The Blue Nile with its spatial extent ranging from the Ethiopian highlands down to Sudan, receives highly spatially distributed rainfall amounts, with the highlands receiving mean annual rainfall amounts in excess of 1,200mm and some areas in Sudan receiving less than 400mm. There is also a noticeable variation of these amounts across the years.
The main control on rainfall over the Blue Nile basin is the east circulation flowing over the Blue Nile and the extent and timing of the seasonal migration of the Inter Tropical Converge Zone (ITCZ). This gives the rainfall distribution over the year in the Blue Nile basin a strong seasonal character. The mean annual rainfall over the Blue Nile sub basin ranges from 1 000 mm in the northeast to 2 000 mm in the southeast (Ribbe and Ahmed 2006). The average annual precipitation over the sub-basin is 1,346 mm, making it the highest among all the sub-basins of the Nile. Changes in temperature and rainfall would affect the flow of the Blue Nile mainly through runoff variability and changing upstream demand.