Part 4

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 _Determined by._ (metres)._

Wadi Natrun, surface of − 23 Ball, Trigonometric lakes levelling, 1914.

Birket el Qarun, surface of − 45 Survey of Egypt, 1926. lake Based on spirit levelling from Alexandria.

Moghara, surface of lake − 23 Walpole, Trigonometric levelling, 1924.

Qattara Depression, various − 80[23] „ „ „ points on salt-marsh, the lowest being

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 + 70 Beadnell, Spirit levelling, wells) 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 + 99 Ball, Spirit levelling from wells 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.

[Illustration: _Outline map of the Libyan Desert, showing the points where the static water-levels are known, and the deduced contours of the underground static water-surface, on the hypothesis of a continuous hydraulic connection between the points_]

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.

[Illustration: _Pottery Hill, possible site of “Zerzura,” from the south-west_]

[Illustration: _Bir Kassaba, a watering-place on the Darb el Arba’in_]

[Illustration: _The waterless stretch of the Darb el Arba’in between Bir Murr and Kharga_]

[Illustration: _Bir Sheb, a well on the Darb el Ar ba’in_]

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.”_