The year 1888 was one of very low Nile flood, but even in that year, from about 20th July to 15th November, the Nile level at Magnûnah mouth was above R.L. 23·00, and reached R.L. 26·00 at the top of the flood.
The quantity of water required to fill the lake from R.L. 19·50 to 22·50, and to allow for its evaporation for six months, is calculated as follows:—
| Million cubic metres. | |||
|---|---|---|---|
| Volume required to raise lake 3 metres =1600 million square metres × 3 | 4800 | ||
| Volume required to make good 6 months’evaporation = 1600 million square metres × 1·30 | 2080 | ||
| Volume required for irrigation of 25,000feddans reclaimed, for 6 months | 100 | ||
| Total volume required | 7580 | ||
The greater part of this would be poured in during the three months of flood, say 5000 million cubic metres in 100 days, or an average of 50 million cubic metres a day.
There now remains to be calculated the discharge that this reservoir would give back to the Nile during the low water months.
| Million cubic metres. | |||
|---|---|---|---|
| The content of the stratum of waterbetween R.L. 22·50 and 19·50 is 1600 million square metres × 3 | 4800 | ||
| Of this there would be lost byevaporation during the 6 months 1600 million square metres × 1 | 1600 | ||
| There would thus remain availablefor purposes of irrigation | 3200 | ||
| The reclaimed land round Arsinoë (about25,000 feddans) would require about 50 million cubic metres for itsirrigation during the 6 months of winter and summer | 50 | ||
| The balance available for theNile Valley would be | 3150 | ||
If we suppose this water husbanded and made use of only during the 100 days of summer, when its want is most felt, the lake would give an average daily discharge to the Nile of 31½ million cubic metres, that is, the low Nile discharges would be doubled and raised from 30 millions to 60 millions. But by a careful distribution of this stored-up water between the months of April, May, and June, and ten days of July, it would have been possible to keep up the discharges constantly to seventy millions, as shown in the table below:—
| Month. | Average Nile Discharge, without reservoir. | Supplied by reservoir. | Total Increased Discharge. | ||
|---|---|---|---|---|---|
| March | 70,000,000 | Nil | 70,000,000 | ||
| April | 45,000,000 | 25,000,000 | 70,000,000 | ||
| May | 34,000,000 | 36,000,000 | 70,000,000 | ||
| June | 34,000,000 | 36,000,000 | 70,000,000 | ||
| July 1 to 8 | 45,000,000 | 25,000,000 | 70,000,000 |
The supply from the reservoir will thus be
(36 million × 61 days) + (25 million × 38 days) = 3146 million cubic metres.