Abstract:
Soil erosion is dramatically increasing and accelerating in developing countries like
Ethiopia. It has worrisome economic and environmental impacts and causes nutrient loss on
agricultural land, sedimentation in rivers and reservoirs, clogged canals and other water
supply systems. Determination critical erosion prone area and calculating soil loss rate in
upper Blue Nile sub-basin watershed is an important priority for prioritizing the area for
watershed management practices in order to reduce soil erosion. Watershed prioritization
has gained importance in natural resources management, especially in the context of
watershed management. Basin morphometric analysis is a means of mathematically
quantifying different aspects of a drainage basin. In present study, Morphometric analysis
and prioritization of the sub-watersheds and soil loss estimation for significantly affected
area have been done. The model has been used to assess the hydrological characteristics and
soil erosion potentials based on the morphological characteristics. The study was carried out
using DEM data 30mX30m resolution in GIS environment, rainfall, soil data, and land use
land cover. Morphometric analysis was carried out for linear, shape and relief aspects.
Under linear aspect bifurcation ratio, drainage density, stream frequency, drainage texture
ratio and length of over land flow were analyzed. Under shape aspect; elongation ratio,
circularity ratio, form factor and constant channel maintenance were evaluated. Finally,
under relief aspect; basin relief, relief ratio, ruggedness number and relative ratio have been
analyzed. The compound factor for sub-watersheds of both Guder and Dabus sub-basin have
been calculated and classified into erosion tolerance class. Accordingly, the G-7(one of the
sub-watersheds) has been the first ranked and classified under high erosion severe class with
compound factor value of 2.82.
To estimate annual soil loss of the G-7 sub-watershed, Revised Universal Soil Loss Equation
(RUSLE) with the ArcGIS 10.3 integration have been used. RUSLE parameters such us
rainfall erositivity(R-factor), soil erodibility(K-factor), slope length and slope steepness(LS factor), cover management (C-factor) and support practice(P-factor) have been calculated
and used as data input in annual soil loss calculation. By integrating these five map layers in
GIS raster calculator, the required spatially distributed annual average soil loss rate was
determined. Accordingly, the result of the analysis for the existed conditions depicted that the
amount of soil loss ranges from 0 to 167.47ton ha-1 yr-1 with average annual soil loss rate
of 15.34ton ha-1 yr-1 from the whole catchment. Totally the annual soil loss of the watershed
was found to be 3,617,172ton. Such losses could threaten the sustainability of land
productivity in the study area and at the same time, excessive sedimentation and
eutrophication problem at the downstream proposed reservoirs on Guder River and also on
Ethiopian Great Renaissance Dam.
