HYDROLOGY, hi-drol'-o-ge. Although etymologically hydrology covers the entire science of water, its properties, phenomena, laws and distribution, the application of the term usually is restricted to investigation and description of water in or on the earth, that is, undergrdund waters and stream flow or the increment of water from rainfall. Water that comes to the earth from the atmosphere in the form of rain or snow is in large part gathered into streams or lakes while a smaller part passes underground. Some of it evaporates so rapidly that it does not add to the increment and there is also considerable loss by evapora tion from all water bodies and from ground water brought to the surface by vegetation and capillary action. Many factors influence the volume and movement of surface waters and the passage of waters underground. Stream flow depends on conditions of precipitation, light showers often yielding no flow and heavy storms resulting in floods. The most import ant factors are climate, vegetation, topography, the works of man and various geologic condi tions, especially those affecting the absorption of surface water by soils and rocks. The passage of water into the ground is controlled by the texture of surface materials and of the rocks below, and in large measure by the con figuration of the land and conditions of rainfall or snow melting. Much underground water comes again to the surface in the general seep age on slopes and also in springs which often draw from deep sources, and the water in the ground is depleted by vegetation and capillary action. The science of underground water is sometimes designated hydrogeology because it is a branch of geology. Extensive investiga tions of underground waters have been mane by geologists in various parts of the world with most important economic results. The basis of the work is geologic because the water is mostly contained in sand, sandstones and various other rocks which occur in succession among the strata constituting the earth's crust. The water-bearing beds are carried to great depths by downward dips of monoclines or basins. Locally they are cut by faults and igneous masses and generally they are affected by variations in texture both of themselves and of adjoining strata. The controlled by height of intake is an important factor in de limiting the area in artesian flows may be expected. Mineral or saline components affect the quality of the water and variations in texture and thickness of the beds control the volume of water available. The study of most problems of hydrology relating to artesian water and their prospects require the determina tion of geologic conditions in wide areas, for the evidence is often many miles away from the place where the water is desired. The rate of movement of water through various rocks and sands and the capacity of these materials for water have been studied extensively by King, Slichter and others. For many years a branch of the United States Geological Survey has been devoted to studies of problems of hydrol ogy. The function of this work is to investi gate water supply from wells and streams for domestic use, manufacturing and irrigation and consideration is also given to water powers and methods of pumping and storing water. The determination of volume of water in sur face streams is an important branch of hydrol ogy. Ordinarily streams are gaged by current meters taking the velocity on a measured cross section but on many rivers vertical gages are set and observations are made of the height of Cie water while in some cases water stage.re
corders are used which give a continuous record. Volume of flow is calculated from these records by a rating table computed from observations made at various stages of flow. The amount of water flowing in a stream, termed the discharge or run off, is express either in rate of flow or volume of increment. Rate of flow is given in second feet (cubic feet per second) but some records are given in miner's inches or in second feet per square mile. A miner's inch is the discharge through an orifice one inch square under a head which varies locally and is defined by State laws. In California the miner's inch is .187 gallons a second or .0248 second feet; in Colorado it is .195 gallons or about 5 per cent more. A second foot is 7.48 United States gallons a second, 1.98347 acre-feet a day. One acre-foot is 325,851 United States gallons, (43,560 cubic feet) the amount covering an acre one foot deep. It is equal to 50 miner's inches in Idaho, Kansas, Nebraska, New Mexico, North Dakota and South Dakota, 40 miner's inches in Ari zona, California, Ilontana and Oregon and 38.4 miner's inches in Colorado. A second foot fall ing 8.81 feet is equal to one horse power. The United States Geological Survey alone and in co-operation with various state surveys maintains many stations, where volume of stream flow is determined as often as may be necessary to indicate the total yearly flow and its variations from year to year. By this means in the last 25 years or more there has been ascertained the amount of water available for various pur poses including water power in all the prin cipal streams of the country. Similar observa tions have been made in Canada for the past 10 years. The methods have been largely de veloped by engineers in the water resource branch of the United States Geological Survey and they are generally accepted as standard. Records of rainfall such as those systematically obtained by the weather bureau of the United States Department of Agriculture throughout the country afford important hydrologic data., for their averages show the amount of pre cipitation that may be expected in various areas.
The determination of rates of evaporation throw light on the prospects for water storage. See GEOLOGY ; IRRIGATION ; PHYSIOGRAPHY ; RIVERS; ARTESIAN WELLS.
Bibliography.— Darton, N. H., 'Report on Artesian Waters of the Dakotas' (in United States Geological Survey, 17th Annual Report, pt. 2, Washington 1896); id., 'Geology and Underground Waters of the Central Great Plains) (United States Geological Survey, Prof. Paper :32) ; Hoyt, J. C., and Grover, N. C., 'River Keilhack, K., (Lehrbuch der Grundwasser and Quellenkunde' • King, F. H., 'Principles and Conditions of the Move ments of Ground Water' ; Slichter, C. S., 'In vestigations of Movements of Ground Waters' ; id., 'Motions of Underground Waters' (Wash ington 1902) ; id., 'Field Measurements of Rate of Movement of Underground Waters' (ib. 1905); Woodward, H. B., 'Geology of Water Darton, N. H., 'Geology and Under Waters of the Arkansas Valley in Southeastern Colorado' (Washington 1905); Fuller, M. L.., 'Summary of Controlling Fac tors of Artesian (United States Geo logical Survey, Bulletin 319, Washington 1908).