Fuel moisture monitoring in Mediterranean forests using sentinel-1 imagery

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University of New Brunswick


In fire-prone Mediterranean regions, fire danger is monitored using the Canadian Forest Fire Danger Rating system. The system’s Fire Weather Index moisture codes are interpolated from single-point weather data. An alternative is to use remote sensing imagery that can provide spatially-explicit data at a repetitive time. The current study investigates the capability of multi-temporal Sentinel-1 C-band imagery to map Drought Code values for a study area of 502.42 km2 in North-western Portugal. Backscatter values from 276 Sentinel-1 C-VV and C-VH images acquired between January 2018 and December 2020 were compared with five Drought Code classes in unburned and burned sites. The Drought Code values used in the study were estimated with the JRC’s High-Resolution model. In addition, confounding variables such as tree cover, and incidence angle were considered. Our results show that: a) C-VV and C-VH backscatter correlate significantly (p<0.001) with the Local Incidence Angle with R2 = 0.324 to 0.438, and the highest coefficients of determination occurred for the wet soils (DC class = 0 to 1), b) CVV and C-VH backscatters can discriminate wet to dry forest environments, but they are less sensitive to the transition between dry (DC classes=1 to 10, 10 to 100) and extremely dry environments (DC classes=100 to 1000), c) the C-VH backscatter has a higher degree of prediction between the Drought Code classes in the unburnt, pre-fire and post-fire areas than the C-VV backscatter, d) the C-VH polarization captures post-fire recovery after an average minimum period of 360 days after the fire event, although with less distinction for extremely wet soils e) Mean backscatter values decrease with DC classes in unburnt, pre-fire and post-fire (after 360 days) areas. Further studies are, however, required using high-resolution DC values in other Mediterranean regions that contain other tree species.