Assessing soil erosion and control factors by the radiometric technique in the Boussouab catchment, Eastern Rif, Morocco

Abstract

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 137Cs 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 along six selected transects within the Boussouab catchment. The transects were representative of the main land uses and physiographic characteristics of that Rif sector. The reference inventory for the area was established at a stable, well preserved, matorral site (value of 4250 Bq m-2). All the sampling sites were eroded and 137Cs inventories varied widely (between 245 and 3670 Bq m-2). The effective soil losses were also highly variable (between 5.1 and 48.8 t ha-1 yr-1). Soil losses varied with land use. The lowest average values were on matorral and fallow land (10.5 and 15.2 t ha-1 yr-1, respectively) but much higher with alfa vegetation or cereal crops (31.6 and 27.3 t ha-1 yr-1, respectively). The highest erosion rate was on a badland transect at the more eroded part of the catchment, with rates exceeding 40 t ha-1 yr-1 and reaching a maximum of 48.8 t ha-1 yr-1.The average soil losses increased by more than 100 % when the slope increased from 10º (17.7 t ha-1 yr-1) to 25º (40. 8 t ha-1 yr-1). Similar results were obtained when comparing erosion rates in soils that were covered by matorral with respect to those under cultivation. Lithology was also a key factor affecting soil loss. Soils on marls were more erodible and the average erosion rates reached 29.36 t ha-1 yr-1, which was twice as high as soils on the glacis and old fluvial terraces (average rates of 14.98 t ha-1 yr-1 ). The radiometric approach was very useful to quantify erosion rates and to examine the pattern of soil movement. The analysis of main erosion factors can help to promote rational soil use and establish conservation strategies in the study area.