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

Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen
1 Introduction
Modelling the results of the processes of soil erosion by water is an important task for
geomorphologic research. Soil erosion models are also used to evaluate land use systems, and
derive guidelines for land use management. Model concepts can be empirical (e.g. USLE;
Wishmeier and Smith, 1965, 1978) or based on physical process description (e.g. WEPP;
Laflen et al., 1997; EUROSEM; Morgan et al., 1998; LISEM; De Roo et al., 1996). There are
also stochastic approaches found in the literature for describing and modelling sediment
detachment and transport (Einstein, 1937, Mirtskhoulava, 1988, Wilson, 1993a, Govindaraju
and Kavvas, 1994, Lisle et al., 1998, Hairsine and Rose, 1991, Govers, 1991, Nearing, 1991,
Shaw et al., 2008, Sidorchuk, 2002, Sidorchuk et al., 2004, Sidorchuk, 2005 a, Sidorchuk,
2005 b, Sidorchuk et al., 2008, Sidorchuk, 2009). However to date no no practical, field-scale
or catchment-scale, model exists which uses stochastic approaches. Different approaches are
currently under construction; RillGrow 1 and RillGrow 2 are examples for model
developments which represent detachment from a distributional perspective (Favis-Mortlock
et al., 1998, 2000).
What is the background of process based soil erosion models?
Many current soil erosion models are described as physically based, or process-based. Knapen
et al. (2007) distinguish between excess shear stress models and excess stream power models.
In the first case, transport and detachment capacities are calculated using shear stress alone
and in the other, shear stress is replaced by stream power which is the product of shear stress
and flow velocity. In both cases, a critical soil parameter (critical shear stress or critical
stream power) is exceeded by a hydraulic factor (shear stress or stream power) which enables
an entrainment of soil particles. Shear stress is therefore a fundamental factor for the process
based soil erosion models, describing the drag force exerted by the flow on the bed (Giménez
and Govers, 2002). For inferring shear stress and the derived detachment and transport
capacity in soil erosion modelling, a set of input parameters are typically used in slightly
varying combinations: slope, liquid density, flow velocity, hydraulic radius, wetted perimeter,
flow cross section and water depth (Knapen et al., 2007). Due to the deterministic nature of
process based models, the same input parameters will always deliver the same results.
Moreover, these models often assume a linear relation between shear stress and soil
detachment (Lyle and Smerdon, 1965, Torri et al., 1987, Ghebreiyessus et al., 1994, Nearing
et al., 1997).
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