Identifying Areas of High Risk to Wildfires

ArcGIS Spatial Analyst, ArcGIS Geostatistical Analyst, and ArcGIS 3D Analyst Working Together

Values, hazards, and occurrences need to be addressed to accurately identify areas at high risk of wildfires. In the western states, local communities are becoming more concerned about fire risks in the wildland-urban interface. As more and more people leave traditional urban areas to live in the mountains and countryside, effective resource management plans must balance the needs of the ecosystem along with our human values.

A community risk assessment map can be created by addressing three components:

• The human values we place on an area
• The hazards
• The probability of occurrence

By mathematically combining these three components, an analyst can obtain a final risk assessment—in this case, of the Boulder County area in Colorado.

Values

The first step is to define the human values or, more specifically, where people live. The building density function in ArcGIS Spatial Analyst is used to identify where most people live and to map their proximity to wildland areas for determining wildfire risk.

From the ArcGIS Spatial Analyst menu, the analyst chooses Density, selects the variable that contains the number of buildings on each parcel, and sets the search radius to 3,000 feet.

With this map, the analyst identifies the areas with low and high numbers of residents.

Hazards

To gain a better understanding of which areas have the highest wildfire hazard risks, an analyst needs to create a slope grid and an aspect grid from the elevation data.

A slope grid identifies a maximum rate of change from each cell to its neighbors. An output slope raster dataset can be calculated as either a percentage of slope (for example, 10 percent slope) or a degree of slope (for example, 45-degree slope).

Aspect identifies the steepest downslope direction from each cell to its neighbors. The value of the output raster dataset represents the compass direction of the aspect: 0 is true north, a 90-degree aspect is to the east, and so forth.

In general, wildfires typically move faster within areas of high slope. Also, fire mitigation experts can more accurately identify the direction the fire may move based on the aspect or steepest upslope direction.

These problems can be modeled using the slope and aspect functions in ArcGIS Spatial Analyst, which create raster surfaces for aspect and slope.

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