Threshold conditions for initiation of valley-side gullies in the Middle Veld of Swaziland

Gullied and ungullied catchments in four areas of deeply weathered saprolite in the Middle Veld of Swaziland can be successfully separated (p<0.01) using discriminant analysis based on area and slope of their basin heads. The discriminant scores for the catchments indicate that those on the Manzini land system differ in their behaviour from those on the Lobamba land system. No differences exist within each land system with respect to land use when the basins are classified as arable, rangeland or mixed. The discriminant functions indicate that gullies on the Lobamba land system have developed by Hortonian overland flow whereas those on the Manzini land system should be attributed to subsurface processes. Since no evidence exists for subsurface activity, it is suggested that gullying on the Manzini land system has been more active and, though initiated by overland flow, has developed to a stage where seepage at the base of the saprolite is now the main process. Erosion control measures should be based on dense ground vegetation to reduce flow velocity on the Lobamba land system and deep-rooted vegetation with high evapotranspiration rates to reduce flow volumes on the Manzini land system.