HYDROLOGIC CYCLE

cycle that involves the continuous circulation of water in the Earth-atmosphere system. Of the many processes involved in the hydrologic cycle, the most important are evaporation, transpiration, condensation, precipitation, and runoff. Although the total amount of water within the cycle remains essentially constant, its distribution among the various processes is continually changing. Evaporation, one of the major processes in the cycle, is the transfer of water from the surface of the Earth to the atmosphere. By evaporation, water in the liquid state is transferred to the gaseous, or vapour, state. This transfer occurs when some molecules in a water mass have attained sufficient kinetic energy to eject themselves from the water surface. The main factors affecting evaporation are temperature, humidity, wind speed, and solar radiation. The direct measurement of evaporation, though desirable, is difficult and possible only at point locations. The principal source of water vapour is the oceans, but evaporation also occurs in soils, snow, and ice. Evaporation from snow and ice, the direct conversion from solid to vapour, is known as sublimation. Transpiration is the evaporation of water through minute pores, or stomata, in the leaves of plants. For practical purposes, transpiration and the evaporation from all water, soils, snow, ice, vegetation, and other surfaces are lumped together and called evapotranspiration, or total evaporation. Water vapour is the primary form of atmospheric moisture. Although its storage in the atmosphere is comparatively small, water vapour is extremely important in forming the moisture supply for dew, frost, fog, clouds, and precipitation. Practically all water vapour in the atmosphere is confined to the troposphere (the region below 6 to 8 miles [10 to 13 km] altitude). The transition process from the vapour state to the liquid state is called condensation. Condensation may take place as soon as the air contains more water vapour than it can receive from a free water surface through evaporation at the prevailing temperature. This condition occurs as the consequence of either cooling or the mixing of air masses of different temperatures. By condensation, water vapour in the atmosphere is released to form precipitation. Precipitation that falls to the Earth is distributed in four main ways: some is returned to the atmosphere by evaporation, some may be intercepted by vegetation and then evaporated from the surface of leaves, some percolates into the soil by infiltration, and the remainder flows directly as surface runoff into the sea. Some of the infiltrated precipitation may later percolate into streams as groundwater runoff. Direct measurement of runoff is made by stream gauges and plotted against time on hydrographs. Most groundwater is derived from precipitation that has percolated through the soil. Groundwater flow rates, compared with those of surface water, are very slow and variable, ranging from a few millimetres to a few metres a day. Groundwater movement is studied by tracer techniques and remote sensing. Ice also plays a role in the hydrologic cycle. Ice and snow on the Earth's surface occur in various forms such as frost, sea ice, and glacier ice. When soil moisture freezes, ice also occurs beneath the Earth's surface, forming permafrost in tundra climates. About 18,000 years ago glaciers and ice caps covered approximately one-third of the Earth's land surface. Today, about 12 percent of the land surface remains covered by ice masses. Figure 2: The present-day surface hydrologic cycle. The numbers in parentheses refer to volumes of water in millions of cubic kilometres, and the fluxes adjacent to the arrows are in millions of cubic kilometres of water per year. hydrologic cycle Figure 4: The detailed hydrologic cycle emphasizing processes involved in the transfer of water in the cycle. Numbers on arrows show relative water fluxes. hydrologic sciences the fields of study concerned with the waters of the Earth. Included are the sciences of hydrology, oceanography, limnology, and glaciology. In its widest sense hydrology encompasses the study of the occurrence, the movement, and the physical and chemical characteristics of water in all its forms within the Earth's hydrosphere. In practice hydrologists usually restrict their studies to waters close to the land surface of the Earth. Water in the atmosphere is usually studied as part of meteorology. Water in the oceans and seas is studied within the science of oceanography; water in lakes and inland seas within limnology; and ice on the land surface within glaciology. Clearly there is some overlap between these major scientific disciplines; both hydrologists and meteorologists, for example, have contributed to the study of water movement in the lower boundary layers of the atmosphere. All are linked by the fundamental concept of the hydrologic cycle, according to which the waters of the sea are evaporated, are subsequently condensed within the atmosphere, fall to the Earth as precipitation, and finally flow in the rivers back to the sea. Water is the most abundant substance on Earth and is the principal constituent of all living things. Water in the atmosphere plays a major role in maintaining a habitable environment for human life. The occurrence of surface waters has played a significant role in the rise and decline of the major civilizations in world history. In many societies the importance of water to humankind is reflected in the legal and political structures. At the present time rising populations and improving living standards are placing increasing pressures on available water resources. There is, in general, no shortage of water on the Earth's land surface, but the areas of surplus water are often located far from major centres of population. Moreover, in many cases these centres prove to be sources of water pollution. Thus, the availability and quality of water are becoming an ever-increasing constraint on human activities, notwithstanding the great technological advances that have been made in the control of surface waters.