| Metres. | |||
|---|---|---|---|
| January | ·071 | ||
| February | ·074 | ||
| March | ·172 | ||
| April | ·193 | ||
| May | ·252 | ||
| June | ·299 | ⎫ ⎬ ⎭ | Average per diem for threehottest months, ·0107. |
| July | ·370 | ||
| August | ·310 | ||
| September | ·226 | ||
| October | ·179 | ||
| November | ·120 | ||
| December | ·098 | ||
| Total | 2·364 | metres per annum. | |
The average per diem for the three hottest months of the year, according to these observations, is ·0107, as against ·01 deduced from observations of the levels of Lake Qurûn. The figure given for July in the above list is higher than I should have expected, as the evaporation of that month I should estimate to be very slightly in excess of that of June or August; that is, about 1 centimetre a day.
Plate I.
THE BAHR YÛSUF, SKIRTING THE LIBYAN DESERT, IN THE NILE VALLEY.
The Bahr Yûsuf outside the Fayûm.—The Bahr Yûsuf (Canal of Joseph) is the watercourse that carries the Fayûm supply. It is not an artificial canal, but a naturally formed sinuous channel, resulting from the Nile flood water draining off the lands and following the line of least resistance along the low-lying part of the Nile Valley on the side of the Libyan Desert. [Plate I.] shows where the Bahr Yûsuf touches the desert and forms a line of separation between the fertile land of the Nile Valley with its grateful shade of trees and the barren desert sands under a scorching sun.
As is usual in the case of rivers which periodically overflow their banks, the land is highest alongside the Nile, and slopes away from it to the hills on either side. The high margins of the Nile are known in Egypt as the “Sahel.” As the Bahr Yûsuf has for a great many years been given an artificial connection with the Nile, and been used as a channel to carry flood water to inundate the lands along its course on both sides, it has imitated the Nile in its action on its borders, and raised a “Sahel” of its own on a smaller scale. The cross-section of the Nile Valley is thus roughly represented in the following diagram.
A former artificial connection of the Bahr Yûsuf with the Nile is plainly visible at Derût, 200 kilometres farther up the Nile Valley than Lahûn, the point at which the Bahr Yûsuf turns westward into the Fayûm. The artificial part is easily distinguished from the natural channel, as the former is straight with high spoil banks, resulting from the earth excavated to form the channel, whereas the latter is sinuous without any signs of spoil banks. The length of the channel from Derût to Lahûn, measured along its windings, is 270 kilometres, as against 200 kilometres measured as the crow flies.
A second artificial connection with the Nile farther south was made at Manfalût by the excavation of a channel 30 kilometres in length. The part of this that remains now is called the “Manfalûtîyah” or “old Bahr Yûsuf.” About twenty years ago a large canal, called the Ibrahimîyah, was made for the irrigation of the Khedive Ismail Pasha’s large sugar-cane plantations. It was made to take off from the Nile at Asyût, 30 kilometres above Manfalût and 61 kilometres above Derût. It absorbed part of the Manfalûtîyah Canal from Beni Qora to Derût. At Derût, regulators of a fine description were constructed for the distribution of the water, and a regulator of five openings of three metres’ span, with a lock 8·50 metres wide, was made as the head work of the Bahr Yûsuf, which under the new nomenclature became a branch of the Ibrahimîyah Canal. When the Ibrahimîyah Canal was first opened, it appears from the Mémoires on Public Works, published by Linant de Bellefonds Bey in 1872-73, that its discharge was small compared with its present discharge, and the Fayûm summer irrigation must have been limited. M. Linant states that the bed width of the main Ibrahimîyah was 35 metres, and its reputed depth in summer 1·50 metres; but, in consequence of the inefficient means of dredging, a depth of one metre at most was all that could be obtained at the lowest level of the Nile; and he calculates that the minimum discharge, which theoretically should have reached Derût, was 666,840 cubic metres per 24 hours (273 cubic feet a second), but, in consequence of the inefficient dredging, no more than 369,624 cubic metres per 24 hours (151 cubic feet a second) were delivered. Under present conditions in the worst years the minimum has never fallen below two million cubic metres per 24 hours (818 cubic feet a second) since, at any rate, 1883.