Autumn is the main rainy season, extending from May to October with precipitation ranging between less than 50 mm in the extreme north to more than 1500 mm in the extreme south. The rainfall,

however, is characterized with significant variations in distribution as well as in timing and location thereby magnifying the risks of localized crop production. To avert this risk mechanized rainfed production schemes have been spread all over central Sudan. Apart from agriculture the rains replenish the underground reserve and provide the scattered wadis and water points with annual quantities to support the enormous wealth of livestock and wild life.

Sudan is a meeting point of river tributaries that emanate from the Ethiopian plateau and the region of the Great lakes. The Blue Nile with its tributaries Dinder and Rahad together with the Atbara flow from the east annually providing some 66 milliards cubic metres (md. c. m.). On the other hand Bahr El Jebel commences from Lake Victoria with permanent rains, but the greater part of the runoff is lost in the Sudd area inside the Sudan, bringing some 15 md. c.m. at Malakal. The Sobat River which joins the Nile at Malakal flows from the Ethiopian plateau and is being fed from tributaries inside and outside the Sudan. However, about 8 md. c.m. of Sobat runoff estimated at 13 md. c.m. are lost in the Sudd area of Sobat and Mashar. Almost all the water flow of Bahr El Ghazal River estimated at 14 md. c.m. are lost in the Sudd area of Bahr El Ghazal, leaving only half a md. c.m. to join the White Nile at lake No.

The big variation in the Blue Nile flow between the high river during the flood season and the low river during the months from March to May have necessitated the construction of dams to store water for irrigation and for the generation of hydro-electrical power. At present there are three dams: Sennar (1 md. c.m.), Roseires (3.4 md. c.m.) and Khashm El Girba (1.3 md. c.m.). However the accumulated silt in the dam lakes has reduced the storage capacity by 25% in Roseires Dam and by 40% in both Sennar and Khashm El Girba dams. A project is now being implemented to heighten Roseires dam to increase the storage capacity to 7.3 md. c.m. and also to construct Siteit Dam across upper Atbara River to install additional storage capacity for irrigation projects.

Sudan is now utilizing about 14.6 md. c.m. of its share of Nile water for irrigation, of which 9.5 md c.m. are from the Blue Nile, 1.7 md. c.m. from Atbara River, 1.8 md. c.m. from the White Nile and 1.6 md. c.m from the main Nile. The heightening of Roseires Dam and the construction of the new dams will enable the country to fully utilize its share of the Nile water which stands currently at 20.5 md. c.m. at Sennar, according to the Nile Water Agreement of 1959. During the early eighties Sudan and Egypt launched a joint project to excavate Jonglei canal and bypass part of the Sudd region, thereby sparing some 4 md. c.m. to be divided equally between the two countries. However, the project was hampered by the civil strife which started in 1983.

These include El Gash seasonal river which has an annual runoff of 600 million cubic metres (m. c.m.), Khor Baraka with 500 m. c.m., in addition to some 40 smaller riverlets or wadis scattered all over the central plain, providing together about 6.7 md. c. m. so far not utilized, with the exception of about 160 m. c. m. used for domestic purposes. This has been made possible by constructing 63 barrages across the wadis to store 130 m. c.m., and by digging 840 hafirs to store about 26 m. c.m.

The water bearing rock strata comprise the Nubian Sandstone which covers 28.1% of the total area of the Sudan, Um Rwaba series covering 20.5% of the total area and the basement complex which covers 9.1% of the total area. The preliminary surveys of the underground reserve quote the figure of nine md. c.m.

However, there is need for more research to ascertain the actual figures for the reserve and the replenishment rate. At present only about 1.3 md. c.m. of underground reserve are utilized, of which about 450 m. c. m. are used for domestic purposes, while about 850 m. c.m. are used to irrigate about 67.2 thousand hectares.