A row of houses and buildings along a riverbank sitting on piles in the water.

Exposure

Exposure is defined as the representative value of buildings (in dollars), population (in both people and population equivalence dollars), or agriculture (in dollars) potentially exposed to a natural hazard occurrence. Exposure is a natural hazard consequence factor for Expected Annual Loss, the natural hazard component of the National Risk Index. A higher exposure value results in higher Expected Annual Loss and Risk Index scores.

Depending on the hazard type, exposure areas were determined by historical events, hazard-susceptible zones, or probabilistic modeling.

Source Data

Exposure data sources were selected for their accuracy, long period of record, and spatial components based on the best available, national-level data per natural hazard. Sources were identified through public knowledge, subject matter expert recommendations, and research, and came from several federal government agencies and academic institutions. Data for territories were excluded based on the availability, compatibility, and reliability of source data for those geographies.

Consequence Types

Exposure is based on three consequence types: buildings, population, and agriculture.

Note: Not all consequence types are considered for all hazard types. Building and population exposure are modeled for all hazard types except Drought, which only modeled agriculture exposure. Agriculture exposure was also modeled for Cold Wave, Hail, Heat Wave, Hurricane, Riverine Flooding, Strong Wind, Tornado, Wildfire, and Winter Weather.

Buildings

Building exposure is defined as the dollar value of the buildings determined to be exposed to a hazard according to a hazard-specific methodology. The maximum possible building exposure of a Census tract, or county is its building value as recorded in Hazus 4.2, Service Pack 01 (SP1), which provides 2018 valuations of the 2010 Census.

Population

Population exposure is defined as the number of people determined to be exposed to a hazard according to a hazard-specific methodology. The maximum possible population exposure of a Census tract, or county is its population as recorded in Hazus 4.2, SP1. Population exposure is also monetized using a value of statistical life (VSL) approach in which each fatality or ten injuries is treated as $7.6 million of economic loss.

Agriculture

Agriculture exposure is defined as the dollar value of the crops and livestock determined to be exposed to a hazard according to a hazard-specific methodology. This is derived from the United States Department of Agriculture 2017 Census of Agriculture county-level value of crop and pastureland.

Exposure Methodology

Exposure is typically calculated at the Census block level for each consequence type and then aggregated to the Census tract and county level by summing the Census block exposure values within the parent Census tract and parent county.

While there are significant differences in the nature of the hazard types and diversity of source data formats, each hazard type can be thought of as having a footprint or exposure area where the hazard can occur and result in loss. Exposure areas associated with each hazard type are modeled in one of three ways:

  1. Widespread exposure areas were used for hazard types where, if the hazard were to occur, it could happen anywhere in the community with equal likelihood (e.g., Lightning, Hail, Strong Wind).
  2. Hazard susceptible zones were used for hazard types where there are specific areas within the community where the hazard can occur (e.g., flood zones along a river).
  3. Fixed exposure sizes were used for the Tornado and Avalanche hazards representing the average area impacted by a hazard occurrence.

Once the exposure areas were defined for each hazard type using one of the approaches, the National Risk Index estimated the exposure values for building, population, and agriculture using a hazard-specific methodology.

For comprehensive details about exposure in the Risk Index, see the National Risk Index Technical Documentation.