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Land degradation mapping in Mediterranean ecosystems

Mapping and continuous monitoring of vegetation and soil conditions has been undertaken in the Mediterranean member states of the European Union, along with identification of the extension and time dynamics of degradation processes, and standardized interpretation of satellite based indices for producing land degradation indicators. The suitability of spectral mixture analysis was investigated as a tool for mapping soil conditions and vegetation abundance under controlled experimental conditions. Based on the excellent results obtained during these experiments it was decided to analyse the possibility of employing this approach as a standardized processing scheme for land degradation monitoring. The Landsat image from 1985 was chosen as reference image, and atmospherically corrected through a radiative transfer model. The approach proved its ability to retrieve ground reflectances with an accuracy of about 10% even under difficult atmospheric conditions. The scene from 1990 was then radiometrically rectified to the 1985 scene by using linear transformations which were obtained through a regression analysis of pixel spectra from nonvegetated surfaces, and both scenes spectrally unmixed. The resulting fraction images were normalised for shade, in order to analyse the changes in green vegetation abundance independently from illumination effects. Soil conditions were also examined. It was found that 40% of the study region had been affected by a considerable reduction of natural vegetation cover. This was confirmed by the fact that the soils within these areas are dominated by degraded categories suggesting that accelerated soil erosion processes are linked to the reduced vegetation cover density. Recent field checks have confirmed that fire impact is an important cause of the degradation processes. This an approach has been developed allowing each image to be viewed as the result of a combined analysis of soil and vegetation conditions. This system needs to be refined and developed but has the advantage that any updating of validation rules is easily incorporated.

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