No abstract

No abstract

Gully erosion is a degradation process affecting soils in many parts of the World. 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 and discontinuous gullies. First, gullies (totaling 5 273 ha) visible from SPOT 5 imagery were mapped for a catchment (nearly 5 000 km 2 ) 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 usecover interactions. These were utilized in a zonal approach in order to determine the extent factors differ between continuous and discontinuous gullies. Factors leading to the development of continuous gullies 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 continuous gully conditions, more discontinuous gullies 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 continuous and discontinuous gullies.

In an assessment of the influence of internal rock moisture content on Schmidt hammer readings, rebound (R) values are found to decrease with increasing moisture content. For samples of basalt, sandstone and dolerite the maximum decrease in R-values is found between oven dry values and saturated rock rebound values, the magnitude of which varies from 2 to 10 points on the R-scale. A quartzite block has the greatest decline of 6 points at 60 per cent saturation. For certain rock types under differing site-to-site field moisture conditions the moisture effect can be a significant factor in the interpretation of the relative state of weathering from rebound values.

Erosion is a major soil degradation problem in South Africa, confronting both land and water resource management throughout the country. Given the increasing threat of soil erosion, a need to improve techniques of estimating the soil-erosion risk at a national scale was identified by the National Department of Agriculture and forms the basic premise of this study. Principles and components of the Revised Universal Soil Loss Equation are applied here since the model combines sufficient simplicity for application on a national scale with a comprehensive incorporation of the main soil-erosion factors. Indicators of erosion susceptibility of the physical environment, including climate erosivity, soil erodibility and topography were improved over earlier assessments by feeding current available data into advanced algorithms. Two maps are presented: an actual erosion-risk distribution, and a potential erosion-risk map that excludes the vegetation cover factor. Actual soil-erosion risk, which relates to the current risk of erosion under contemporary vegetation and land-use conditions, was accounted for by regression equations between vegetation cover and MODIS-derived spectral index. The area of land with a moderate to severe potential risk is found to total approximately 61 m. ha (50%). Although more than 91 m. (75%) are classified as having only a very low to low actual risk, approximately 26 m. ha (20%) of land is eroded at a rate greater than a soil-loss tolerance of 10 t/ha•yr, showing the potential to target erosion control to problem areas. The Eastern Cape, Limpopo and KwaZulu-Natal Provinces have the highest erosion potential. Comparison of potential and actual erosion risk indicates that over 26 m. ha (>30% of national land) could be subject to high erosion risk without maintenance or careful management of the current vegetation cover and land use. Although the distribution of the actual erosion risk broadly follows that outlined previously, this study provides an advance on previous assessments of erosion; results are validated more comprehensively than before, and show an overall accuracy of 77%. The paper also describes many of the limitations inherent in regional erosion studies.

The formation of a pronival (protalus) rampart on sub-Antarctic Marion Island is investigated. Morphological attributes show debris at the angle of repose on the rampart's proximal slope and at a lower angle on the distal slope. Relative-age dating, based on the percentage moss cover and weathering rind thickness of the clastic component, indicates accumulation mainly on the proximal slope and rampart crest, implying upslope (retrogressive) accumulation. This contrasts with a previously published model for pronival ramparts, which proposes rampart growth by addition of material to the distal slope. Development of the Marion Island rampart is suggested to result from the control exerted by a relatively low-angled surface and a shrinking snowbed. A small debris step formed on the proximal slope appears to be a response to decreased snowfalls due to changing climate over the last c. 50 years. Growth rate of the rampart is considered to be variable during the Holocene in response to changes in climate and debris supply.

Wetting and drying is an acknowledged yet still poorly understood rock weathering process. Previous experiments documented in the literature measure physical changes or mass loss in relation to moisture oscillations but only one study directly compares different moisture amplitudes by using different modes of moisture application. In this experimental study, four sets of sandstone tablets are subjected to 48 h wetting and drying cycles at set moisture content fluctuations of 29, 42, 56 and 63% respectively. A common moisture application, full immersion, is used for all four sets. Mass loss after 52 cycles averaged 0·27%, some three times more than that of the control samples. Average porosity of the samples increased, while water absorption capacities and the saturation coefficients of the samples decreased. No discernable difference in mass loss or change in physical properties was found between the different sample sets. When compared with previous studies, results suggest that the effect of moisture application type may be more of a controlling factor on the weathering effect than actual moisture content achieved. A need to move towards a more standardized approach in wetting and drying experimental studies is emphasized.

Coarse slope deposits, frequently grading into blockstreams, are common throughout the Lesotho highlands, southern Africa. This paper describes one such blockstream. It is interpreted as a typical lag deposit derived from valley‐wide colluvial mantles, which contains superimposed and incorporated Late‐Pleistocene blocky material. Blocks are largely derived from local scarps but may contain a component of corestones from mobilized regolith. The widespread mantle argues against landscape‐scale glaciation in the Lesotho highlands in the Late Pleistocene. Rather, the environment appears to have been conducive to deep seasonal frost during the period of block production. No unequivocal evidence for permafrost is found. Copyright © 2002 John Wiley & Sons, Ltd.