of Al Jazirah and Khashm al Qirbah for irrigated cultivation. East of the Blue Nile, large areas are used for mechanized rainfed crops. West of the White Nile, these soils are used by traditional cultivators to grow sorghum, sesame, peanuts, and (in the area around the Nuba Mountains) cotton. The southern part of the clay soil zone lies in the broad floodplain of the upper reaches of the White Nile and its tributaries, covering most of Aali an Nil and upper Bahr al Ghazal states. Subject to heavy rainfall during the rainy season, the floodplain proper is inundated for four to six months — a large swampy area, the Sudd in South Sudan, is permanently flooded — and adjacent areas are flooded for one or two months. In general this area is poorly suited to crop production, but the grasses it supports during dry periods are used for grazing. The sandy soils in the semiarid areas south of the desert in northern Kurdufan and northern Darfur states support vegetation used for grazing. In the southern part of these states and the western part of southern Darfur are the so-called qoz sands. Livestock raising is this area's major activity, but a significant amount of crop cultivation, mainly of pearl millet, also occurs. Peanuts and sesame are grown as cash crops. The qoz sands are the principal area from which gum arabic is obtained through tapping of Acacia senegal (known locally as hashab). This tree grows readily in the region, and cultivators occasionally plant hashab trees when land is returned to fallow. Hydrology Except for a small area in northeastern Sudan, where wadis discharge the sporadic runoff into the Red Sea or rivers from Eritrea flow into shallow evaporating ponds west of the Red Sea Hills, the entire country is drained by the Nile and its two main tributaries, the Blue Nile and the White Nile. The longest river in the world, the Nile flows for 6,737 kilometers from its farthest headwaters in central Africa to the Mediterranean. The importance of the Nile has been recognized since biblical times; for centuries the river has been a lifeline for Sudan. The Blue Nile flows out of the Ethiopian highlands to meet the White Nile at Khartoum. The Blue Nile is the smaller of the two rivers; its flow usually accounts for only one-sixth of the total. In August, however, the rains in the