Sources of Water

In practice, there are four genera1 sources of water available to man: surface water, ground water, atmospheric water and the oceans. The most important of these is surface water in the form of rivers, streams and lakes. Of the other three sources, ground water is increasing rapidly in importance, particularly in those areas which lack surface drainage. The use of pure water derived artificially from the atmosphere and the oceans may become significant if and when technological advances make it available on а sufficiently large scale and at an economic cost.

Ground water or, as it is sometimes called, underground water occurs below the surface of the ground in а zone of saturation, that is, the zone in which permeable rocks are saturated with water under hydrostatic pressure. Water moves down from the surface by gravity to enter this zone, the upper surface of which is called the water-table or phreatic surface; for this reason, ground water is sometimes called phreatic, subsurface or subterranean water. The lower 1irnit of the zone is the point at which the underlying rock formation becomes so dense that water cannot penetrate it. It may vary in depth from а few feet to hundreds of feet and there are isolated examples of porous rock having been found at depths of more than а mile. The zone of saturation is very important because it supplies all wells and maintains the normal, relatively uniform flow of streams. It acts as а gigantic reservoir which retains water during wet periods, causing а rise in the watertable.

Ground water has been laid down very unevenly beneath the surface and moves towards the oceans like surface water, only much more slowly.

The chief uses of ground water are for irrigation and domestic purposes. It is of no direct importance for the generation of hydroelectricity but of considerable indirect importance in that the flow of streams is primarily sustained by it.

Atmospheric water, or water vapour, has two major advantages over water contained in the oceans: it is to be found everywhere above the land surface and it is free of salt. Unfortunately, nо large-scale, successfu1, economic method has yet been devised to tap this water-supply and direct it to places where it is most needed. One serious initial obstacle lies in the fact that clouds are not necessarily water-bearing and may be 'dry'. If, however, they do contain appreciable amounts of water vapour, this may either dry out or condense and fall as rain or snow. The most that we have been able to do is to cause а particular humid cloud – one that would almost certainly sooner or later have precipitated – to shed its moisture at а time and place of our choosing.

This is achieved in one or two ways. The first method involves the "seeding" of clouds from aeroplanes or rockets with small particles of various chemicals, which cause waterdroplets to form and precipitation to take place. The second method is to create artificial convection currents by heating а large air mass near the ground. The air thereupon rises rapidly into the cloud, upsets the equilibrium, and causes precipitation. Some local successes have been achieved by these methods, but both are expensive and both depend upon the presence of water-filled clouds.

The oceans remain by far the largest potential source of water and together with the island seas contain 92.7 per cent of the earth's water. This water could be made potable if its saline content were reduced from about 35,000 parts per million to 500 parts per million or less. We have known for а long time that it is possib1e to produce fresh water by heating salt water and so promoting distillation.

Each of these processes depends upon the use of energy, which may be thermal, mechanical or solar. They each suffer from the disadvantage that the cost of desalination is very high.

It is clear therefore that although а great deal of water is available for use by man, the supply is not infinite. While it is likely that the quantity of usable water will be increased by desalination and the creation of artificial precipitation, it is certain that for some time to come the greater proportion of our water supply will be derived primari1y from surface runoff and to а lesser extent from subterranean sources.

Water is put to а great variety of uses: for irrigating crops, for the generation of hydroelectricity: for canals and waterways, for controlling pollution and as а source of food. All these uses are interdependent. For example, the construction of а modern dam promotes irrigation, the generation of hydroelectricity, flood control and recreation. Frequently, however, the uses of such natural resources as water may be mutually exclusive and а choice must sometimes be made between one, use and another.

Date: 2015-10-19; view: 666; Нарушение авторских прав