Résumé
This study of desertification is based on ground measurement of reflectance properties of the different land degradation levels recognised by field ecologists in Southern Tunisia. Landsat MSS images from 1972 onwards recorded have been superimposed after geometrical correction. Using the field reflectance data ground features of low temporal variability were taken as radiometric references. This allowed to adjust for the differences in radiometry of the images and to detect temporal variations of image-derived indices. These indices, i.e. brightness, vegetation and colour were found to be correlated with land surface parameters such as roughness, green vegetation density and soil surface composition. As a result fluctuations of areas with degraded soil and mobilised sand could be monitored as well as areas treated by wind barriers or exclosure which appear darker. The overall trend is currently a significant recovery of the ecosystems after the severe drought and eolisation of the 80's. This experiment demonstrates the feasibility of long term monitoring of arid ecosystem changes, and its potential for the implementation of desertification control programmes. (Résumé d'auteur)