Historical gully erosion growth in the Lower Thina Catchment, Eastern Cape

Abstract

Gully erosion is a complex phenomenon initiated by various erosion-related processes. Due to the complexity of drivers contributing to this degradation process, it has received, compared to sheet and rill erosion, less scientific attention on large spatial scales. In South Africa, erodible duplex soils coupled with inefficient land management result in gully erosion, with the Eastern Cape and KwaZulu-Natal the two provinces most prone to gullying. One of the greatest challenges in rural areas is the limited information regarding the actual location of gully systems. This study quantifies historical gully erosion for six time periods in the Lower Thina Catchment in the Eastern Cape of South Africa using parameters recognised as having the greatest impact on gully erosion (climate variability; land use change). A desktop-based research approach was followed to determine the effect of climate variability and land use on historical gully development and growth in the selected catchment. Soils in this catchment are prone to extreme soil erosion and although several soil erosion studies have been conducted in the upper regions of the study area, no studies have been recorded in the lower parts of the catchment to date. The first part of this study made use of GIS and remote sensing techniques to identify, map and quantify the lateral growth rate of four selected gullies within the catchment using high-resolution aerial and satellite imagery. The second part involved determining the effects of various land use activities in the catchment, while the third part entailed using rainfall records from three weather stations (Umthatha, Cengcane and Papane) to calculate inter- and intra- annual rainfall variability (CV and PCI respectively), rainfall erosivity (MFI) and temperature patterns within the study area. Furthermore, regression analyses and ANOVA were conducted. Outcomes of the second and third parts were used to determine the drivers of gullying within the catchment. Results show that all investigated gullies are permanent in nature, with lateral growth rates of gullies ranging between 0.07 and 0.1 ha.yr-1 across the 68-year time period. A clear increment in surface area is evident for all investigated gullies. Changing land use patterns and anthropogenic factors are associated with an increase in gully growth. A low inter-annual rainfall variation (CV < µ - σ) and a moderate (15 ≥PCI>10) to irregular (20 ≥PCI>16) intra-annual distribution is evident across the years. Rainfall erosivity (MFI) values for Umthatha and Papane are moderate (90<MFI<120) and low (60<MFI<90) respectively, while that of Cengcane are high (120<MFI<160). Irregular and moderate intra-annual rainfall patterns (indicating seasonality of rainfall), coupled with heterogeneous temperature data suggest that investigated climate parameters can affect gully erosion with respect to altitude and the seasons. However, linear trends (p > 0.05) show little control of climate parameters on gully erosion at an inter-annual level. Furthermore, since risks of soil erosion parameters are directly proportional to erosivity potential within the catchment, based on PCI, CV, MFI, and temperature variability, the risk of gullying in the Lower Thina Catchment across the 68-year period is low to moderate. While factors such as underlying geology, unconsolidated and duplex soils, topography, as well as some climatic fluctuations contribute to gully erosion, agricultural practices, land use cover and proximity to roads and residential areas are identified as the main causes of gully erosion. Irrespective of this, no factor is solely responsible for gullying.