Suitability of SWAT Model for Sediment Yields Modelling in the Eastern Africa - Advances in Data, Methods, Models and Their Applications in Geoscience - page 274

curve were bias corrected. Sediment load correction factors were derived from both
statistical bias estimators (Ferguson, 1986) and actual sediment load approaches (Ndomba et
al ., 2008a). It was important to do this as it is known that uncorrected rating curves,
developed by Ordinary Least Square (OLS) tend to underestimate sediment loads
(Ferguson, 1986; Ndomba et al., 2008b). The SWAT model simulation was validated with
long term reservoir sediment accumulation and/or sediment loads.
Parameter uncertainty in this study was reduced by placing emphasis on the most sensitive
parameters and reformulating the model. This was achieved in one case by estimating some
important parameters outside the model using proposed equations/estimators for the
Eastern Africa and tropics. This approach was also suggested by Melching (1995). In some
cases, the degree of parameter estimation uncertainty of the catchment sediment yield
model was reduced by calibrating the parameters during the wet years' period for which
most of the hydro-climatic and sediment flow data required by the model are available, as
suggested by Yapo, et al ., (1996) who observed that the hydrographs of wet years produce
more identifiable parameters.
The performance of the model using filled and raw rainfall was evaluated in order to assess
input data uncertainty (Ndomba et al ., 2008b). Model performances were mainly evaluated
based on Nash-Sutcliffe Coefficient of Efficiency (CE), Relative Error (RE), and Total Mass
Balance Controller (TMC). CE provides a normalized estimate of the relationship between
the observed and predicted model values. The simulation results were considered good for
CE values greater than 0.75, while for values of efficiency between 0.75 and 0.36 the
simulation results were considered to be satisfactory. CE values between 0.36 and 0 were
considered to be fair. A value of zero would indicate that the fit was as good as using the
average value of all the measured data. RE was estimated as the ratio of the absolute error to
the true value and expressed in percentage. RE of less than twenty percent (20%) is
considered acceptable for most scientific applications.
SWAT applications in this study assume a number of things. Although the principal
external dynamic agents of sedimentation are water, wind, gravity and ice (Vanoni, 1975)
only the hydrospheric forces of rainfall, runoff and streamflow were considered. The
computed sediment yield in SWAT is solely a result of sheet erosion processes in the
catchment (Shimelis et al ., 2010).
3. Results and discussions
3.1 Sensitive parameters controlling sediment generation and routing
Seven (7) out of nine (9) SWAT parameters that directly govern the sediment yield and
transport in the study cases NYM, SRC and KRC were found to be sensitive (Table 3). It
should be noted that rank 10 signifies that a parameter is not sensitive/influential at all.
These parameters can be categorized into two groups: upland and channel factors. The
former group includes parameters such as P USLE , C USLE , K USLE , Biological mixing efficiency
(BIOMIX), and Initial residual cover (RSDIN); whereas Linear re-entrainment parameter for
channel sediment routing (Csp), Channel cover factor (CCH), Channel erodibility factor
(KCH) and Exponential re-entrainment parameter for channel sediment routing (SPEXP)
parameters belong to the latter group.
However, it should be noted that only channel routing parameters with serial numbers 1, 2,
4, and 5 in the Table 3 were calibrated in all cases. As described in Neitsch et al . (2005),
SWAT upland factors according to the MUSLE equation are formed based on regression