EVALUATION OF TOPOGRAPHIC INDICES FOR EPHEMERAL-GULLY EROSION ASSESSMENT

Soil erosion by concentrated flows in agricultural areas is an important process affecting soil losses and landscape degradation. The main factors controlling concentrated flow erosion include the erodibility of materials, soil use and management, climate, and watershed topography. In this paper,two topographic indices, closely related with mathematical expressions suggested by different authors, are used to characterize the influence of watershed topography on gully erosion. The AS1 index is defined as the product of the watershed area and the partial area-weighted average slope.The AS2 index is the product of the watershed area and the length-weighted average swale slope.Using different ephemeral gully erosion databases, a high correlation was found between the topographic indices and the volume of eroded soil. The accuracy of different methods for field measurement of ephemeral gullies was evaluated to ensure that the relation between erosion and topographic indices is not affected by assessment errors. The resulting relation are useful to assess soil losses from gully erosion, to identify the most susceptible watersheds within large areas, and to compare the susceptibility to gully erosion among different catchments. This information also can be important to study the response of natural drainage network systems to different rainfall inputs.