Assessment of Soil Erosion in the Blue Nile Basin

Abstract

Water erosion in the form of sheet and rill erosion and lack of good land management practices lead to accelerated land degradation. Rill erosion is a major contributor to sediment detachment and transport from agricultural fields leading to both on-site and off-site effects of erosion. Farm-runoff drainage ditches have also contributed to the same effects. This chapter, thus, presents assessment results of rill erosion and farm ditches in the Lake Tana subbasin of the Blue Nile basin in order to understand their contribution to the overall soil loss. Monitoring the spatial and temporal rill erosion development on agricultural lands of 6, 15, and 300–500 m slope lengths has revealed the greater role of tillage-induced surface roughness, soil texture, slope shape and length, and barriers along the topo-sequence of the catchment. Similarly, quantitative assessment of the farm-runoff drainage ditches has made a clear picture that ditches serve as both a source of erosion and a sediment transport channel for the soil eroded from the inter-ditch areas. The changes in depth and width of ditches constructed on moderate to steep slopes vary between 8 and 20 mm, and 20 and 30 mm during two months rainfall period, respectively. Eroded sediment from inter-ditch areas is transported at a rate between 0.5 cm2 and 4.0 cm2 per meter of single ditch. This study indicated the role of rills and ditches for total soil loss and the importance of protecting and controlling both rill and ditch erosion without compromising the need to drain excess water.