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analysis of runoff plots-based data. Mulengera and Payton (1999) attempted to define K USLE
values for tropical regions. One may note from Table 3 that the same set of important
parameters for all the cases is retained. There are few cases of swapping of the ranking of
the parameter importance, e.g ., for BIOMIX in SRC. The influence of these parameters in
sediment yield is well documented in Neitsch et al . (2005) and Ndomba (2007). Besides, the
independently performed simulation results for catchment sediment management scenarios
in the study cases indicate that all sorts of farming practices as captured by the P USLE and
C USLE parameters (Table 3) are the main determinants in reducing soil loss and sediment
yield in the upland catchments and subsequent sedimentation problems in the downstream
reservoirs (Endale, 2008). The results also suggest that micro river channels also act as
important sources of sediment as represented by the high rank of the linear re-entrainment
parameter for channel sediment routing factor, Csp. It should be noted that many other
factors affect the sediment yield estimations. For instance, the hydrological parameters seem
to have a high effect as well on the sediment computations (Ndomba, 2007). This may be
explained by the fact that one of the parameters/factors in the sediment yield equation, the
MUSLE, used in the SWAT model is the surface runoff, Q surf . Experience also shows that the
resolution of DEM and the monthly average rainfall intensities, which are provided in the
weather generator database are fundamental and crucial. The results in Table 3 below
compare well with that of Shimelis et al . (2010) who worked in Angeni Gauged watershed,
Ethiopia. In their case the ranking for Csp and CCH are first and second, respectively, with
C USLE ranked in the sixth position.
NYM
Rank
SRC
Rank
KRC
Rank
SN
Parameter
Description of parameter
Csp
Linear re-entrainment parameter for
channel sediment routing
1
2
1
1.
2.
CCH
Channel cover factor
2
5
2
3. P USLE
USLE support practice factor
3
3
3
4.
KCH
Channel erodibility factor [cm/h/Pa]
4
6
4
SPEXP
Exponential re-entrainment parameter
for channel sediment routing
5
4
5
5.
6. C USLE
Minimum USLE cover factor
6
7
6
7.
BIOMIX
Biological mixing efficiency.
7
1
7
K USLE
USLE soil erodibility factor
[t.ha.h./(ha.MJ.mm]
10
10
10
8.
9.
RSDIN
Initial residue cover [kg/ha]
10
10
10
Table 3. Sensitivity analysis results of sediment component of SWAT for three study cases,
i.e., NYM, SRC and KRC
3.2 Model performance
The discussion in this section focuses on the study cases where there was relatively
adequate data and where more modelling efforts were applied (Table 4). For instance, in
KRC the results of the model performance according to CE for flow calibration and
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