MAPPING SOIL DEPTH CLASSES IN DRY MEDITERRANEAN AREAS USING TERRAIN ATTRIBUTES DERIVED FROM A DIGITAL ELEVATION MODEL

Modem)and management increasingly demands quantitative information on spatially variable soil properties. Traditional soil survey maps do no t provide this information. In this paper we report on the application of terrain attributes to mapping soil depth classes at high spatial r esolution over large areas under dry Mediterranean conditions. Soil da ta were collected in 111 georeferenced field plots of 30 m x 30 m, mor e or less equally distributed over three lithological units, phyllites , shales and limestones. Topographic attributes were computed from a d igital elevation model at 30 m resolution. A principal components anal ysis was carried out on the map overlays of the terrain attributes in order to obtain uncorrelated topographic factors that enabled us to ap ply a probability approach. Using the maximum likelihood classifier wi th the field plots as a dispersed training area, predictions were made of mean soil depth class and the probability of occurrence of shallow or deep soils. A cross-validation revealed a 65%, 81% and 61% accurac y for the three maps of the shale area, a 50%, 55% and 40% for the map s of the phyllite area, and a 78%, 72% and 75% accuracy for the maps o f the limestone area. Explanations for both the good results in the sh ale and limestone areas, and the poor results in the phyllite area, fo cus on the effects of the spatial scale of topographic variation, sedi ment transport mechanisms, and the impact of land use.