Influence des systemes de culture sur les risques d'erosion par ruissellement concentre. I.- Analyse des conditions de declenchement de l'erosion

French. On a cherché à mettre en évidence les conditions d’apparition des rigoles et ravines temporaires typiques de l’érosion par ruissellement concentré qui prévaut dans les régions limoneuses du Nord-Ouest de l’Europe. Pour cela, on a observé pendant 3 campagnes, le comportement de 9 petits bassins-versants du Pays de Caux. L’apparition et l’aggravation des figures d’érosion sont mises en relation avec les caractéristiques des pluies et avec les états de surface des parcelles constituant l’impluvium. Il apparaît que la probabilité d’érosion dépend étroitement du degré de développement des croûtes de battance, de la rugosité de surface, et de la présence d’empreintes de roues à la surface du sol. Lorsque l’érosion est provoquée par le ruissellement concentré, c’est donc sur la base de ces critères qu’il faut apprécier l’influence des systèmes de culture. En effet, ceux-ci induisent une forte variabilité spatiale et chronologique des propriétés hydrauliques du terrain.

English. In the context of the intensive agriculture of North-West Europe, runoff generation and concentration are fundamental processes that must be taken into account in order to understand and predict soil erosion. For assessing the impact of cropping systems (defined as plant sequences plus cropping management applied to each crop), it is necessary to consider their influence on the hydrodynamic properties and the microtopography of the soil surface. This is not explicitly done in the U.S.L.E. standard procedures, which in contrast put a major - and perhaps exaggerated stress on the effect of soil covering by the crop canopy. For a catchment area composed of several cultivated plots, the cropping systems applied to each one induce different types of spatial and temporal variation affecting infiltrability and surface storage. Some of these systematic variations are progressive, others discontinuous. Furthermore, the cropping system contributes to the creation of a specific network of channels for overland flow, this network also evolving with time. In order to reduce this complexity, a qualitative typology of soil surface states is proposed. 9 elementary watersheds, ranging from 0.6 to 13 ha, with gentle slopes and loamy soils, were observed in the Pays de Caux area during 3 cropping seasons. Gully initiation and development was not related with rainfall characteristics, but was clearly explained by the soil surface state of the agricultural plots constituting the catchment. This could lead us to an improvement of the system used for characterizing and grading cropping systems according to their influence on erosion.