Estimation of surface roughness and stone cover in burnt soils using SAR images

Abstract

Abstract In many quartzitic slopes in the NW of the Iberian Peninsula there is a high topsoil stoniness which produces a random surface roughness. The surface roughness is one of the control parameters in the signal of Synthetic Aperture Radar (SAR) images. In this work we test the use of the SAR images for distinguishing and mapping areas with different surface roughness which, in this study, correspond with areas with different surface stoniness. In accordance with the geographic location, the relief of the study area and the aims of the project JERS and ENVISAT SAR images were used. The images are characterized by the high view angles, between 33° and 42°, alternating polarization (HH/HV, VV/VH) data and ascending and descending passes. In order to describe the soil roughness various measuring methods were applied in plot parcels (direct measurements of height of clasts and laser scanning) and different roughness models were tested. The results show that there are relation between SAR signal (DN) and the soil roughness classes: DN = aθb and the b parameter is correlated with the roughness. The best fit between roughness model and SAR data was found for mean standard deviation (rms) height of clasts. Once verified that the correlation between SAR signal and surface roughness exists, the ENVISAT images were classified into three successive surface roughness classes. In order to facilitate the classification Principal Components Analysis were carried out over all polarization bands and the object oriented classification was applied. To validate the accuracy of the SAR images classification we elaborated by photointerpretation a surface condition map for an area of 10,700 ha. The SAR classification and photointerpreted map were compared for three classes: surface without stones, surface with stones (low and medium stoniness) and total cover of loose clasts. The overall coincidence between the maps is about 70%. Besides, large areas located in the northwest of the Iberian Peninsula suffer soil degradation caused by wildfires and the increase of surface stoniness is one of the effects described in the postfire erosion. The influence of wildfires and the associated erosion is particularly important on quartzitic slopes partially covered by coarse grain-supported deposits, like the ones considered in this study. The available data for the area of study suggest the high correlation between the different classes of surface roughness and stoniness obtained from SAR imagery, and the gradual different status of postfire soil erosion.