When Wildfire Damage Threatens People, Landsat Provides Answers

A wildfire's damage rarely ends when the last embers die. In the western United States, rain on a scorched mountainside can turn ash into mudslides, and debris flows unleashed by storms can threaten homes and send people running for safety. Landsat satellite imagery gives vital early warning of those dangers.

The shortwave infrared band

Landsat's sensors include a shortwave infrared (SWIR) band, which few other satellites carry. It speeds up how quickly federal emergency teams can map fire damage and advise on stabilizing burned land. Burn teams once needed days to reach remote sites and trace a fire's footprint; Landsat 7 and 8 now supply data for mapping soil burn severity within hours of passing overhead.

More than 50 years ago, when Interior Secretary Stewart Udall first pushed to turn space technology toward Earth's natural resources, he could hardly have imagined a SWIR band. Today it is one more tool on a multiband system that images the Earth broadly at 30-metre resolution, with nearly half a century of continuous records.

SWIR responds to the moisture in soil and vegetation. Paired with Landsat's near-infrared (NIR) band, which is highly sensitive to growing plants, it produces accurate images of burn scars.

The 2015 Cougar Creek fire, northeast of Portland, Oregon, seen by Landsat 8. Left: Mount Adams, Washington, before the fire, snow cyan on its peak. Centre: after the fire, charred vegetation south and east of the mountain in red. Right: the burn severity map, made with infrared bands including SWIR — dark green unburned, light blue low, yellow moderate and red high severity. USGS.

Triage after the fire

The real value of SWIR is in post-fire triage. Landsat 7 and 8 deliver infrared data at least every 8 days, so USGS fire science staff can supply burn maps less than 24 hours after an image for Interior Department lands — for the Bureau of Land Management, National Park Service, Bureau of Indian Affairs and U.S. Fish and Wildlife Service.

Burn severity maps tell rehabilitation managers where to focus: felling dead trees along the contour, mulching, and placing sandbags or concrete barriers can slow and divert flash floods on a mountainside. The maps also give a view of whole watersheds, helping decide where larger culverts are needed to carry debris sliding down burned slopes.

• In 2018 USGS staff mapped 35 fires covering 1,018,917 acres for the Interior Department.
• From 2001 to 2018 the USGS and the U.S. Forest Service together mapped 2,008 wildfires totalling 54,842,681 acres.
• The USGS also mapped several fires in Sweden in 2018.

Other satellites give finer images — of city growth, even of cars on a highway — but Landsat and its SWIR sensor are among the few that can efficiently image the broad sweep of scorched soil and vegetation after a large fire. In the critical time between the last smoke and the first rain, that may save a home, or a life.

Sources

• U.S. Geological Survey, When Wildfire Damage Threatens Humans, Landsat Provides Answers, Fact Sheet 2016–3044, version 1.1, September 2019. https://doi.org/10.3133/fs20163044
• USGS Earth Resources Observation and Science Center: https://www.usgs.gov/centers/eros