Understanding Wildfire Behavior Across Alaska Using Remote Sensing of Fire Intensity
After a wildfire, land managers need timely information about its impact to develop treatment strategies. USGS scientists investigated the utility of instantaneous satellite-derived estimates of fire intensity in Alaska, where the remoteness and inaccessibility of many wildfire sites make ground-based investigations difficult.
Measuring a wildfire’s impact
The environmental impact of a wildfire can be measured in multiple ways. Fire severity, which indicates how damaging a fire is to the landscape, is probably the most common. But severity can take time to assess, as it is typically determined from satellite imagery by measuring the presence of plants before and one year after a fire. In addition, satellite imagery cannot directly show how much of the soil layer has been consumed by fire. This is especially important in places like the Alaskan boreal region, where fire impact is largely determined by how much of the soil layer burned.
An additional wildfire characteristic is its intensity, or energetic output. Satellite-derived fire intensity—fire radiative power (FRP)—is a measurement of the rate at which a fire is emitting radiative energy. A higher FRP indicates faster fuel consumption and a more vigorous fire. Unlike fire severity, FRP is obtainable instantaneously, as the fire burns. FRP also incorporates all biomass burned, including the soil layer, so it can potentially provide more information about wildfire impact than fire severity alone.
However, in order to use satellite-derived FRP to evaluate fire impact, land managers need more information about the utility of this indicator. That is where USGS research comes into play.
A novel USGS study
To gather more information about the utility of FRP for land managers, USGS scientists investigated how the parameter varies across Alaska in the last two decades, as well as the relationship between FRP and factors that are known to influence fire dynamics. A key question motivating this study was: what are the patterns of fire intensity in relation to the vegetation burned and to how many times that area had previously burned? By creating a catalog of FRP informed by knowledge of the landscape and its fire history, the scientists expected to understand when and why certain areas experience unexpectedly intense fires, allowing land managers to better anticipate future fire behavior. The USGS team combined data collected across Alaska by NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) satellite from 2003-2022 with information about vegetation type and a historical (1940 – present) record of fire extents.
The results of the study
The scientists found that fire intensity is a promising diagnostic and predictive indicator of fire behavior and post-fire effects. They determined that ecosystems with lower amounts of above-ground fuel, like tundra and peatlands, burn with distinctly lower FRP values than those with higher fuel loads, like coniferous and deciduous forests. Even individual forest types show distinct FRP differences; white spruce forests, which form the densest stands in Alaska, burn more intensely than black spruce and deciduous, which have relatively smaller trees and sparser coverage.
The results are summarized in a publication freely available online. Next efforts will expand the study to other ecosystems in the U.S. to determine how satellite estimates of fire intensity can complement and enhance estimates of fire severity.