EROS Burn Mapping Crucial in Post-Fire Risk Assessments

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It began with a tree falling onto a power line.

There were sparks, then very quickly a wilderness ablaze. In its first 13 hours, the Las Conchas Fire that erupted just west of Bandelier National Monument in New Mexico on June 26, 2011, spread at an acre per second. By the time the last embers died weeks later, it had grown into what then was the largest wildfire in New Mexico history.

Color map of burn scar in New Mexico

This Landsat image acquired on June 29, 2011, shows the Las Conchas fire in Bandelier National Monument in New Mexico. Active fire and the burn scar (dark red) are readily evident in the image

(Public domain.)

In the smoldering aftermath, a Burned Area Emergency Response (BAER) team of hydrologists, soil scientists, engineers, biologists, archeologists and more stepped in to assess the damage done. One of the first things the team did was turn to the USGS’ Earth Resources Observation and Science (EROS) Center for assistance.

Staff at EROS rely on the combined shortwave infrared (SWIR) and near infrared (NIR) bands on Landsat to acquire information on moisture content in soil and vegetation, and on growing vegetation, after a wildfire ends. They then come up with a Differenced Normalized Burn Ratio (dNBR), which uses pre- and post-fire Landsat images of the burned area to capture a range of values signifying either an uptick or decrease in greenness across the fire’s footprint.

The more negative the values, the more greenness. The more positive the values, the more severe the fire impacted the surface cover.

Those values then are categorized into four loosely defined classes: unburned, low burn severity, moderate severity, and high severity. That information is transformed into a Burned Area Reflectance Classification (BARC) map that is handed off to BAER teams in the field as a guide for more in-depth field measurements and observations of fire damage.

“Fires will burn in a mosaic. Not every patch is going to be equal as far as the burn severity,” said Rich Schwab, the post-fire program director for the National Park Service. Still, with BARC data in hand after the Las Conchas Fire, the BAER team was able to assess those places where a loss of vegetation would most likely expose soil to erosion and other risks, Schwab said.

With the BARC maps in hand, the BAER team went looking for a loss of vegetation that would expose soil to erosion. The team knew that water runoff from a mountainside that is no longer forested can lead to flooding and mudslides. Sediments run downhill and downstream much less impeded, damaging houses or filling reservoirs, which can put endangered species and community water supplies at risk.

All the major watersheds within Bandelier National Monument were heavily impacted by the fire, including Frijoles Canyon, where the historic Visitors Center and the main archeological sites are located. Over 75 percent of Frijoles Canyon lay within the fire's footprint, much of it burned with high severity. Using burn severity maps and flood models, the BAER team developed a solid understanding of what potential dangers existed should the skies open up.

As it turned out, heavy rain did in fact begin falling that following August, sending huge torrents of water rampaging through the watershed and toward the Visitors Center at the bottom of the canyon. Based on the burn maps and ground assessments, the BAER team knew that there wasn’t time for seed to germinate on the slopes and slow any potential flooding, Schwab said. Nor would using fallen trees to channel floodwaters work in this situation.

So, after relying heavily on the burn maps and studying the burned areas, researchers “came back and said, ‘Your best bet is evacuation and point protection,’ ” Schwab said

They put down modular concrete Jersey barriers to divert any potential floodwater away from the Visitors Center and parking lot. They also put sandbags out by the corner of the Visitors Center to help protect it.

It worked, and the Visitors Center was saved.

That’s the goal every time, EROS Geographer Birgit Peterson said.  “The maps we produce really are meant for them to use to help dictate where responses need to be made,” she said. “What mitigation activities do they need to undergo to keep worse things from happening? Our maps help to get them to that point.”

And not just at Las Conchas. EROS burn mapping products have been used heavily across Department of Interior-managed lands through the years. From 2001 through 2016, 609 wildfires on DOI-managed lands were mapped. That accounts for almost 25 million acres.

“The maps are an important part of what we do, what the BAER teams do,” Schwab said. “We need to assess the risks that exist after a fire as quickly as we can. Those maps help us do that.”