Identifying gullies in the Mediterranean environment by coupling a complex threshold model and a GIS

Soil erosion by water is an important physical process that leads to land degradation. In particular, gully erosion may contribute, in some cases, to 70% of the total soil loss. Water erosion occurs, forming a channel, when the erosive forces of overland flow exceed the strength of the soil particles to detachment and displacement. A relationship between local slope, S, and contributing area, A, is supposed to exists as runoff is proportional to the local catchment area. Therefore, an approach to interpret the physical process of gully initiation can be based on the concept of "geomorphologic threshold". Already in the seventies, PATTON & SCHUMM (1975) and BEGIN & SCHUMM (1979) began modeling gully erosion as a threshold process. They suggested that an equation could be derived by considering that, to excavate a gully channel, the overland flow should produce shear stresses in excess of a critical value. This approach was further organized and systematized by MONTGOMERY & DIETRICH (1994). Similarly, a gully can be assumed to end when there is a reduction of slope, or the concentrated flow meets more resistant soilvegetation complexes.