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

Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen
1 Introduction
Soil erosion is a complex process, which involves different processes at variable spatial and
temporal scales. Rill erosion is considered to be the most important process affecting soils.
Not only because of the large amounts of eroded soil material, but also due to the generation
of more or less persisting forms, which can develop into gullies and disable further land use.
(OOSTWOUD WIJDENES et al. 2000, VANDEKERCKHOVE et al. 1998, WOODWARD
1999, HANCOCK et al. 2008)
The outstanding role of linear erosion in geomorphological, hydrological and economical
aspects can be explained by the fact that the runoff water reaches its maximal power as both,
eroding and transporting medium, when concentrated in a rill (MERZ 1993). Runoff depth
and flow velocity reach higher values than within diffuse sheet flow, thus the forces affecting
soil particles are multiplied as compared to sheet flow (POESEN 1987). Consequently, more
energy for mobilization and transport of soil material is available (MERZ 1993) leading to a
clear increment in erosion rates. MEYER et al. (1975) noticed a triplication of the erosion
rates due to rill development. Corn fields in Bedfordshire, England for example produce 9-21
times as much sediment with rill erosion under corn as a comparable field only affected by
sheet erosion (MORGAN et al. 1987). According to CERDAN et al. (2002) up to 90 % of soil
loss in several heavy rainfall-events in Normandy was caused by rill erosion.
Despite the efforts made for understanding rill erosion processes and building models to
explain the retreat of headcuts (e. g. FLORES-CERVANTES et al. 2006) or to model gully
erosion by identification of stochastic components (SIDORCHUK 2005) there is still much
work to do for a comprehensive modelling of rill erosion and to the development of field
methods for understanding and quantifying the rill erosion processes. The lack of rill erosion
models is all that weighty, if is compared with the approaches for modelling sheet erosion
(BRUNO et al. 2008).
BRUNO et al. (2008) tested the evolution of a rill network under natural rainfall on testplots
in Sicily. Investigating the effect of initial conditions, rill headcut development and the most
important hydraulic variables, they tried to compile simple mathematical models. Their results
are remarkable, because they demonstrate that the erosion occurs within the upper part of the
rill, but the eroded material is only passed through the lower rill section.
Correct and reproducible measurement of rill erosion processes are needed for the
understanding and the correct model building of a rill erosion model. CASALI et al. (2006)
tested three frequently used methods to quantify the intensity of rill erosion processes and
showed that different methods offer clear differences in resulting data. Depending on the
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