If, however, the Wadi Raiân were to be made a reservoir, the reclamation of the lands along the south and south-west borders of the Fayûm would be made comparatively easy. What this area amounts to is rather uncertain.

In Chapter III. the conclusion was arrived at, that the Wadi Raiân depression had never hitherto acted as a regulator to control the Nile floods and supplement the Low Nile, and that its past history shows no record of useful work, so far as the irrigation of Egypt is concerned. But this fact does not affect the question of its possible uses in the future, for which its physical features and geographical position may fit it. It is a depression, separated by a short width of hill from the Nile Valley, and if filled with water up to R.L. 24·00, would become a lake, having a surface area of about 600 million square metres, and a greatest depth of 64 metres. There is no doubt that the communication could be made; the only question is, would it be worth the expense, and could not better results be obtained for the same expenditure by the adoption of other rival projects. This question is now being considered by the Ministry of Public Works.

There are four uses which the Wadi Raiân depression might be made to serve, if a communication with the Nile Valley were established.

It might be used,

(1)As a reservoir of control for the Nile floods.
(2)As a reservoir of storage to supplement the Low Nile.
(3)As an area to be brought under cultivation.
(4)As a receptacle for the drainage of the Nile Valley during the flood season.

(1) It would not make an efficient regulator for the control of the Nile floods, unless it were to be expressly reserved for this object, and its level kept low until all fear of the necessity of relief arising had passed. If it were considered necessary to provide for the relief of the Nile to the extent of 100 million cubic metres per day for 30 days, the lake, having an area (at R.L. 24·00) of 600 million square metres, should be kept at such a level as would allow of its receiving the 3000 million cubic metres without checking the inflow in consequence of its surface level becoming too high. An addition of 3000 million cubic metres to the reservoir would raise it about 4¾ metres, allowing for evaporation for 30 days. This is about the extreme duty the Wadi could perform as a reservoir of control, if it were expressly reserved as such; but, if it is to serve this object alone, the expenditure, which would be incurred in fitting it to do so, would certainly be considered out of proportion to the benefits to be obtained. An attempt to combine the two duties of controller of Nile floods, and feeder to Low Niles would probably result in failure, as the necessity of keeping the reservoir level low to fit it to act as an escape-valve during September might make it impossible to raise the level afterwards to a sufficient height to render it an efficient feeder to the Low Nile.

(2) It would, however, make an efficient feeder for supplementing the Low Nile, if the control of the flood Nile were neglected. If connected with the Nile about Beni Suef, and also with the Bahr Yûsuf (which could continue to flow into it after the Nile ceased to do so), the reservoir could easily be filled to R.L. + 26·00. Assuming R.L. 21·00 as the level to which the water in the reservoir would fall in summer, and allowing one metre for evaporation for six months, the volume required to fill the lake from R.L. 21 to 26 would be 600,000,000 square metres × (5 + 1) = 3600 million cubic metres. From November to January, say 90 days, the Bahr Yûsuf could supply an average of 12 million cubic metres a day at least, or 1080 million cubic metres, leaving 2520 million cubic metres for the direct Nile feeder during the 90 days of flood, or an average of 28 million cubic metres a day.

The reservoir would return to the Nile Valley 600,000,000 × (5 - 1) = 2400 million cubic metres. Allowing for a loss of 10 per cent. in the distributing canals outside the reservoir, we get a supply of (2400 - 240 =) 2160 million cubic metres available for irrigation.

Now 60 days is given as the critical period in Lower Egypt, when the Nile supply is generally insufficient. Subtracting 160 million cubic metres for the Fayûm, the 2000 million cubic metres remaining would therefore give an average discharge of 33 million cubic metres a day to supplement the Low Nile, and, if distributed in increasing quantities in proportion as the Nile fell, it might be so arranged as to prevent the Nile minimum discharge ever falling below 50 million cubic metres a day in the very lowest years of summer Nile.

But calculating with a period of 100 days, which is the length of the critical period for Upper Egypt, we obtain a mean discharge of 20 million cubic metres a day, which might be so distributed as to prevent the minimum Nile falling at any rate below 45 million cubic metres a day, as for instance below:—