The rainfall stations within the Bahr el Ghazal exhibit high inter annual variation together with spatial variation in amounts. Stations in Sudan record mean annual values of about 500mm whereas those in South Sudan receive on average 900mm of rain.
The Bahr el Ghazal basin is a large and highly complex, where evaporation in its downstream swamps makes it almost an endoergic system. The rainfall of 1200-1400mm in the upper basin is the highest in the Sudan and gives rise to a number of seasonal tributaries, which converge towards the confluence of the Bahr el Ghazal with the White Nile. While collecting a large amount of runoff, the flows of the different tributaries are mostly spilled into swamps and floodplains along the river course where they evaporate. Only a negligible amount of flow reaches the Bahr el Jebel at Lake No. The Bahr el Ghazal is therefore unique among the Nile tributaries in that its outflow to the White Nile is almost negligible.