R. J. Rickson

A rainfall simulation study of soil erosion on rangeland in Swaziland

Rainfall simulation studies on rangeland in the Ntondozi area of Swaziland showed that soil loss decreased exponentially with increasing vegetation cover. Vegetation exerted an important hydrological control by increasing the infiltration capacity of the soil which, in turn, influenced the time to and duration of runoff. The expected effects of vegetation on soil protection and soil strength were not demonstrated. Instead, the amount of soil loss occurring in an individual storm appeared to depend on the supply of loose material on the surface which could be transported by the runoff.

Modelling methodology for soil erosion assessment and soil conservation design: the EUROSEM approach : Soil management and conservation

Increasing concern about the effects of agriculture on the environment has drawn attention to the inadequate scientific base on which many policy decisions are made. Decisions on how soil should best be protected from erosion demand knowledge of the risk of erosion and of the relative effectiveness of different conservation measures. Process-based erosion models are designed to provide this information. This paper outlines one such model, the European Soil Erosion Model (EUROSEM), which is being developed jointly by scientists from ten European countries. The model incorporates some of the best process-based erosion research being carried out in Europe and, compared with other similar models, contains novel features in the way rill–interrill interactions and the role of vegetation are described. Tests of the model in the UK show that it performs rationally and gives reasonable predictions of storm soil loss and runoff. Some examples of how the model might be used to evaluate conservation measures are provided.

Soil erosion survey of the central part of the Swaziland Middleveld

Abstract Soil erosion has worsened considerably in the Middleveld of Swaziland over the last 20 years. Most of the erosion has occurred on deep colluvial and saprolitic materials and is associated with convex-slope breaks and rejuvenation shoulders on valley sides. Although the risk of erosion seems to reflect natural instability in the landscape, the way in which the land is used determines the ultimate severity of the problem. Land systems can serve as functional units as far as erosion assessment is concerned. The most severely eroded land occurs within the Manzini, Jabuleni, Lobamba and Chibidze land systems. The universal soil loss equation (USLE) and the soil loss estimator for southern Africa (SLEMSA) give vastly different estimates of the rate of soil loss but no information is available to validate or refute the predictions.

Concise overview of European soil erosion research and evaluation

Abstract Soil loss by erosion is a major threat to European soil resources. It is linked to most other threats to soils and is estimated to incur substantial costs to society. To monitor and evaluate soil erosion comprehensively, measurements and modelled estimates of soil loss by erosion need to integrate water, wind and tillage erosion, as these are common throughout Europe. Tolerable rates of soil erosion (thresholds) in Europe can be set equal to estimated soil formation rates, that is, ~1 t ha−1 yr−1, but defining this threshold will require further research in the context of soil functions. There is a strong need for improving the knowledge base of current erosion rates, especially to develop and validate reliable erosion prediction models. This should include depositional environments (e.g. flood plains) where greater erosion rates than those on hill slopes may be tolerable, depending on the consequent effects on all relevant ecosystem goods and services provided by soil. Moreover, in considering erosion at catchment (watershed) level, the distance between erosion sites and areas of deposition should be taken into account.

Soil erosion in Swaziland: a synthesis

Abstract The main conclusions of the research project on soil erosion and sedimentation in Swaziland are as follows. (1) Soil erosion has worsened over the last 20 years. The proportion of an 1800 km 2 study area in the Middleveld classified as ‘high erosion class’ has increased from 6.7 to 13.6% between 1972 and 1990. (2) Gully erosion is the main process and is concentrated within the Manzini, Jabuleni, Lobamba and Chibidze land systems, all characterised by soil-saprolite complexes. (3) Overgrazing and compaction along paths and tracks lower the infiltration rate of the clay-rich ferralitic soils, promoting surface runoff and the formation of rills. (4) Once the rill deepens and cuts through the soil on to the underlying saprolite, gullies develop rapidly because of the low shear strength of the material. (5) Soil erosion problems are compounded by the existing system of land tenure and increasing pressure of livestock and population on the land. (6) In some instances, population pressure can lead to improvements in land management and better soil protection. Recommendations for controlling soil erosion and for further research are presented.

EROSION AS AFFECTED BY TILLAGE–INDUCED ROUGHNESS AND SURFACE MULCH IN A SOIL WITH A SIMULATED SUBSURFACE PAN

Soils with subsurface pan are subject to more rapid erosion compared with soils without a pan. The effect of tillage–induced roughness and surface mulching on soil loss and runoff in a sandy loam soil with a simulated subsurface pan was investigated under laboratory condition. A soil tray 0.75 m long × 0.5 m wide × 0.05 m deep, which was underlain by a plastic sheet, simulated an impermeable pan beneath the soil. A wire mesh was used to simulate treatments without a pan. The soil was manually tilled to create two roughness conditions, which were classified as harrowed (fine seedbed condition) and plowed (rough seedbed condition). Dried wheat straw was applied at a rate of 3 ton/ha in the mulched treatments. Simulated rainfall with intensity of 128 mm/h was applied to test the treatment effect. In soil with a pan, increased tillage–induced roughness led to a 50% reduction in soil loss compared with harrowed, bare soil. Similarly, application of surface mulch to the harrowed soil led to a 65% reduction in soil loss compared with the unmulched equivalent. Combining a rough surface with surface mulch led to 85% reduction in soil loss from the soil with a pan compared with harrowed, bare treatment. This reduction in soil loss from the plowed soil with a pan was comparable with that obtained from a similar treatment without a pan. Runoff from the panned soil was not affected by tillage or surface mulching. On the other hand, runoff was less in the soils without a pan compared to the panned soils. This preliminary investigation shows that through appropriate tillage with a good residue cover, sediment losses from panned soil can be reduced to the level obtained in soils without a pan. However, excessive runoff may still constitute a problem in panned soil unless a better drainage method is put in place.

Soil erosion and sedimentation in Swaziland: an introduction

Abstract Although it is widely recognised that gully and sheet erosion threaten sustained agricultural production in much of Swaziland, very little information is available about the current status of erosion and whether the situation is worsening. Since the country is very active geomorphologically, it is not clear whether an inherently erodible environment or human impact is the major causal factor explaining the severity and spatial distribution of erosion. Soil erosion is associated particularly with deeply weathered saprolite but few studies have been made on applied issues related to the physical and chemical properties of this material.

Assessment of slope stability influenced by vegetation cover and additional loads applied

Abstract Assessment of slope stability influenced by vegetation cover and additional loads applied. The article presents the results of research conducted in order to create nomographs allowing assessment of slope stability. The proposed recommendations involve graphs and charts, where factor of safety (FOS) is computed in according to six different characteristics: slope height and angle, slope surface vegetation cover, applied surcharge, position of water table, soil mechanical conditions. For the purpose of the research several geotechnical models and approaches were used and became a basis for developing simplified method of predicting the failure of natural and engineered slopes. The advantage of the solutions presented is their simplicity of use; as further FOS calculations are not required during the complex assessment of slope stability Streszczenie Ocena stateczności skarp przy uwzględnieniu wpływu okrywy roślinnej oraz przyłożonego obciążenia. W artykule przedstawiono wyniki prac podjętych w celu stworzenia nomogramów służących ocenie stanu bezpieczeństwa skarp. W wyniku podjętych prac badawczych i obliczeniowych opracowano zestawy tabel i grafów przedstawiających zależność współczynnika stateczności od wybranych sześciu parametrów skarpy: wysokość i nachylenie, wpływ okrywy roślinnej, przyłożone dodatkowego obciążenia, warunki geotechniczne i wpływ wody. Zaletą nomogramów jest prostota ich użytkowania, gdyż ocena stateczności nie wymaga dodatkowych obliczeń

Factors affecting changes in erosion status in the Swaziland Middleveld

Abstract At sample locations in the Swaziland Middleveld comparison of erosion severity class at two dates (1972 and 1990) shows where the incidence of erosion has decreased (stabilising sites) or increased (destabilising sites). The changes in erosion status are analysed in relation to on-site changes in vegetation, land use, path density, settlement density and land management practice (including soil conservation measures). The results show that it is impossible to identify simple cause and effect relationships. Criticisms of the methodology are presented. More research is recommended on the processes and mechanisms which influence the observed trends in erosion severity.

The Role of Soil Erosion in the Movement of Pollutants

An overview is given of the changing emphasis in studying the impacts of soil erosion, from those concerned with the effects of soil erosion on soil productivity to those linking soil erosion with environmental damage. Eroded sediment can be perceived as a pollutant, but it is the chemicals adsorbed onto erodible particles that cause most damage both on-site and off-site. Knowledge of erosion processes and sediment delivery relationships gives an insight into pollutant removal, transport and relocation or deposition. Pollutants associated with eroded sediment include agro-chemicals and heavy metals, which may originate from agricultural, industrial or municipal sources. The management of these sources can thus affect the potential environmental impacts off-site. Data from erosion studies are used to illustrate the selective removal of chemically active sediment and runoff by erosion processes. The way forward is outlined, concentrating on modelling as the most efficient methodology for further studies. Physical, rather than empirical soil erosion prediction models are able to simulate mathematically the processes by which soil and any associated pollutants are detached, transported and re-located off-site.