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

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Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen Furthermore, the article points out the vulnerability of these shrublands against grazinginduced erosion. Basically, two main theses on the impacts of grazing can be found in literature. The studies with a more ecological or botanical focus regard grazing animals as ‘ecosystem engineers’ and emphasize the increased biodiversity of grazing areas with different biospheres for other biota, as compared with non-grazing areas (e.g. JONES et al. 1994, SCHAICH 2007, STAVI et al. 2009). The more soil scientific, geomorphologic and hydrologic studies point out the physical compaction phenomena and, accordingly, the reduced infiltration rates of the soils (MULHOLLAND & FULLEN 1991, PARIENTE 2002, PIETOLA et al. 2005, PROFFITT et al. 1995, STAVI et al. 2008a, b, STAVI et al. 2009, STEWART & CAMERON 1992, TABOADA & LAVADO 1993, MWENDERA & SALEEM 1997a). In contrast to that, the immediate influence of the trampling of sheep and goat hooves on the soil surface are overlooked. The destruction of crusts and the loosening and mobilization of soil material for further transport through wind and water are possible effects. Correlating geomorphologic rates are also unknown, so far. Only few studies suggest that the impact of animal trampling is significant and should not be neglected any more (e.g. UNGAR et al. 2009, RIES 2005, 2010). The issue of rock fragment translocation is treated in even fewer publications. GOVERS & POESEN (1998) investigate the translocation of rock fragments by sheep and goat trampling in a study area with steep, debris-covered slopes in Turkey, and discovered significant rock fragment movements. Particularly, the long-term effects of animal trampling on debriscovered slopes should be subject to further investigation (GOVERS & POESEN 1998). UNGAR et al. (2009) determine considerable rock fragment movement regardless of their size on grazed areas as opposed to the reference area without grazing influence in the Negev desert. The vegetation cover is regarded to be the most important factor influencing rockfragment movement by animal trampling. On existing trails more rock fragments are moved than on densely vegetation-covered areas. NYSSEN et al. (2006) emphasize rock fall and material movement as being the most important geomorphologic process induced by animal trampling in Ethiopia. On grazed test-areas, up to 95 % of the rock fragments on the soil surface were moved in the course of 20 months. Only a small number of experimental studies focus on the influence of grazing on vegetation cover and geomorphodynamics (RIES et al. 2003, ELDRIDGE 1998, MWENDERA & MOHAMED SALEEM 1997a). RIES et al. (2003) observe that sheep and goat trails in dense Matorral with intensive grazing can be regarded as the most active surfaces due to the 227
Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen concentrated trampling impact. They show a tendency to spatial progression, even when shrub density increases. By means of rainfall simulations, runoff coefficients of up to 98 % and sediment discharges of up to 328 g m -2 could be measured on trails using 40 mm h -1 rainfall intensity and a simulated rainfall duration of 30 minutes (RIES 2010). These values are very high compared to runoff coefficients and sediment discharges on other surfaces in the same study area (runoff up to 76 %, sediment discharge up to 68 g m -2 ). ELDRIDGE (1998) determine a higher erodibility, decreasing infiltration rates, increasing overland flow generation, a decrease in surface crust-cover and an increase of loose, bare soil, also using rainfall simulations on areas with crusted soil surfaces and simulated animal trampling (artificial sheep hooves). Nevertheless, livestock-induced erosion rates are hitherto practically unknown. The fundamental significance of trampling erosion is, however, undisputed. There are few quantitative field studies that acquire their measurements from animals in their natural surroundings. This lack of research is likely to derive from the complexity of the processes involved, being far from thoroughly understood, and non-existing adequate methods. In addition, handling live animals and using them as research objects can be difficult and should not be underestimated. Particularly, data are missing on: the impact of hooves on dry coherent crusted surfaces; the influence of trampling on the translocation of fine material in downslope as well as in and against the animals’ moving direction; the influence of slope and moving speed on those translocation rates; and the impact of trampling on the translocation of rock fragments. In this context there is need for comparative studies for the rock translocation experiments in the Taurus conducted by GOVERS & POESEN (1998). Even though this phenomenon is ubiquitous in nearly all semi-arid and arid regions, the impact of trampling livestock on stonecovered surfaces is clearly understudied. Above all, quantitative studies on the distance and direction of translocated rock fragments and fine material are missing. Finally, the impact of trampling livestock on fluvial erosive forms is of particular interest since little is known about how detached material is eroded when flushing occurs, for instance in rills or gullies. Following those points, the main objectives of this study are 1. to develop methods for the quantification of translocation rates of soil-material and rock fragments by sheep and goat trampling on in-situ sites, 2. to present first results concerning ranges of moved rock fragments and soil material under these specific experimental setups and 3. to quantify the effect of animal trampling on sediment availability in rills. 228
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