In the Eastern Rif of N Morocco, soil conservation is seriously threatened by water erosion. Large areas of soil have reached an irreversible state of degradation. In this study, the 137 Cs technique was used to quantify erosion rates and identify the main factors involved in the erosion process based on a representative catchment of the Eastern Rif. To estimate erosion rates in terms of the main factors affecting soil losses, samples were collected taking into account the lithology, slope and land use

The soil in the Rif, Morocco, is at serious risk because increasing anthropogenic pressures are gradually transforming large natural areas into farmland. The distribution of magnetic minerals within the soil profi le can be used to assess soil development and degradation. The soils in the study area are severely eroded because of a combination of highly erodible soils, intense rainstorms and scarce vegetation cover. To sample of representative soil profi les, lithology, slope gradient and land use were considered. The ranges of magnetic susceptibility in the soil profi les distinguished between two primary soil groups. Magnetic susceptibility varied in the soil profi le and along the soil toposequence, and the variations were related to the differences in the original magnetic composition and the infl uence of main erosion factors. Lithology is the main factor contributing to the variation in magnetic susceptibility. The magnetic susceptibility values in soils on Tertiary marls (χ = 13·5 × 10 −8 m 3 kg) differed significantly from those on Quaternary terraces (χ = 122·1 × 10 −8 m 3 kg −1). Slope affected the distribution of magnetic susceptibility because of the continuous loss of topsoil in some parts of the slope and the deposition of eroded soil in others. Elimination of the natural vegetation cover and a shift to cultivated land for cereals has had a negative impact on soil development and, on similar slopes and substrates, magnetic susceptibility decreased signifi cantly in cultivated soils. The soils on steep slopes that had natural vegetation cover retained the magnetic minerals better than did those on gentler slopes that were under cultivation. Grazing, clearing and, especially, tilling has weakened the soil and made it much more vulnerable to erosion. An analysis of the main factors causing erosion will help to promote rational use of the land and to establish conservation strategies in such fragile agroecosystems.

The purpose of this study is to understand and quantify the relationships between current Cs inventories and the soil properties and the physiographic characteristics. A total of 36 cores were taken in seven transects with different slopes, lithology and land use. The analysis focused on theCs mass activity as well as inventories and its relationship with soil properties as grain size and organic matter. The mass activity of Cs and the inventories varied between 3.6 and 63.7 Bq kg and between 521.7 and 3304.4 Bq m, respectively. At uncultivated soils, high concentrations of Cs are located in the top 10 cm and decreased exponentially in depth. Cultivated soils record disturbed and heterogeneousCs profiles with values of mass activity and inventories up to 38.7 Bq kg and 2510.8 Bq m, respectively. The Cs inventories were significantly higher in uncultivated soils (mean: 2086.9 Bq m) compared to cultivated soils (mean: 1397.1 Bq m). The Cs mass activity showed a significant positive correlation with organic matter, silt and clay, but it was negatively correlated with sand and pH respectively. High levels ofCs were found at altitudes between 500 m and 640 m a.s.l, and at slopes from 8° to 11°, as well as in dense scrub land and Fersialitic soils. A principal component analysis showed that more than 86.7% of the variance in Cs mass activity was explained by the organic matter content and the land use. The results of this study provide insights into the effects of soil properties and physiographic factors on the behaviour ofCs in soils of Mediterranean environments of North Africa and strengthen the reliability of this radioisotope as an erosion tracer.

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Le bassin versant de l'oued Amzaz (Maroc, Rif central) est caractérisé par un climat semi-aride. Des pluies irrégulières, le plus souvent à caractère orageux combinées à une déforestation importante provoquent une érosion sévère. Les sols sont de plus en plus dégradés et les sédiments issus de cette érosion contribuent à l'envasement des retenues de barrages. L'intégration des cartes thématiques des différents facteurs de l'Equation de Gavrilovic (EPM) dans le Système d'Information Géographique (SIG) avec leurs bases de données a permis d'une manière rapide et efficace de démêler la complexité et l'interdépendance des facteurs dans l'analyse des risques d'érosion, de mieux cerner l'impact de chaque facteur et d'évaluer sa contribution aux pertes en sol. L'intégration dans le SIG des formules de Gavrilovic a permis de hiérarchiser les différentes zones du bassin versant en produisant une carte synthétique de répartition des degrés de sensibilité à l'érosion, de déterminer le taux d'érosion par le ruissellement en nappe (10483m3/km²/an en moyenne) et d'établir les facteurs décisifs qui contrôlent l'érosion hydrique qui sont par ordre d'importance la pente, l'érodibilité des sols et la couverture végétale.