Browsing by Author "Van Zijl, George Munnik"
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- ItemAn investigation of the soil properties controlling gully erosion in a sub-catchment in Maphutseng, Lesotho(Stellenbosch : University of Stellenbosch, 2010-03) Van Zijl, George Munnik; Ellis, F.; Rozanov, Andrei Borisovich; University of Stellenbosch. Faculty of Agrisciences. Dept. of Soil Science.ENGLISH ABSTRACT: Lesotho is a country with an international reputation for the severe degree of soil erosion in its landscape. Despite several national soil conservation projects, soil erosion continues at an astounding rate. One of the reasons for this is possibly that the interactions between soil properties and erosion in Lesotho are not understood. Soil erosion is a site specific, cyclic phenomenon, controlled by geomorphological thresholds. To control soil erosion, the processes and soil properties which influence soil erosion in the specific place must be understood. In this study the soil properties of a highly eroded sub-catchment in Maphutseng, Lesotho was investigated. The gully extent in the sub-catchment, in 1957 and 2004 respectively, was mapped from aerial photos. These maps show where in the landscape gullies developed during this time. The gully maps were superimposed on maps of several soil erosion factors, to correlate the spatial distribution of the erosion factors with that of the gully distribution. A soil map was especially drawn for this. The spatial analysis shows that gully development between 1957 and 2004 was primarily confined to the area where duplex soils occur. The rest of the sub-catchment underwent negligible differences in gully extent during this time. The initiation of the gullies on the duplex soil area is ascribed to tunnel erosion. The high dispersibility of the duplex soil samples, sink holes which occur in this area and previous observations by researchers in this area gave evidence to this hypothesis. In the second part of the study the soil properties of seventeen soil profiles from across the study site were analysed. The difference in gully distribution between the duplex soils area and the rest of the catchment is ascribed to the high dispersibility of the duplex soils. No strong correlations could be found between the dispersion index and other determined soil properties. Segmented quantile regression was used to analyse the data further. Soil samples with moderate levels of total carbon (1.17%), iron oxide (0.9%) and effective cation exchange capacity (13.7 cmolc/kg), have below average dispersibility. When none of these stabilising agents are present in moderate amounts, soils with even low exchangeable sodium percentage values (0.68%) are dispersive. Furthermore, soils which have developed in colluvial material from basaltic origin were found to be less dispersive, presumably because of the amorphous clay minerals present in the volcanic material. The colour and increase in clay content between the A and B horizons of a soil can indicate the tunnel erosion potential of the soil. Dark coloured soils (values less than 4 and chromas less than 3) were found to have low dispersibility and free water can accumulate in the subsoils where the B horizon has much more clay than the A horizon. The accumulation of free water in the subsoil is necessary for tunnel formation. Thus soils with dark colours and/or a low clay accumulation index have low tunnel erosion potential.