University of Bonn, Institute of Soil Science, Germany
The Drâa is situated in the dry zone of the southern part of the High Atlas Mountain in southern Morocco. A large N-S-transect (> 400 km) along an extreme gradient of altitude (3.200-445m) and aridity (600 > 50 mm rainfall per year) at 6°30' W longitude is taken. From periglacial up to desert landscapes 13 study sites are chosen. Their soil cover was investigated 2001-2003 to detect differences in their potential use and degradation risk. This was in the framework of the project IMPETUS -- an integrated approach to the efficient management of scarce water resources in West Africa. So-called soil(land)scapes are build up by several polypeda or pedocomplexes and correspond to the pedochore after the choric idea of soil geography (SCHLICHTING 1970). The different soils of a soilscape result in a soil associations. Different soilscapes are presented here using the World Reference Base for Soil Resources (ISSS-ISRC-FAO 1998).
Calcisol-soilscapes on steep slopes and a transmountain basin of the calcareous High Atlas are rich in carbonates (up to 58%). The silty and loamy silty texture has high contents of gravel (20-86%). The soil cover of the Leptosol-Regosol-soilscape in the crystalline Anti-Atlas is generally very thin (5-25cm soil depth) and incompletely conserved between volcanic outcrops. The Leptosol-Regosol-Calcisol-soilscape of the escarpment relief of sedimentary rocks of the middle Drâa is developed with deep Calcisols and their eroded relict as free-rinsed calcrets of a total eroded escarpment. All these soilscapes on slopes indicate a strong degradation by water erosion.
The Arenosol-Fluvisol-Anthrosol-soilscapes of some Drâa oasis include important arable land. These alluvial deposits suffer rising salinisation with the South up to 16,dSm and are no more used there. The occurence of a low ground water table less quality, a finer soil texture and probably more salty alluvial deposits of the southern terrace can explain this situation. The drying former lake Iriqui (end of the Drâa river) is characterised by a Solonchak-soilscape. The Arenosol-Calcisol-soilscape of a neighboured dayet is used as rainfield.
Keywords: Aridity, IMPETUS, soilscape, Southern Morocco