Part 3

But alas! an investigation of the northern borders of the depression showed that the hoped-for traces of a former connection with the Mediterranean Sea do not exist. The depression is entirely shut in from the north, either by great cliffs, or by ground lying so high that the cutting of a canal to the sea is utterly impracticable. Of the 56½ kilometres from the sea-level contour of the depression to the coast along a line running 12° east of north from Moghara Lake (which is the easiest line hitherto found for the cutting of a canal), only 16 kilometres are at less than 50 metres above sea; 31 kilometres lie at altitudes between 50 and 100 metres; and 9½ kilometres are above the 100-metre level. An open channel being thus put out of the question, it was next natural to inquire whether a tunnel, or a channel partly open and partly tunnelled, could be excavated to serve for the conveyance of sea-water into the depression. But even this, though perhaps not impracticable, would be a very costly undertaking. The conveyance of the requisite quantity of water, even at a relatively high velocity (which of course implies a considerable slope), would necessitate a tunnel or tunnels of very large dimensions. To convey 36 million cubic metres a day at a velocity of 5 kilometres per hour would require a sectional area of 300 square metres—_i.e._ if two tunnels were made, each would have to be 14 metres in diameter. The cost of boring and lining such tunnels would certainly be very great, and it is doubtful whether the value of the power generated could justify the capital expenditure involved in the excavation and other works which would be necessary. Until more is known of the nature of the strata through which the aqueduct would have to pass, and as to the length of tunnelling and amount of open cutting which would be required, it is impossible to form even an approximate estimate of the expense which the undertaking would involve. All that can be said at present is that the utilization of the Qattara Depression offers a possible means of obtaining power on a large scale for the drainage of the low-lying lands of the Delta, with at the same time a prospect of improving in some measure the agricultural resources of the Mediterranean Littoral; much further investigation will have to be accomplished before any judgment can be formed as to whether such a project would be an economically sound one.

In formulating any scheme for improving the drainage of the Delta, it is of course important to consider, not only the manner in which the drainage water could be ultimately disposed of, but also the modifications which would have to be made in the existing drains and irrigation canals—modifications which would need to be carried out without serious interruption to existing agriculture. A scheme which otherwise appeared attractive might easily prove to be impracticable by reason of the heavy expenses and inconveniences of the subsidiary works which would be required to make it effective.

[Illustration: THE LIBYAN DESERT Showing Surface-Relief, Contours of Static Underground Water-levels, Distribution of Sand-dunes, and Routes of Principal Exploratory Expeditions in the West and South by Dr. JOHN BALL

THE GEOGRAPHICAL JOURNAL JULY 1927]

4. _The Artesian Water Supplies._

The origin of the artesian water supplies of the Egyptian oases of Baharia, Kharga, Dakhla, and Farafra (Siwa seems hitherto generally to have been left out of consideration) has been a much-discussed question. Some geologists, myself amongst them, have always regarded the water as being derived from rainfall in the western Sudan, flowing underground in permeable beds towards the Mediterranean. Others have held the view that the oasis waters are merely Nile water which has penetrated more or less laterally into the adjoining deserts. The arguments that have been urged in support of the former view are, firstly, the high temperature of the water in many of the oasis wells; and, secondly, that the levels of the springs and wells are often much higher than those of the Nile in the same latitudes. To these arguments it has been justly replied that neither of them is conclusive; the high temperature of the outflowing water merely testifies that it has descended to considerable depths at some part of its underground path not very remote from the point of outflow, but really tells us nothing as to its place of origin; and the high level of the springs in Baharia, for instance, as compared with that of the Nile in the same latitude, might be accounted for by the seepage from the Nile taking place fairly high up in the river’s course. There the question remained until 1925, when I was able to visit and determine the positions and levels of a number of water-sources farther in the interior than any of those on which the “Nile” argument was based. Amongst other level-determinations, I ascertained that the Sheb well is 228 metres above sea-level, and that Merga Lake, lying far to the south-west (in lat. 19° 3′, long. 26° 18′), is at an altitude of no less than 509 metres above the sea. Shortly before my tour in the Sudan, Hassanein Bey had confirmed Rohlfs’ level of 400 metres for the Kufra water-sources, and I had found that of Abu Mungar (north-west of Dakhla Oasis) to be 117 metres. At all these places the water-supplies are derived from underground sources in the same rock—namely, the Nubian sandstone, which covers such vast areas in the Sudan and Egypt.

I had thus four well-determined natural water-levels at the corners of a great quadrilateral whose sides averaged over 500 kilometres in length, and embraced more than 20 square degrees of the Earth’s surface. Now just as in solid geometry the levels of any three points determine the inclination of an oblique plane to the horizontal, so on the Earth any three levels will determine the inclination of a surface to the geoid (of course assuming both geoid and surface to have the same curvature). And on making the calculation, I found that I obtained practically the same degree and direction of inclination for the natural water-surface, whichever three of the four known points I utilized for the calculation. In other words, I found that if I took, say, the levels of Kufra, Abu Mungar, and Sheb, and deduced from them the inclination of the water- surface to the horizontal, I could _calculate_ the level of Merga pretty exactly. Extending the trial, I found that I could do the same with a fairly close approximation for the other wells in the Sheb neighbourhood, and also for wells in the oases of Dakhla and Kharga. The conclusion seemed irresistible that all the wells considered were fed from a continuous sheet of underground water; and it was evident that this water did not come from the Nile, firstly because of the high level of Merga, which is above that of the swamps of the Bahr el Ghazal and other western feeders of the White Nile, and secondly because of the direction of the downward slope of the underground static water-surface, which is from the south-west, instead of from the south as it would have been had the water been derived from the Nile in the Bahr el Ghazal region. The true source of the water must be somewhere more or less nearly on a line drawn south-west from Dakhla, for this is the direction of upward slope of the underground static water-surface; and if such a line be drawn on a map of Africa, it will be found to lead towards the Erdi and Ennedi region, on the borders of the Chad basin. It is in the highlands of Eastern Erdi and Ennedi, therefore, that we must look for the source of the artesian water of the Egyptian oases. What is known of this region from the recent explorations of Colonel Tilho lends good support to our conclusion.[17] It is a bare and rugged sandstone country, where, in spite of a rainfall which is by no means negligible, permanent water-sources are scanty, and where, in consequence, there must be a considerable absorption of moisture by the rocks; and it lies at so high an altitude as to give sufficient “head” for the absorbed water to percolate through the porous sandstones and thus to reach Egypt.

Being convinced that I had at last arrived at the true origin of the artesian water, I next began to entertain the idea of attempting to make a map which would show the contours of the underground water-sheet, and from which, in conjunction with the contour-map of the surface which I had already prepared, I might be able to predict the depth of boring required to tap the underground water at any point in the desert. But a little consideration showed that this idea was an impracticable one, by reason of our ignorance of the underground geological structure over the greater part of the desert. The underground water would naturally pass along permeable sandstone beds, often confined between impermeable clays above and below. And although the general structure of the Libyan Desert is doubtless one of simplicity as compared with that of other parts of Egypt, yet we know, from observations in the oases and in the Owenat region, that the beds are in some places folded and faulted, and that in others they have been uplifted and entirely removed by denudation, with the exposure of large areas of the underlying ancient crystalline rocks. The only parts of the desert for which the boring-depths could safely be predicted would be certain small areas within the oases, where wells have been sunk in sufficient numbers to give us definite information as to the local underground structure; and underground-water maps of these small areas, though they might usefully systematize our knowledge concerning them, would not be of any use for predictions at points situated elsewhere in the deserts.

But while it was thus impracticable to prepare maps showing everywhere the depth at which underground water actually exists, I conceived that it might be quite possible to prepare maps showing _static water- levels_; that is, the levels to which the water would anywhere rise hydrostatically when once it _was_ tapped by borings. For the slope of the static water-surface between known points will be largely independent of the underground structure of the intervening country. Apart from any physical changes which may still be going on in the underground rocks themselves through geological agencies—changes which, if taking place at all, must be so slow as to be negligible except in the course of centuries—the only factors which can affect the slope of the static water-surface, once it has been established, are variations either in the rate of supply of water to the beds, or in the rate at which it is removed from them. As regards variations in the rate of supply, it is obvious that variations in the annual rainfall of the Erdi and Ennedi region must cause very considerable variations from year to year in the amount of water received by the underground beds. But the resistance of friction to the flow of water through the pores of the sandstones is so great, that the annual oscillations of pressure must be rapidly damped out as the distance from the place of influx increases; consequently the levels of the water in the wells of the Egyptian oases (and even, so far as is known, that of the lake at Merga) show little or no annual variation. And with regard to variations in the rate of removal of water from the beds (by outflow to the Nile, or to the sea, or into lakes wherein it evaporates, or by the exploitation of wells and springs for irrigation purposes), these changes, though possibly in some cases they may be progressive, and in restricted localities very sensible, can exercise but little influence from year to year on the general distribution of water-pressure within the underground strata. We may therefore conclude that the gradient of the static water-surface will everywhere have assumed practically a steady state. Unlike the actual water bearing beds themselves, which may be much folded, the static water-surface will in general have simple gentle slopes everywhere in the open desert. In the oases, of course, where numbers of wells yielding large outputs have been bored in proximity to each other, the static water surface will be wrinkled; but over the vast bulk of the desert the contours may be expected to be smooth curves. The diagrammatic section below will, I think, make clear this point about the general non-dependence of the shape of the static water-surface on the geological structure:

[Illustration: _Diagrammatic section showing that the static water-level is largely independent of the underground structure_]

In the diagram, FEKHG represents a water-conveying stratum, folded throughout its course and faulted at HK, but having a general downward slope from F to G. A and B represent points at which the water just rises to the ground-level, either through natural fissures or in artificial borings. The straight line drawn through A and B represents very approximately the static level at any point between A and B, that is, the level to which the water would rise in bores carried down to the water-bearing bed. A boring at C, for instance, would have to go down to E to tap the water, but once the bed was tapped the water would rise in the bore as far as D. At the fault HK, the pressure of the water at K will cause it to rise through the crushed rock at the fault-plane and re-enter the porous stratum at H. If there is a considerable outflow at B, and the fault-plane is a very narrow fissure, we may expect some drop in the line AB over the fault, by reason of the extra frictional absorption of head at this place. But unless the thickness and degree of permeability of the fault-rock are markedly different from those of the sandstone bed itself, the drop of pressure will not greatly disturb the general slope AB. In any case it is apparent that the level of the static water-surface at any place between A and B is capable of being estimated with a far closer degree of approximation than is the level of the water-sheet itself. We may therefore justifiably assume a uniform gradient for the static level between points at which that level is known, disregarding folds in the strata; and though we cannot entirely allow in detail for unknown faults and variations in permeability, it must be borne in mind that the _total_ effect of all the unknown factors between any two known points is already automatically allowed for in our data. It is only the variations from uniformity, due to the unknown distribution of the faults and of the departures from the average permeability, which can affect us; and these variations and departures are probably but small in most of the great unexplored areas of the south-west of Egypt, where the geological structure, from all we know of it, appears generally to be remarkably uniform.

The first requisite for the construction of a map showing the contours of the static water-surface was, of course, a sufficiency of well- determined positions of points where the static water-level of the artesian supply was fairly exactly known. Such points are the springs and wells of the various oases, the surfaces of lakes occupying depressions and presumably fed by underground supplies, and any places on the Nile where the river taps artesian beds.

In regard to the wells and springs, it was obvious that only those known to derive their supplies from artesian sources could be utilized as giving points on the static water-surface. This consideration cut out from the discussion the springs of Owenat and Arkenu, which are known to be fed by local rainfall, and also the small water-sources of Kurkur, Dungul, Nakheil, and Ain Amur, which occur in situations where percolation from occasional local rainfall seems to be the only possible source of supply. And for reasons which will appear presently, none of the wells and springs situated to the north of the Siwa-Qattara-Faiyum chain of depressions could be considered as entering into the problem. With these exceptions, every water-source situated within the area of the Libyan Desert covered by the map, and whose level was known, was utilized; but in the greater oases and in the Wadi Natrun the wells and springs are so numerous and so close together that in these localities it was necessary to select one or two wells as representatives of a group. I had no hesitation in including the wells of Siwa and the Wadi Natrun, because the temperature of some of the wells and springs of Siwa, and the quantity of the output of water at both places, seem to me to afford conclusive evidence of the artesian character of their supply.[18] The wells of the little oasis of Lageita, to the east of the Nile near Qena, were included, for although they are not in the Libyan Desert, they most probably derive their supplies from the same underground flow which feeds the western oases. I have included only one well in which the water does not rise nearly to the ground-level. That well is one which was bored by the British Army during the Great War, at a place called B6, some 40 kilometres to the east of Baharia Oasis. The level of the ground at this point is 112 metres above sea-level, and as the water was stationary in the bore at 78 metres below the ground, the static level here is 34 metres above sea.[19] The well is said to have yielded some 800 gallons per hour without the water-level in the bore being sensibly changed.

The level of the well at Sarra has recently been determined by Prince Kemal el Din; but I have not included it in my data, because he informs me that the water-level fluctuates by 20 metres or more in different years, while the level of the artesian water of the Egyptian oases and Merga is very nearly constant. The inference I draw from the great fluctuations in the water-level at the Sarra well is that it is dependent on percolation from a more or less local rainfall rather than on the same flow which feeds the Egyptian oases.

5. _Permanence of Lakes._

In regard to lakes and salt-marshes, the permanence of those occupying the depressions of Areg, Bahrein, Sittra, and Qattara can only, I think, be adequately explained by regarding them as fed, at least in part, by underground supplies coming into them from the south. The total area of the lakes of Bahrein, Nuemisa, Sittra, and Moghara is nearly 20 square kilometres, and that of the salt-marshes (_sabakha_) is not less than 5000 square kilometres. The depressions are situated in a region which is nearly rainless; in Siwa the mean annual rainfall is only about a quarter of an inch, and that in the depressions farther south, such as Bahrein and Sittra, is doubtless even smaller. The mean daily evaporation from the lake-surfaces cannot well be less than some 4 mm., which would mean a lowering of the lake-levels by evaporation of 1½ metres each year unless there was some inflow to make up for the loss. And though the rate of evaporation from the salt-marshes, area for area, is doubtless very much smaller than that of the lakes, the 250-fold greater extent of the marshes makes it certain that the total quantity of water annually evaporated from them must far exceed that from the lakes.

It appears unlikely that the loss by evaporation in the lakes and marshes can be entirely made up from local rainfall and by seepages from the northern slopes. The rocks forming the surface of the great Miocene plateau, 200 metres high, which separates the depressions from the sea, are chiefly limestones and clays; the beds are nearly horizontal, but such slight dips as exist are believed to be towards the sea. The average annual rainfall on the coastal portion of the plateau is about 6 inches; but it falls off rapidly inland, till it is only about a quarter of an inch near Siwa. The heaviest rainfall on the plateau thus occurs along a strip parallel to the coast, where it is largely drained off towards the sea by the gullies which indent the plateau-edge. Of that which falls on the plateau-surface farther inland comparatively little is absorbed, owing to the generally non-porous nature of the uppermost rocks; after a heavy shower, water lies on the surface in shallow pools for a few days and is soon evaporated. So impervious to water is the limestone in this region, that the Romans excavated chambers in it to form reservoirs, of which many hundreds still exist. Nor can we think that much surface drainage-water from the country to the south ever finds its way into the depressions; for there is an almost complete absence of drainage-lines entering them. At the feet of the northern scarps of the Qattara depression, and along the north-eastern shores of the lakes in the Wadi Natrun, there are, indeed, small springs which show that some of the rain falling on the plateau does actually penetrate the rocks and escape by seepage into the depressions. But the amount of this seepage appears to be insignificant compared with the volume of water which must annually disappear from the lakes and marshes by evaporation. A further consideration bearing on this point is that whatever may have been the agency by which the depression of Siwa was formed, that same agency almost certainly operated to produce the other depressions of the northern chain; and it seems most unlikely that a connection should have been opened up with the underground water-bearing beds in Siwa, and not also in the larger and much deeper depression of Qattara.

It would be a difficult matter to estimate the relative proportion of the water entering the depressions by underground flow from the south, to that contributed by local rainfall and seepage from the northern slopes. But that is not necessary for our immediate purpose. It is sufficient to show that there must be _some_ influx into the depressions from the same source as that which supplies the wells of the greater oases, to establish the existence of that underground water-connection which is all that we need to justify us in regarding the levels of the lakes and salt-marshes as furnishing us with points on the static water- surface; and from the considerations mentioned above it seems to me certain that some influx of underground water really does take place.