The yield of drainage areas is expressed in a variety of ways, depending somewhat upon the uses to which the water is put. For preliminary studies the yield May he stated in inches of depth, or in percentage of the total rainfall. The former is readily eonverted into cubic feet per second. hour. or day. per mile of drainage area, or into millions of gallons per square mile. The gallon is the most Convenient unit where water works are involved. and the cubic foot where water ik to he applied for power or navigation. In the ease of irrigation either pubic feet or aere feet may be employed. (See IRRIGATION.) ream gougings we primarily. in culde feet per second.
springs may be developed and sometimes have their flow increased by digging a basin or well around them, or by driving a tunnel or gallery into a hillside. By either of these means the 11,,w of a number of springs may often be united. The walling up and covering in of springs is fre quently desirable to exclude foreign matter of all sorts and to keep the water cool.
Underground waters other than springs are developed by sinking wells, building infiltra tion galleries, driving tunnels, and, occasionally by the construction of submerged dams. The latter have been employed to intercept per eolatirg waters flowing through valleys in the West, chiefly for use in irrigation. Well-sink ing, dent-building and tunnel-driving are de scribed under their proper heads. Infiltration galleries are generally formed by digging a trench :find walling up its sides with timber, brick, or stone. laid with open joints. The top is tightly covered and the bottom left open.
works and irrigation are described finder those heads, and the various elasses of pumps are treated under AND l'umeING .MacuiNLEY.
Estimates of the probable yields of drainage areas should be made only after due considera tion of all the factors reviewed above. This is particularly true of underground supplies. It is a rare thing to find a single group of wells that yield, year after year, more than 5.000, 0110 to 10,000.11110 gallons a day, but sometimes a number of groups may he made tributary to one water-supply system 'by proper pipe connections and pumping plants.
The yield of surface supplies, as has been stated, is far better known than that of under ground waters. The accompanying table, taken front Turnenure's Public Water 8app1ics, gives the drainage area and yields of 15 streams in the United States, differing widely in size and location. The records, as will be seen, are mostly for quite long periods and include the average yearly, minimum yearly, and average half-yearly _ .
The water of running streams is sometimes diverted to the canal, bead-race, or intake pipe by means of a dam, or in the ease of a large stream by a wing dam, extending only part way across. Intake pipes are often laid on the bed of large streams or lakes until a point of sufficient depth or remoteness from the shore is reached, where it terminates in a crib or other arrangement for the protection of the exposed end and the strainer frequently placed upon it. Beneath rivers pod for a number of
the cities on the Great Lakes. the water sup ply is sometimes drawn through tunnels. These terminate at the shore end in the pumping eta rainfall, run-off, and percentage of run-off to rainfall. A fall of I inch of rain on a square mile of area is equal to 2,323.200 cubic feet. or about 17,375.000 gallons. A total flow of 1 cubic foot per second is equal to 646,300 gallons a day. A more detailed study of the records would show that after making allowance for evapora tion from water surfaces, there would be a negative yield in some months; o•, in other words. the river would run dry were it not for artificial o• natural storage. It is to meet just such contingencies that storage reservoirs are provided. There are two ways of considering storage: (1) The amount reqnired to stipple tion and at the river or lake end in a vertical shaft, protected above by a timber and stone crib and tower.
Pumping plants are often important parts of water-supply systems. Their relations to water meat the natural flow so as to give the required daily supply; (2) the economic limit of the storage. development. The deficiency is then to be ma de good by establishing the storage capa city indicated. provided the expense involved is not so great as to render some other source of supply cheaper. The amount of storage will range all the way from nearly the whole run-off of very small streams to a few months' supply for medinm-sized ones and nothing for large rivers. But whenever an attempt is made to utilize a large percentage of the total run-off. storage will also have to be provided in large quantities. With adequate storage an average daily yield of 500,000 to 700,000 gallons per square mile may be expected in the New Eng land States, New York, and New Jersey, where all the problems involved have been most care fully studied.
In a brief paper on Storage Water Supplies presented to the American Water-Works Asso ciation in May, 1901, L. J. Le Conte submitted the preceding estimates as to the capabilities and requisite storage of the drainage areas which supply Boston, New York, San Francisco, and Oakland, Cal.
BIBLIOGRAPHY. Consult: F. E. Turneaure, Bibliography. Consult: F. E. Turneaure, Public Water Supplies (New York, 1901), and other general reference-books under articles on I REIcATIox and XVATER-WORKS: Water Supply and Irrigation Papers, and recent annual Reports of the United States Geological Survey; and Vol. iii. Power") of the Final Report of the State Geologist of New Jersey (Trenton, 1891i. For further information on the natural sources of water supply, see WATER; ARTESIAN WELLS; RAIN; RIVER; SPRINGS. For the devel opment of water supplies and their useful ap plication. see AQUEDUCTS; DAMS AND RESER VOIRS ; IRRIGATION; WATER POWER; WATER PURIFICATION; WATER-WORKS; and WELL-SINK ING.