Wildfire

Forest and rangeland fires (wildfires) are sources of important ecological and economic damage, as well as serious risks to human lives and settlements. A wildfire is an environmental conflagration that [Page 1000]burns out of control and threatens people, buildings, or resources. In Australia, where wildfires are a particularly important natural hazard, they are known as bushfires. About 40 percent of Australia is susceptible to bushfire hazards (i.e., it has enough combustible vegetation), which are a particularly strong risk at the urban-wildland interface. Although it is the hottest part of Australia, the desert at the heart of the continent is not at risk; rather, it is the vegetation in less arid areas, especially those that are settled.

Wildfire is a natural phenomenon that has always existed where dry vegetation is present. The causes of wildfires can be divided into natural and anthropogenic. Lightning strikes are a natural cause whose importance varies from place to place with meteorological and vegetational conditions. Human negligence and vandalism have come to be dominant causes, especially at the urban-rural interface. It is thought that vegetative fermentation leading to spontaneous combustion may cause fires, but the evidence for this is extremely scarce. The frequency of wildfires in the past can be calculated by assessing damage to the xylem (wood) of trees and dating it by counting the annual rings, a process known as dendrochronology. Indications may also be present in the sedimentary record, as fires are often followed by episodes of accelerated erosion and deposition.

The essential ingredients of a wildfire are combustible material (fuel), heat, and oxygen. The dimensions, duration, and intensity of wildfires vary according to several group factors. The first of these includes the type, flammability, and water content of combustible material (Table 1). Together, these represent the fuel loading factor. Second are meteorological conditions, in particular temperature, ambient humidity, and the speed, direction, and variability of winds. Finally, the topography and ground slope in the local area interact with the other factors to exert some control on rates and directions of burning. In fact, a wildfire can climb a slope, propelled by convection of wind and radiation of heat.

Table 1 Characteristics of fuels in relation to wildfire occurrence

In geographical terms, fires tend to spread in a nonlinear manner. Large expansions in area occur during the early phases as abundant fuel is consumed, and asymptotically smaller expansions are characteristic of later periods as fuel is consumed or the fire is brought under control. Owing to their demand for oxygen, large fires create their own drafts, which can lead to fire-induced convection and a positive feedback situation in which the fire creates substantial winds, effectively a firestorm.

There are various classifications of wildfires. The simplest and most common has six categories. A “ground fire” involves combustion without flames in the roots and undergrowth. A “surface fire” may occur in rangelands, bushes, and plant litter. In a “dependent crown fire,” the crowns of trees catch fire but the fire is propagated in the undergrowth. In contrast, a “running crown fire” propagates the flames rapidly through the tree crowns. This may lob burning brands ahead on the wind and thus cause a localized, concentrated “spot fire.” Finally, if the wind changes direction and the combustible material has not been completely consumed, the result may be a “backfire.”

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