WATER SUPPLY. Water for irrigation, navigation, water-works, and other industrial and domestic purposes, derived primarily from the rainfall and secondarily from running streams and lakes, or ssurface water; and from springs, wells, tunnels, and infiltration galleries or under ground sources. The quality of water supplied for irrigation, water power, and navigation is of comparatively small consequence, except that a badly polluted stream might be a general nui sance, and that the mineral contents of certain waters are harmful to crops. The water made available by water-works should be above all question as to sewage pollution and should pos sess other qualities, natural or artificial, dis cussed under WATER-WORKS and WATER PURIFI CATION. A gravity flow is generally an essential in the case of supplies for navigation and water power. The same is often but by no means al ways true of water for irrigation. These limita tions are due to the cost of pumping and (:o not apply to supplies for domestic and manufactur ing purposes. Besides volume, there is a further essential to water for power, namely, sufficient fall to yield the requisite amount of energy to accomplish the desired work. (See WATER POWER.) As has been stated, the common source of all water supplies is the rainfall. Some of this finds its way to the streams, ponds, and lakes; some sinks into the soil to be utilized by plants, or to percolate through the soil to lakes and streams: some is stored in the soil and the permeable layers below it ready to he tapped by wells; and a large part is evaporated directly from the surface of the ground, only to fall to the earth again, in the form of rain, dew, or With a given rainfall in any locality the amount of water naturally available will de pend upon the extent. surface slope, and geo logical formation of the gathering-ground, or drainage area, and the nature of the vegetation en that surface. The steeper the slope and the greater the proportion of the rainfall reaching the streams, the less the amount available as underground supplies. With very steep and im pervious drainage areas, the streams rise and fall suddenly, leaving comparatively little water available for nay useful and threaten ing the stability of such water-supply as arc built. and other well-covered areas retain the rainfalls for longer periods. but, in
common with all vegetation, they make their own demands upon the water stored in the soil. Nevertheless, such areas, and also flat, sandy stretches of country, tend greatly to equalize the seasonal yield of drainage areas. Temperature and humidity are important factors in all water slimily calculations, high temperatures and low humidities increasing the evaporation from both land and water. When the ground is frozen water cannot percolate into it and the surface flow, or run-off, is so rapid that but little of the rainfall is available, unless there is ample stor age. The percentage of water surface to total drainage area plays a very important part, since the evaporation from water surfaces, particu larly in hot and dry climates, is much greater than that from land areas.
The excellent maps and reports of the United States Geological Survey and sonic of the State geological surveys often afford sufficient data relating to the extent, surface, and 'subsurface character of drainage areas for at least pre liminary studies of water supplies. Where these maps are lacking or insufficient, special topo graphical surveys are necessary, but it may not be so easy to secure geological data. In like manner, meteorological statistics (see PAIN) may be secured from the United States, State, and local weather bureaus or meteorological sta tions. Still further aids are the observations on the flow of streams made for a number of years past by the llydrographie Department of the United States Geological Survey, by some of the State engineering bureaus, by municiind water departments, and by owners of private water powers. The New Jersey Geological Survey, the State Engineer of New York. the cities of Bos ton and New York, the private corporations which control the water power of Lawrence and Lowell, Mass., the East Water Company. ins northeastern New Jersey. and the Spring Val ley Water Company at and near San Francisco, Cal., afford the most notable examples of valuable records of this sort. Observations on the amount of water required for various crops. and various phases of ground-water investigation, have been conducted for years past by the United States and the various' State agricultural ex periment stations, under the control or leader ship of the United States Department of Agri culture.