We have first to determine the lowest level to which the Lake would have been lowered by the outflow and evaporation at the end of the summer, and before the next rise of the river commenced. The Nile may have flowed at a lower level then (that is, in very early prehistoric times) than now, the summer volume was probably greater then than now, as the unbreached barriers in the upper reaches would have ponded up the water into reservoirs, which, slowly emptying themselves, would have helped to raise the summer level; the flow-out also from the Fayûm Lake would have raised at its exit the level of the summer Nile, at least during the winter months, but not necessarily during the summer months, as it may have expended itself sooner. There are thus three unknown elements in the problem, and nothing to witness to the former minimum levels in the same way that the Nile deposit does to the maximum levels. We are therefore forced to base the calculations on existing levels, and to suppose that the effect on the water surface of the former lower level of the Nile bed was counterbalanced by the increased volume of water flowing in the river bed. At any rate at some period sooner or later the present minimum level of the Nile must have been reached.
The probability is that the exit channel of the lake joined the Nile at or near Wâstah. Up to the end of April the rate of fall of the river exceeds the rate at which evaporation would lower a lake surface, but in May the river falls about 15 centimetres, and not at all on an average in June. Hence, up to the end of April the fall of the river would determine the rate of fall of the lake, but in May and June the fall due to evaporation would rule the rate.
| R.L. | ||||
|---|---|---|---|---|
| The level of the Nile at Wâstahat the end of April may be taken to be | 18·75 | |||
| The distance from Wâstah to Lahûn is | 30 | kilometres. | ||
| „ Lahûn to Hawârah | 15 | „ | ||
| Total | 45 | „ | ||
| Allowing a water surface slopefor the outflow of ¹⁄₂₅₀₀₀, the difference in water surface levelbetween the lake at Hawârah and the Nile at Wâstah would be | 1·80 | |||
| Hence the water surfaceof the lake at the end of April would be at | 20·55 | |||
| Evaporation would still furtherlower the surface in May by | ·25 | |||
| „ „ „ June „ | ·30 | |||
| 0·55 | ||||
| The water surface in the lakewould thus become | 20·00 | |||
| Probably the commencement of theflow into the lake would not take place till a few days after themiddle of July, which may be taken as the time when the lakereaches its lowest level for the year. Evaporation for this periodof July must therefore also be allowed for, say | 0·20 | |||
| The lowest level of thelake would therefore be | 19·80 | |||
The mean surface area of the Lake Fayûm between R.L. 19·80 and 25·00 may be taken as 2000 million square metres.
Evaporation during the ninety days of flood would tend to lower the level 70 centimetres.
The quantity of water required to raise the lake from R.L. 19·80 to 25·00 would therefore be 2,000,000,000 × (5·20 + 0·70) = 11,800 million cubic metres, or a daily average for ninety days of 131,111,111 cubic metres.
At the commencement of these ninety days the inflow would be small, increasing rapidly to the maximum; and again, as the lake level rose and the Nile began to fall in October, the inflow would gradually decrease to nothing by the end of the ninety days. Hence it would probably be necessary to suppose a maximum daily discharge into the lake of about 200 million cubic metres a day for part of the time.
| 1 | 2 | 3 | 4 | 5 | |||||
|---|---|---|---|---|---|---|---|---|---|
| Date. | Number of Years out of 16 in which the Supply given in Col. 3would be available for abstraction. | Balance available after allowing for LowerEgypt. | Corresponding Gauge, Beni Suef. | Corresponding Gauge, Magnûnah Mouth on Nile. | |||||
| million cubic metres per day. | metres. | metres. | |||||||
| Sept. | 1 | 11 | 67 | ⎫ ⎬ ⎭ | Average 112 | 28·08 | 26·64 | ||
| „ | 11 | 11 | 108 | 28·11 | 26·67 | ||||
| „ | 21 | 11 | 162 | 28·14 | 26·70 | ||||
| Oct. | 1 | 13 | 157 | ⎫ ⎬ ⎭ | Average 192 | 28·14 | 26·70 | ||
| „ | 11 | 12 | 174 | 28·15 | 26·71 | ||||
| „ | 21 | 14 | 245 | 28·12 | 26·68 | ||||
| Nov. | 1 | 15 | 372 | 28·05 | 26·61 | ||||
| „ | 11 | 11 | 372 | 27·05 | 25·61 | ||||
| „ | 21 | 11 | 67 | 26·40 | 24·98 | ||||
| Dec. | 1 | 12 | 63 | 25·88 | 24·40 | ||||
Colonel Western, in a note on the Wadi Raiân, shows that for a certain number of years out of sixteen years (1872 to 1887, of which he had the statistics to work with) there is a surplus discharge in the Nile which might be abstracted without in any way interfering with the ordinary irrigation of the Delta. In his calculations he takes 565 million cubic metres in twenty-four hours as the discharge required by the Delta during September and the first half of October. He is, of course, referring to the conditions of the Delta at the present time. He gives the table which appears on the preceding page, to which I have added the corresponding gauge levels at Beni Suef and Magnûnah mouth for reference further on.
The month of August has not been given, as the conclusion was come to that there was no water to spare in August under present conditions.