Water

WATER Water is probably the natural resource that is most essential to man's existence on the earth. It furnishes him food and drink and the means of sanitation, transportation, and the production of power. It is unique among the earth's natural resources in that it is recurring by precipitation, is pure in each recurrence, and is not conserved by non-use. The water of precipitation travels over and through the ground, is transpired in part by vegetation, flows in part to the oceans and inland seas, is evapo rated from both land and water surfaces, and is again precipitated. This never-ending circulation of water through the earth and air leads to its continuing recurrence on the earth in essentially the same average annual quantities. Although water is polluted by washing the land or by sewage and industrial wastes, the pol luting substances are not carried into the air, and the water of precipitation is always essentially pure. Man's activities do not diminish the quantity or impair the quality of water in its sub sequent precipitation, do not affect materially its rate of circula tion through the earth and air and have no effect on the supply for use by future generations. Conservation of water consists in its legal control in the interest of the public and in its greatest proper use for multiple purposes and to conserve the exhaustible nat ural resources.

Precipitation varies widely from time to time and place to place and the quantities of water in the ground and on its sur face vary accordingly, resulting in periods of high water and of low water—of floods and of droughts. These and other phases of the occurrence of water are beyond the control of man, although their effects may be modified by hydraulic works to augment low water flows and to protect flood plains from floods, and by vegeta tion to reduce erosion. The operation of reservoirs for storage and controlled release of water changes the regimen and there fore the utility of the rivers below them. Since the supply of water in any drainage basin is limited by precipitation, it sets the limits of development: it determines the amount of land that may be irrigated, the size of water-power and steam-power plants, the capacities of industrial establishments, and the growth of cities. In many parts of the world the limits of supply have

been reached or are being approached, and water is imported from long distances and at great expense for municipal and ir rigation uses.

Surface Water.

Run-off results directly from precipitation, but the rapidity with which run-off follows precipitation depends upon various conditions. In tropical and subtropical lands the sequence is rapid. In the areas from which run-off is supplied mainly by melting snow it is retarded by the length of time water is held in the form of snow. Such areas embrace in general all polar regions and all lands above about 5,000 ft. in the latitude of northern United States and central Europe, and above 1 2 , 000 to 15,000 ft. in the tropics. In other areas, if the rainfall is fairly uniform throughout the year as it is in eastern United States and most of western Europe, the distribution of run-off is nearly uniform, but there is considerable reduction during the summer due to greater evaporation. In areas situated in warm temperate and subtropical lands which receive precipitation dur ing a definite rainy season, the run-off is likely to be extremely variable. Streams that flow from areas having permeable soils and subsoils or are underlain by cavernous rocks are likely to have relatively well maintained run-off.

The utility of any stream for power, irrigation or municipal supply, depends in large measure upon the adaptability of its run-off to the demand. Storage must ordinarily be provided to assist in this adaptation. Natural sites available for use as storage reservoirs may contribute largely to the value of the water re sources of a drainage basin. Areas that were covered during the glacial period by continental ice sheets or by extensive alpine gla ciers generally abound in lakes, which in their natural condition regulate run-off in some degree, and some of these lakes may be converted into reservoirs of large capacity. High rates of run-off and attendant- possibilities of power and irrigation development are generally associated with mountainous areas.