CATCHMENT WATER BALANCE FOR BLUE NILE RIVER BASIN

Abstract

The Blue Nile River is one of the major tributary for the Nile River. It provides the greater portion (about 62%) of the total flow of the main Nile. Blue Nile originates from Ethiopia high lands at about an elevation of 2000 m.s.l. Despite enormous potential to the countryaswell as regional countries such as Sudan and Egypt, the information availability and low level of studies make\Blue Nile as one of obscure river. The proper water balance and sub basins water yield has not been properly estimated .. This study is an attempt to estimate the total annual flow at Sudan border through estimation of water yield at each sub basins using water balance modeling. A conceptual rainfall -runoff model known as MOWBAL has been used for Calibration and validate of model parameters. Application of the MOWBAL model for 43 catchments shows that more than 52 % of the total gauged catchments give outstanding performance with R2 greater than 80%, and more than 45 % of the total catchments also show good performance with R2 of more than 50 %.This shows that MOWBAL appears to be a suitable model for catchments of BNRB.(see table 5.2) The estimate of water yield for each of 14 sub basins have been generated using estimation of areal average model parameters and running the model The total flow from the Blue Nile Basin at the border to Sudan is estimated to be 52.9BCM. This figure is comparable with WAPCOS study (1990) which is estimated to be 52.6BCM.However recent estimates by Sutcliffe and Park (1999) and BECOM (1998) gives a figure of 48.7BCM. In the case of seasonal variation, the bulk of the runoff (89%) on average occurs between June and October which are the rainy season for most of the sub basins. In all sub basins except Tana the peak outflow occurs in August where for Tana sub basin the maximum appears in September. The long term mean annual flow of 48.66 km3 at Sudan border from 1912 masks a variation from low annual totals of 20.89 Km3 in 1913 and 29.65km3 in 1984, to high totals of 69.67 km3 in 1917 and 69.85 km3 in 1929 (Sutcliffe,etal 1999).ln this study also which uses recent input records gives the total outflow of 52.9km3 at the border. This shows the annual variation of flows of Blue Nile. In general the out puts of the model are comparable with the previous works. As this study is more in depth and the accuracy of rainfall-Runoff relation ship for the sub catchments are found to be higher, the overall water yield looks reasonable.