Exploring the Potential of C-Band SAR in Contributing to Burn Severity Mapping in Tropical Savanna

Abstract

The ability to map burn severity and to understand how it varies as a function of time of year and return frequency is an important tool for landscape management and carbon accounting in tropical savannas. Different indices based on optical satellite imagery are typically used for mapping fire scars and for estimating burn severity. However, cloud cover is a major limitation for analyses using optical data over tropical landscapes. To address this pitfall, we explored the suitability of C-band Synthetic Aperture Radar (SAR) data for detecting vegetation response to fire, using experimental fires in northern Australia. Pre-and post-fire results from Sentinel-1 C-band backscatter intensity data were compared to those of optical satellite imagery and were corroborated against structural changes on the ground that we documented through terrestrial laser scanning (TLS). Sentinel-1 C-band backscatter (VH) proved sensitive to the structural changes imparted by fire and was correlated with the Normalised Burn Ratio (NBR) derived from Sentinel-2 optical data. Our results suggest that C-band SAR holds potential to inform the mapping of burn severity in savannas, but further research is required over larger spatial scales and across a broader spectrum of fire regime conditions before automated products can be developed. Combining both Sentinel-1 SAR and Sentinel-2 multi-spectral data will likely yield the best results for mapping burn severity under a range of weather conditions.

Near Real-Time Automated Early Mapping of the Perimeter of Large Forest Fires from the Aggregation of VIIRS and MODIS Active Fires in Mexico

“…Although the capacity to detect active fires, especially of VIIRS, can reduce false alarms due to high tree cover, cloud shadows, sensor failures or other reasons [11,31,45], errors are not totally excluded, affecting the number of valid observations [9]. In fact, this can be the main limitation for analyses using optical data, especially in tropical landscapes [8,16,74], as illustrated in Figure 6c, where active deforestation causes high ignition densities resulting in numerous small fires of short duration not detected at a satellite pass [22], or covered by clouds [25,29]. To overcome this latter impediment by cloud obscuration, which is a common limitation to both active fires and also reflectance-based algorithms of even high-resolution optical sensors [5,73], the use of radar technologies for fire detection, potentially useful in areas with permanent cloud cover [5,[74][75][76][77], should be further explored.…”

Quantifying the Response of German Forests to Drought Events via Satellite Imagery

“…The second index in this study is the Normalized Burn Ratio (NBR) that includes the red and Short Wavelength Infrared (SWIR) 2 band (Formula ( 8)) [72]. The index has been frequently used to study fire severity and for mapping burnt areas [73][74][75][76], but also for forest degradation and recovery analysis [77][78][79].…”

Remote Sensing of Environmental Drivers Influencing the Movement Ecology of Sympatric Wild and Domestic Ungulates in Semi-Arid Savannas, a Review

“…However, mapping fire severity is more challenging than just mapping the occurrence of fire. One major limitation of all optical SRS approaches is the presence of cloud cover that hinders the temporal continuity of the follow-up [325]. For animal movements studies, the severity of a given fire event is more relevant than its frequency and timing alone.…”

“…To bypass such limitation, SAR images could be used. Philipp and Levick (2020), for example, demonstrated that C-band SAR data can contribute to effectively map fire severity in tropical savanna [325]. Characterizing savanna fire severity in addition of being able to locate fire events could also be useful for measuring more accurately the influence of human land use practices [326] and how it potentially affects animal movements.…”