The rate of flow of water through an underground sandstone depends upon a number of conditions, the most important being the size of the pores or spaces between the component grains, the porosity or water-holding capacity of the sandstone, the temperature of the water, and the pressure acting on it. The yield of a well will depend, of course, not only on all these factors, but also on the diameter of the bore, its depth into the water-stratum, the size and number of fissures passed through, and, last and most important of all, on the absolute height of its outlet. Large pores, high average porosity, and high temperatures make for strong flows, though in the absence of pressure greater than that due to a column of water equal in height to the distance between the water-stratum and the outlet of the well, they are in themselves of no avail in the production of an artesian flow. Moreover, although some of the above conditions may be known beforehand, the resistance to flow of the strata immediately surrounding a bore can never be more than approximately conjectured, as the size and mode of arrangement of the individual grains of any sedimentary rock must always vary, both horizontally and vertically, to a very great extent, and on these factors depends in very large measure the capacity of the strata to transmit water.

Data are as yet far too insufficient to warrant an attempt to calculate the supply which can safely be drawn from a given area without unduly reducing the pressure, lowering the average static head, and endangering the continuance of the artesian supply. In some parts of the oasis there are bores many hundreds of years old still pouring forth their hundreds of gallons a minute; such wells are probably situated in exceptionally favourable localities, and are very possibly fed to a great extent by fissures. At the same time it must not be forgotten that there are throughout the oasis scores of wells which have ceased to run, either through local exhaustion of the water-bearing strata, or through failure to keep the bore-channels open; possibly through a combination of both circumstances. In some cases time seems to have remedied matters, as it is not uncommon to meet with instances where new bores, sunk in the immediate neighbourhood of long extinct wells, have produced strong discharges of considerable volume.

As I have already stated, the rate of flow of water is largely influenced by both the size of the pores and the porosity of the rock, the capacity to transmit water being very much greater for large than for small pores, and for high than for low porosity. It must, however, be pointed out that large pores and high porosity do not necessarily go together, and that small pores in a rock frequently accompany high holding capacity. For instance, a fine-grained sample of Nubian Sandstone will absorb from 25 to 28 per cent. of water, a medium-grained sample 20 per cent., while a very coarse sample may take up as little as 15 per cent. The pores and transmitting capacity of the coarse-grained variety will, however, be very much greater than in the case of either of the others.

In order to arrive at some sort of idea as to the holding capacity of the artesian-water strata of the oasis I made an examination of between sixty and seventy samples of sand brought up from varying depths from a few selected bores in the oasis. As, however, owing to the methods of drilling employed, only powdered samples were available, it was necessary in the first instance to ascertain the relative porosity of sandstone in its ordinary state and broken up into the form of sand. For this purpose I collected eight specimens of the Surface-water Sandstone from various points in Northern Kharga, and subjected them to absorption tests, both in the whole and in the powdered states. In six out of the eight examples the absorption was more when powdered than when whole, the average for the eight rock samples being 22·44 per cent.; for the same when powdered, 23·55 per cent. There is, moreover, no reason to suspect that in ordinary lithological characters the Surface-water Sandstone differs in any important respect from the Artesian-water Sandstone, so that if we estimate the porosity of the latter from powdered samples, we shall obtain a figure only about 5 per cent. too high.

The average porosity of sixty-four samples, collected from Bores 14, 16, 18, 31, 39, and 44, at various depths in the water-bearing strata, was found to be 19·45 per cent. If we confine our attention to a single bore, and examine a representative sample from every stratum of the water-bearing sandstone, and, in deducing the porosity, take into account the thickness of each individual bed, we shall obtain a still more reliable figure. This was done in the case of Bore No. 18, which passed through 122 metres of water-bearing strata, thirty-one samples being collected and subjected to examination. The absorptions obtained varied from 15·3 to 25·5 per cent., the average porosity of the whole column being calculated as 19·6 per cent. I believe this figure may be accepted without misgiving as a satisfactory working value for the porosity of powdered Nubian Sandstone, that of the solid rock being taken as 5 per cent. lower, or, say, 18·5 per cent.

The Artesian-water Sandstone has been proved to reach a thickness of 122 metres, and probably its total thickness is considerably more. Assuming, however, a vertical extent of only 122 metres, the water-bearing beds under 1 square kilometre would, if fully saturated, hold 4,965,000,000 gallons, which is the equivalent of the water which would be discharged in ninety-four years by a well flowing at the rate of 100 gallons a minute. When we consider that the area of the floor of the depression is some thousands of square kilometres, and that the water-sandstones, except in the immediate neighbourhood of the existing wells, are probably fully saturated, we realize the vast amount of water which is stored under the depression alone, irrespective of the still greater quantities which underlie the surrounding plateaux. The movement of the water through the sandstones, except along fissures and through particularly porous beds, is, however, very slow, so that the amount of water which can economically be made available at the surface is more or less limited, self-flowing wells being only obtainable over that portion of the area lying below the general static head.

CHAPTER XI
THE ORIGIN OF THE ARTESIAN WATERS

Flow of Water through Porous Rocks — Importance of Pressure, Porosity, and Temperature — Intermittent Flows — Abundance of Extinct Wells — Former Prosperity — Possibility of increasing Present Total Discharge — Local Traditions regarding Origin of Artesian Waters — Possible Sources of Origin — Seepage from Nile into Nubian Sandstone — Sandstones as Storage Reservoirs — The Oasis Waters of Meteoric Origin — Fissures — Rate of Flow — Strongly-flowing Wells not necessarily dependent on Fissures — Local Pressure from Variation in Level of Water-Table — Rise of Water due to Hydraulic Pressure — Points requiring Investigation — High-Level Springs: Ain Amûr; on Escarpment near Beris; at Nakhail.

The rate of flow of water through porous rocks has been investigated by a number of engineers and geologists, among whom may be mentioned Darcy, Hazen, King, Slichter, Knibbs, and Baldwin-Wiseman. The subject is an extremely complicated one, and its study requires a combined knowledge of mathematics, physics, and geology. Various formulæ for determining the rate of flow under varying conditions have been devised, but it will be sufficient for our present purpose to remark that the average velocity of water in sands does not appear to be more than 3 or 4 kilometres a year. The importance of porosity, pressure, and temperature on the rate of flow can be illustrated by utilizing tubes containing equal columns of sand of different degrees of coarseness, and noting the volumes of water passed under different conditions of pressure and temperature. The coarse sands will be observed to permit the passage of water at a far greater rate than the finer varieties, while any sand can be made to markedly quicken its rate of discharge by increasing the head of water in the tube above. Moreover, the rate of flow will be found to increase with the temperature of the water. In the case of the ordinary water of the oasis, a difference of only a few degrees was found to cause a very great difference in the rate of flow; it seems, indeed, as if the water, when below a certain temperature, deposits its mineral contents in the pores of the sandstone, so as to block the passages to a large extent. I have not yet had an opportunity of making the necessary experiments to ascertain if this explanation is correct, but if it should prove to be so, the importance of temperature on the rate of underground flow can hardly be over-estimated.

We have already described the intermittent character of the flow in the case of some of the larger wells in the southern part of the oasis. The circumstance is apparently due to the temporary blocking of the bore-hole by sediment, and the consequent increase of water and gas pressure below, which at intervals forces the sediment from the channels and restores the normal flow. Something of the same nature was also noticed in Bore No. 42, in the course of experiments carried out to determine the height to which the water would rise in an open pipe fixed to the end of the casing. At intervals a distinct gurgling and bubbling took place in the pipe, the water at the same time rising to as much as 8 centimetres above its normal level, with distinct oscillations of a pressure-gauge attached to the well. The ebb and flow did not, however, take place at regular intervals, but at periods varying from four to nine minutes. I think a sufficient explanation of these phenomena is the probable variation in the amount of gas finding its way into the bore; and certainly, after several years’ observation of the flows of a large number of bores, I cannot admit that there is the slightest evidence in favour of the view that the flows of the wells have a periodicity dependent on the rise and fall of the Nile.