The northern third of the country is covered by a sandy desert with mobile and fixed sand dunes in the north-western part which is considered to be an extension of the eastern outskirts of the Great Desert. The climate of Sudan varies from continental in the northern parts, through savannah in the centre, to equatorial in its most southern parts. Average annual rainfall is 436 mm. Water resources Water used in Sudan derives almost exclusively from surface water resources, as groundwater is used in only very limited areas, and mainly for domestic water supply. There are large areas in Sudan where the exploitation of groundwater has been hampered by cost, as the water table is very deep.
Surface water is provided mainly by the Nile river.
Both the Atbara and the Blue Nile rivers originate in the Ethiopian plateaux, while the White Nile originates from the Equatorial Lakes Plateau. The rivers of the Ethiopian catchment are marked by the extreme range in discharge between the peak and low periods, while the flow from the Equatorial Lakes Plateau is more uniform. The agreement does not consider possible future reduction in water flowing from upstream countries. Apart from the Nile system, there are also the seasonal rivers of Gash and Baraka in eastern Sudan. The water balance of Sudan is very complex, due in part to extensive evaporation from the swamps, the best known being the Sudd or Jonglei area on the White Nile in the southern part of Sudan where only half the water entering the region is estimated to flow out of it.