In tropical regions, soil structure degradation is an important agronomic problem. A study of the variability of the topsoil structure in a small region of the Sudano-Sahelian zone was undertaken in order to separate the variations due to natural factors from those induced by agricultural activity. A model of porosity distribution in the landscape was established, and regression equations enabling scientific forecasting of porosity for .sites where laboratory investigations were not carried out, but for which information on soil profile and cropping system are available were established. This model was obtained from limited laboratory investigations, and is a good illustration of the value of the statistical method employed.