Soil erosion susceptibility assessment through morphometric analysis and morphotectonic implications in Rietspruit sub-basin, South Africa

Soil erosion is a basic subject with major global consequences, which usually affect soil productivity. Identifying and delineating the susceptible soil erosion zones is essential for applying preventive measures in any basin. The current case study research documents a morphometric, morphotectonic and soil erosion hotspot study of the Rietspruit sub-basin, which falls under the Vaal Basin, South Africa, considering the standard and integrated frame of methodology. A Shuttle Radar Topography Mission (SRTM) produced digital elevation model (DEM) was analyzed on a Geographical Information System (GIS) platform in order to evaluate the sub-basin’s morphometric parameters. Rietspruit sub-basin has an outlet of 6th order. The mean bifurcation ratio (Rbm) (3.88) suggest that sub-basin has a lithologically and structurally controlled terrain. The low value of the Rho coefficient (0.46) indicates small hydrologic storage in the sub-basin. The value of drainage texture (0.76) infers very coarse texture of the sub-basin. The computed values of circularity ratio (Rc), form factor (Ff) and elongation ratio (Re) depict an oval shape with homogenous geological terrain that is dominated by late youth to early mature stage of basin development. The values of drainage intensity (Di) (0.64) and drainage density (Dd) (1.09) are very low for this sub-basin, which suggest surface erosion that is linked to gully erosion and permeable soil with thick vegetal cover. The relief parameters of Rietspruit sub-basin suggest a moderate morphology, low basin relief and basin is slightly rugged with less prone to sediment transport. The soil erosion susceptibility analysis was performed at sub-watershed level with the help of ten selected parameters and by using analytic hierarchy process (AHP) and technique for order of preference by similarity to ideal solution (TOPSIS) method in ten sub-watersheds. The tectonic implications on the basin were performed by computing various tectonic parameters. This study supports the control of morphotectonics over the genesis and development of various scale landscapes. The drainage basin asymmetry factor (AF) (80.40) and the transverse topographic symmetry factor (TF) (0.64) infers the tectonic inclination and left side tilting of trunk stream with the asymmetric nature of the drainage basin. The hypsometric integral (Hi) (0.30) suggests a more evenly dissected drainage basin. It also suggests an association with the monadnock phase of geomorphic development. The basin shape index (BS) (1.24) is suggestive of a moderately elongated shape. The study could assist to provide a much-needed plan for water utilization to South Africa’s Department of Water and Sanitation. Currently, the sub-basin is critically threatened from mine and general domestic pollutants, thus rendering some of the water resources unusable for normal processes.