Quantifying wildfire risk to the built environment in rural rangelands of the western U.S.

Data and preliminary analyses

The skinny

Invited paper for Philosophical Transactions of the Royal Society B themed issue Novel fire regimes under climate changes and human influences: impacts, ecosystem responses and feedbacks

The pitch

The following mini-Abstract included in the original themed issue proposal and accepted by guest editors:

Two global change processes that create challenging, novel conditions for fire management include alterations in the frequency and seasonality of wildfire, and the expansion of the human population into areas dominated by natural vegetation (known as the wildland-urban interface). Research and media about the wildland-urban interface has focused on human-driven land use changes in forest and chapparal-type ecosystems, while open ecosystems like rangelands have received much less attention and are generally viewed as rural and empty. But in the western United States, human populations sprawl from mid-size towns and small cities; meanwhile, woody plant encroachment threatens to increase the intensity of, and difficulty to control, fire in these areas. We use publicly-available, spatially-explicit data to characterize spatial and temporal trends in wildfire threat to human development in rural US rangelands.

Objectives

• Describe extent of interface between wildland and built environment (WUI) that occurs in rangeland

  • Total area
  • Total number of buildings
  • Overall risk to properties (risk \(\times\) number of properties)

• Describe rural extent of WUI (distance from town = emergency response time)

• Describe socio-economic trends of rangeland WUI

  • Do areas with greater number of non-whites have greater risk?
  • Are lower-income folks at greater risk in rural areas?

Available data

• Vegetation & fire risk
  • Extent of US rangelands (30 m resolution raster)
  • WUI designation (30 m resolution raster)
  • Property risk ratings (count of properties per risk class by US Census tract)
• Remoteness
  • Rural/Urban Commuting codes (by US census tract; shapefile)
  • Distance from urban areas > 2500 ppl (calculated)
• Socio-economic
  • Population, incl. proportion Black/White/Native American (2020 US census)
  • Median household income (2020 American Community Survey)
  • Rural Capacity Index (2023 Headwaters Economics)

Study area

EPA Level I ecoregions in the portion of the Interior West included in the analysis.

Overview of results

Substantiating the premise

First off: is this inquiry even worth it?

Is rangeland WUI a thing?

Yes. There are over 1 million km² of rangeland designated as WUI, the largest of all land cover classifications.

Is rural WUI a thing?

Yes. Strictly rural census tracts add up to the greatest WUI area in the Interior West, with nearly 80,000 km^-2.

General patterns

Rangeland and WUI

5% of all rangeland in the Interior West is identified as WUI (103,301 km²). Among the three rangeland classifications, those undergoing succession to woody species–both those currently undergoing increasing shrub cover (transitional) and those with >25% tree cover (afforested)–are more likely to be identified as WUI than regular rangeland.

Greater woody cover in rangeland WUI likely increases severity of fire risk and control costs.

Relative area of three rangeland classifications identfied as WUI. ‘Transitional’ rangelands are dominated by herbs and shrubs and are at risk of transition to woody dominance; ‘Afforested’ rangelands those with historically < 25% cover of tall shrubs and trees but woody cover currently exceeds 25%.

Across the Interior West, most of the rangeland designated as WUI (61%) is more than 10 km from the nearest town. 10% of it is between 50-75 km from town, while only 4% is further than 75 km.

Tribal areas are more remote, with 55% of rangeland designated as WUI more than 25 km from a town.

Distribution of rangeland area by distance from nearest town of at least 2500 persons. Distance categories derived from USDA ERS Frontier and Remote codes.

Fire risk

Rangeland and WUI distribution

Proportion of each county classified as Rangeland in USFS Reeve’s Rangeland product.

Proportion of each county classified as WUI in USGS product.

Available data

Maps showing the types of data included in the project, ranges, and known geographical gaps

Census data

Population

Total population by county and population density by county, plotted as discrete groups due to huge spread in the ranges of both datasets.

Percentage of each county’s population identifying as one of three single-race demographic groups measured by the US Census.

Top: Median household income from the US Census; Center: The Rural Capacity Index from Headwaters Economics ‘is based on 10 variables that can function as proxies for community capacity. The variables incorporate metrics related to local government staffing, community education and engagement, and socioeconomic trends’; Bottom: County-level Rural/Urban distinctions from USDA ERS based on 2010 census data,

Properties and wildfire risk

County-level data on wildfire risk to properties from First Street.