Hydrologic cycle

The natural circulation of water on the earth is called the hydrologic cycle. Water cycles from bodies of water, via evaporation to the atmosphere , and eventually returns to the oceans as precipitation, runoff from streams and rivers, and groundwater flow. Water molecules are transformed from liquid to vapor and back to liquid within this cycle. On land, water evaporates from the soil or is taken up by plant roots and eventually transpired into the atmosphere through plant leaves; the sum of evaporation and transpiration is called evapotranspiration .

Water is recycled continuously. The molecules of water in a glass used to quench your thirst today, at some point in time may have dissolved minerals deep in the earth as groundwater flow, fallen as rain in a tropical typhoon, been transpired by a tropical plant, been temporarily stored in a mountain glacier, or quenched the thirst of people thousands of years ago.

The hydrologic cycle has no real beginning or end but is a circulation of water that is sustained by solar energy and influenced by the force of gravity. Because the supply of water on the earth is fixed, there is no net gain or loss of water over time. On an average annual basis, global evaporation must equal global precipitation. Likewise, for any body of land or water, changes in storage must equal the total inflow minus the total outflow of water. This is the hydrologic or water balance.

At any point in time, water on the earth is either in active circulation or in storage. Water is stored in icecaps, soil, groundwater, the oceans, and other bodies of water. Much of this water is only temporarily stored. The residence time of water storage in the atmosphere is several days and is only about 0.04% of the total freshwater on the earth. For rivers and streams, residence time is weeks; for lakes and reservoirs, several years; for groundwater, hundreds to thousands of years; for oceans, thousands of years; and for icecaps, tens of thousands of years. As the driving force of the hydrologic cycle, solar radiation provides the energy necessary to evaporate water from the earth's surface, almost three-quarters of which is covered by water. Nearly 86% of global precipitation originates from ocean evaporation. Energy consumed by the conversion of liquid water to vapor cools the temperature of the evaporating surface. This same energy, the latent heat of vaporization, is released when water vapor changes back to liquid. In this way, the hydrologic cycle globally redistributes heat energy as well as water.

Once in the atmosphere, water moves in response to weather circulation patterns and is transported often great distances from where it was evaporated. In this way, the hydrologic cycle governs the distribution of precipitation and hence, the availability of fresh water over the earth's surface. About 10% of atmospheric water falls as precipitation each day and is simultaneously replaced by evaporation. This 10% is unevenly distributed over the earth's surface and, to a large extent, determines the types of ecosystems that exist at any location on the earth and likewise governs much of the human activity that occurs on the land.

The earliest civilizations on the earth settled in close proximity to fresh water. Subsequently, and for centuries, humans have been striving to correct, or cope with, this uneven distribution of water. Historically, we have extracted stored water or developed new storages in areas of excess, or during periods of excess precipitation, so that water could be available where and when it is most needed.

Understanding processes of the hydrologic cycle can help us develop solutions to water problems. For example, we know that precipitation occurs unevenly over the earth's surface because of many complex factors that trigger precipitation. For precipitation to occur, moisture must be available and the atmosphere must become cooled to the dew point , the temperature at which air becomes saturated with water vapor. This cooling of the atmosphere occurs along storm fronts or in areas where moist air masses move into mountain ranges and are pushed up into colder air. However, atmospheric particles must be present for the moisture to condense upon, and water droplets must coalesce until they are large enough to fall to the earth under the influence of gravity.

Recognizing the factors that cause precipitation has resulted in efforts to create conditions favorable for precipitation over land surfaces via cloud seeding. Limited success has been achieved by seeding clouds with particles, thus promoting the condensation-coalescence process. Precipitation has not always increased with cloud seeding and questions of whether cloud seeding limits precipitation in other downwind areas is of both economic and environmental concern.

Parts of the world have abundant moisture in the atmosphere, but it occurs as fog because the mechanisms needed to transform this moisture into precipitation do not exist. In dry coastal areas, for example, some areas have no measurable precipitation for years, but fog is prevalent. By placing huge sheets of plastic mesh along coastal areas, fog is intercepted, condenses on the sheets, and provides sufficient drinking water to supply small villages.

Total rainfall alone does not necessarily indicate water abundance or scarcity . The magnitude of evapotranspiration compared to precipitation determines to some extent whether water is abundant or in short supply. On a continent basis, evapotranspiration represents from 56 to 80% of annual precipitation. For individual watersheds within continents, these%ages are more extreme and point to the importance of evapotranspiration in the hydrologic cycle.

Weather circulation patterns responsible for water shortages in some parts of the world are also responsible for excessive precipitation, floods, and related catastrophes in other parts of the world. Precipitation that falls on land, but that is not stored, evaporated or transpired, becomes excess water. This excess water eventually reaches groundwater, streams, lakes, or the ocean by surface and subsurface flow. If the soil surface is impervious or compacted, water flows over the land surface and reaches stream channels quickly. When surface flow exceeds a channel's capacity, flash flooding is the result. Excessive precipitation can saturate soils and cause flooding no matter what the pathway of flow. For example, in 1988 catastrophic flooding and mudslides in Thailand caused over 500 fatalities or missing persons, nearly 700 people were injured, 4,952 homes were lost, and 221 roads and 69 bridges were destroyed. A three-day rainfall of over nearly 40 in (1,000 mm) caused hillslopes to become saturated. The effects of heavy rainfall were exacerbated by the removal of natural forest cover and conversion to rubber plantations and agricultural crops.

Although floods and mudslides occur naturally, many of the pathways of water flow that contribute to such occurrences can be influenced by human activity. Any time vegetative cover is severely reduced and soil exposed to direct rainfall, surface water flow and soil erosion can degrade watershed systems and their aquatic ecosystems.

The implications of global warming or greenhouse effects on the hydrologic cycle raise several questions. The possible changes in frequency and occurrence of droughts and floods are of major concern, particularly given projections of population growth . Global warming can result in some areas becoming drier while others may experience higher precipitation. Globally, increased temperature will increase evaporation from oceans and ultimately result in more precipitation. The pattern of precipitation changes over the earth's surface, however, cannot be predicted at the present time.

The hydrologic cycle influences nutrient cycling of ecosystems, processes of soil erosion and transport of sediment , and the transport of pollutants. Water is an excellent liquid solvent; minerals, salts, and nutrients become dissolved and transported by water flow. The hydrologic cycle is an important driving mechanism of nutrient cycling. As a transporting agent, water moves minerals and nutrients to plant roots. As plants die and decay, water leaches out nutrients and carries them downstream. The physical action of rainfall on soil surfaces and the forces of running water can seriously erode soils and transport sediments downstream. Any minerals, nutrients, and pollutants within the soil are likewise transported by water flow into groundwater, streams, lakes, or estuaries.

Atmospheric moisture transports and deposits atmospheric pollutants , including those responsible for acid rain . Sulfur and nitrogen oxides are added to the atmosphere by the burning of fossil fuels . Being an excellent solvent, water in the atmosphere forms acidic compounds that become transported via the atmosphere and deposited great distances from their original site. Atmospheric pollutants and acid rain have damaged freshwater lakes in the Scandinavian countries and terrestrial vegetation in eastern Europe. In 1983, such pollution caused an estimated $1.2 billion loss of forests in the former West Germany alone. Once pollutants enter the atmosphere and become subject to the hydrologic cycle, problems of acid rain have little chance for resolution. However, programs that reduce atmospheric emissions in the first place provide some hope.

An improved understanding of the hydrologic cycle is needed to better manage water resources and our environment . Opportunities exist to improve our global environment, but better knowledge of human impacts on the hydrologic cycle is needed to avoid unwanted environmental effects.

See also Estuary; Leaching

[Kenneth N. Brooks ]

RESOURCES

BOOKS

Committee on Opportunities in the Hydrologic Sciences, Water Sciences Technology Board. Opportunities in the Hydrologic Sciences. National Research Council. Washington, DC: National Academy Press, 1991.

Lee, R. Forest Hydrology. New York: Columbia University Press, 1980.

Postel, S. "Air Pollution, Acid Rain, and the Future of Forests." Worldwatch Paper 58. Washington, DC: Worldwatch Institute, 1984.

Van der Leeden, F., F. L. Troise, and D. K. Todd. The Water Encyclopedia. 2nd ed. Chelsea, MI: Lewis Publishers, 1990.

PERIODICALS

Nash, N. C. "Chilean Engineers Find Water for Desert by Harvesting Fog in Nets." New York Times, July 14, 1992, B5.

OTHER

Rao, Y. S. "Flash Floods in Southern Thailand." Tiger Paper 15 (1988): 1–2. Regional Office for Asia and the Pacific (RAPA), Food and Agricultural Organization of the United Nations. Bangkok.