An investigation of the soil properties controlling gully erosion in a sub-catchment in Maphutseng, Lesotho

Van Zijl, George Munnik (Stellenbosch : University of Stellenbosch, 2010-03)

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.

AFRIKAANSE OPSOMMING: Lesotho is ‘n land met ‘n internasionale reputasie vir die ernstige graad van gronderosie waaronder die landskap gebuk gaan. Ten spyte van verskeie nasionale grondbewaringsprojekte duur die erosie teen ‘n verstommende tempo voort. Een van die redes hiervoor is heel moontlik dat die interaksies tussen grondeienskappe en erosie in Lesotho nie verstaan word nie. Gronderosie is ‘n plekspesifieke, sikliese verskynsel, wat deur geomorfologiese drempelwaardes beheer word. Om gronderosie te bekamp moet die prosesse en grondeienskappe wat gronderosie in die spesifieke plek beïnvloed, geïdentifiseer en verstaan word. In hierdie studie is die grondeienskappe van ‘n hoogs geërodeerde opvanggebied in Maphutseng, Lesotho ondersoek. Die dongaverspreiding in die opvanggebied, in 1957 en 2004 respektiewelik, is vanaf lugfoto’s gekarteer. Die kaarte wys waar in die landskap dongas gedurende hierdie tyd ontwikkel het. Die dongakaarte is op kaarte van verskeie gronderosie faktore gesuperponeer om die ruimtelike verspreiding van die erosie faktore met die donga verspreiding te korreleer. ‘n Grondkaart is spesiaal vir hierdie doel opgestel. Hierdie analise het gewys dat donga-ontwikkeling tussen 1957 en 2004 hoofsaaklik op die area waar dupleks gronde voorkom plaasgevind het. Die res van die opvanggebied het weinig verskille in donga verspreiding in hierdie tyd ondergaan. Die ontstaan van die dongas in die dupleksgronde word toegeskryf aan tonnelerosie. Die hoë dispergeerbaarheid van die dupleks grondmonsters, sinkgate wat in die area voorkom en vorige waarnemings deur navorsers in die area verleen bewyse aan hierdie hipotese. In die tweede deel van die studie is die grondeienskappe van sewentien grondprofiele van regoor die opvanggebied ontleed. Die verskil in donga verspreiding tussen die dupleksgrond area en die res van die opvanggebied is toeskryfbaar aan die hoë dispergeerbaarheid van die dupleks gronde. Geen sterk korrelasies is tussen die dispersiwiteits indeks en ander bepaalde grondeienskappe gevind nie. Gesegmenteerde kwantiel regressie is gebruik om die data verder te ontleed. Hierdie ontleding het gewys dat grondmonsters met matige vlakke van totale koolstof (1.17%), ysteroksied (0.9%) en effektiewe katioonuitruilkapasiteit (13.7 cmolc/kg), ondergemiddelde dispergeerbaarheid toon. Waar nie een van hierdie stabiliserings agente in matige hoeveelhede voorkom nie, is selfs gronde met baie lae uitruilbare natriumpersentasie waardes (0.68%) dispersief. Daar is ook gevind dat gronde wat vanuit kolluviale basaltiese afsettings ontwikkel het, minder dispersief is. Die kleur en verskil in klei persentasie tussen die A en B horison van ‘n grond kan as aanduiding dien van die grond se potensiaal vir tonnelerosie. Donker grondkleure (waarde laer as 4 en chroma laer as 3) wys op ‘n lae dispersiwiteit terwyl vrywater in die ondergrond van gronde waar die B horison veel meer klei as die E horison bevat kan akkumuleer. Die aansameling van vrywater in die ondergrond is noodsaaklik vir tonnelvorming. Dus het donker gronde en gronde met ‘n lae klei akkumulasie indeks ‘n lae potensiaal vir tonnelerosie.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/4197
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