How are community wildfire risks determined?

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Learn about the methods used to calculate a community’s risk of wildfire.

Wildfires can have an indirect impact on you even if your home is not burned, by impacting the broader social, commercial, and community infrastructure surrounding your home. The First Street Foundation Wildfire Model is used to determine the likelihood of direct wildfire impact or indirect exposure to embers from nearby wildfires. The probability of being in a wildfire and exposure to embers is used to determine which structures are at risk, the number of structures at risk, and the severity of risk within a community. Use Risk Factor to find community-level wildfire risk assessments.

Community Risk

Wildfire risk impacts more than individual property. Community risk measures the impact wildfires can have on day-to-day life within the surrounding community. Wildfires that impact infrastructure can cut off access to utilities, affect emergency services and transportation, and can impact businesses and the finances of homeowners. Community Risk shows how wildfires can impact various community levels, such as neighborhoods, zip codes, municipalities/cities, and counties. Because community risks can vary for different levels of geographies, Fire Factor® allows you to compare risks between these levels. 

Communities available through community risk overview

  • Neighborhoods: Encompasses macro neighborhoods, neighborhoods, sub-neighborhoods, and residential districts (e.g. subdivisions and apartment complexes) are not an official designation and vary in size depending on local usage
  • Zipcodes: are zip code tabulation areas as provided by the U.S Census Bureau. 
  • Cities: Place as provided by the US Census Bureau. Refer to a village, town, or city typically governed by a mayor and council.
  • Counties: are territorial divisions of a state and are typically government units that sit below the state level. County or county equivalents as provided by the US Census Bureau.

Wildfire Risk

The First Street Foundation Wildfire Model is used to determine the probability of a facility being in a wildfire or its exposure to embers. 

Some communities may be located in an area that is directly exposed to wildfire from nearby vegetation and fuel sources. Anything that can burn is fuel for a fire – various plants such as grass, shrubs, trees, fallen trees, and dead leaves. As this vegetation piles up, the chance of it burning, or fueling a fire, increases. The type of fuel source impacts how intense a fire is and how quickly it can spread under different weather conditions. The location and intensity of historic fires are used to determine the most likely potential ignition locations for future wildfires.

Other communities are located in an area indirectly exposed to wildfire from embers. Embers are small pieces of material that remain after a fire, radiate a substantial amount of heat, and are light enough to be carried by the wind for long distances without being extinguished. Facilities in an urban area with little vegetation can be indirectly exposed to distant wildfires from embers carried aloft by the wind.

In order to provide the most accurate current and future wildfire risk assessments, the Model simulates over 100 million wildfires for the 2022 and 2052 simulations to see which of these wildfires grow and become damaging. It outputs over 8 million significant fires per simulation that are then used to calculate risk. Publicly available and 3rd party data is used to identify property boundaries and buildings. The building’s location is then used to determine the likelihood of a specific facility being in a wildfire based on how many times the 8 million simulated fires reached the structure

Calculation of Community Risk

To determine the Community Risk of wildfires, the number of facilities in each category with direct and indirect exposure to wildfires this year and in 30 years is separately calculated. The average probability of being in a wildfire for all facilities in a community is calculated for each category: (1) Infrastructure (utilities, airports, ports, and emergency services); (2) Residential properties; (3) Commercial properties; (4) Social facilities (churches, schools, museums). 

This average probability of being in a wildfire or exposed to embers is used to create a relative ranking across communities for each category from “minimal” to “extreme”. Once risk is determined for each of the 4 categories, the overall risk for a community is calculated by averaging the likelihood for all 4 categories. Community risk reflects the weighted number of properties or facilities with direct and indirect exposure to wildfires. Within a community residential homes make up the majority of land use and provide a very important part of daily life. As such residential homes are weighted the highest. Other types of infrastructure, including non-residential properties, and public and social infrastructure, have smaller weights.  

Unlike flooding, which can cause severe damage without destroying property, wildfires are a uniquely devastating and destructive natural disaster. In addition to damaging properties, wildfires can also cut off access to utilities, and emergency services, impact evacuation routes, and may impact the overall economic well-being of an area. In communities with many different types of infrastructure, the weighted percent of non-residential properties, roads, and public and social infrastructure may add up to be bigger than the residential weight. Compared to a rural community, developed areas with a booming economy have more variety of infrastructure types. 

The overall Community Risk encompasses the social risk, residential risk, commercial risk, and risk to infrastructure for a given area. Minimal risk is a case where no facilities within a category have wildfire risk. Weighting each category allows wildfire risk to be community-specific. 

Consider the level of infrastructure risk within a community. The facilities in this category include power stations, wastewater treatment facilities, superfund sites, fire stations, police stations, hospitals, seaports, and airports. The percent of infrastructure facilities that fall into each probability threshold is used to determine a community’s level of infrastructure risk.

Changing Risk

Community risk also takes into account how wildfire risks are likely to change over time with rising temperatures and changing weather patterns over the next 30 years. As the atmosphere warms, surface temperatures increase, which increases the rate of evaporation in dense wilderness areas, causing soil and vegetation to dry more quickly and more completely than in past seasons. In the next few decades, this warming trend will continue, with temperatures expected to rise over 2.5°F compared to current averages. 

Further, a warming planet and increased evaporation also mean changing weather patterns. As a result of changes in weather patterns across the US, many areas already prone to wildfires will see changes in precipitation that will result in more severe storms with longer dry spells between them during hot summer months. These weather pattern shifts can also result from changes in wind direction, blowing wildfires into areas, or allowing them to spread to areas, where they may not have spread before.  For this reason, risk scores are reflective of both current and future wildfire risks.

Risks to each category -- social risk, residential risk, commercial risk, and risk to infrastructure -- account for the effect climate change will have on a given community. The climate is highlighted by adjusting these risks into the future as “Increasing” or “Decreasing”. Increasing risk due to the changing climate may increase the number or percent of structures expected to be in a wildfire, increase the probability of structures burning, or a combination of both. 

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