Abstract:
Soil erosion and land degradation are important phenomena in the Blue Nile Basin. Beles
sub-basin is one of the major sub-basins of the upper Blue Nile which is used for water
resources development through irrigation and hydropower. This development could be
challenged by sedimentation and irrigation systems leading to water shortage. Therefore, it
is important to investigate the level of sediment yield in this watershed and the aim of this
paper is to estimate the sediment yield of the upper main Beles catchment by coupling
simplified sediment model with HBV model. A hydrological model, HBV was utilized to
simulate the stream flow.
The watershed was first calibrated to HBV for the period from 1994 to 2003 and then
validated from 2004 to 2010. Afterwards, the erosion model was coupled with the HBV by
assuming all part of the watershed is runoff producing area. The erosion was simply
calibrated the few available measured sediment concentration at the out let by the MoWIE.
The performance of the hydrology model was ENS = 0.66 for calibration and 0.64 for
validation respectively, which were found in the acceptable range. The sediment modeling
resulted in ENS = 0.8, R
2
=0.9 and RMSE= 0.23 for simulation.
In the sediment model we assume that the concentration from the runoff areas is at the
transport limit (i.e., H=1, H is the fraction of runoff producing area that contributes
sediment) for the first four weeks after the first rainfall event. Then for another month a few
more areas of the watershed are being covered well by vegetation limiting the availability of
sediment and the H decreases from 1 to zero. Around August 1, the sediment concentration
from the runoff areas is at the source limit. The annual sediment load of upper main Beles is
found to be 5.1 ton per ha per year, which is below the soil loss tolrenace of 6 t/ha defined
by Hurni Huni (1993) for Etiopian highlands. The total mean annual sediment yield that was
drawn from main Beles predicted by sediment model was found 1,765,386.5 ton/year.
