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hydrologic cycle[¦hī·drə¦läj·ik ′sī·kəl]
on the earth, the continuous process of the movement of water in the earth’s geographic shell, accompanied by phase conversions of the water. The hydrologic cycle consists primarily of the evaporation of the water, the carrying of the water vapor over distance, its condensation and the precipitation of clouds, the seepage of the fallen water, or infiltration, and runoff.
Water evaporates from the surface of bodies of water, soil, and plants and enters the atmosphere in the form of water vapor. In the atmosphere, the water vapor spreads upward by way of turbulent diffusion, and the air currents carry it into other regions on the earth. As the temperature of the humid air drops, both adiabatically and as a consequence of giving up heat, the water vapor condenses, changing into a liquid or solid state. Clouds and fogs are formed in this manner. In particular, condensation of water vapor leads to the formation of ground hydrometeors. Clouds are also transported by air currents. As clouds precipitate, the water is returned to the earth’s surface, it again evaporates, and so forth. In this process, a portion of the water that has fallen on land runs off into bodies of water. Along with heat exchange and general atmospheric circulation, the hydrologic cycle is one of the basic climate-forming processes.
The total quantity of water on the earth in the modern geological and, in any event, the historical era, has remained constant. The mean level of the world’s oceans and the mean moisture content of the atmosphere also do not experience changes. This means that over a long period of time, precipitation has equaled evaporation on the entire planet. The mean depth of the layer of precipitation over the entire planet equals 1 ,000 mm per year. This corresponds to 511,000 cu km of water (approximately 7 times the amount of water in the Black Sea). Some 21 percent of this amount (108,000 cu km) falls over land and 79 percent (403,000 cu km) over the oceans. Almost half of the total precipitation falls in the region between 20° N lat. and 20° S lat; just 4 percent of the precipitation occurs in both polar regions. For the oceans of the world, evaporation exceeds precipitation, and for land, precipitation exceeds evaporation. On the average, a layer of water 1,250-mm deep (450,000 cu km) is evaporated annually from the surface of the world’s oceans. Of this amount, 1,120 mm are returned to the ocean in the form of precipitation and 130 mm in the form of runoff from land. On the average, a layer of water 410 mm deep (61,000 cu km) evaporates annually from the surface of the land, with 720 mm of precipitation falling on land. In addition to evaporation, land loses 310 mm through runoff (47,000 cu km); this is somewhat more than one-half the amount of water in the Black Sea. For individual zones and areas of the earth, the ratio of the components in the hydrologic cycle can differ sharply from the average conditions. There are regions where the total precipitation is much greater or less than the evaporation.
The water that evaporates from the surface of the oceans precipitates not only over the oceans but also over the continents where the water vapor has been carried by air currents. A large portion of the precipitation that falls over land is of oceanic origin. Having fallen on land and reevaporated, this water may again fall on the same continent or over the same area of the continent. This is the so-called internal hydrologic cycle. The precipitation from the internal hydrologic cycle constitutes a small portion of the total precipitation; for example, it constitutes only 10 percent for the European USSR.
REFERENCESDrozdov, O. A., and A. S. Grigor’eva. Vlagooborot v atmosfere. Leningrad, 1963.
Alpat’ev, A. M. Vlagooboroty v prirode i ikh preobrazovaniia. Leningrad, 1969.
S. P. KHROMOV