| Month. | Minimum Nile Discharge without Reservoir. Monthly Average. | Supplied by Reservoir. | Total Increased Discharge. | ||
|---|---|---|---|---|---|
| March | 40,000,000 | 5,000,000 | 45,000,000 | ||
| April | 35,000,000 | 10,000,000 | 45,000,000 | ||
| May | 25,000,000 | 20,000,000 | 45,000,000 | ||
| June | 20,000,000 | 25,000,000 | 45,000,000 | ||
| 10 days of July | 30,000,000 | 15,000,000 | 45,000,000 |
This disposes of 1975 millions, whereas 2000 millions was the quantity calculated as being available after loss by evaporation in the lake, and by absorption and evaporation in the distributing canals outside the lake. I think, therefore, I have not overstated the capabilities of the reservoir as a feeder to supplement the Low Nile.
I have said nothing about the first filling of the lake, which is a question of no small difficulty. To fill it to R.L. 21·00, its low summer level when once in working order, would require a volume of 15,000 million cubic metres plus the quantity required to meet loss by evaporation during the time of filling.
(3) The idea that the depression might be converted into a cultivated basin is, I think, not likely to get beyond the stage of suggestion, as, with the object only of extending cultivation, the expense of connecting the Wadi Raiân with the Nile will not be incurred, since there are so many other projects of reclamation, which would need less expenditure and give a better return.
(4) The last use to which the Wadi Raiân might be put, and which has lately been suggested, is to adapt it for the reception of the drainage waters of the Nile Valley, after the basin area of at least Middle Egypt has been converted into Sêfi (summer) irrigation by means of the increased supply provided by the assumed existence of reservoirs in the Upper Nile Valley and a regulating dam at Asyût.
When all these basin lands are converted into tracts under perennial irrigation, there will be a great difficulty in the disposal of their drainage water during the time of high flood, and the Wadi Raiân affords a possible means of solving this problem. Even though the drainage might gain on evaporation and the Wadi Raiân become eventually full, its water surface could be annually so far lowered by allowing a flow out into the Nile during the summer months, as to prepare the basin for the reception of all the drainage it would be called upon to receive during the next flood season.
It would further be possible to combine the uses Nos. 2 and 4, and make the depression serve both as a receptacle of the drainage during the floods and a reservoir to supplement the Low Nile during summer. But it might be objected that the admixture of drainage with the reservoir waters, returned to the Nile in summer, might render the river water unfit for irrigation. Supposing the drainage discharge, which must be received into the reservoir, amounts to 15 million cubic metres a day for 80 days (probably a high estimate), the total volume of drainage water would amount to 1200 million cubic metres, or one-third of the quantity of water (3600 million cubic metres) required to fill the lake from R.L. 21 to 26, or half of the 2400 million cubic metres returned to the Nile Valley. This would be further diluted by the summer discharge of the Nile itself, to which it would be added.
If the reservoir were filled to R.L. 25 during the flood months by the drainage and flood waters together, the remaining metre could be added by a canal discharging 7 million cubic metres a day for 100 days in winter, and fed from the Ibrahimîyah Canal, or a new branch of it, which would replace the Bahr Yûsuf, when the latter was converted into the main drainage line consequent on the basin lands being brought under perennial irrigation.
If then the drainage water should not be found salt enough to seriously affect the quality of the reservoir water, the Wadi Raiân might be made to serve both the purposes stated.
It has been assumed in previous notes on the subject, that such a reservoir alongside the Fayûm would be capable of giving that province its summer supply, but there would be a difficulty in the way of doing this. Under present arrangements the water-level at the end of the Bahr Yûsuf at Medineh is maintained throughout the summer at R.L. 21·70. If the level were to be lowered, lands now commanded by the water would cease to be so. The Wadi Raiân reservoir, whose level has been assumed to fall to R.L. 21·00, while a length of canal of at least 40 kilometres would be required to convey the water to Medineh, could not, during part of the summer (after its water surface had fallen below R.L. 23·50) deliver water at the level at present maintained.