INDEXES FOR THE ESTIMATION OF INTERRILL ERODIBILITY OF MOROCCAN SOILS

Replicated runoff and soil loss measurements were made on nine Morrocan soils having 9 to 10% slopes using a rainfall simulator. Soil physical, chemical and mineralogical properties were measured at each site. Forty-two individual soil properties or combinations of properties were related by regression analysis to soil loss. Variation in erodibility of the major soils studied are explained by the resistance of aggregates to dispersion and interrill transportation. The high relative erodibility of the calcareous soils was partly attributed to the occurrence of CaCO3 in the silt size fraction. The smectitic nature of the clay minerals in the vertisols and their low organic matter content are important factors in their low infiltration capacity and higher soil loss rate. The slaking process resulted in strong small aggregates that were vulnerable to transport and removal from the soil surface. Agreement was found between the relative erodibility obtained with the use of the rainfall simulator and the relative magnitude of the erosion problems as observed in the field and reported by other authors in the region. A multiple regression model using only three soil variables, i.e. % soil > 2 mm + % sand, electrical conductivity, and % silt and clay size so-called "active" CaCO3, predicted soil loss with an R2 = 0.980. This regression model used as a relative erodibility index could represent a valuable tool for areas in Morocco where limited information is available on soil erosion.