Les processus hortoniens et leur importance dans la genèse et le développement des crues en zones semi-arides

Hortonian overland flow by rainfall excess seems to be the main runoff process in semi-arid areas as demonstrated directly through observations and measurements of rainfall-runoff events, and indirectly from isotopic, geochimical and biological tracer tests. Examples taken from semi-arid tropical zone (Burkina Faso and Senegal) and the Mediterranean zone (Tunisia and Southern France) illustrate this hypothesis. Several spatial scales, varying from few square meters (m 2 ) for rainfall simulations, to plots of hundreds of m 2 and small catchments of few km 2 are studied. Horton's empirical model is applied and validated at various spatial scales. The model is suitable for crusted soils and for events where the hydraulic conductivity at saturation of the soil surface is low compared to rainfall intensity. Simple relationships are used to relate horton's model prameters to the hydraulic conductivity at natural saturation and initial water content.