If the area of the lake were more accurately determined, there are several problems of interest connected with evaporation and the “duty” of water which might be calculated out from the facts known about the fall of this lake, the level of which is recorded daily. The discharge entering the Fayûm throughout the year is known, as also approximately the areas under crop; but while the area of the lake remains so vaguely guessed at, the results of the calculation would be of little value, as the figure representing the lake area is an important factor in the calculations.

But the following figures give a measure of what the daily evaporation is in June and July, at least approximately:—

In 1889 and 1890, when there were exceptionally short summer supplies, and the drainage into Lake Qurûn must have been as little as it ever is, the lake levels were as follow:—

Date.Level of Lake.
R.L.		Fall in 30 Days.Daily Average.
1st June, 1889- 41·63
1st July „- 41·87·24·008
31st July „- 42·11·24·008
1st June, 1890- 42·38
1st July „- 42·66·28·0093
31st July „- 42·89·24·008

The evaporation, then, could not have been less than ·0093 of a metre per diem in June 1890; or less than ·008 per diem in June and July 1889 and July 1890.

Allowing for a slight amount of drainage finding its way into the lake, an estimate of 1 centimetre a day for evaporation during the three hottest months of the summer would seem to be nearly correct.

We have also the following figures:—

R.L.
On 14th March, 1890, the lake level was- 42·00
On 18th September, 1890 „- 43·13
The fall in 188 days was therefore1·13
Giving a daily average of·006

As, for half this period, the flow of water into the lake must have been considerable, the average fall per diem due to evaporation for these six months must have been certainly over 6 millimetres.

Observations on evaporation made in the Abassieh Observatory in Cairo give the following figures as the measure of the average evaporation for each month of the year:—