Gully erosion processes: monitoring and modelling

Soil erosion on hillslopes has been divided traditionally into sheet, rill, and gully erosion.In sheet erosion, a relatively shallow overland fl ow acts on a hillslope and removes sediment particles uniformly from the land surface.The erosion processes can be very gradual and result in an imperceptible change in the landscape during individual storm events.In rill and gully erosion, overland fl ow is concentrated, which markedly increases the erosivity of the fl ow and causes signifi cant incision into the landscape during individual storms.Usually, the relatively smaller rills occur on sloping surfaces, whereas the relatively larger gullies occur within topographic swales or hollows.There remains much discussion on discriminating rill erosion from gully erosion, yet the impact of gully erosion on soil losses deserves special attention.Permanent or classical gullies, edge-of-fi eld or bank gullies, and ephemeral gullies now are the foci of many soil erosion studies for a number of reasons.Gully erosion represents an important -if not the dominant -sediment source within watersheds.Gullies are effective links for transferring runoff, sediment, and other potential materials from source to sink, thereby increasing landscape connectivity.Gullies may be triggered by land-use change or extreme hydrologic events, thus they may serve as critical indices of land mismanagement and desertifi cation.Finally, gullies, especially ephemeral gullies, can grow rapidly in size within individual storm events, as such they represent an area of intense, localized scour that may not be predictable using current numerical or modelling paradigms.Despite its importance, gully erosion has received much less attention from the scientifi c community.For instance, a brief survey of published papers shows that less than 10% of these soil erosion studies address gullies directly.More research is required to better understand the physical mechanisms involved in this important erosion process, with the ultimate aim of developing prediction technology and effective management strategies.To this end, international symposia have been organized to address advances in the study of gully erosion from a variety of environments and perspectives.The fi rst of these meetings was held in Leuven, Belgium, in 2000, the second in Sicuani, China, in 2002, and the third in Oxford, Mississippi (USA) in 2004.