Suitability of SWAT Model for Sediment Yields Modelling in the Eastern Africa - Advances in Data, Methods, Models and Their Applications in Geoscience - page 278

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Simulate d se dime nt load by SWAT Sus pe nde d s e dime nt load by Rating curve

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Fig. 5. SWAT simulations vs “observed”, rating curve based sediment loads at 1DD1
(annually), between January, 1969 - December, 2005 (Ndomba et al ., 2008b)

For the case of the NYM Reservoir catchment the SWAT model captured 56 percent of the
variance (CE) and underestimated the observed daily sediment loads by 0.9 percent
according to TMC performance indices during a normal wet hydrological year, i.e ., with a
calibration period set between November 1, 1977 and October 31, 1978 (Table 4). The SWAT
model predicted satisfactorily the long-term sediment catchment yield with a relative error
of 2.6 percent (Table 4 & Figure 5). It should be noted that the “observed” sediment loads
were estimated from the bias-corrected suspended sediment rating curve. Authors are
aware that the SWAT model simulates bed-material load ( i.e. bed and suspended load)
while the rating curve computes only suspended sediment load. However, it should be
noted that the sediment loads delivered to streams are characterized as fine (Ndomba, 2007).
They would mostly be transported as suspended sediments loads. The accuracy achieved by
using the SWAT model was expected because it was hypothesized that correct estimation of
surface runoff would lead to a better prediction of the sediment yield. Other researchers
such as Garde and Ranga Raju (2000) are of similar opinion. Also, the model has identified
erosion sources spatially and has replicated some erosion processes as determined in other
studies and field observations in the NYM (Ndomba et al ., 2008b). This result suggests that
for catchments where sheet erosion is dominant, the SWAT model may substitute other
sediment yield estimation methods such as the sediment-rating curve. However, SWAT
model simulations results for sediment storage in a reach such as the 1DD1 gauging station
in the Myumba Ya Mungu Reservoir catchment differ from the findings based on other