The Blue Nile River originates at Lake Tana, in the Ethiopian Highlands and provides a vital source of freshwater to the downstream riparian users, Sudan and Egypt. The Blue Nile is one of the most important tributaries of the Nile River. However, only a few studies in its upper basin within Ethiopia have been conducted and the information available on the climatic, hydrologic, topologic, and hydraulic characteristics of the river and the basin is incomplete. However, global datasets are becoming increasingly available from the Internet and can often be used to assess the hydrologic characteristics of such basins. Using climatic and topographic data from the public domain a simple water balance model was used to generate monthly surplus for each 0.5°×0.5° grid. A GIS based approach has been used for deriving hydrologic parameters from the girded representation of the land surface terrain of the basin. A fraction of precipitation is extracted and declared runoff, before remaining precipitation is passed to the soil. The runoff for each grid is then routed directly from its source of generation to the outlet with a response function. The function includes a measure of travel time, storage effects and losses due to evaporation and infiltration along the flow path. The approach is also known as source to sink (STS) routing method and recently been used in the global scale flow routing studies. The subsurface flow component from each grid is simulated using a linear reservoir model. Total streamflow at the sink at any time is considered as the sum of the contributions from all sources in the watershed. The model was verified by comparing simulated flows with the recorded monthly flows at the outlet of four sub-catchments and the total simulated flow with the recorded monthly flow measured at the outlet of the catchment at El/Deim station located near Sudan Ethiopia border
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