Contemporary climate and hydrological regime of the Nile
From the climatological point of view, Egypt is the northeastern edge of the Sahara belt (Fig. 4.1) and its extreme aridity is caused by the descent of tropical air masses, which causes them to become hot and dry. Along the Mediterranean coast, a narrow strip of its land is influenced during winter by the cyclonic system of the westerlies. The annual average precipitation over most of Egypt is less than 50 mm and only along the coast does it reach about 100 mm. Yet Egypt, since ancient times, has been inhabited by an agricultural society because of the water brought by the Nile (Fig. 4.1). This river is the longest in the world, measuring about 6600 km from its headwaters in Rwanda to the northern edge of its delta. It has a relatively high constancy of flow and of periodicity of its fluctuations. There are several reasons for this. The Nile is fed by precipitation falling on the subequatorial countries of east Africa, the source of this moisture being the southwesterly air streams from the Indian Ocean and the equator during the northern hemisphere summer. These summer rains affect mainly the Blue Nile and the Atabara rivers, which originate in the north and central highlands of Ethiopia. The resultant floods start in June and reach their peak in August. The White Nile is fed by rains falling on equatorial Uganda and southern Ethiopia, where there are two rainy seasons, thus providing more evenly distributed runoff. Another very important storage and regulating factor is the gigantic swamps of the Sudd into which the White Nile flows; here the suspended silt of the river is deposited and the supply of water is regulated. In terms of relative contributions, the Blue Nile contributes about 60% of the average annual water supply, the White Nile about 30% and the Atabara about 10%.
Continue reading here: Climate during the Upper Pleistocene and Holocene
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