Gully Erosion: Importance and Model Implications

Abstract

This paper deals with gully erosion in agricultural environments and concentrates in particular on types of gullies, importance of gully erosion with respect to sediment production in agricultural environments and modelling needs. Two major gully types can be often observed in agricultural lands: ephemeral gullies and bank gullies. Field measurements combined with aerial photo analysis in three contrasting European environments indicate that ephemeral gully erosion rates represent 44 per cent of total sediment production in central Belgium and up to 80–83 per cent of total sediment production in Mediterranean conditions (Portugal and Spain). In contrast, bank gully erosion rates are about one order of magnitude less than ephemeral gully erosion rates in central Belgium. Nevertheless, there is a need to model these erosion features, which are complementary to interrill and rill erosion. More particularly, there is a need for more detailed monitoring, experimenting and modelling of the development and infilling of both ephemeral gullies and bank gullies in a variety of agricultural environments. In other words, there is a need to better predict the location, the total length and the cross-section of gullies. The threshold concept could be a useful tool to help locate ephemeral gullies in the landscape. However, threshold conditions for incipient gullying in a variety of climatological, topographic, pedological and land-use conditions first need to be established. Existing erosion models need to be refined to incorporate the effects of the resistance of various soil horizons to concentrated flow erosion and the effects of other soil detaching mechanisms in gullies such as soil fall, slumping and headcutting. Improved gully models are needed to predict more accurately the effects of environmental change on the intensity of this soil degradation process.