Topographic thresholds for gully head formation in badlands

Gully erosion is a particularly damaging process which is not yet sufficiently under- stood and parameterized. Gully head topographic threshold relative to Hortonian runoff have been studied in cropland, rangeland and forest. This study extends such modelling approach to badlands. Different badlands (eight sites) have been studied in the Mediterranean environment in Italy and Spain, characterized by diversified cli- matic, lithological, and geological settings under different anthropogenic condition- ing. Many badlands have been characterized by their specific human history in addition to their geomorphological properties. Land use, as part of the human history, strongly affected many badland formation and development, through extremely impacting land exploitation (usually overgrazing). The effect of geological and geo- morphological processes are usually particularly well visible. While the weakening effect of joints is confirmed, the different geological layer bedding orientation with respect to the slope aspect generates a different development of badland morphol- ogies and different values of gully head thresholds values (as shown in two badlands sites on the same geological material and climate. The selection of Curve Number values, at the base of the introduction of land use into the gully head thresholds, has been more objectively defined in order to reduce arbitrariness in threshold application. The study additionally revises some of the physical basics behind the gully head threshold concept, requiring a description of the soil resistance in terms of frictional and cohesive components. This implies the explicit inclusion of rock fragment into the grain size distribution, which cannot be limited to fine grains. It results into an enriched threshold formulation that allows to describe the condition for gully head initiation and retreat as the result of the tra- deoff between the frictional and cohesive components of the soil resistance forces. Eventually, the gully head threshold concept is confirmed and extended to include badlands.