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

Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen
1 Introduction
About 40 % of the earth's land-surface is used for grazing and 80 % of this grazing area is
situated in landscapes with semi-arid and arid climates (e.g. BRANSON et al. 1981,
MONFREDA et al. 2008, 2009, RAMANKUTTY et al. 2008). Over-grazing is considered the
main cause of 35 % of world-wide soil degradation (OLDEMANN et al. 1991). On the
African continent, 49 % of the soil surface is already degraded due to grazing and in Australia
the affected area is 81 % (EVANS 1998, OLDEMANN et al. 1991). Accordingly, grazinginduced
soil degradation is one of the most widespread land-degradation phenomena.
Numerous field surveys in literature focus on the degradation of vegetation due to browsing
damage by animals (e.g. ALADOS et al. 2004, ANDRESEN et al. 1990, MWENDERA &
MOHAMED SALEEM 1997b, MWENDERA et al. 1997, NOY-MEIR 1975, 1978, 1995,
PEARSON et al. 1990, PEGAU 1970, RIES et al. 2003, VALENTIN 1985, WANG et al.
2002). There are also many studies about the formation of specific grazing-induced vegetation
patterns (e.g. BARBIER et al. 2006, BOONKORKUEA et al. 2010, BORGOGNO et al. 2009,
DEBLAUWE et al. 2008, GALLE et al. 2001, GOLODETS & BOEKEN 2006,
HILLERISLAMBERS et al. 2001, MONTANA 1992, TONGWAY et al. 2001, TONGWAY
& LUDWIG 2001, WORRALL 1959). Vegetation density and distributions patterns play a
crucial role in soil erosion processes. According to basic literature (ELWELL & STOCKING
1976), a vegetation cover of 30-40 % is sufficient to protect soil surfaces against degradation.
SEUFFERT (1999) confirms these values. Other studies, which deal specifically with
sediment yields in low open matorral shrublands in Mediterranean mountain regions, assume
that a considerably higher degree of vegetation cover is needed to contain erosive processes
(MOLINILLO et al. 1997, RIES 2005). They ascribe this to the patchy pattern of tussocks.
Spots densely covered with Stipa tenacissima have high infiltration rates and low runoff
coefficients, while the bare areas around them experience sheet flow and rill erosion. Grazing
in these areas leads to a higher contrast between tussock patches and bare soil (CERDÀ
1997).
ROSTANGO et al. (1991) found a remarkable distinction in terms of infiltration rates and
chemical and physical soil properties on comparing shrub mounds and inter-mound areas for
soils in northeastern Patagonia. Similar results were presented by DUNKERLY & BROWN
(1995) for banded vegetation covers in patterned chenopod shrublands in New South Wales,
Australia. They divided their areas of investigation into source sections, characterized by high
runoff rates, and sink sections with vital importance for the redistribution of water.
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