THE NILE AND THE IRRIGATION SYSTEMS.

I am indebted for a great deal of information on this head to an article by W. C. MACKENZIE, D.Sc., entitled, "The Nile in Relation to Egyptian Agriculture." (Year Book of the Khedivial Agricultural Society, 1905, p. 233.)

The White and Blue Nile join close to Khartoum and form what is known as the Egyptian Nile. From Khartoum to the Mediterranean it is 3,000 kilometers in length, but receives only one tributary, the Atbara. The Blue Nile and the Atbara have their sources in the mountains of Abyssinia. The White Nile rises in the large lakes of Central Africa. Whilst the Blue Nile and the Atbara are in flood from August to November, flowing quickly and carrying with them the soil from the Abyssinian highlands, where owing to the heavy fall the water does not soak into the soil, the White Nile has its flood time in the early summer. It flows slowly through marshes, deposits there the mud it may bear away from its sources, filters through the marshes, and carries away with it large amounts of vegetable matter, such as particles of roots of grasses from the marshland. On the other hand, the Blue Nile and the Atbara are full of silt and mud. It is the deposit of this silt which in the course of many thousand years has formed the Nile Delta.

Egypt, being almost a rainless country, is dependent for its water supply upon the Nile.

Up to 1882 the country was supplied with its water by means of the system known as basin irrigation. The country was divided up by banks into large plots varying from 500 to 50,000 acres; these were flooded by the Nile at the usual flood times, and the silt year after year deposited itself in these basins; the amount of such deposit has been estimated to be about one ton per acre, equal to a layer of one millimetre in thickness. In other words, it would take one thousand years to raise the level of the basins one metre. The filling of the basins begins. about the 10th to 15th of August. Early in October the water from the basins is discharged. Where the soil depends entirely

upon the flood for its supply of water, it is not possible to grow summer crops, such as cotton, sugar cane, etc.

This system of basin irrigation has been to a large extent replaced by the modern system known as perennial irrigation. Canals were cut from the Nile to supply a larger area, and these canals supply water for irrigation all the year round; but it was soon found that these canals began to silt up, and they had to be cleared after each flood. With the view to keeping a constant supply of water in the Delta canals at a higher level, thus saving the annual clearing of the silted canals, the Cairo Barrage a kind of enormous sluice or weir (see illustration page 183) was constructed at the branching of the Nile into its two effluents twelve miles below Cairo, its object being to hold up the level of the water in the Nile after the floods and enable the water to be turned into the canals at a higher level. The result has been that the levels of the canals were gradually raised, and the farmer has now in many cases no need to lift the water from the canals, but simply waters his fields by free

flow irrigation. In other cases the difference between the level of the canal is not so great as before, and the farmer's task of lifting the water into his field is rendered much lighter. Various appliances, of which illustrations are given, are used for this purpose.

Lower Egypt has no basin irrigation at all; during the flood time the numerous canals take the water directly from the Nile, but in summer, when the supply is limited, the whole of the

water is arrested at the Cairo Delta Barrage and distributed by three great main canals to their respective irrigation areas. Each main canal is supplying smaller canals, which again are sub-divided, until the water runs in numberless arteries all over the Delta. During eight months (September to April) the water is abundant, whilst from May to August the water is dealt out in "rotations," according to which canals or sections of canals receive water for a certain number of days and no water during so many more days. The extent of the restrictions depends on various circumstances; the extreme is six days water and eighteen days none.

In order to guarantee a sufficient supply of water in the Nile during the summer before the flood came, a huge storage reservoir was constructed at Assuan, and a barrage at Assiut. This has rendered possible the conversion of the basins of

Assuan, which together with the first Nile Cataract forms this reservoir, was constructed by John Aird & Company. It has a height of 160 metres, and a length of 2,000 metres; there are 140 sluices below, each 14 metres square, and 40 upper sluices of 71⁄2 metres square. The bottom sluices allow the silted water to pass when the river is in flood. The dam is at present being raised by 6 metres in order to allow of sufficient additional water being collected for a further 1,500,000 feddans in the north of the Delta, which at present lie unreclaimed. When the flood ceases the sluices in the Assuan dam are closed. This is generally about the first of December, and it takes 100 days to fill the huge reservoir. Towards the beginning of May the water of the Nile becomes too low to feed the extensive network of canals, and from that time the water from the dam at Assuan is used, the contents of which, amounting to 1,000,000,000 cubic metres, are necessary for supplying about half a million feddans in Upper Egypt with water. The cost of the Assuan dam and