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

Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen
moistures, the sediment concentrations were lower. In three of our rill experiments, the first
run (dry conditions) showed higher sediment concentrations than the second run (wet
conditions). Only in the experiment at Salada, were higher sediment concentrations reached in
the second run. The differences between run a and run b are caused by the differences in the
supply of loose material, which is normally higher in the first run, and the detachment, which
is normally higher in the second run (higher flow velocity).
The ranges of transport capacity calculated are within the same orders of magnitude within
the rills as reported by Giménez and Govers (2002). Plotting the sediment flow rate Q s against
the transport capacity T c shows some differences between the tested rills (Fig. 7 - 10). In
Negratin and Belerda the measured values are below the calculated transport capacity, due to
a limitation in sediment supply or sediment detachment. This is especially accentuated in the
Belerda experiment. Both Negratin and Belerda have loam soils, however Belerda has in
addition a high coarse fragment content and the higher organic matter content which may be a
reason for the lower detachment rates. Within the tested rills of Freila and Salada, the
observed sediment load is at least partially above the calculated sediment transport capacity.
Here, transport of sediments within the rill should be limited by the transport capacity
however gravitational processes like bank failure and headcut retreat at knickpoints are the
main sediment producing processes rather than erosive deepening of the rill's bottom. As a
result it is possible for the sediment load to surpass the transport capacity. As Q s and T c have
been calculated based on the inflow into the rill, both values are doubtless lower than the
values given here, but as T c decreases more quickly (flow shear stress with an exponent 1.5,
Q S with an exponent 1) the relationship is expected to shift towards exceeding the transport
capacity as runoff decreases. However a common observation in all experiments was the fact,
that at each measuring point and at each run we found a wide range of values for the transport
capacity and the sediment load. We even found shifts between exceeding the transport
capacity and dropping below at the same measuring point. This is a clear indicator of the
spatially (along the rill) and temporally (during different stages of the water flow) variable
processes and process intensities. The variability of erosion processes was tested from
different research groups. Nearing (1998) tested the variability between replicated soil erosion
field plots under natural rainfall and determined the principal factor or factors which correlate
to the magnitude of variability. The coefficient of variation ranged on the order of 14 % for a
measured soil loss of 20 kg m -2 to grater than 150 % for a measured soil loss of less than 0.01
kg m -2 . Ruttiman et al. (1995) analysed statistically the data of four sites, each with five to six
reported treatments; each of them with three replications. The coefficients of variation in soil
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