Comment on "Lightning as a geomorphic agent on mountain summits : evidence from southern Africa" by Knight and Grab (2014)

Insights into the possible effect of lightning strikes on rock breakdown are presented by Knight and Grab (2014) froma summit area in the Lesotho highlands. Based on their findings, the authors challenge the association of angular debris with frost shattering and use this as a platform for directing critique against palaeo-geomorphic studies.While the lightning strike data are not questioned directly here, the palaeo-environmental contextwithin which the paper is set, the portrayal of former findings and the assumptions regarding weathering mechanisms in Lesotho are commented on. Frost shattering is the centre of Knight and Grab's weathering critique but, contrary to that stated in their text, none of the cited authors invoke this process in Lesotho. Other weathering processes that are speculated upon are not specific to cold climates either and thus cannot be used in support of their argument. In terms of debris and block distribution, lightning will not account for the preferential location of relict blocks and debris below the summits on south-facing slopes, or for the extensive valley floor accumulations that are documented in Lesotho. Knight and Grab also falsely portray former studies by implying that palaeo-environmental inferences in the area are drawn fromblock origin or morphology alonewhen the climatic signatures were derived from integrated assessments. In a palaeo-context, the relative contribution of lightning to debris production under dryer and colder conditions, when convective thunderstorm activity in the highlands was likely reduced, is also questioned. The weathering context, as well as the critique that Knight and Grab direct at other studies on relict landforms, is thus shown as inappropriate.http://www.elsevier.com/locate/geomorphhb201

Factors controlling gully development : comparing continuous and discontinuous gullies

Gully erosion is a degradation process affecting soils in many parts of theWorld. Despite the complexity of a series of collective factors across different spatial scales, previous research has not yet explicitly quantified factor dominance between different sized gullies. This factorial analysis quantifies the differences in factor dominance between continuous gullies (cgs) and discontinuous gullies (dgs). First, gullies (totaling 5273 ha) visible from SPOT 5 imagery were mapped for a catchment (nearly 5000 km2) located in the Eastern Cape Province of South Africa. Eleven important factors were integrated into a geographical information system including topographical variables, parent material-soil associations and land use–cover interactions. These were utilized in a zonal approach in order to determine the extent factors differ between cgs and dgs. Factors leading to the development of cgs are gentle footslopes in zones of saturation along drainage paths with a large contributing area, erodible duplex soils derived from mudstones and poor vegetation cover due to overgrazing. Compared to cgs conditions, more dgs occur on rolling slopes where the surface becomes less frequently saturated with a smaller contributing area, soils are more stable and shallow. Factorial analysis further illustrates that differences in factor dominance between the two groups of gullies is most apparent for soil factors. A combination of overgrazing and susceptible mudstones proves to be key factors that consistently determine the development of cgs and dgs.The ARC-ISCW, as well as Mr. D. J. Pretorius and his colleagues at the Department of Agriculture Forestry and Fisheries (DAFF), Directorate Land Use and Soil Management.http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1099-145X

Water erosion risk assessment in South Africa : a proposed methodological framework

With the increase in human impacts on the environment, especially in terms of agricultural intensification and climate change, erosion processes need to be assessed and continually monitored. In many countries, but particularly in developing countries such as South Africa, standardized methodological frameworks that deliver comparable results across large areas as a baseline for regional scale monitoring are absent. Due to limitations of scale at which techniques can be applied and erosion processes assessed, this study describes a multi-process and multiscale approach for soil erosion risk assessment under South African conditions. The framework includes assessment of sheet-rill erosion at a national scale based on the principles and components defined in the Universal Soil Loss Equation; gully erosion in a large catchment located in the Eastern Cape Province by integrating 11 important factors into a GIS; and sediment migration for a research catchment near Wartburg in KwaZulu-Natal by means of the Soil andWater Assessment Tool. Three hierarchical levels are presented in the framework, illustrating the most feasible erosion assessment techniques and input datasets that are required for application at a regional scale with proper incorporation of the most important erosion contributing factors. The methodological framework is not interpreted as a single assessment technique but rather as an approach that guides the selection of appropriate techniques and datasets according to scale dependency and modelled complexity of the erosion processes.National Research Foundationhttp://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1468-0459hb201

A baseline climatology of sounding-derived parameters associated with heavy rainfall over Gauteng, South Africa

Irene weather office sounding data are considered a proximity sounding for the Gauteng province of South Africa. Sounding-derived parameters are analysed for 35 austral summers from 1977 to 2012. The goal of this study is to provide a climatology of sounding-derived parameters commonly used as ingredients to forecast heavy rainfall. The emphasis is placed on identifying those variables that distinguish between climatology and heavy rainfall events. Special attention is given to how the critical values associated with heavy rainfall change from early to late summer. During early summer (October to December), the atmospheric circulation over Gauteng is markedly extra-tropical in nature. Heavy rainfall occurs in a conditionally unstable atmosphere and is associated with conditions conducive to the development of severe storms, such as large wind shear and convective available potential energy (CAPE) values, strong upper tropospheric winds and large temperature lapse rates. In late summer (January to March), the atmosphere takes on distinct tropical characteristics and becomes increasingly convectively unstable. During this time of year there is abundant moisture in circulation and the storms that develop are highly efficient in producing precipitation. Forecasting heavy rainfall in early summer requires different techniques than in late summer. Sounding parameters, which provide information about the moisture content of the atmosphere, are capable of distinguishing between climatology and heavy rainfall during all summer months. The only other variables capable of doing this are the average meridionial wind direction in the 800 to 600 hPa layer, the mean layer equivalent potential temperature, the Showalter Index (SI), the K-Index (KI) and the Elevated K-Index (EKI). However, critical values associated with heavy rainfall for all these parameters change month by month.http://onlinelibrary.wiley.comjournal/10.1002/(ISSN)1097-00882016-01-31hb201

Southern African geomorphology : looking back, moving forward

No abstract availablehttp://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1468-0459hb201

Intra-event characteristics of extreme erosive rainfall on Mauritius

Mauritius is a typical tropical volcanic island with a raised interior where extreme rainfall events generate the bulk of the erosivity. Intra-event characteristics of the 120 highest erosive events at six selected locations between 2004 and 2008 were analysed to provide the first detailed intra-storm data for a tropical island environment. On Mauritius, spatial variation is evident in the characteristics of the extreme erosive rainfall recorded at the stations, with a noticeable increase in rainfall depth, duration, kinetic energy and erosivity of extreme events with altitude. Extreme events in the raised interior (central plateau) show a high variability in peak intensity over time as well as a higher percentage of events with the greatest intensities in the latter part of the event. Intra-event distribution of rainfall in the interior of the island shows that rainfall has a higher potential to exceed infiltration rates as well as the ability to generate high peak runoff rates and substantial soil loss. The study suggests that even though the within-event rainfall characteristics is complex it has implications for soil erosion risk, and that in tropical island environments the within-storm distribution of rainfall must be incorporated in soil loss modelling.National Research Foundation (NRF)http://www.tandfonline.com/loi/tphy202017-05-31hb2016Geography, Geoinformatics and Meteorolog

Temporal sensitivity analysis of erosivity estimations in a high rainfall tropical island environment

The Erosivity Index (EI) and the Modified Fournier Index (MFI) are two commonly used methods in calculating the R factor of the universal soil loss equation/ revised universal soil loss equation formula. Using Mauritius as a case study, the value of high-resolution data versus long-term totals in erosivity calculations is investigated. A limited number of four Mauritius Meteorological Services stations located on the west coast and the Central Plateau provided the study with detailed rainfall data for 6 years at 6-min intervals. Rainfall erosivity for erosive events was calculated using different set interval data. In this study, within the EI, the use of 6-min rainfall intervals during erosive rainfall gave estimates of around 10% more erosivity than the 30-min time intervals and 33% more rainfall erosivity than the 60-min rainfall measurements. When the MFI was used to determine erosivity through annual and monthly rainfall totals, substantially higher erosivity than the EI method was calculated in both regions. This stems from the large amount of non-erosive rainfall that is generated on Mauritius. Even when the MFI was used to calculate erosivity through monthly and annual rainfall totals derived purely from erosive rainfall, erosivity calculations were not comparable to those from high-resolution data within the EI. We suggest that for the computation of erosivity, rainfall data with the highest possible resolution should be utilised if available and that the application of annual and monthly rainfall totals to assess absolute soil erosion risk within a high rainfall tropical environment must be used with caution.http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1468-0459hb201