7. Are the Artesian Water Supplies of the Oases diminishing?

The native cultivators in certain parts of Kharga and Dakhla have for some years past found that their wells no longer discharge at so high a level as formerly, and in consequence some of their land has gone out of cultivation. From this fact, and from the evidence of the former greater prosperity of the oases which is afforded by the various ruins of temples, forts, and villages, by the large areas of formerly cultivated lands, and by numerous sanded-up wells, it has sometimes been inferred that the total yield of the oasis-wells is now but a fraction of what it formerly was. But, as Mr. Beadnell has pointed out,[24] the remains of the past which exist in the oases belong to successive generations, so that we cannot fairly draw such a conclusion from them; and the reduction or cessation of the discharge of certain wells does not necessarily imply any falling-off in the total water-output of the oases. Mr. Beadnell’s experiments on flowing wells in Kharga have clearly shown how the opening of a new well at a slightly lower level will affect the discharge of an old well, even one at a distance of a kilometre or more, by lowering the static head in its vicinity.[25] And since a large number of wells have been bored in recent years both in Kharga and Dakhla, it is most likely that the discharge from these wells has caused a falling-off in the yield of older ones situated at slightly higher levels. In this connection it will be well to note that it is not the mere existence of a new well that affects the static head, but the discharge from it. If the new well is securely closed so as to discharge nothing, it has then no effect on the static head and therefore none on the neighbouring wells. But for this to hold, it is important that the well which is closed should be closed throughout its entire depth; it is not sufficient merely to close its mouth so that it does not discharge any water on to the ground, for there may still be rapid leakage somewhere in the bore (unless effectively cased) into porous unsaturated underground strata. Owing to the rapid rate at which iron pipes are corroded in the wells of the oases, leakage of this kind is more likely to happen with an abandoned modern well cased with iron piping and plugged near the top, than with old wells which were filled up with clay and sand. These factors, the mutual interference of wells and the importance of preventing underground leakage, especially from abandoned wells, being now thoroughly understood, steps are being taken towards ensuring that future sites for new wells shall be judiciously selected, and that leakage and waste from abandoned wells shall be as far as possible arrested.

The method of carrying out measurements of well-discharges in the oases is so inaccurate, and the records of the past output so defective, that it is not possible to gather from them whether the total yield of artesian water is at present diminishing or not. Mr. Beadnell considers it likely, however, that the general average water-pressure in the oases has been very much reduced within the historical period, owing to the long-continued exploitation of the artesian supplies.[26] The general study of the Libyan Desert which I have made in the last few years suggests that a gradual reduction in the static water-pressure in the oases may possibly have been brought about by other agencies than the exploitation of the water in the oases themselves.

The first and most important of these other agencies is the withdrawal of artesian water by the Nile in the neighbourhood of Dakka. As mentioned on p. 113, it is practically certain that sandstone beds carrying artesian water are cut through by the Nile along a distance of several kilometres in that locality, and the influx of artesian water into the Nile may far transcend in quantity that removed by the wells and springs of the oases. The Nile has probably deepened its channel in this region by a few metres within historical times, and thus cut through a greater section of the water-bearing beds. An increase in the sectional area of the beds cut through would naturally mean an increase in the quantity of artesian water passing into the Nile, and hence a lowering of the static water-surface extending perhaps to the oases.

The second possible other cause operating to diminish the static head of the artesian water of the oases is the progressive desiccation of a lake which may once have occupied a part of the Qattara depression. As mentioned on p. 110, the floor of this great depression, large areas of which are 80 metres and more below sea-level, is partly covered by a salt-marsh, which is so soft and watery that it can only be crossed at a few places. The hundreds of great water-cisterns cut in the limestones of the plateau to the north of the depression—cisterns most of which are now dry—as well as other ruins along the coast indicative of a considerable former population, seem to show that the rainfall in the littoral region has within the historical period been greater than it is at the present day. When the rainfall in the coastal region was greater, there must have been more drainage into the Qattara depression, and what is now salt-marsh was thus possibly once a lake of some depth. Assuming, as I think is likely, that an underground water-connection exists between the marsh occupying the bottom of the depression and the artesian water of the oases, it is obvious that any progressive lowering of the lake-level consequent on the change of climate must have lowered the static water-surface in the country extending southwards towards the oases. In the oases themselves the lowering of the static surface would of course be much less than at the lake; but it is quite conceivable that even in the oases the lowering may have amounted to the few metres which would cause some of the older and higher-lying wells to cease to flow.

As both the deepening of the Nile channel in Lower Nubia and the desiccation of the Qattara depression are probably still slowly progressive, it is possible that these causes may to some extent account for any slow lowering of the static water-surface in the oases which may be still going on.