PREDICTING SEDIMENT LOAD FROM BELES SUB-BASIN IN BLUE NILE BASIN BY COUPLING SIMPLE EROSION MODEL WITH HBV MODEL

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.