Abstract:
Savannahs are mixed woody-grass communities where low-intensity surface fires are
common, a ecting mostly the grass layer and rarely damaging trees. We investigated the e ect of
surface fires in a savannah system in the Kruger National Park, South Africa, on the backscatter of
synthetic aperture radar (SAR) C-band Sentinel-1A images. Pre-fire and post-fire dual polarized
(VH, VV) C-band backscatter values were examined for 30 burn events. For all events, a systematic
backscatter decrease from pre-fire to post-fire conditions was observed, with mean backscatter
decreases of 1.61 dB and 0.99 dB for VH and VV, respectively. A total of 90% and 75% of the burn
events showed a decrease in VH and VV backscatter greater than 0.43 dB, the overall absolute
radiometric of Sentinel-1A products. The VH data were, overall, 1.7 times more sensitive to surface
fire e ects than the VV data. C-band data are likely sensitive to a reduction in grass biomass typical
of surface fires, as well as in grass/soil moisture levels. Early season fires had higher backscatter
decreases due to greater early season moisture conditions. For region with more than 30% woody
cover, the e ect of fire on the C-band backscatter was reduced. Denser woody communities tend
to produce lower grass fuel load and less intense surface fires, and limit the penetration of C-band
microwaves to the ground where most savannah fires and associated e ects occur. This research
provides evidence that C-band space-borne SAR is sensitive to the e ects of surface-level fires in
southern African savannahs. The unique availability of frequent and spatially detailed C-band data
from the Sentinel-1 SAR constellation provide new opportunities for burned area mapping and
systematic monitoring in savannahs systems, for instance, for fine-scale fire propagation studies.
