When the great extent and depth of the Qattara depression were established by Mr. Walpole’s explorations of last year, hopes were immediately entertained that the depression might be made to serve some useful economic purpose. Any idea of its being of use as a Nile reservoir was of course at once cut out, not only by reason of its position and distance from the Nile, but also by its immense size; for even if we could turn the whole Nile flood into it, some twenty years or more would be occupied in filling it to a sufficiently high level,[16] and the loss by evaporation from so large an area would be enormous. As a receptacle for drainage water from the Delta it appeared equally impossible of consideration, because of the great length and depth of the channels which would have to be cut to reach it. A more reasonable prospect seemed to be that of admitting sea-water from the Mediterranean into it by means of a navigable canal from the Arabs Gulf; this would only have to be about 56 kilometres in length to reach the nearest point of the depression; and once the canal was made and the depression filled, ships might sail almost to Siwa. Other advantages which might accrue from the formation of this inland sea were an increased humidity of the climate along the Mediterranean Littoral of Egypt, leading to increased rain-crops in that region, the establishment of a valuable fishery, and a possible increase in the water-supply of the oases by the causing of a slight rise in the static water-level there. Another idea, which rapidly followed the first one, was to utilize the depression as a source of electrical power for driving pumps by which the drainage of the northern parts of the Nile Delta might be improved. At first sight this latter prospect looked a very attractive one. The salt-marsh which covers much of the floor of the depression appeared suggestive of a former sea-connection, and if we could trace out this old connection, the cutting of a canal along it might not, it was thought, be a very expensive matter. Moreover, it was apparent that evaporation from an inland sea or lake of so large an area would keep pace with quite a large influx from the Mediterranean, so that if the influx were restricted to such a quantity as would permit of the lake-surface being maintained at a level considerably below that of the Mediterranean, power could be generated continuously. Suppose, for instance, that we maintained a permanent water-level in the lake of 50 metres below the sea; the estimated area of the lake at this level being 9000 square kilometres and the mean evaporation assumed to be 4 millimetres a day, an influx of no less than 36 million cubic metres of water per day could be passed into the lake without altering its level, and this with a fall of 50 metres would suffice theoretically for the continuous generation of over 270,000 horse-power. Of course the lake would gradually get more and more saline, but the power might continue to be maintained for very many years before the lake became as rich in salt as the Dead Sea.

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.

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:

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.

I have also thought it justifiable to include the Birket el Qarun in my collection of static water-level data, because although that lake was probably first formed by an overflow of the Nile into the Faiyum, and is even now being fed by Nile water through the Faiyum drains at the rate of some 350 million tons a year, there is a certain amount of evidence suggesting that it has some underground water-connection with the Qattara depression. That evidence, to which attention was first drawn by Professor Schweinfurth,[20] consists in the relatively low salinity (1·3 per cent.) of the lake, notwithstanding the long period through which it has been subject to evaporation and the fact of its having shrunk to dimensions very much smaller than it possessed in ancient times. Unless there has been a large underground efflux of salt water from the lake, it appears impossible to account for its present degree of freshness. In Professor Schweinfurth’s day, of course, the existence of the Qattara depression was unknown, and it was puzzling to suggest where the salt water had gone to.[21] An underground leakage from the Birket el Qarun into the Qattara depression is quite conceivable, for although the two places are separated by some 200 kilometres, there is a very considerable fall between them. Thus the salt in the marshes of the Qattara depression may possibly have come in part from the Birket el Qarun. The present rate of discharge of the Faiyum drains into the lake is, however, just sufficient to make up for an average daily evaporation from the lake-surface of a little over 4 mm., which is about the rate we might expect; and although the level of the lake-surface has fallen some 5 metres since observations of it were first made in 1886, it is now nearly stationary; hence it does not appear likely that there is much underground leakage at present. If the former leakage from the lake took place by lateral flow into porous strata near its surface, of course the leakage may have been arrested by the lowering of the lake-level uncovering the porous beds into which it took place; but I think a more likely explanation is that the leakage occurred at or near the bed of the lake, and has gradually been reduced by the continued deposition of Nile mud on the lake-bottom, and by the diminution of head due to the fall in the water-level.

In regard to the tapping of the artesian waters by the Nile, there is only one locality in which this is known to take place; but the quantity of underground water which is there withdrawn by the river is probably very considerable. When I was surveying the Nile Valley between Aswan and Korosko in December 1898, I observed that in the vicinity of the temple of Dakka (about 105 kilometres south of Aswan) the lands on the west bank of the river were being irrigated with warm water, drawn by “sakias” (water-raising machines) from pits sunk in the alluvial flat which extends between the river and the edge of the sandstone desert. The length of the tract over which the warm water was being withdrawn for irrigation was found to be about 16 kilometres, stretching from 2 kilometres north of Dakka temple southwards to the temple of Maharraga; and the width of the alluvial tract at Dakka, where it is widest, was about 1300 metres. Some of the water-pits were more than a kilometre from the river. Levelling from the Nile (the surface of which was then about 99 metres above sea) across the cultivation to one of the sakia-pits 750 metres west of the river, I found the level of the ground at the sakia-pit to be 7·9 metres above that of the Nile, and the water-surface in the pit to be 8·4 metres below the ground-level; there was 1·2 metres depth of water in the pit. The temperature of the water in the pit I found to be 83° F., while that of the Nile was 60° F. and that of the air was 67° F. The headman of Dakka told me that the exploitation of this warm underground water had begun about 1887; they dig out the sandy mud, and then see the water oozing rapidly into the pit out of the sandstone below. On crossing to the east bank of the river, I found that there also the warm water was being similarly raised for irrigation, though to a smaller extent, because on that side the sandstone desert approaches more closely to the river and there is much less cultivable land. The exploitation of the water on the east side of the Nile extended only over a distance of about 5 kilometres along the bank, with a maximum width of alluvial plain of 600 metres, just at the place where the great Wadi Alagi debouches into the Nile Valley. As the sandstone bed from which the warm water issues is less than 2 metres below the level of the water-surface of the Nile, and the water occurs on both sides of the river, it is certain that the water-bearing bed is cut through by the Nile channel itself; the seepage into the river along the stretch of 16 kilometres must therefore be very considerable. It seems evident that the water is not derived from the bed of the Wadi Alagi, great drainage-channel though that wadi is; for we could not then account for the temperature of the water, nor for its appearing to a larger extent on the west bank than on the east, with the river in between. Moreover, the water appeared to be much more free from salts than we should expect it to be if it were merely drainage from the Wadi Alagi. It strongly resembles, in fact, both in temperature and character, the artesian water of the greater oases, and there can hardly be the smallest doubt that at Dakka the Nile is not only continually abstracting artesian water from the same underground water-sheet that feeds the oases, but is abstracting it in far larger quantities than those yielded by all the oasis wells and springs put together.[22] It is certainly remarkable that the place where considerable supplies of warm underground water enter the Nile should coincide with the embouchure of what is perhaps the greatest drainage channel of the Eastern Desert of Egypt; but I think it is likely that the explanation of the coincidence may be a tectonic one; the water-bearing beds may have been brought up by a local fold in the strata, and the same fold may in some way have conditioned the formation of the primitive drainage-line which was ultimately to become the Wadi Alagi.

Having now indicated briefly the grounds for their acceptance, I give below a table showing the various points which I have adopted as furnishing data for constructing the contours of the static water-surface underlying the Libyan Desert, together with the altitudes of the points above or below sea, and the sources of these level-data. The levels are doubtless in some cases slightly inaccurate; but a few metres of error are immaterial to the object in view, and it is believed that even those levels which rest on barometric determinations are sufficiently accurate for our purpose.

List of Adopted Points on the Static Water-Surface
Place.	Level (metres).	Determined by.
Wadi Natrun, surface of lakes	− 23	Ball, Trigonometric levelling, 1914.
Birket el Qarun, surface of lake	− 45	Survey of Egypt, 1926. Based on spirit levelling from Alexandria.
Moghara, surface of lake	− 23	Walpole, Trigonometric levelling, 1924.
Qattara Depression, various points on salt-marsh, the lowest being	− 80[23]	„ „ „
Sittra, surface of lake	− 16	„ „ „
Areg, surface of lake	− 25	„ „ „
Siwa	− 17	„ „ „
Jaghbub	+ 32	Hassanein, Barometric observations, 1923.
Jalo	+ 61	„ „ „
Bir Butaffal	+ 98	„ „ „
El Harrash	+ 310	„ „ „
Awadel (Kufra Oasis)	+ 434	„ „ „
Ezeila (Kufra Oasis)	+ 389	„ „ „
Bawitti (Baharia Oasis)	+ 129	Ball, 1917, and Walpole, 1924. Trigonometric levelling.
El Hez (Baharia Oasis)	+ 134	„ „ „
B6 Well (water surface in)	+ 34	Walpole, Trigonometric levelling, 1924, and military records of depth, 1916.
Lageita (Eastern Desert)	+ 121	Murray, Trigonometric levelling, 1921.
Farafra	+ 90	Ball, Barometric observations, 1924.
Abu Mungar	+ 117	„ „ „
Mut (Dakhla Oasis)	+ 119	„ „ „
Kharga (average of numerous wells)	+ 70	Beadnell, Spirit levelling, 1909.
Ain Ismail (Kharga Oasis)	+ 67	Ball, Barometric observations, 1925.
Bir Murr	+ 156	„ „ „
Bir Abu Hussein	+ 182	„ „ „
Bir Kassaba	+ 176	„ „ „
Sheb Well	+ 228	„ „ „
Safsaf	+ 230	„ „ „
Bir Terfawi	+ 244	„ „ „
Merga, surface of lake	+ 509	„ „ „
Dakka, water-surface in wells	+ 99	Ball, Spirit levelling from the Nile, 1898.

To prepare a map showing the contours of the static water-surface, I took a graticuled sheet and plotted the above-scheduled points on it in their ascertained geographical positions, affixing the adopted level to each. To get points on the various contours at vertical intervals of 100 metres, I joined each pair of points on the map by a pencil line, and then, by interpolation from the terminal levels, found the points on this line where the various contours crossed it, on the assumption of a uniform gradient between the terminal points. Many of the lines thus drawn of course crossed each other, so that interpolation of the static level at the point of their intersection gave two values for the same place. But their agreement was wonderfully close, considering the fewness and the scattered nature of the datum-points, and this went a long way to encourage me in the belief that the hypothesis on which I had been working, namely, that of an underground water-connection between all the points included in my list, was correct. I found that the contours of the static water-surface could be approximately represented by a series of smooth curves, as shown (on a reduced scale) in the outline map below.

Apart from the general smoothness of the curves, especially in the south-west, where it may in part be due to the scantiness of control-points, the most striking thing on this outline map is the north-eastward projection of the 100-metre static contour, where it runs out so as to include Baharia Oasis. The reasons for this projection are obviously the efflux of water, on the one hand north-westwards into the great Qattara depression, and on the other hand into the Nile at Dakka. The indentation of the 400-metre contour near Kufra is likewise explained by the withdrawal of water from the wells of that oasis. The general parallelism of the curves in the south-western part of the map, showing a gradual rise in a south-westerly direction towards the Erdi and Ennedi country (which, as I have already stated, is the most probable source of the underground water) is strikingly apparent. I have not been able to extend the contours far to the east and west of Merga, for lack of control-points. It is much to be hoped that some future traveller will determine the water-levels at Selima and Lagia, which would enable the static contours to be extended into the region between Merga and Dongola; provided, of course, that an examination of the water-sources at these places proves their supplies to be artesian.

The most effective way of testing any working hypothesis in natural science being the prediction of hitherto unobserved facts, I venture to forecast that if, as is most likely, the water-sources of Selima and Lagia are artesian, their levels when eventually determined will not be found to differ very much from 270 and 390 metres above sea respectively. These are the approximate levels deduced by prolonging the static water-contours of my maps into the localities of these wells, assuming the contours to continue as smooth curves.

Another interesting prognostication which I think may fairly be deduced from the map is that if ever the well at Sarra is considerably deepened, a much more abundant water-supply will probably be obtainable. The ground-level at Sarra, according to observations made by Prince Kemal el Din, is 461 metres above sea, and the water-level in the well varies in different years from about 390 to about 410 metres above sea. But an examination of the static contours of the map shows that the static level of the true artesian water in the neighbourhood of the well is probably somewhere about 500 metres above sea, though an exact estimation of the static level at that spot is not possible because of the lack of data farther west. As already remarked on p. 110, I think the present supply at Sarra is derived from more or less local rainfall, conveyed by higher-lying permeable strata than those which convey the main artesian supplies of Kufra and the Egyptian oases; by deepening the well considerably, lower-lying beds might be reached so as to tap the main supply, and the water might even be expected to overflow at the surface.

Interesting as are the static water-level contours in themselves, they become vastly more so when superposed on the ground-contours, as is done in the larger map (G.J., July, following p. 96). From the two sets of contours on that map we can estimate at any point the approximate depth of the static water-level below the ground; and this information affords new light on some of the most interesting, but hitherto the most difficult, of the problems connected with the Libyan Desert.

6. Can the Present Water-supplies of the Mediterranean Littoral be supplemented by Artesian Borings?

The present water-supplies of the Egyptian portion of the Mediterranean littoral, derived mainly from shallow wells dependent on the local rainfall, are neither very abundant nor of very good quality. At one or two of the most important settlements along the coast, such as Matruh and Sollum, attempts have been made to improve the supplies by sinking wells to a considerable depth in situations where it appeared likely that the drainage from the inland plateau would be specially abundant. But these have met with little success; the yield has been found to be very moderate in quantity, and of poor quality owing to dissolved salts. The question has often been raised as to whether very deep borings, carried down right through the Tertiary strata and into the Nubian sandstone, might result in the procuring of an artesian supply of the same excellent water as occurs in the oases. Hitherto it has not been possible to give a definite answer to this question, and geologists have been reluctant to recommend deep borings, which would entail great expense, without feeling some assurance that they would be successful. The depth to the Nubian sandstone is unknown, but is certainly great; and if borings were carried down into the sandstone, it was not known whether the water would rise to anything like the ground-level. From our new map, however, we obtain a very decisive verdict on the matter. The Nubian sandstone, even if reached, would not be found to be charged with artesian water under anything like the pressure that it is in the oases; leakage into the Qattara and other depressions will have depleted the beds of much of the water coming from the south-west, and will have lowered the static head to such an extent, that the water left in the sandstone will have too little pressure to rise far into the bores. Any idea of sinking deep artesian wells along the coast to tap the Nubian sandstone can consequently be definitely abandoned. We are driven to the conclusion that in any attempt to improve the local water-supplies of the littoral settlements, we can count only on local rainfall for our primary source, and we must do our best to collect the run-off before it has had an opportunity to absorb much salt. The Romans evidently understood this when they excavated the large rock-cisterns on the plateau, of which there are hundreds. We cannot do better than imitate their example, and arrange for the collection and storage of a sufficient volume of rainwater as it runs from the rocky surface of the plateau. We may do this by restoring to use the old reservoirs; or we might possibly achieve our end by damming some of the rocky gullies which bring down the run-off from the plateau to the plain. Now that the artesian idea is shown to be out of the question, there is justification for a thorough investigation as to the best method of collecting and conserving the local rainfall.

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.

8. “Lost” Oases—“Zerzura.”

Of all the questions asked by intending travellers in the Libyan Desert, none is more frequent than that as to the most likely whereabouts of undiscovered oases, and especially as to the possibility of finding the mysterious “Zerzura, or Oasis of the Blacks.” Hitherto the only aid which I have been able to render to such inquirers has been to acquaint them with the various statements which have been made by Arabs at different times as to the situation of Zerzura, with the routes which have been followed by others (including myself) who have sought in vain for it, and with the indications of old tracks which have been encountered by these previous travellers. So contradictory have been the various Arab statements, and so numerous the vain attempts to find the place, that I have at times felt almost convinced that “Zerzura” is a myth. But Owenat and Merga were little more than traditions until a year or two ago, and I think there is a sufficient possibility of the existence of undiscovered springs or oases to encourage a further look-out being kept for them, more especially as a consideration of the general surface-contours and static water-contours which are now available may furnish a new aid in the matter by narrowing down the field of search.

As regards Arab traditions concerning Zerzura, the earliest account of them which I have been able to trace is that of Sir Gardner Wilkinson in his ‘Topography of Thebes and General View of Egypt,’ published in 1835, p. 359. Wilkinson’s book is now rather scarce, and his statement concerning Zerzura is so short that I quote it in full: