Suitability of SWAT Model for Sediment Yields Modelling in the Eastern Africa - Advances in Data, Methods, Models and Their Applications in Geoscience - page 276

validation are 68 and 63%, respectively. For sediment calibration and validation, CE is 66
and 68%, respectively. The calibration and validation results have shown that measured and
simulated values were closely related with RE of 7.5 %.
The sediment yield calibration results for SRC for the period from 1970 to 1975 in daily and
monthly aggregated outputs are presented in Table 4 and Figure 2. The result from the
SWAT model at a daily time step is fair with the model performance of CE= 24%. The
sediment loads in the peak flood events such as those in 1970, 1972 and 1974 are over-
predicted. However, the performance of the model when looking at the monthly sediment
loads is good, with CE= 83%. The sediment modelling was validated for the period between
1976 to 1978 (Figure 3). Daily sediment load is fairly simulated with a model performance of
CE = 16%. Sediment loads during the peak flood events are over-predicted such as in the
months of April, 1976 and November, 1978. On the other hand, it under-predicted the loads
in January and April of 1977. However, the performance of the model in simulating monthly
sediment loads is good with CE = 80%. The improvement of the SWAT model performance
when aggregating the outputs over a longer time period has also been observed by other
researchers in the region and elsewhere (Schmidt and Volk, 2005; Shimelis et al ., 2010).
Since the routing models for the SWAT model as well as the coupled SWAT-SOBEK models
have shown to give acceptable and comparable results, the uncertainty of the sediment
routing component has been evaluated to be minimal (Abdelhamid, 2010). The performance
was measured against total sediment loads transported to the main outlet of the catchment.
A similar result was obtained by Andualem and Yonas (2008) in Ethiopia where they
applied SWAT and CCHE1D sediment transport models together in tandem. In Figure 4
shows that simulated and observed annual sediment loads are comparable for Case study 1,
the Koka Reservoir catchment.

450000

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0

1/1/1970 1/1/1971 1/1/1972 12/31/1972 12/31/1973 12/31/1974 12/31/1975

Ti me (D a y s )

Observed Sediment Calibrated sediment

Fig. 2. Comparison between “observed” and simulated Simiyu daily sediment for
calibration period from 1970and 1975 at Main Bridge outlet (Abdelhamid, 2010).