The discharge of this river below the Bahr El Zeraf junction varies from 300 to 500 metres cube per second according to the season of the year and the nature of the flood. It is probable that the last figure is a maximum and is never surpassed.[22] At El Dueim, 637 kilometres below the Sobat junction, the summer supply varies between 350 and 500 metres cube per second. The minimum levels are generally attained in the month of April and the first half of May. The discharge, owing to the Sobat water, gradually increases until the Blue Nile flood exceeds the volume of 5,000 metres cube per second at Khartoum. As soon as this figure is passed the discharge of the White Nile is reduced by an amount varying from 30 to 60 per cent., and this holding back continues until the Blue Nile falls again below the figure above given. This reduction of the White Nile discharge takes place in the months of August and September. As soon as the Blue Nile discharge has fallen below 5,000 metres cube per second that of the White Nile rises very rapidly, attaining its maximum in the months of November and December, when as much as from 1,500 to 1,700 metres cube per second have been recorded. This increased discharge is, of course, partly due to the Sobat, but also to the draining off of the water which has been ponded up for so long a period. It seems safe to assume that the White Nile discharge at Khartoum never under any circumstances exceeds 1,800 cubic metres per second.
To sum up:—The White Nile is at its lowest from March to May. It rises in June, is checked again in August and September, and attains its maximum during the months of November and December. Its limits in a low year are from 300 to 1,500 metres cube per second, and in one of high flood from 400 or 500 to 1,700 metres cube per second.[23]
7. The Blue Nile.
The supply of this river is chiefly derived from the drainage of the basin through which it runs and from the large tributaries which enter it downstream of the point where it issues from the Abyssinian hills. The Tsana lake has but a small influence upon its supply at any period of the year. It is at its lowest in May, when its discharge at times shrinks to nothing. It begins to rise in June and attains its maximum about the end of August. Its discharge in a year of good flood is as much as 10,000 metres cube per second, and it seems probable that in a year of exceptional flood 12,000 metres cube may pass Khartoum. In September it falls very rapidly, and during the winter months rarely discharges more than from 200 to 400 metres cube per second. The Khartoum gauges prove that a higher reading is recorded for a given flood discharge when the river is falling than is the case when the river is rising. This is probably due to the filling of the valley between Khartoum and the Shabluka Pass.
8. The Atbara.
The first water from this river reaches the Nile in the last week of June, and the maximum is usually reached in the last days of August, or in the first week of September. The Atbara generally attains its maximum before the full flood from Khartoum has arrived at the junction of the two rivers. After the maximum has been reached, the fall of the Atbara is rapid, and by the end of the year the river reverts to its summer state of a series of pools. The maximum discharge of the Atbara, measured in 1903, was 3,088 metres cube per second, but this is probably surpassed in a year of very high flood.
9. The Nile north of Khartoum.
The discharges of 1903 record a maximum of 10,500 metres cube per second in an average year. If to this be added the volume of the Atbara, a total of nearly 14,000 cubic metres per second is reached. As in 1903 the levels at both Halfa and at Cairo did not pass those of a very ordinary flood supply, it would seem probable that in very high flood a volume of quite 16,000 metres cube per second must pass Berber.
In conclusion it may be stated with confidence that the White Nile contributes practically nothing to the flood which reaches Egypt. This is entirely derived from the Blue Nile and from the Atbara. On the other hand, the supply passing Aswan during the spring and early summer is due, almost entirely, to the water of the great lakes brought down by the White Nile.
The following are the water-slopes of the two rivers, as worked out from the discharges:—