Integrating geographical information systems, remote sensing, ground truthing and modelling approaches for regional erosion classification of semi-arid catchments in South Africa

Water resources management is an important issue in southern African countries. With respect to water quality problems, the understanding of the dynamics of integrated soil erosion processes in river basins is of crucial importance. This study is on the delineation of response units (RUs) in the Mkomazi River catchment (KwaZulu/Natal; South Africa). It was carried out within the framework of an interdisciplinary EU-funded project aimed at developing an integrated water resources management system for water resources analyses and prognostic scenario planning in semi-arid catchments of southern Africa. Within this more general framework, particular attention was focused on the identification of sediment source areas. For this purpose, the RUs concept was applied to delineate erosion RUs (ERUs). The ERUs were subsequently used to identify heterogeneously distributed areas within a river catchment characterized by specific hydrological and erosion dynamics and features. Therefore, they can be applied as modelling entities in physically based hydrological and erosion models. Spatially distributed input data from the catchment were derived by remote sensing techniques and geographical information systems analyses. Taking into account the high amount of sediments produced by gully erosion, not considered in USLE-type models, special attention was focused on gully erosion using a dynamic gully erosion model. The example from southern Africa shows that the methods applied are able to identify areas affected by gully erosion. Furthermore, it was possible to estimate the entire amount of soil loss, including gully erosion.