Citation: Chaplot, V.; Mutema, M. Impact of Overgrazing on Diffuse and Concentrated Erosion: Case Study in the Sloping Lands of South Africa. Hydrology 2022, 9, 121. https://doi.org/10.3390/ hydrology9070121 Academic Editors: Gabriel Minea, Manuel Pulido Fernádez, Teodor Rusu and Tammo Steenhuis Received: 5 May 2022 Accepted: 28 June 2022 Published: 2 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). hydrology Article Impact of Overgrazing on Diffuse and Concentrated Erosion: Case Study in the Sloping Lands of South Africa Vincent Chaplot 1,2, * and Macdex Mutema 2,3 1 Institut de Recherche pour le Développement (IRD), Laboratoire d’Océanographie et du Climat, Expérimentations et Approches Numériques, UMR 7159, 4 Place Jussieu, CEDEX 05, 75252 Paris, France 2 SAEES, Rabie Saunders Building, University of KwaZulu-Natal, Private Bag X01, Scottsville 3209, South Africa; macdexo@gmail.com 3 Agricultural Research Council-Agricultural Engineering, Private Bag X519, Silverton, Pretoria 0127, South Africa * Correspondence: vincent.chaplot@ird.fr Abstract: Soil erosion is one of the most critical threats to cultivated land. Yet little information is available in Sub-Saharan Africa, especially on the relative contributions of various forms of erosion. Therefore, this study’s objective was to quantify soil loss by sheet and linear erosion. The study was carried out on the sloping land rangeland of the Potshini catchment of KwaZulu-Natal, South Africa, with an annual average rainfall of 766 mm. The average sheet erosion computed using a network of 1m 2 microplots was 7.7 ton ha −1 y −1 with standard error of 1.97 ton ha −1 y −1 (which corresponded to an ablation rate of between 0.35 to 1.32 mm y −1 ) while linear erosion, mainly the retreat of gully banks, removed 4.8 ton ha −1 y −1 , i.e., 38.4% of total soil losses. Despite removing a lower amount of soil, sheet erosion by depleting fertile, carbon- and nutrient-enriched soil horizons has a great impact on most ecological functions associated with soils. Keywords: land degradation; natural resources; smallholders; Africa 1. Introduction Erosion by water is one of the main causes of soil loss and impacts food production [1,2]. Soil erosion by water occurs as sheet and linear erosion. Sheet erosion removes a thin layer of soil from the soil, which has long been unnoticed, but since the early 1990s, it has been identified as one of the major threats to the sustainability of soils [3]. Linear erosion takes place when runoff concentrates and can lead to the formation of rills and gullies. Most often, sheet and linear erosion have been investigated independently [4], and several questions have arisen: (1) What is the relative contribution of splash, sheet and gully erosion to the overall soil erosion? (2) What are the mechanisms involved and controlling factors? (3) What are the interactions between the different types of water erosion? (4) What are the appropriate remediation techniques at landscape level? In South Africa, as in many areas of the world, the evaluation and prediction of soil erosion has often been conducted using the universal soil loss equation (USLE) method [5]. However, this method, even in its revised version, does not account for the contribution of linear erosion as pointed out by [6], thus leading to incorrect estimations. When based on field investigations, fluxes of sediments are evaluated within rivers such as by [7] reporting soil erosion rates of 306 g m −2 y −1 at 23 ha level and 0.29 g m −2 y −1 at 1000 ha level, but with no information on the respective contributions of sheet and linear erosion. Only a few studies in South Africa gathered quantitative data on sheet erosion alone [8–16]. Ref [15], using 1 m 2 micro-plots installed within the sloping lands of KwaZulu- Natal (South Africa), indicated that splash and little surface runoff remove from its initial place an average of 6.45 ton ha −1 y −1 , with values varying from 3 to 13 ton ha −1 y −1 Hydrology 2022, 9, 121. https://doi.org/10.3390/hydrology9070121 https://www.mdpi.com/journal/hydrology