Gully erosion in South Eastern Tanzania: spatial distribution and topographic thresholds

Though gully erosion is often mentioned as a major process of land degradation in South Eastern Tanzania, little information is available on its distribution. The Makonde plateau and adjacent inland plains are of particular concern as they are densely populated and are major areas of cashew nuts production. The occurrence of gully erosion was assessed in 66 villages selected by stratified random sampling in an area of 13,000 km(2). Difference in susceptibility to gully erosion between landscape units was assess by determining topographic threshold parameters of 22 gullies on the Makonde plateau and 14 in the inland plains. Overall, gullies are common and spread equally over the different landscape units. Their occurrence is positively associated with terrain roughness (Cramer's V = 0.30; P = 0.05) and negatively with population density (V = 0.44; P < 0.01). On the Makonde plateau occurrence of gully erosion is associated with the presence of roads, while on the inland plains it is predominantly found in fields (V = 0.37; P < 0.05). This association is explained by the high susceptibility of the Makonde plateau to gully erosion and is due to the particular nature of its deep, highly weathered, sandy soils. Appreciating differences in susceptibility to gully erosion between landscape units is most relevant for targeting soil conservation measures.