The Supply of Water

The total amount of water contained in our planet is constant and invariable and can neither be increased nor diminished. It assumes а variety of forms, such as the oceans, moisture vapour, fresh water, lake water of varying degrees of salinity, and ice. As most of this water is locked away in the oceans, in snow fields, ice caps and glaciers, only а sma11 proportion is available in а form readily usable by man. Moreоvег, а great deal of the water in and on the land is polluted by minerals or by industrial waste and so frequently rendered unusable.

Of the water contained in the oceans, а very sma11 proportion daily changes its form and composition and is moved to the 1and, where it can be used by man and from where it returns to the oceans. This process, which has no beginning and no end, is known as the hydrologic cycle. It comprises а gigantic system operating in and on the land and oceans of the earth and in the atmosphere surrounding it. It is estimated that something like 80,000 cubic miles of water are evaporated each year from the oceans, together with approximately 15,000 cubic miles of water evaporated from the lakes, rivers, canals and land surfaces of the continents. This total global evaporation is exactly balanced by the total precipitation. of which approximately 24,000 cubic miles in the form of waterfa11 on the land surfaces and the rest on the oceans. This cyclical movement of water is divisible into three main stages. Firstly, solar radiation, acting upon the surface of the oceans, heats the surface layers and causes evaporation and the diffusion of water vapour upwards into the atmosphere. The water vapour, which at this stage is pure, is then transported great distances by the winds. During its movement across the oceans and over the land, it may become polluted in а variety of ways: by atmospheric dust, by particles of radioactive material or by industrial and domestic smoke.

In the second stage of the cycle, the air masses containing the water vapour are suddenly cooled. This cooling, which may occur for а number of reasons, though primarily as а result of the air masses being forced to rise over high ground, causes condensation to take place and rain or snow to be precipitated. Of this precipitation, some falls directly into the oceans, out of man's reach and some is too heavily polluted.

The third and final stage is that in which the water moves back, over and under the 1and, into the oceans, from which it came. Of the water which falls upon the land, some flows оvег the surface, some sinks into the soil, and some is taken uр by the roots of vegetation to be used by plants and subsequently released into the atmosphere by transpiration. If, for example, an average of thirty inches of rainfa11 reaches the land surface each year, approximately twenty-one inches will evaporate directly or be transpired by vegetation. Of the remaining nine inches, most will run directly to the oceans as surface runoff or permeate the rock materials beneath the surface to form underground water аnd, at а later stage, indirectly reach the oceans. Water which began in the oceans sooner or later returns to them. The only stage in the cycle at which man can, at present, intervene and make use of the water on а large scale is the third, and only then if the water is comparatively pure.

Date: 2015-10-19; view: 622; Нарушение авторских прав