Stefan Wirtz Vom Fachbereich VI (Geographie/Geowissenschaften ...

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Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen another but from one measuring point and run to another. Thus, sediment producing processes change with high spatial and temporal variability. This is the fourth reason for model problems. It is very difficult to propose a single factor that always describes the soil detachment satisfactory. The high variability of erosion processes, even under controlled experimental conditions, has been shown in different studies, measured variability shows a wide range between 3.4 and 173.2 % [78-83]. This is partially the result of non-homogeneous parameters concerning soil characteristics and rainfall. On experimental plots, infiltration rates and soil aggregate stability can be highly variable [84] as well as the rainfall shows a high spatial and temporal variability [85]. Therefore, the input parameters to the different measurements reflected in the mentioned studies were not really comparable. Nevertheless, the results also make clear that modelling soil erosion has to include uncertainty in model input as well as in the data used for model calibration and validation. In the field, the spatial and temporal variability of soil conditions cannot be avoided, and is, furthermore, part of the investigations. Thus, additional input parameters as rainfall or flow should be maintained constant in the experiments to generate reproducible data. The high variability in soil erosion processes cannot be represented by a single factor like shear stress. The results show that there is not a simple linear correlation between a certain hydraulic parameter and soil detachment rate. Depending on model purpose and scale, the factors can be used to predict the magnitude of rill detachment but they are not applicable for the simulation of rill erosion with high-resolution spatial and temporal change in processes. A newer approach is the use of probability density functions to predict soil detachment [86, 87]. Sidorchuk gives two sources of stochasticity in erosion modelling: (1) the necessity of spatial and temporal averaging when determining deterministic equations, which describe concentrated flow erosion and (2) the fact that the main erosion factors, if these can be determined anyway, can only be measured with limited accuracy. This is not the first attempt to model erosion by relating the probability of soil detachment with the excess of erosion driving forces over soil erosion resistance forces. Other papers were published by Nearing [88], Wilson [89] and Sidorchuk [90-95]. One of the earliest publications about stochastic in erosion processes has been published by Einstein [96]. These stochastic models reduce the number of empirical components, but this approach seems to be too complicated [14] – yet. 5 Conclusion The results show that simple linear correlation between hydraulic parameter and soil detachment is not suitable, at least in natural rills. The reason for that is the combination of 197
Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen different processes that cause different amounts of soil erosion. Shear stress only describes one process. The results clearly show that there is not one fixed parameters that always predict soil detachment best. Applicability of one certain hydraulic parameter to predict the sediment concentration changes at a certain point in time within few minutes because the temporal and spatial distribution of the different erosion processes is highly randomly determined. Therefore, it might be more useful to formulate results in probalistic terms. This is required since the 1990s [89, 91] and implemented since the 2000s especially by the working group of Sidorchuk [90, 94-98]. But first, much more field experiments are needed to provide the required data. Acknowledgement The research was supported by the ‘Internationale Graduiertenzentrum’ of Trier University and the ‘Freundeskreis Trierer Universität e.V.’. Additionally, we thank all participants of the field trip to Andalusia in September 2009 who supported the performance of the experiments and Olli and Seta for revising the whole paper. References 1. Lyle WM, Smerdon ET (1965) Relation of compaction and other soil properties to erosion resistance of soils. Transactions of the ASAE 8: 419-422. 2. Torri D, Dfalanga M, Chisci G (1987) Threshold conditions for incipient rilling. Catena Supplement 8: 97-105. 3. Ghebreiyessus YT, Gantzer CJ, Alberts EE, Lentz RW (1994) Soil erosion by concentrated flow: shear stress and bulk density. Transactions of the ASAE 37 (6): 1791-1797. 4. Nearing MA, Norton LD, Bulgakov DA, Larionov GA, West LT, Dontsova KM (1997) Hydraulics and Erosion in Eroding Rills. Water Resources Research 33 (4): 865-876. 5. Giménez R, Govers G (2002) Flow Detachment by Concentrated Flow on Smooth and Irregular Beds. Soil Sci Soc Am J 66(5): 1475-1483. 6. Bagnold RA (1977) Bed load transport by natural rivers. Water resources Research 13: 303-312. 7. Hairsine PB, Rose CW (1992) Modeling water erosion due to overland flow using physical principles, 2. Rill flow. Water resources Research 28: 245-250. 198
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Dipl. Geogr. Stefan Wirtz; Physisch
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