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in hydrology, the flow of rain and meltwater into seas and low-lying areas. Runoff occurs along the earth’s surface, in which case it is called surface flow, and in the layer of soils and rocks, in which case it is called subsurface flow. The process is a constituent part of the earth’s water cycle. Thus, runoff, which is primarily a product of climate, influences the shaping of the relief, geochemical processes in the earth’s crust, the development of the soil cover, and the distribution of vegetation. In turn, the quantity and regime of runoff depend on the amount and regime of precipitation, evaporation, and temperature conditions, as well as on the nature of the region’s relief and geological structure, soil cover, and vegetation. Erosion, natural drainage and irrigation, and the transport and deposition of products of denudation are associated with runoff.
The proportion of atmospheric precipitation that does not evaporate and is not consumed by plants but forms runoff is inversely related to the average air temperature. On plains maximum runoff is usually observed in the wettest forest zone on the western and eastern coasts of the continents, and in mountains the amount of runoff increases up to a certain elevation, primarily on the slopes of mountain ranges facing humid airstreams. The typical periods in formation of runoff are high water, freshet, and low water (summer and winter).
The amount of runoff in any period of time (year, season, month) is expressed by the depth (in mm or cm), total volume, average discharge, modulus of runoff, and modular coefficient of runoff. The total volume of runoff (in cu m or cu km) is the amount of water that flows through the hydrometrically closed segment of a drainage basin. The total volume of runoff is usually determined graphically by constructing a hydrograph, that is, a diagram of change over time (t) of discharge (Q) for a year. Using the hydrograph it is possible to construct the integral curve of runoff, which gives a representation of the progressive accumulation of water volume (in a reservoir) with the passage of time. The modulus of runoff is the volume of runoff in a unit of time per unit of area of the watershed; it is usually expressed in //sec-km2 or //sec-hectare. The modular coefficient of runoff is the ratio of the quantity of runoff changing in time to its average value. The coefficient of runoff shows the ratio of the amount of runoff to the amount of precipitation that has fallen on the area for the same period of time; in other words, it specifies what proportion of precipitation goes into runoff. In addition to water (liquid) runoff, the runoff of suspended sediment and sediment drawn along the bottom (solid runoff) as well as dissolved mineral substances are also determined.
Observations are made at hydrologic posts and stations, and experimental investigations are conducted at runoff sites and in laboratories to study the runoff process and make a quantitative determination of all the factors that affect it. The determination of the numerical parameters of runoff, including where it is a regulated flow, can be done at reservoirs by means of hydrologic calculations and forecasts.
The total volume of annual river runoff into the world ocean is 35,000–40,000 cu km; of this volume roughly one-third is formed by subterranean waters drained by river valleys (a certain part of the subterannean runoff enters the ocean directly). The volume of annual river runoff into enclosed interior basins on land is roughly 750 cu km, of which 320 cu km goes to Middle Asia and the Caspian Sea basin. The total annual runoff of rivers in the USSR is about 4,700 cu km. Runoff from the rivers of Brazil (3,200 cu km), China (2,600 cu km), and the United States (2,100 cu km) is also significant.
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Muraveiskii, S. D. Rekiiozera: Gidrobiologiia, Stok. Moscow, 1960.
Zhelezniakov, G. V., and B. B. Danilevich. Tochnost’ gidrologichesakikh izmerenii i raschetov. Leningrad, 1966.
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Rukovodstvo po opredeleniiu raschetnykh gidrologicheskikh kharakteristik. Leningrad, 1973.
Mirovoi vodnyi balans i vodnye resursy Zemli. Leningrad, 1974.
N. I. MAKAVEEV and K. G. TIKHOTSKII