Process-based Erosion Models Research Articles

Spatially distributed data for erosion model calibration and validation: The Ganspoel and Kinderveld datasets

Recent experience in distributed modelling has revealed that the performance of process-based erosion and hydrological models are extremely sensitive to parameter estimations and that predictions are often poor. It is also observed that quite different parameter sets may lead to very similar results and that no ‘best’ parameter set can be identified. In this study, we describe a dataset that offers possibilities for improved evaluation and parameterisation of spatially distributed soil erosion models. The dataset combines rainfall, runoff and sediment discharge data collected at the outlet and field surveys within the catchments that describe soil surface characteristics and soil erosion features. This offers clear advantages over traditional model evaluation as not only the simulated overall system response, integrated over time and space, but also the simulated internal system dynamics can be compared with measured data. The paper discusses and illustrates the use of the dataset to narrow uncertainties associated with model predictions.

Sediment delivery ratios: a misaligned approach to determining sediment delivery from hillslopes

*Correspondence to: P. I. A. Kinnell, School of Resource, Environmental and Heritage Sciences, University of Canberra, Canberra, ACT 2601, Australia. E-mail: peter.kinnell@canberra.edu.au Considerable interest exists in the impact of land management on water quality throughout the world. A large number of non-point source (NPS) pollution or water quality models exist. In many cases, these models use the universal soil loss equation (USLE; Wischmeier and Smith, 1978) or the revised version of the USLE (RUSLE; Renard et al., 1997) to model erosion on hillslopes in conjunction with sediment delivery ratios (SDRs) to determine the sediment delivered from the hillslope to water bodies. The SDR can be defined as the ratio of the erosion upslope of a point in the landscape to the sediment delivered from that point. Erosion involves detachment, transport and deposition processes that operate all the time. The net loss or gain of soil material depends on the balance between the rates of particle uplift and deposition. In the context of modelling sediment delivery on hillslopes with respect to water quality, erosion refers to the situation where there is a net loss of soil material from the soil surface, and deposition to the situation where there is a net gain. The use of SDRs owes its origin to the observation that using erosion predicted by the USLE overestimates the amount of sediment delivered from hillslopes, because sediment deposition often occurs on hillslopes and the USLE does not account for deposition. The USLE was developed on planar surfaces and was not designed to apply to situations where there is a net gain of soil material on the area being considered. However, SDRs are essentially ‘performance’ factors that simply relate observed or modelled amounts at the plot scale to observed amounts at a larger scale. Using them to predict sediment delivery from hillslopes can produce erroneous results. One of the features of the USLE/RUSLE–SDR approach is that the predicted amount of sediment delivered from the hillslopes varies directly with the predicted erosion for any given hillslope profile. Halving the erosion results in a halving of the amount of sediment delivered from the hillslope. However, the deposition effect results from sediment in or entering the flow exceeding the transport capacity of that flow at a point on the hillslope. As a result, sediment delivery is directly related to erosion if the transport capacity of the flow is not exceeded (i.e. SDR = 1·0), but it is limited by the transport capacity if erosion exceeds the capacity of the flow to transport the eroded material (i.e. SDR < 1·0). This concept is encapsulated in Figure 1 and in so-called process-based erosion models like WEPP

Effect of cattle treading on interrill erosion from hill pasture: modelling concepts and analysis of rainfall simulator data

Damage to pasture as a result of intensive grazing and treading by cattle increases the erosion of sediment and associated nutrients. In this study we present a model for the effect of treading damage on interrill erosion. The model formulation is based on the interrill erosion components of conventional process-based erosion models, with some modifications. The model was developed using data from small rainfall simulator plots (0.5 m2) on hill-country pasture in New Zealand with varying amounts of treading damage. Treading increased the amount of bare ground, and this resulted in increased concentrations of sediment in runoff. Concentration was found to increase linearly with the amount of bare ground. Treading resulted in increased runoff, which had some effect on sediment concentration for the high runoff rates in the experiments, but would have a negligible effect on concentration for typical natural runoff rates. However, the increased runoff results in greater sediment loads because a larger volume of sediment-laden runoff is produced. The increase in runoff was modelled through a reduction factor for the hydraulic conductivity which varied linearly with the fraction of damaged ground. For 100% damaged ground the hydraulic conductivity was approximately halved. The recovery of bare ground and damaged ground over time after treading was also assessed. Bare ground resulting from treading typically halved after 2 months, while damage took longer to recover. Such treading-related effects are superimposed on seasonal variations in erodibility, bare ground, and infiltration capacity. Based on the results of the study, some measures to reduce treading-related erosion are suggested.

Sediment transport capacity and erosion processes: model concepts and reality

Sediment transport capacity, Tc, defined as the maximum amount of sediment that a flow can carry, is the basic concept in determining detachment and deposition processes in current process-based erosion models. Although defined conceptually and used extensively in modelling erosion, Tc was rarely measured. Recently, a series of laboratory studies designed to quantify effects of surface hydrologic conditions on erosion processes produced data sets feasible to evaluate the concept of Tc. A dual-box system, consisting of 1·8 m long sediment feeder box and a 5 m long test box, was used. Depending on the relative magnitudes of sediment delivery from feeder and test boxes, five scenarios are proposed ranging from deposition-dominated to transport-dominated sediment regimes. Results showed that at 5 per cent slope under seepage or 10 per cent slope under drainage conditions, the runoff from the feeder box caused in the additional sediment transport in the test box, indicating a transport-dominated sediment regime. At 5 per cent slope under drainage conditions, deposition occurred at low rainfall intensities. Increases in slope steepness, rainfall intensity and soil erodibility shifted the dominant erosion process from deposition to transport. Erosion process concepts from the Meyer–Wishmeier, Foster–Meyer and Rose models were compared with the experimental data, and the Rose model was found to best describe processes occurring during rain. A process-based erosion model needs to have components that can represent surface conditions and physical processes and their dynamic interactions. Copyright © 1999 John Wiley & Sons, Ltd.

Sediment transport capacity and erosion processes: model concepts and reality

Sediment transport capacity, Tc, defined as the maximum amount of sediment that a flow can carry, is the basic concept in determining detachment and deposition processes in current process-based erosion models. Although defined conceptually and used extensively in modelling erosion, Tc was rarely measured. Recently, a series of laboratory studies designed to quantify effects of surface hydrologic conditions on erosion processes produced data sets feasible to evaluate the concept of Tc. A dual-box system, consisting of 1·8 m long sediment feeder box and a 5 m long test box, was used. Depending on the relative magnitudes of sediment delivery from feeder and test boxes, five scenarios are proposed ranging from deposition-dominated to transport-dominated sediment regimes. Results showed that at 5 per cent slope under seepage or 10 per cent slope under drainage conditions, the runoff from the feeder box caused in the additional sediment transport in the test box, indicating a transport-dominated sediment regime. At 5 per cent slope under drainage conditions, deposition occurred at low rainfall intensities. Increases in slope steepness, rainfall intensity and soil erodibility shifted the dominant erosion process from deposition to transport. Erosion process concepts from the Meyer–Wishmeier, Foster–Meyer and Rose models were compared with the experimental data, and the Rose model was found to best describe processes occurring during rain. A process-based erosion model needs to have components that can represent surface conditions and physical processes and their dynamic interactions. Copyright © 1999 John Wiley & Sons, Ltd.

The use of in-situ produced cosmogenic radionuclides in glaciology and glacial geomorphology

AbstractThe usefulness of in-situ produced cosmogenic radionuclides in constraining glacial chronologies through exposure-age dating has been demonstrated in numerous studies. However, an understanding of cosmogenic radionuclide techniques and their uncertainties opens up a wide range of other potential applications in glaciology and glacial geomorphology. Recently developed applications include: estimation of spatial and temporal variations in the depth of glacial erosion from cosmogenic radionuclide inheritance, which provides important constraints on process-based erosion models; and burial dating, which can provide chronological control for glacial advances and the onset of till deposition. An interesting new application, currently at the level of theoretical model development, concerns the unraveling of complex exposure and burial histories. Overall, in-situ produced cosmogenic radionuclide techniques provide a means with which to constrain a wide range of ice-sheet and glacial models over time-scales of a thousand to a few million years.

Modelling Methodology for Soil Erosion Assessment and Soil Conservation Design: The EUROSEM Approach

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.