On May 4,
2000, a prescribed fire
on National Park Service land near Los Alamos,
New Mexico, was blown out
of control by erratic, gusty winds. The fire
exploded into nearby Santa
Fe National Forest, swept across lands at Los
Laboratory, and destroyed hundreds of homes in the town
of Los Alamos.
Even as firefighters were struggling to contain the
problem loomed in the minds of the people involved.
Some of the burned
land was in a watershed that fed into canyons where
the Los Alamos
National Laboratory, birthplace of the atomic bomb, had
potentially radioactive waste before environmental regulations
as strict as they are today. What would happen when the
season began in less than two months and thunderstorms drenched
Answering that kind of question is
the work of a multidisciplinary
group of scientists and natural resource
management experts called a
BAER team, for Burned Area Emergency
Rehabilitation. Members can comefrom several federal and state agencies
and can consist of hydrologists,
wildlife biologists, archaeologists,
soils scientists, landscape
architects, geologists, ecologists,
engineers, foresters, botanists, and
Geographic Information System (GIS)
specialists. Usually on the ground
before a fire is even fully
contained, a BAER team evaluates the burned
area for threats to life,
property or natural resources due to post-fire
Forest fires attract the most attention when they’re actively burning, but a threat remains after the flames have died down. After a fire, damaged vegetation, scorched underbrush, and soils that shed water can lead to severe erosion and flash floods. Scientists now use satellite data to aid rehabilitation teams in locating severely burned areas, helping to prevent post-fire disasters. (Photograph copyright Kari Brown, National Interagency Fire Center Image Portal)
Why is flooding after a fire such a big threat? For one thing, flames consume leaf litter and decomposing matter on the ground that normally soak up water. Additionally, after a fire, the soil itself has the potential to become hydrophobic, or water repellant. Plants and trees have numerous protective chemicals with which they coat their leaves to prevent water loss. Many of these substances are similar to wax. Vaporized by the heat from fires, these substances disperse into the air and then congeal over the soil surface when the fire begins to cool. Like the wax on your car, these substances coat the soil, causing water to bead up and run off quickly. In general, the greater the fire intensity and the longer the fire’s residence time, the more hydrophobic the soil becomes.
|Burned Area Emergency Recovery (BAER) teams move in soon after a fire sweeps through. Composed of wildlife biologists, soil scientists, foresters, and other natural resource specialists, the teams are responsible for locating areas that are susceptible to post-fire erosion, and doing what they can to prevent soil loss and flooding. (Photograph courtesy Rob Sohlberg, University of Maryland)|
In the short term, loss of vegetation and hydrophobic soil contribute to flash flooding and erosion—sometimes to the point of landslides. When fallen trees and other fire debris are added to the scenario, floods can become severe. Excessive rainwater can sweep ash and debris into streams and rivers, contaminating natural and manmade water sources. This past summer, the Hayman Fire in Colorado resulted in erosion problems that fed ash and sediment into Cheesman Reservoir, which supplies 15 percent of Denver’s water. Several other reservoirs were affected by the blaze as well. Humans aren’t the only ones affected. Mudslides resulting from the Missionary Ridge Fire near Durango, Colorado, washed mud and debris into the Animas River, killing hundreds of fish in the award-winning trout stream.
|A hot fire can even change the composition of soil, making it hydrophobic. Hydrophobic soils repel water, which will bead up and quickly run downslope. Instead of being absorbed by vegetation and forest litter, rain in a severely burned area will stay on the surface, potentially causing floods, erosion, and mudflows. Assessing how hydrophobic the soil has become is an important BAER team activity. [Photographs courtesy USDA Forest Service (USDAFS) Agricultural Research Service (left) and Annette Parsons, USDAFS (right)]|
Beyond this immediate cause for concern, water-repellant soil poses a longer-term problem for new plant growth. As seeds and undamaged plant structures try to regenerate in a burned area, they need water. But rainwater has difficulty soaking down into water-repellent soil, sometimes slowing the healing process in severely burned areas.
In assessing such threats, BAER teams are under intense pressure: they must make an assessment report with seven days of a fire’s containment. If threats exist, such as the potential for a landslide on a fire-charred mountain slope to descend upon nearby homes, the team must include mitigation measures that can be carried out before the first damaging storm. In the case of the Cerro Grande Fire, those measures included the construction of a small dam in Pajarito Canyon to prevent any potentially radioactive-contaminated sediment from entering the watershed and being carried away from the site.
|These photographs show the results of post-fire erosion. The boulder-filled channel (left) was created after the Cerro Grande fire. The eroded material was deposited further downstream (right). The water during the flood reached several feet higher, as shown by the trees stripped of bark. (Photographs courtesy John A. Moody, USGS Hydrologic and Erosional Responses of Burned Watersheds)|
Satellites Do It Faster, Cheaper
Given the time constraints, remote sensing data have great potential to assist BAER teams in their assessments. Over the past decade, scientists at the USDA Forest Service’s Remote Sensing Applications Center (RSAC) in northern Utah have been developing techniques for using airborne and satellite-based instruments to map out burned landscapes. Mark Finco is a senior scientist specializing in remote sensing and GIS for the RSAC. He described the group’s BAER activities as a long-running project that began 8 or 9 years ago. The group’s initial efforts were in the development of a digital infrared color camera that could be mounted on an aircraft platform and flown over fires. Although the camera produces good imagery, it’s expensive, and it takes a lot of effort to map a large area. Over the last three years or so, the RSAC has been investigating how satellite-based images could be used in the BAER analysis.
The Multi-sensor Advantage
The burn scar of the Missionary Ridge fire appears dark red in this Moderate Resolution Imaging Spectroradiometer (MODIS) image from August 10, 2002. With a maximum resolution of 250 meters per pixel [the image above is 135 km (84 miles) across] and coverage every 1 to 2 days, MODIS provides frequent observations of large areas. The scientists use higher-resolution satellites like Landsat to map the burned area in detail. The white box in the image above indicates the subsetted Landsat scene shown below. (Image by Robert Simmon and Jesse Allen, NASA GSFC)
Geographer Rob Sohlberg with the University of Maryland describes how multiple satellite sensors are used in satellite mapping of burn severity. “In general, the higher the sensor’s spatial resolution, the less likely it is that it will be observing the area we need at exactly the right time for burn severity mapping of a specific fire.” Higher resolution data are also inevitably more expensiveto obtain. “With a sensor like MODIS,” Sohlberg continues, “the coverage is much better—we see almost the entire surface of the Earth every day—but at coarser resolution.” These coarser- resolution data aren’t ideal for mapping out such detail as which hillside, creek, or ravine is especially damaged, but they can be useful for very preliminary mapping of burned areas, and perhaps more importantly, they can help scientists to decide in which areas high-resolution imagery is most needed.
The Enhanced Thematic Mapper plus (ETM+) aboard Landsat 7 has a true-color resolution of 30 meters per pixel—much better than MODIS—but only covers an area every 9 to 16 days. Smoke partially obscures the burn scar in this true-color image, making it difficult to see the patchy nature of the burn. Infrared wavelengths of light measured by Landsat penetrate smoke, and false-color images made using data from the infrared portion of the spectrum reveal the structure of the scar. (Image by Robert Simmon, based on data provided by Andrew Orlemann, USDAFS)
Annette Parsons has worked for more than 20 years as a soil scientist and mapping specialist for the Forest Service, and has been involved with BAER activities for more than a decade. Parsons was quick to see the potential of the RSAC satellite-based products and is now serving as a liaison between the RSAC and the BAER teams that operate on the ground. Says Parsons, “One of our main goals is to get local forest officials to notify the RSAC as soon as they think a fire might end up needing a BAER assessment. The earlier we know, the better our window of opportunity for finding the best satellite imagery for the area and having it in the team’s hands when they arrive at the field location.” Parsons agrees that MODIS’ coarser resolution provides less detail than Landsat or SPOT, but she says, “You can count on it almost every day. And in very large fires, such as this summer’s Biscuit Fire in Oregon, coarser-resolution data may be all we get for some areas.”
Finco identifies another use for the MODIS products. “All these burn products have the potential to be used for carbon budget investigations.” Fires release into the atmosphere carbon that is stored in trees, plants, and even the soil if the fire is intense and long-lasting. Regrowth draws carbon back in and stores it in plant matter. “People tend to think that a burn perimeter on a map means everything within that perimeter was totally burned. But the vast majority of the terrain is a mosaic of areas showing different burn severity. If you understood this mosaic, it would improve carbon emission estimates.” MODIS data are well suited for this application, especially when the fires are in the several-hundred-thousand-acre category.
Detail down to the level of individual trees is revealed by Space Imaging’s IKONOS satellite, which produces color images at 4-meter resolution. This true-color image, acquired June 23, 2002, shows dark gray, burned area in the upper left, and green, unburned area in the lower right. The bright blue-white smoke plume is caused by a smouldering hotspot, and thin smoke covers much of the rest of the scene. (Image copyright Space Imaging, based on data provided by Andrew Orlemann, USDAFS)
Using a combination of sensors allows the RSAC team to make burn severity maps quickly and relatively cheaply—especially compared to the cost of hours of helicopter time, which is the traditional reconnaissance source. It may seem trivial, but satellites have another advantage over sketch mapping from helicopters, which Finco and Parsons are both quick to point out. Leaning out of a moving helicopter as it weaves back and forth over a burned area, looking out to the terrain and down to the map in your lap not only makes it hard to be sure where you are, but also makes it hard to keep down your lunch!
Burn severity maps are produced by assigning the burned landscape into one of four categories based on characteristics like damage to trees and other vegetation, soil hydrophobicity, and ash color and depth. This map of burn severity for the Missionary Ridge Fire is based on preliminary satellite data. Before making a final map, scientists must carefully compare the post-fire effects to the unique pre-fire landscape. (Image courtesy Monte Williams, USDAFS)
Assessing Burn Severity from Space
This aerial view shows the differences between low, moderate, and high burn severity. Green trees are in low burn severity areas, brown trees with dead needles are located in moderately burned areas, and black, needleless trunks have been severely burned. Gound level images of similar areas are on page 3. (Image courtesy Annette Parsons, USDAFS)
The way in which the maps are produced depend largely on what sensor provides the data, but most involve using vegetation characteristics as an indicator of soil conditions. Generally, the maps are based on an indicator called NDVI, for Normalized Difference Vegetation Index, or one called NDBR, for Normalized Difference Burn Ratio. NDVI is based on the principle that vegetation absorbs red light and reflects near-infrared light. Changes in the amount of each kind of energy reflected from the surface can signal how severely the pre-existing vegetation has been transformed by fire. NDBR is based on the observation that vegetation reflects energy in the near-infrared, while soil tends to reflect energy from the mid-infrared part of the spectrum. After a burn, the vegetation within the burned area has been reduced and the soil has been exposed. As a result, the near-infrared values are lower than before the fire, while the mid-infrared values are higher. These NDVI and NDBR relationships are used to classify areas in a satellite image into one of four burn severity categories: high, moderate, low, and unburned.
Scientists estimate burn severity with satellite data by comparing the signal from different wavelengths of light. Near-infrared and red light are used for the Normalized Difference Vegetation Index (NDVI), which indicates the density and health of vegetation. Dark areas in the NDVI image (left) represent unvegetated or burned areas. Normalized Difference Burn Ratio (NDBR) uses the same math as NDVI, but near-infrared and shortwave wavelengths. NDBR highlights areas of exposed soil [bright areas in the NDBR image (right).] Data collected after a fire are compared with pre-fire data to produce burn severity maps. Before these maps are finalized, teams must survey the burned areas from the ground. (Images by Robert Simmon, based on Landsat 7 data provided by Andrew Orlemann, USDAFS)
But How Good Are They?
While the BEAR teams revise the satellite maps based on field observations, their primary concern is conducting their own emergency assessment. They have neither the time nor the resources to provide any kind of systematic feedback to the satellite mappers to help them improve their maps. In search of something more scientifically defensible, the RSAC has partnered with scientists at the University of Maryland and obtained funding from the National Interagency Fire Center’s Joint Fire Science Program to collect their own field data on burn severity conditions, which they are using to check the accuracy of the satellite classification, a process called validation.
Sohlberg is one of the scientists who spent some time in the field this summer. Since BAER teams are often on the ground before fires are totally contained, Sohlberg had to become certified as an entry-level firefighter in order to do this work. A lot of the training was in the classroom, said Sohlberg, but they also had to learn how to dig fire lines, operate pumps and hoses, and how to maintain their tools. They had to participate in timed drills to see how quickly they could set up an emergency fire shelter using the distinctive shiny Mylar bags. Said Sohlberg, “We were out there to collect data, but we always had to remember to be on guard; and we had to carry full personal protective equipment: fire-resistant pants and shirt, 8-inch logging boots, fire shelter, compass, water, goggles, helmet and leather gloves. Although we were never in contact with active fire, there were still lots of hazards—damaged trees that are prone to falling, called snags; rolling rocks; and smoldering stump holes.”
field, the team uses a mapping-grade Global Positioning System
receiver that has a built-in data dictionary. Once the GPS device
pinpointed the scientists’ location, they can access the
dictionary, which prompts them with a series of preset questions
the burn severity at the site. Is there any ground cover left? How
is the ash, and what color is it? How long does a drop of water sit
the surface before soaking in or running off? Are there any green
left? Are there any needles or leaves left on the trees? What is
overall assessment of the burn severity at this location? Since
satellite maps the whole area, the team tries to collect data to
up with all the different burn categories within a fire perimeter.
the physical geography of the terrain is highly variable, for
because of topography or ecology, the team tries to collect data
each burn category in each of the different physiographic regions
the fire perimeter. That’s a lot of different
BAER teams enter burned areas so soon after a fire has rolled through that they need to be trained as wildland firefighters. In the field they carry full gear, including fire resistant clothing, helmet, and a fire shelter. In addition to the threat from hot spots, snags of dead trees and unstable slopes contribute to the danger. (Photograph courtesy Rob Sohlberg, University of Maryland)
differences can be tricky for satellite classification.
“You have to be initially cautious about the
classifications, particularly in areas of diverse terrain or
geology. Fire may quickly pass through a rocky area with already
vegetation, or grassland, and blacken everything. Or it might
a forested ridge, consuming acres of trees and all the
vegetation. From a satellite, all these areas might look
same—just black—and might be classified as high
severity. But the actual
severity as far as the impact on the watershed
would be very different.
In the first case, the post-fire runoff and
erosion would probably not
be substantially increased over pre-fire
conditions, and the vegetation
would probably recover in a season. The
effects could hardly be called
severe. In the second case, the impacts
would be much more
The RSAC has provided satellite-based burn severity maps for BAER teams for two fire seasons: 2001 and 2002. Parsons says that their experience suggests that the remote-sensing-based assessments are most accurate in more forested ecosystems. In that kind of terrain, they can often use the satellite maps from the RSAC without any major modification. In shrubby or rocky ecosystems, they require more fine tuning from ground-truth. Even so, Parsons says, her main criteria for the project’s success is less in how technically accurate the maps are and more in their utility to field crews. “We have produced satellite maps for at least 70 fires in the 2002 fire season, and the word we have gotten back from BEAR field teams is overwhelmingly positive. They may not be 100 percent accurate at fine scale, but they have been very useful in pointing teams to areas of greatest concern.” Using those criteria, the project can already be called a success.
Though the flames seem menacing, grassland fires are rarely considered severe. Grassland ecosystems are well adapted to fire, and while above-ground grass stems may be consumed, roots often remain, allowing the ecosystem to regenerate rapidly. In these cases, the potential for flooding and erosion is not very different from pre-fire conditions, and the burn can hardly be considered severe—despite the fact that the satellite sees a blackened surface. Scientists are collecting validation data in the effort to increase the sophistication of the satellite classifications. (Photograph copyright Kari Brown, National Interagency Fire Center Image Portal)
That won’t stop the scientists from continuing to tweak their approach. Over the winter, Forest Service and University of Maryland scientists will look for ways to increase the accuracy of the satellite maps using data collected this past summer at the Missionary Ridge Fire in Colorado, the Rodeo-Chediski Fires in Arizona, the East Fork Fire in northern Utah, and the Biscuit, Winter, Toolbox and Eyerly Fires in Oregon. NASA is supporting that work as part of its Natural Hazards program. The team will also be preparing for another season of data collection in the summer fire season of 2003.
The final burn severity maps developed by BAER use satellite data combined with information gathered by the ground teams. These maps are then used to prioritize efforts to stabilize soil, preventing later erosion and mitigating possible floods. (Image courtesy Monte Williams, USDAFS)
The group also
take advantage of its involvement in NASA’s newly created
Response Team to coordinate the logistics of acquiring imagery
airborne sensors. The team has their eye on the MAS (MODIS
Simulator) on NASA’s ER-2 aircraft and the AIRDAS
Infrared Disaster Assessment System), developed by
Research Center and the Forest Service to fly on a
variety of aircraft.
They also hope to make use of another Terra
called ASTER (Advanced Spaceborne Thermal Emission
Radiometer). ASTER offers unique capabilities in sensing
infrared radiation, which makes it particularly useful for
through smoke. Increasing the number of data sources increases
team’s opportunity to produce useful images and burn severity
to BAER teams and also increases opportunity for cross-comparisons
all the remote-sensing data, which serves as means of validation
Satellite data were used to map many fires in the summer of 2002, including the 200,000-hectare (500,000 acre) Biscuit Fire near Grants Pass, Oregon (above). In this image, red shades indicate burned areas and green indicates unburned forest. Low and moderately severe burns are a mixture of red and green. The image combines shortwave-infrared, near-infrared, and green light as red, green, and blue, respectively. Landsat 7 acquired the data on August 30, 2002. (Image by Robert Simmon, NASA GSFC, based on data provided by Andrew Orlemann, USDAFS)
This partnership is not the first for the RSAC, NASA, and University of Maryland. The three originally teamed up as part of the MODIS Land Rapid Response Project, in which daily imagery and fire detections from MODIS were relayed from NASA’s Goddard Space Flight Center, to the University of Maryland, and on to the RSAC for use by the National Interagency Fire Center as it allocated firefighting personnel and resources across the country. That collaboration ultimately led to the installation of a MODIS Direct Broadcast Receiving Station at the RSAC and paved the way for the current work. With solid validation data, the satellite-based burn severity maps produced in coming seasons will only get better, saving BAER crews precious time as they assess the dangers left behind in the wake of wildfires.