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

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Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen We measured distance and direction of the rock fragment translocation in experiment 3. The mean translocation distance of the 3 experiments only reaches 8.8 cm, thus being much lower than the mean displacement distance of 38 cm on Lesvos (Greece) that was determined by OOSTWOUD WIJDENES et al. (2001). However, this rather short distance is a consequence of the considerably flatter slopes of 4° - 11° in the present study vs. 2° - 24° in OOSTWOUD WIJDENES et al. (2001). It is important to notice that OOSTWOUD WIJDENES et al. (2001) measured the 38 cm after one year, the grazing peak is between April and June. It is not reconstructible how far the rock fragments were displaced after a certain number of sheep. Additionally, they corrected the negative distances (upslope movement) by adding the largest negative distance plus one centimeter to all negative distances. This transformation resulted in a slightly overestimated mean. Our smaller rock fragments are probably not the reason for these different results, as OOSTWOUD WIJDENES et al. (2001) showed that the rock fragment displacement distance is not depending on mass. The study started in May and in the first 7 months the displacement distances are lower than in the second 5 months (OOSTWOUD WIJDENES et al. (2000). In the 5-month period, only one of the grazing peak months is included; in the 7-month period two of them. That means despite longer measuring times and despite a higher trampling intensity, the values are higher in the 5-month period. A reason could be the winter months December and January, in which other erosive processes influence the displacement distances. This fact emphasizes the need for short-term experiments to isolate the influence of animal trampling. GOVERS & POESEN (1998) measured an average horizontal displacement distance of 38 cm for rock fragments on slopes between 27° and 31°. But the test sites had different widths (between 1.4 m and 4.1 m) and the number of sheep and goats ranged between 22 and 100. To make the measured values comparable, they calculated a unit displacement distance in cm per animal and meter. They presented values between 0.47 cm and 5.6 cm. The highest value was calculated for the smallest plot, the lowest value for the largest plot. Again, we used a plot width of 0.6 m and 45 goats and measured an average distance of 8.8 cm. Our test plot was thus smaller than the smallest one used by GOVERS & POESEN (1998). Calculating the same unit displacement distance (displacement distance / (number of animals / plot width)) we receive a value of 0.1 cm. Once more, slope is probably responsible for this much lower value; its effect covers the fact that the plot width of the smallest Govers-Poesen-plot was twice as large as our plot width. In our experiments, the marked rocks were positioned on top of given overlying or embedded rock fragments. This means our rocks were probably easier to move than naturally occurring rock fragments. Even after considering this, our values are still 249
Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen much lower than those of GOVERS & POESEN (1998). It is hard to mark existing rock fragments and measure their translocation distances because their original position cannot be marked as easily. Knowing the exact original position of each rock fragment is paramount, however, for recording translocation distances and angels. The values of mobilized material indicates severe exceeding of the expectable tramplinginduced erosion rates for the whole study area, especially considering the fact that a mediumsized flock only crossed our test plots for a single time. The effects of the mobilization of soil material by the animals’ hooves has scarcely been regarded in literature until now. There is a plethora of studies, however, on the decrease in vegetation cover by grazing animals and the consequent increase in bare soil areas that are unprotected from erosion (e.g. ALADOS et al. 2004, ANDRESEN et al. 1990, DUNKERLEY & BROWN 1995, 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). CERDÀ (1997) showed that runoff and soil loss increase with decreasing vegetation cover. He used rainfall simulations on bare plots (0-3 % vegetation cover), on plots vegetated by herbs (30-85 % vegetation cover) and on plots vegetated by tussocks (75- 100 % vegetation cover). The mean runoff coefficient after 60 min on bare plots was 31 %, on herbs plots 2 % and on tussock plots 0.3 %. The mean erosion rate on bare plots was 37 g m -2 h -1 , on herbs plots 1.44 g m -2 h -1 and 0 g m -2 h -1 on tussock plots. Accordingly, the reported processes of mobilization of soil material and rock fragments by animal hooves need to be investigated in different areas in future studies because our preliminary findings show that the impacts of these processes on total erosion rates should be tremendous. Nevertheless, the experimental designs should be animatedly discussed and need to be further developed. 5 Conclusions Independent from the discussible extrapolation of the measured values to a larger area of several hectares, the determined values have to be evaluated as high, regarding the land-use of extensive grazing in the test area. Also in comparison to erosion rates on agricultural fields, the measured values are considered to be too high for a sustainable land use of Mediterranean rangelands. Regarding our experimental results, the following conclusions can be drawn: - Slope and running speed influence the translocation rates by the factor 2.5 to 5.5. 250
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Stefan Wirtz Vom Fachbereich VI (Ge
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Danksagung Danksagung Die vorliegen
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