which for the most part are in Uganda and Ethiopia.
From there it takes its course northwards to the Mediterranean over a total
distance of 6,671 km. In northern Sudan and Egypt it flows over a distance
of more than 2,700 km without receiving any tributaries, since in this area
a hyper-arid climate prevails. The Nile passes through various climatic and
vegetational zones from tropical rainforest and mountain forest with ten to
twelve humid months and more than 1,600 mm of annual rainfall, through
the high-rainfall woodland savannah, the dry savannah and the thorn-scrub
savannah, until it finally reaches the eastern part of the Sahara Desert, one
of the world’s driest regions, for which evaporation rates between 2,078 mm
(Fentzloff 1961) and 9,300 mm (Schendel 1967) have been calculated.
Nile sources belong to two main hydrological systems which characteristi-
cally determine the Nile discharge.