The Study area: Dinder and Rahad river basins 2.1 THE DINDER AND RAHAD RIVER BASINS (D&R) The Dinder and the Rahad are the lower sub-basins of the Blue Nile river basin located between longitude 33°30' E and 37°30' E and latitude 11°00' N and 15°00' N (Figure 2.1). The Blue Nile basin collects flows of eight major tributaries in Ethiopia besides the two main tributaries in Sudan: the Dinder and the Rahad rivers. Both tributaries receive their water mainly from the runoff generated in the Ethiopian highlands approximately 30 km west of Lake Tana (Hurst et al., 1959). The Dinder river joins the Blue Nile at the village Al-Rabwa, 64 km downstream of Sennar reservoir, while the Rahad river joins the Blue Nile at the village Abu Haraz below Wad Medani town. The D&R generate around 7% of the Blue Nile basin’s annual flow. The Rahad river supplies water to the Rahad irrigation scheme (126,000 ha), while the Dinder river supplies water to the diverse ecosystem of the Dinder National Park (DNP). The catchments areas about 34,964 and 42,540 km2 for the Dinder and the Rahad, respectively, resulting in a total area of about 77,504 km2. However, in the Ethiopian highlands where rainfall is relatively high (about 1,400 mm/y), the catchment area of Dinder (18,000 km2) is two times that of the Rahad river (8,758 km2). The total catchment has varied topography with elevation ranging between 384 m at the catchment outlet and up to 2,731 m at the Ethiopian plateau (Figure 2.1). The basin boundary and the streams network have been delineated from a 90 m x 90 m digital elevation model database of the NASA Shuttle Radar Topographic Mission (SRTM) acquired from the Consortium for Spatial Information of the Consultative Group for International Agricultural Research (CGIAR_CSI) website (http://srtm.csi.cgiar.org). The main soil types in the D&R according to the Food and Agriculture Organization (FAO) classification are: vertisols 71%, luvisols 9%, nitisols 8%, leptosols 5%, cambisols 4%, alisols 2% and fluvisols 1%. The vegetation cover is characterized by grasslands, shrublands, croplands and woodlands. 12 .