Wildfire

A wildfire is an unplanned, unwanted wildland fire. Wildfires include unauthorized human-caused fires, escaped wildland fire use events (where appropriate management response to naturally-ignited wildland fires escape), escaped prescribed fire projects, and all other wildland fires where the objective is to put the fire out (Botts, et al., 2015). While this section’s emphasis is on wildfires as an unwanted hazard, it also discusses wildfire in the context of how and why wildland fires occur.

A wildland fire is any non-structure fire that occurs in areas of vegetation or natural fuels, and can be either prescribed fire or wildfire. Wildland fire occurs when vegetation, or “fuel,” such as grass, leaf litter, trees, or shrubs, is exposed to an ignition source and the conditions for combustion are met, resulting in fire growth and spread through adjacent combustible material. Wildland fires are either ignited by lightning or by some consequence of human activity. In Colorado, lightning accounts for only 17 percent of wildfires, with human ignitions accounting for the remainder (Colorado Natural Hazards Mitigation Plan, 2013). Human causes vary and can include escaped debris pile burning, campfires, fireworks, construction sparks, downed transmission lines, and arson.

Wildland fires can occur during any time of year. Although there are frequent references to a “fire season,” ignitions are a result of the ability of fuels to support combustion. In addition to an ignition source, the fuel type, amount of fuel, distribution pattern, and moisture content—coupled with weather and topography—will determine the conditions for combustion and resulting fire behavior. Fire behavior characteristics, often referred to as “outputs,” include intensity, residence time (i.e., the time required for the active flame zone to pass a stationary point at the surface of the fuel), rate of spread, ember production, ember transport distance, and fire size. These fire behavior outputs determine the influence the wildfire has on adjacent and surrounding fuels through radiant, convective, and conductive heat.

Wildland fire is a natural ecological disturbance process, and in many cases it is necessary ecosystem health. Historically, "natural" fire varied in size, intensity, and severity, creating a mosaic of native vegetation communities across different landscapes. Multiple fire events will occur over time and the frequency and length of the fire return interval is dependent upon the vegetation type and climatic conditions. This natural variation of fire has declined in North America over the past two centuries due to a number of human influences. These influences have significantly altered the natural fire regime and created extensive areas of homogeneous forests (forests of the same composition including trees of the same age, size, species etc.), causing a significant and widespread change in fire effects and fire’s influence on ecosystems and people.

Wildfires become wildland-urban interface fires when they transition from natural areas of vegetation to a combination of vegetation and the built environment, such as the Waldo Canyon Fire in 2012. Fire Adapted Communities, Waldo Canyon Fire 2012, National Interagency Fire Center Photo Gallery. Kari Greer/US Forest Service

 

The introduction and increasing growth of development adjacent to and intermixed within the natural vegetation across the landscape poses additional risk to people and property. In the context of wildfire, the combustible components of buildings, infrastructure, and associated accessories make them susceptible to ignition and are also considered fuel for the fire. A fire burning in this situation has transitioned from a wildfire to a wildland-urban interface (WUI) fire, where a combination of vegetation and man-made structures provide fuel for the fire. This situation increases the complexity, cost, and risk of wildfire in Colorado. In most WUI fire situations, fire suppression resources are quickly overwhelmed and multiple structures are lost.

 

The terms wildfire hazard and wildfire risk are distinctly different. Wildfire hazard refers to the fuels in a given location and represents the intensity with which an area is likely to burn if a fire does occur there. Wildfire risk is the probability and consequence of a wildfire burning in an area (based on the wildfire hazard, potential losses, and weather conditions). Identifying wildfire hazard is an important first step in assessing the risk of wildfires. Wildfire risk assessments can be analyzed on different spatial scales, depending on the intended use of the assessment.