Wildfires: Risk and Hazard

Analysis of the threat, hazard, and risk relative to wildfire is a spatial problem: Where does a wildfire occur relative to resources valued by humans? In the natural or built environment? Wildfires burn uncultivated vegetation in natural environments, whether mountainous forests, rural woodlands, brush lands, or prairie grasslands. In common usage, the terms threat, hazard, and risk are often interchanged and confound understanding of the important distinctions between them.

Any wildfire may be considered to pose a threat, to present the potential to do harm. Threat refers to the source that could cause damage. A wildfire threat exists where heat from burning vegetation could ignite other combustible material regardless of whether burning that material would constitute a loss. Whether a wildfire poses hazard or risk depends on whether any valued resources, natural or human-made, that could be damaged by fire are potentially in a path of fire spread. Unless some resource determined to be of value is in harm's way, no hazard or risk is associated with a wildfire.

The term hazard may be most effectively used when planning to mitigate potential losses in some future, nonspecified time. Efforts to thin forest fuels around homes well in advance of the wildfire season constitute hazard reduction. This action is taken in the absence of an immediate threat. Fuel reduction usually occurs where it is recognized that the potential for loss would be high if a wildfire were to occur some time in the coming years.

Analysis of risk explicitly requires assessment of the probability that a loss will occur as a result of wildfire. What is the chance or likelihood that a fire will reach and damage or destroy a valued resource? The probability statement may be qualitative, based on some form of actuarial analysis, or derived from quantitative techniques such as numerical fire spread modeling. Strictly applied, risk analysis is the calculation of expected loss, the value of the threatened resource multiplied by the expected value change to that resource.

For example, if a fire is ignited on a heavily forested mountain, the mere presence of an active flame is commonly considered to present a threat. If a communications tower is downwind or otherwise in the vicinity of the flames, the fire presents a hazardous condition relative to the tower. This statement of hazard does not assume one way or the other that the fire will actually reach and damage the tower. If the tower is valued at $250,000 and there is a 50% chance that the fire will reach and destroy the tower, the tower is at risk. The expected loss is calculated at $125,000. This calculation could be extended to loss of services if a value for the loss could be clearly defined.

Analysis of hazard and risk posed by wildfire is fundamentally a two-part spatial problem. It starts with threat analysis: Where is the fire currently located, and where is it headed? The second part analyzes hazard and risk. Hazard assessment identifies the valued resources that wildfire may intersect as it spreads. Risk assessment considers the probability that valued resources will be damaged or destroyed so that the value will be changed. This spatial intersection of the potential area of wildfire spread with valued resources may be implied by observation and judgment or explicitly determined through rigorous geospatial analysis commonly conducted within geographic information systems.

...