In semi-arid Mediterranean regions the pressure on natural resources such as water and soil is increasing. In the Maghreb, soil degradation and reservoir sedimentation are serious problems, particularly in catchment areas with a high proportion of marls. As part of an Algerian-German project of scientific-technical cooperation, the runoff from soils on marls was studied, using modern rain simulators. A description of the rainfall experiments is followed by the presentation of a simple and appropriate mathematical approach to the modelling of runoff from soils on marls, and by a discussion of the influence of cultivation on water infiltration and runoff of these soils.