Classification of the wildland-urban interface for fire risk analysis in Los Angeles County, California

Wildfires in California play an important role in many ecosystems; however, when frequency rises, they can have deleterious effects on native flora and fauna. For humans living in vulnerable areas, wildfire can threaten the property and lives of residents. This is of particular concern for those living in the wildland-urban interface (WUI), where urban and open lands meet. Identifying areas making up the WUI and assessing fire risk is critical for proper land management. However, common definitions of these areas are quite general, often relying solely on a distance from housing structures to wildland vegetation or a threshold population density. In California, where wildfire occurrence has become prolific in recent years, the most recent effort to map high risk areas was carried out in the early '90s and has not been updated since. This study employed a method of WUI delineation developed by Lampin-Maillet et al. (2009) using housing density, vegetation aggregation, and terrain criteria to identify factors that contribute to wildfire risk (ignition density) and severity (burned area). In addition, classification of the interface area based on these factors allowed for an exploration of their interactions and the identification of areas that face particularly high wildfire risk or severity. This was done with the goal of determining where fire prevention resources and practices could be employed with the greatest benefit. It was determined that the WUI area in a subsection of Los Angeles County occupied 208,718 ha. Fire occurrence data including 1,802 fire ignition points and fire perimeters covering 139,178 ha were used to explore the relationship between fire risk and severity between classes of three risk factors. In addition 18 different WUI types were defined by combining classes of each risk factor to identify the combination of risk factors that results in the greatest wildfire risk or severity. It was found that housing density was the most important factor in determining wildfire risk, while slope was the most important in determining severity. Areas of intermediate housing density (1.2 houses/hectare) were at the greatest risk and experienced the greatest severity. Areas of high vegetation aggregation experienced the greatest severity, but the lowest risk. Finally, due to significantly high wildfire risk, the Castaic, Acton/Agua Dulce, and Leona Valley areas were identified as being ideal candidates for fire prevention programs focused on resident education and awareness. Given continued urban growth in Los Angeles County and the spread of urban development into wildland territory, this information will be critical to future decisions in urban planning and land management as well as to the direction of fire prevention resources.