The flood of 1874 was an early one and the basins were discharged on a falling Nile, still they raised the Cairo gauge to 8·7 metres on the 5th October while it would have risen to 8·5 on the 15th September with perennial irrigation. The flood of 1878 was an exceedingly late one and the basins had to be discharged while the river was still very high. By the 10th October, the river had risen to 8·7 metres at Cairo when the banks were breached and all future rise stopped. With perennial irrigation, the maximum gauge of 8·6 metres would have been reached on the 10th October. The flood of 1892 was at Assuân 10 centimetres below that of 1874 and 20 centimetres below that of 1878, and midway between them in point of time. It was also under complete control owing to the new regulating works on the basins. It rose to 8·4 metres at Cairo on the 5th October and fell exceedingly slowly. With perennial irrigation, it would have risen to 8·4 at Cairo on the 30th September and then fallen rapidly.
Speaking generally, we may say that with perennial irrigation the very high floods at Cairo will be 15 days in advance of what they are at present, that they will not rise higher, and that they will fall 15 days earlier than what they do now. With low floods there will be no appreciable difference as to date, but the floods will be slightly higher at Cairo. In ordinary floods, there will be an advance of from 20 to 25 days in the date of the maximum flood, and a maximum gauge at Cairo 50 or 60 centimetres under the maximum gauge at Assuân. We have so far considered Cairo only, as the Delta proper depends on the Cairo gauge. We now turn to the Nile in Upper Egypt itself: south of Sohag, there will be no serious change in levels, but the Sohagia and Ibrahimia canals between them carry at present 750 cubic metres per second in excess of what they would carry if there were perennial irrigation in Egypt, and the greater part of this water is not returned to the Nile until the Kushesha escape is reached. The reach of the Nile from Sohag to Kushesha is the one which will experience the greatest changes, and I calculate that there will be a rise of 40 centimetres as compared with the maximum gauges under basin irrigation.
It will be noted that at the beginning of this paragraph I stated that “the perfection of the perennial irrigation of the Delta north of Cairo will in no way affect the Nile in flood.” This had reference only to the quantity of water taken from the Nile in high floods. There is however one very serious aspect of this question. The regulation on the Barrage in low floods, which has gone on steadily since 1899 when Sir Hanbury Brown used the Barrages in flood for the first time, has enormously increased the value of the works, but it has certainly caused the Main Nile to silt, and probably also the branches, owing to a reduced discharge and velocity of the silt-bearing water, whose capacity to carry on that quantity of silt depends on its velocity. It seems to me that unless steps are taken to insure the scouring out of this silt by the clear water of November, December and January the consequences will be very serious. High floods scour out their beds, but if a very high flood were to come early before the silt had been scoured out, it might overflow the banks near Cairo or in the middle reaches of the Nile branches in the Delta.
Sir William Garstin has estimated the cost of converting the existing basins of Upper Egypt into perennially irrigated land at £7,000,000, thus made up:—
| Conversion of Upper Egypt basins | £ | 5,000,000 |
| Two barrages between Assyut and Kena | „ | 2,000,000 |
| Total | £ | 7,000,000 |
The resulting land tax from the improved irrigation in Upper or Lower Egypt he has estimated as follows:—
| Upper Egypt. | - | Basin land converted | 750,000 | acres | @ | £ | ·50 | = | £ | 375,000 | |
| Land irrigated by pumps | 100,000 | „ | @ | £ | ·30 | = | „ | 30,000 | |||
| Lower Egypt. | Reclaimed land | 800,000 | „ | @ | £ | 1.00 | = | „ | 800,000 | ||
| Total | £ | 1,205,000 | |||||||||
39. Development of the Sudan.
—Lord Cromer’s wise decision to construct the Suakin-Khartoum railway immediately and the Abu Hamed-Dongola railway as soon as possible, is the charter of the development of the Sudan. With these railways and especially the former in working order, we can arrange for irrigation works for the production of cotton and wheat for export, knowing that they can be exported at a cost of transport which will not be absolutely prohibitive. The soil of the Sudan along the Blue Nile, the Atbara, the Main Nile and a great part of the White Nile is the same as that of Egypt itself. It has all come from Abyssinia. When at Khartoum last February, I collected specimens of typical Gezireh soil from points 10 miles south of Khartoum and from near Khartoum itself. They were analysed by Mr. Frank Hughes and reported on by Mr. Foaden.
The specimens were numbered as follows:—