25. THE SEARCH FOR WATER BENEATH THE DESERTS
by Dr Frank Dixey
The question of the water supplies of the arid lands is of great significance because of the alarming increase in world population and the need for a greatly increased production of food. The arid lands include some of the best and most extensive grazing areas of the world, and offer prospects of greater agricultural production where water is available for irrigation. Moreover, the arid countries themselves need larger and better distributed water supplies to provide for their own expanding human and stock populations, to make more efficient use of their lands, and to develop their economy by providing export crops of meat, hides and skins, gum, fruit - particularly dates - and other agricultural products.
But in view of the general paucity or absence of fresh surface waters, new water supplies in arid lands can normally be provided only from ground-water resources - that is to say, from water stored below the land surface. Hence it is of the greatest importance to these countries to find such new resources and to develop them; but it is equally important that such development be carried out efficiently and economically, not only for current use, but also on a well-planned long-term pattern, so that the water supplies may continue to be available for future years. Without such regard for the future, these supplies can be, and often are, greatly reduced in amount or even exhausted by over-use when the new stock and human populations have settled around them.
Since ground-waters are replenished directly or indirectly by rainfall, and since, of course, arid countries owe their aridity to sparse rainfall, it may be expected that ground-waters in arid countries are small in volume - unless they percolate laterally underground from some distant humid source. Moreover, when present at all, they tend to occur in useful amounts only in scattered localities, according to prevailing hydrological, topographic and geological conditions. Thus, in an arid plain distant from highlands, the ground-water supplies may be negligible; but where the arid areas are bounded by highlands, streams flow into the desert before their waters disappear by evaporation and by sinking into their beds or valley floors. There they replenish the ground-waters and are often recoverable from wells and bore-holes. Such ground-waters are known as "free" ground-waters. In other cases the geological structure of the highlands and the plains is such that rain water entering a porous stratum in the highlands is conveyed underground by that stratum to distant parts of the plain whence it may be recovered by "artesian" bore-holes. Such waters are known as "artesian", "pressure" or "confined" waters, because the water of the porous stratum is confined within that stratum by impervious beds above and below, and accordingly, where the stratum reaches a level lower than the intake, the water is held under pressure which may cause it, in a well or borehole, to rise to a level below, at, or above the surface of the ground at that point.
When an area has to be examined for ground-water possibilities, as for pastoral, agricultural or industrial needs, it is necessary first to provide or make use of available air photographs of the area and to use these in conjunction with, or to compile from them, a topographic map on a suitable scale. With the aid of such a map and the photographs, and detailed examination on the ground, a geological survey of the area is made from which is produced a geological map and report showing the distribution, nature and structure of the rocks of the area. Then the surface hydrology is considered in relation to possible areas of recharge of groundwater from streams and of discharge from springs, and the rock structures themselves are considered in relation to absorption, storage, yield and movement of ground-waters. In turn one can deduce the nature, extent, depth and quality of the ground-waters, and the most suitable sites or areas for developing them by means of wells and bore-holes. The data will indicate also whether or not artesian structures are likely to exist in the area, and the sites and depths most suitable for tapping the confined waters.
(from New Scientist, 20th July, 1961)