Sudan has a tropical sub-continental climate, which is characterized by a wide range of variations extending from the desert climate in the north through a belt of summer- rain climate to an equatorial climate in the extreme south. The average annual rainfall is 416 mm, but ranges between 25 mm in the dry north and over 1 600 mm in the tropical rain forests in the south.
Sudan is generally self-sufficient in basic foods, albeit with important inter-annual and geographical variations, and with wide regional and household disparities in food security prevailing across the country. The high-risk areas are North Kordofan, North Darfur, the Red Sea, Butana and the fringes of the major irrigation schemes in addition to the Southern States.
Internally produced water resources in Sudan are rather limited. The erratic nature of the rainfall and its concentration in a short season places Sudan in a vulnerable situation, especially in rainfed areas. Surface water in Sudan comprises the Nile river system (nilotic water) and other, non-nilotic streams. 64 percent of the Nile Basin lies within Sudan, while 80 percent of Sudan lies in the Nile Basin. Local rainfall is the main source of the non-nilotic streams and of the Bahr El Ghazal basin, whereas rainfall over the Central African Plateau (Equatorial Lakes) and over the Ethiopian-Eritrean highlands is the main source of the Nile River system and other transboundary seasonal streams (Gash and Baraka).
Sudan shares parts of the following basins with neighbouring countries:
• The Nile Basin, 1 978 506 km2 (79.0 percent of the area of the country);
• The Northern Interior Basins, covering 313 365 km2 in the northwest part of
the country (12.5 percent);
• The Lake Chad Basin, in the west of the country along the border with Chad
and the Central African Republic, covering 101 048 km2 (4.0 percent);
• The Rift Valley Basin, in the southeast part of the country at the border with
Ethiopia and Kenya, covering 16 441 km2 (0.7 percent). The Nile system within Sudan comprises:
• The Blue Nile, Sobat and Atbara Rivers originating in the Ethiopian
• The White Nile system, upstream of Sobat River, originating on the Lakes
• The Bahr El Ghazal Basin, an internal basin in southwest Sudan.
The characteristics of the Nile system tributaries are the following:
• The Blue Nile: The flow of the Blue Nile reflects the seasonality of rainfall
over the Ethiopian highlands where the two flow periods are distinct.
The White Nile: Due to losses in the Sudd swamp area, the White Nile leaves this area with only about 16 km3, out of 37 km3 on entering it. The river receives about 13 km3 from the Sobat River before joining the Blue Nile at Khartoum. The contribution of the Bahr el Ghazal basin is negligible, estimated at about 0.5 km3. The average annual flow of the White Nile System at Malakal is about 29.5 km3 and the daily discharge fluctuates between 50 million m3 in April to 110 million m3 in November (ratio 1:2). During the flood period the Blue Nile forms a natural dam that obstructs the flow of the White Nile and consequently floods the area upstream of the confluence.
• TheAtbaraRiver:Thisisahighlyseasonalriver,withanannualflowupstream of its confluence with the Nile of about 10 km3 restricted to the flood period of July-October, the maximum occurring between August-September. The river has a steep slope and small catchments, and reflects the rainfall over the upper catchments as runoff at Sudan border within one to two days.
• The Main Nile: The reach of the Nile downstream of the confluence of the Blue Nile and the White Nile Rivers is known as the Main Nile. The Atbara River is regarded as the only and last tributary joining the Main Nile. The average annual flow of the Main Nile at the Sudan-Egypt border at Aswan is estimated at 84 km3. (Source : Irrigation in Africa in figures – AQUASTAT Survey 2005)