R. P. C. Morgan

The EUROSEM model.

The European Soil Erosion Model (EUROSEM) is a dynamic distributed model for simulating erosion, transport and deposition of sediment over the land surface by interrill and rill processes. It is designed as an event-based model for both individual fields and small catchments. Model outputs include total runoff, total soil loss, the storm hydrograph and the storm sediment graph. EUROSEM provides for explicit simulation of interrill and rill flow; the effects of plant cover on rainfall interception, infiltration, rainfall energy and flow velocity; and the effects of rock fragment cover on infiltration, flow velocity and splash erosion. Catchments are represented as a simplified cascading network of elements, which may be either planes (for hillslope segments) or channels. Each plane is considered uniform in its soil, slope, surface microtopography and land cover.

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.

Contour grass strips: a laboratory simulation of their role in soil erosion control

Abstract Using metal rods to simulate the effects of contour grass strips in a laboratory experiment, significant differences were observed in their effectiveness in controlling soil loss for different plant densities, slopes and discharges; there was also a significant interaction effect between plant density and slope. With a simulated plant density of about 3000 plants/m 2 , contour grass strips reduced sediment concentrations to at least one-third of that recorded with a bare soil control on slopes between 5 and 10%. On slopes of 20% the grass strips increased the erosion twofold.

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.

Contour grass strips: A laboratory simulation of their role in erosion control using live grasses

The effectiveness of contour grass strips in controlling erosion was studied in a laboratory experiment involving two grass treatments (Festuca ovina and Poa pratensis) and a bare soil control, five slopes (5, 7, 9, 11 and 13 degrees) and two discharge types (runoff only and rainfall-runoff combined) with two replications. Significant differences were observed between the two grass species with Festuca ovina giving the greater reduction in soil loss. This is explained by the greater root density and the interwoven nature of the stems and leaves.

EUROSEM: An Evaluation with Single Event Data from the C5 Watershed, Oklahoma, USA

For the GTCE model comparison EUROSEM was evaluated against data for the C5 watershed, Oklahoma, USA. Model calibration was carried out using data for events from the training period with similar rainfall patterns and soil conditions to that of the test events. Calibration was conducted by modifying the saturated hydraulic conductivity and initial moisture content of the soil until broad agreement was reached between the runoff simulated by the model and the observed data Once this had been achieved the model was independently calibrated to simulate soil loss by modifying the parameters describing the cohesion and erodibility of the soil, and rill density. During the calibration phase parameter values were maintained within realistic limits for the watershed concerned. Once calibrated, the model was applied to four test events. EUROSEM simulated the total runoff and soil loss from three of these events quite well, but failed to reproduce the hydrographs and sedigraphs. Observed hydrographs and sedigraphs must be supplied at the calibration phase if catchment response is to be successfully simulated.

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.

Reply to discussion on ‘The European Soil Erosion Model (EUROSEM): a dynamic approach for predicting sediment transport from fields and small catchments’

It is considered that EUROSEM is already able, in principle, to replicate all the combinations of detachment and transport considered by Kinnells discussion, with suitable parameter values. Some effects will only be reproducible when EUROSEM contains a more explicit representation of particle sizes. A minor error in the manuscript is acknowledged.

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.