Effect of Rainfall Distribution over the Blue Nile Watershed on Filling the Grand Ethiopian Renaissance Reservoir (GERR)

Abstract

AbstractThe Abyssinian Plateau’s climate varies with latitude, altitude and exposure or slope. The rainy season is extended from June until September (Hurst et al., 1931). The West of Ethiopia receives higher amount of rainfall than the remainder of the country. Highland rainfall normally peaks in August in the administrative regions of Welo and Northern part of Shewa. Rains decreases over most of the highlands during September ( Henricksen, 1986 ).The maximum rainfall amount was extracted for regular years from the Jemma station and found to be 287 mm. On the other hand, the minimum amount of rainfall was acquired and found to be 164 mm for drought years. The effect of these amounts for overall the Blue Nile Basin on the life span of Grand Ethiopian Renaissance Reservoir ( GERR ) were studied before by Moussa et al.,2016 and 2017 with time duration of 24 hours.About 98 percent of the annual sediments are carried by the Nile during the flood season. Most of these sediments are eroded from the Blue Nile high lands (about 82%). Since the Grand Ethiopian Renaissance Dam (GERD) starts to store the flooding water and sediment during this year, then water and sediment that will be stored by this dam will be varied from regular to the drought years especially when rainfall amounts are distributed over different administrative regions of the Blue Nile Basin. The objective of this research is to determine water and sediment discharge of the Nile Basin with respect to the new regulation of rainfall that will fill the GERR. Volume of water and sediment for the Blue Nile basin were determined by using remote sensing techniques and by the aid of hydrological model, watershed storm hydrograph model (WASHMO), that was established by Andy Ward 1986. Hydrological parameters for 15 sub basin were extracted from digital elevation model (DEM) of Shuttle Topographic Radar Mission. Other parameters that concern rainfall, soil characteristics and land cover were collected from Atlas of the Blue Nile Basin (Yilma and Awulachew, 2009) and input in the model in order to determine sediment yield from the Blue Nile Watershed.