Soil erosion assessment using geomorphological remote sensing techniques: an example from southern Italy

The aim of this study is to assess of the distribution and map the geomorphological effects of soil erosion at the basin scale identifying newly-formed erosional landsurfaces (NeFELs), by means of an integration of Landsat ETM 7+ remotely sensed data and field-surveyed geomorphological data. The study was performed on a 228.6 km(2)-wide area, located in southern Italy. The study area was first characterized from a lithological, peclological, land-use and morpho-topographic point of view and thematic maps were created. Then, the georeferenced Landsat ETM 7+ satellite imagery was processed using the RSI ENVI 4.0 software. The processing consisted of contrast stretching, principal component analysis (PCA), decorrelation stretching and RGB false colour compositing. A field survey was conducted to characterize the features detected on the imagery. Particular attention was given to the NeFELs, which were located using a global positioning system (GPS). We then delimited the Regions of Interest (ROI) on the Landsat ETM 7+ imagery, i.e. polygons representing the 'ground-truth', discriminating the NeFELs from the other features occurring in the imagery. A simple statistical analysis was conducted on the digital number (DN) values of the pixels enclosed in the ROI of the NeFELs, with the aim to determine the spectral response pattern of such landsurfaces. The NeFELs were then classified in the entire image using a maximum likelihood classification algorithm. The results of the classification process were checked in the field. Finally, a spatial analysis was performed by converting the detected landsurfaces into vectorial format and importing them into the ESRI ArcViewGIS 9.0 software. Application of these procedures, together with the results of the field survey, highlighted that some 'objects' in the classified imagery, even if displaying the same spectral response of NeFELs, were not landsurfaces subject to intense soil erosion, thus confirming the strategic importance of the field-checking for the automatically produced data. During the production of the map of the NeFELs, which is the final result of the study, these 'objects' were eliminated by means of simple, geomorphologically-coherent intersection procedures in a geographic information system (GIS) environment. The overall surface of the NeFELs had an area of 22.9 km(2), which was 10% of the total. The spatial analysis showed that the highest frequency of the NeFELs occurred on both south-facing and southwest-facing slopes, cut on clayey-marly deposits, on which fine-textured and carbonate-rich Inceptisols were present and displaying slope angle values ranging from 12 degrees to 20 degrees. The comparison of two satellite imageries of different periods highlighted that the NeFELs were most clearly evident immediately after summer tillage operations and not so evident before them, suggesting that these practices could have played an important role in inducing the erosional processes. Copyright (C) 2009 John Wiley & Sons, Ltd.