DEVELOPMENT OF RAINFALL-RUNOFF- SEDIMENT DISCHARGE RELATIONSHIP · IN BLUE NILE BASIN

Abstract

The Blue Nile (Abbay) basin lies in the western part of Ethiopia between 7° 45'-12° 45' N and 34° 05'-39° 45' E. From its geographic location, the Blue Nile region is the main contributor to Nile flood flows. In Ethiopia only it comprises 18 percent of the total surface area of the country (199,812 km2 out of 1.1Mkm2) with mean annual discharge of 48.5 Km3 . Soil erosion is a major problem in Ethiopia. Deforestation, overgrazing, and poor land management accelerated the rate of erosion. Many farmers in Ethiopia's highlands cultivate sloped or hilly land, causing topsoil to wash away. The objective of this study was to determine rainfall, mnoff and sediment yield relationship in Blue Nile basins and specifically to analysis spatiotemporal distribution of sediments in the Blue Nile catchment; moreover, to identify susceptible regions for erosion and deposition. To analysis this, SWAT model was applied with methodology of collecting hydro metrological data, sediment data, topographic, land use and soil map data and by overlying mechanism, the model nm. SWAT was successfully calibrated and validated for measured stream flow at Bahirdar, at near Kessie and at Sudan Border for flow gauging stations and for measured sediment yield at Gilgel Abbay, Addis Zemen and near Kessie gauging stations in the Blue Nile Basin. The model performance evaluation statistics (Nash-Sutcliffe model efficiency (ENs), coefficient of determination (r2)) are in the acceptable rang, (r 2 in the range of 0. 71 to 0.91 and ENs in the range of 0.65 to 0.90). From the model simulated result, it was found that the Guder, N.Gojam and Jemma sub basins are the severely eroded area with 34% of sediment yield of the Blue Nile are from these sub basins. Similarly, the Dinder, Beshilo and Rahad sub basins cover only 7% of sediment yield of the basin. The annual average sediment yield for the whole Blue Nile is 4.26 t/ha/yr and total 91.3 Million tones eroded from the whole Blue Nile Basin in Ethiopia.