Mapping Fire Scars in Global Boreal Forests Using Imaging Radar Data

1. Mapping Fire Scars in Global Boreal Forests Using Imaging Radar Data Written By: L.L. Bourgeau-Chavez, E.S. Kasischke, S. Brunzell, J.P. Mudd, and M. Tukman Reviewed By: Daniel C. Dunning

2. Overview • Fire scars are mappable due to ecological changes that occur post burn including increased soil moisture. • High soil moisture causes an enhanced backscatter signal to be received from burned forests. • Regional ecological differences can affect post fire changes thus impacting appearance of scars in C-band SAR imagery. • C-band European Remote Sensing Satellite (ERS) and Radarsat SAR data was used in this study to detect, map, and monitor boreal forests globally. • Study sites include four areas in Canada and an area in central Russia.

3. Overview Continued… • Fire boundaries were mapped from ERS SAR data without prior knowledge of fire scar locations. • Maps were validated utilizing fire service records and ground-truthing. • C-band SAR data has a high potential for detecting and mapping fire scars globally.

4. Review • SAR – Synthetic Aperture Radar • Uses a 1-2 meter fixed length antenna and synthesizes a much larger antenna that can be hundreds of meters in length and has improved resolving power which achieves very fine resolution from great distances. • C-band Radar – Wavelength of 5.6 cm • Experiences some surface scattering and volume scattering in the heart of a tree stand, does not reach the ground if trees are present. Also has the ability to penetrate clouds.

5. Introduction • Mapping and monitoring fire scars is important for resource and land management. • ERS SAR images of Alaska fire scars were 3-6 dB brighter that adjacent unburned forests. • Burned areas were determined to be detectable due to ecological changes occurring post burn. • Changes included removal of tree canopy, exposure of rough surfaces, and increased ground moisture. • Research revealed phenomenon only occurred when burned areas were wet such as early spring, early autumn, or after rain events. • The enhanced brightness allowed fire scars to be detected with moderate precision.

6. Introduction Continued… • Increased ground moisture is due to reduced surface albedo and the melting of the permafrost layer as well as reduced evapotranspiration. • Study was conducted using C-band SAR imagery collected over four Canadian boreal regions and an area in central Russia.

7. Objectives • Goal of study was to develop technique for mapping and monitoring fire-disturbed boreal forests on a global basis with the following objectives: • Determine if fire scars can be detected and mapped in varying ecological conditions using C-band SAR data; • Determine if mapping fire scars with SAR imagery alone is feasible; and • Identify any geophysical, ecological, or temporal conditions which may affect fire scar detection and area estimation in ecologically different boreal regions.

8. Background on Fire Mapping with SAR • Benefit of using C-band SAR for fire scar mapping is its ability to penetrate ground cover. • ERS SAR sensor is a C-band, 5.6 cm wavelength imaging radar with vertical transmit and receive polarization (C-VV). It has a resolution of 30 m and a footprint of 100 km by 100 km. ERS-1 was launched in 1991 and ERS-2 (still in operation) was launched in1995. • Radarsat launched in 1995, and is also a C-band system. It also has a resolution of 30 m and a footprint of 100 km by 100 km. • Research indicates that SAR C-band data has the potential to be used in conjunction with Landsat TM data for high accuracy fire scar mapping and monitoring. • Image on following slide indicates a before and after of a fire occurring in Alaska. Imagery indicates that both Radarsat and Landsat can be used to map burn extent, but cloud obscurity is a problem with the Landsat TM imagery.

9. Radarsat VS Landsat Clouds

10. Ecology of Canada Study Areas • The North American boreal forest extends from New Foundland to Alaska with the northern limit ranging from 68° N Latitude in the Brooks Range in Alaska to 58° N Latitude at the western edge of Hudson Bay. The southern limit is less distinct and is dependant on precipitation and soil moisture. • Climate ranges from dry with extreme annual temperature variations in the west to relatively warmer, wetter, maritime climate of eastern Canada. • Fire is more frequent in the drier regions of western Canada and Alaska than in the eastern regions. Variations in fire frequency are tied to variations in climate.

11. Study Areas • To determine if fire scars can be detected and mapped in ecologically varying boreal ecozones of Canada three ERS study areas were chosen to capture the west to east differences. • The study areas selected were the Northwest Territories, Ontario, and Quebec. • To capture ecological variation in the north-south direction, a sequence of three to eight adjacent north-south images were obtained from each study swath.

12. Study Areas

13. Climate and Fire Data

14. Fire Scar Detection and Mapping in Canada • Procedure: • SAR data acquisition; • Visual evaluation and rating of data; • Selection of best rated images to be georeferenced and mosaicked; and • Digitization of potential fire scar boundaries.

19. Results of Fire Scar Analysis

20. Results of Fire Scar Analysis

21. Results of Fire Scar Analysis

22. Results of Fire Scar Analysis

29. Discussion and Conclusions • Fire scars are mappable in in boreal ecosystems using C-band SAR imagery. • For Ontario and NWT study sites, fire detection is feasible with SAR. For NWT, SAR data indicates more area burned than was mapped by CFS. • Data availability for Quebec was limited and analysis inconclusive. Further evaluation of SAR data collected over Quebec is necessary for any conclusions to be drawn. • Seasonal variations in fire scar visibility occur globally, with the best viewing season being either spring or autumn. • Major problem distinguishing fires form wetlands. • For boreal regions in Russia the SAR data was limited to only two years over a geographic region, and the areas were not well mapped by fire service agencies. • Manual interpretation was used for this study, but technology exists to automate the process. • An improved method for fire scar mapping and monitoring might use a combination of SAR and multi-spectral data.

30. Potential Applications • Fire scar mapping in deciduous forests. • Fire scar mapping in chaparral. • Fire scar mapping in grasslands. • Forest reduction in Amazon rainforests.