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A significant risk from fire is posed to the people and houses in California, as more structures are built in areas with a significant wildland Fire Threat.

Areas of concern regarding housing and public safety were defined as those areas that have a structure density of 1 house per 40 acres, or denser, as calculated from the 1990 census block data. The census data is resolved into polygons called “blocks”, designed to hold roughly 400 people, and consequently vary widely in size and shape depending on the nature of development in a given area. Often, census blocks include many areas that are not typically developed, so the density of housing is not accurately represented by dividing the number of houses by the acres in the census block. To resolve this problem, staff “migrated” the density from areas of restricted development to areas of non-restricted development. Federal land is considered restricted development land in this analysis (houses in the wildland are on private ownership rather than federal ownership, generally).

The final step in the analysis was to search for all areas identified in the urban- interface layer that were in the vicinity of fire threats. Staff defined vicinity as all areas within 1.5 miles of a fire threat. Consequently, all areas with WUI values from 1 to 3 (i.e., densities greater than one house per 40 acres except those not supporting wildland fuels or in large urban centers) were labeled with the highest threat rank within a 1.5 mile radius. A 0.25 mile high density buffer for the urbanized density class (i.e., greater than 1 house per 0.5 acre) was included to account for the peripheral areas of urban centers abutting wildlands. Hence, high density areas lying immediately adjacent to wildlands would be included, but not those urbanized areas in the central parts of cities. The resultant map of threatened Wildland- Interface areas shows not only the aerial extent of affected areas, but also the relative fire threat to those areas.

A total of 7.8 million acres of California are developed with housing unit densities considered to meet the Wildland-Urban Interface (WUI) criteria. Of this total, 920,000 acres (12 percent) are exposed to an Extreme Fire Threat, 3.4 million acres (43 percent) to a Very High threat, and an additional 1.2 million acres (15 percent) to a High threat. If we consider all WUI lands with threat levels greater than Moderate to be at significant risk to damage from fire, the total area is 5.5 million acres, or 70 percent of the total WUI area.

A total of 11.8 million homes are located in the WUI. Of this, 4.9 million housing units (42 percent) are exposed to High or greater Fire Threat. Furthermore, of these, 4.1 million homes (84 percent) are from urban areas, where density of housing units exceeds one unit. Thus while the land area considered WUI is dominated by areas of relatively low development density, the majority of houses at risk come from urbanized areas.

An additional 8.6 million acres of wildlands surrounding WUI communities pose Very High or Extreme Fire Threat conditions. These lands represent many of the areas requiring mitigation treatments to reduce risks to people and property of the State.