Abstract
Burned area algorithms from radar images are often based on temporal differences between pre- and post-fire backscatter values. However, such differences may occur long past the fire event, an effect known as temporal decorrelation. Improvements in radar-based burned areas monitoring depend on a better understanding of the temporal decorrelation effects as well as its sources. This paper analyses the temporal decorrelation of the Sentinel-1 C-band backscatter coefficient over burned areas in Mediterranean ecosystems. Several environmental variables influenced the radar scattering such as fire severity, post-fire vegetation recovery, water content, soil moisture, and local slope and aspect were analyzed. The ensemble learning method random forests was employed to estimate the importance of these variables to the decorrelation process by land cover classes. Temporal decorrelation was observed for over 32% of the burned pixels located within the study area. Fire severity, vegetation water content, and soil moisture were the main drivers behind temporal decorrelation processes and are of the utmost importance for areas detected as burned immediately after fire events. When burned areas were detected long after fire (decorrelated areas), due to reduced backscatter coefficient variations between pre- to post-fire acquisitions, water content (soil and vegetation) was the main driver behind the backscatter coefficient changes. Therefore, for efficient synthetic aperture radar (SAR)-based monitoring of burned areas, detection, and mapping algorithms need to account for the interaction between fire impact and soil and vegetation water content.
Highlights
Fire has a key role in the global climatic balance modifying the greenhouse gases (GHGs) emission fluxes and the presence of aerosols in the atmosphere [1,2,3]
Over the entire study area, the frequency of pixels not affected by the temporal decorrelation was higher when compared to pixels mapped as burned at some point after the fire (Figure 3)
Since this study considered data acquired less than one year after the fire event, the NDVI increment due to such recovery processes has little influence on the backscatter coefficient increment, as few months are insufficient to substantially change scattering processes in slow-growing Mediterranean vegetation
Summary
Fire has a key role in the global climatic balance modifying the greenhouse gases (GHGs) emission fluxes and the presence of aerosols in the atmosphere [1,2,3]. The most recent global burned area products estimate that around 4 million km are burned every year [10,11]. There is still a remarkable uncertainty about the worldwide burned area extent [12], since the currently available global products are limited by (i) the use of passive remoted sensing datasets, which is associated with limitations in areas of persistent cloud cover (i.e., inter-equatorial latitudes), and (ii) relatively coarse spatial resolutions (250 m), which makes the detection of small fires difficult [13,14]. A recent comparison between burned area products from Sentinel-2 and Moderate Resolution Imaging Spectrometer (MODIS) for Africa indicated that the latter missed almost half of the total burned area, mostly due to small fires omission (
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