each raster cell contains a value that characterizes the degree of vegetation encroachment for that location.
In the current UC workflow, the Encroachment Factor is interpreted as a distance-based value, where the raster value represents the distance from a cell to nearby vegetation. Higher or lower values are used by the model to determine how WUI-specific fire behavior should be represented in that area. Areas with valid encroachment values are treated as locations where the transition between vegetation and the built environment may affect fire spread and impact calculations.
EF supports WUI modeling by helping the system distinguish between areas that are primarily natural fuels, areas that are primarily developed or non-combustible, and areas where vegetation and development are close enough to influence fire behavior. This allows simulations to better account for potential fire movement into or through developed areas and improves the representation of exposure near structures and communities.