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Gully location mapping at a national scale for South Africa

Gully erosion is an important form of soil erosion that contributes greatly to soil degradation and loss in South Africa (SA). It is a process whereby soil is removed through the concentration of surface or subsurface water which results into the formation of incised channels. However, little is known regarding the spatial extent of gullies in SA, especially at a national scale. Previous soil erosion assessment studies indicated the difficulties of modelling or automatically extracting gullies using remote sensing at regional scales due to the temporal and spatial complexity at which the phenomenon occurs. This study created a gully location map for SA by means of visual interpretation and vectorisation from Système Pour l'Observation de la Terre (SPOT) 5 imagery at a scale of 1:10,000 within a geographic information system. Results illustrate the extent of gully erosion in the Eastern Cape (161,517 ha), Northern Cape (160,885 ha), KwaZulu-Natal (92,543 ha), Free State (64,674 ha), Limpopo (58,669 ha), Western Cape (25,403 ha), Mpumalanga (17,420 ha) and North West (10,782 ha) provinces. Additional zonal calculations indicate that gullies are more prominent on land that is suitable for cultivation (4.3%) than on less suitable to unsuitable land (1.5%), subsequently undermining sustainable management of soil resources and food security. This is probably attributed to inappropriate agricultural activities such as clearing of vegetation and overstocking in certain agricultural areas. Despite some problems experienced during the interpretation and mapping phases (such as difficult distinction between gullies and dry river beds/channels, landslides and other erosion forms), results show an overall accuracy of 90% when compared to observations (n = 1019) in the field. Future studies should focus on quantifying different contributing factors in order to ease selection of appropriate management strategies.