The distribution of rainfall in Sudan, despite its generally low value, is also highly seasonal in character. The country is therefore dependent on its river system for perennial irrigation for reliable agricultural production. At present about 4.6 million acres (1.9 million ha) are under irrigated agriculture. In all these areas, the system of irrigation is achieved by a gravity-flow method based on hydraulic principles which take maximum advantage of land contours. It consists of hierarchies of supply and distributive canals. The lower stages of these hierarchies, called Minor-canals, produce ponding conditions and are thus very favourable for extensive growth of aquatic weeds (macrophytes).
Because future agricultural development will be based on gravity-flow, and for economic reasons, attention is focused on the increasing extent of this man-made aquatic habitat in the arid region of central Sudan.