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Hydrologic Cycle

Hydrologic cycle

The hydrologic, or water , cycle is the continuous, interlinked circulation of water among its various compartments in the environment. Hydrologic budgets are analyses of the quantities of water stored, and the rates of transfer into and out of those various compartments. A simplified hydrologic cycle starts with heating caused by solar energy and progresses through stages of evaporation (or sublimation), condensation , precipitation (snow, rain, hail, glaze), groundwater , and runoff .

The most important places in which water occurs are the oceans , glaciers , underground aquifers, surface waters, and the atmosphere. The total amount of water among all of these compartments is a fixed, global quantity. However, water moves readily among its various compartments through the processes of evaporation, precipitation, and surface and subsurface flows. Each of these compartments receives inputs of water and has corresponding outputs, representing a flow-through system. If there are imbalances between inputs and outputs, there can be significant changes in the quantities stored locally or even globally. An example of a local change is the drought that can occur in soil after a long period without replenishment by precipitation. An example of a global change in hydrology is the increasing mass of continental ice that occurs during glacial epochs, an event that can remove so much water from the oceanic compartment that sea level can decline by more than 328 ft (100 m), exposing vast areas of continental shelf for the development of terrestrial ecosystems.

Estimates have been made of the quantities of water that are stored in various global compartments. By far, the largest quantity of water occurs in the deep lithosphere , which contains an estimated 27×1018 tons (27-billion-billion tons) of water, or 94.7% of the global total. The next largest compartment is the oceans, which contain 1.5×1018 tons, or 5.2% of the total. Ice caps contain 0.019×1018 tons, equivalent to most of the remaining 0.1% of Earth's water. Although present in relatively small quantities compared to the above, water in other compartments is very important ecologically because it is present in places where biological processes occur. These include shallow groundwater (2.7×1014 tons), inland surface waters such as lakes and rivers (0.27×1014 ton), and the atmosphere (0.14×1014 tons).

The smallest compartments of water also tend to have the shortest turnover times, because their inputs and outputs are relatively large in comparison with the mass of water that is contained. This is especially true of atmospheric water, which receives annual inputs equivalent to 4.8×1014 tons as evaporation from the oceans (4.1×1014 tons/yr) and terrestrial ecosystems (0.65×1014 tons/yr), and turns over about 34 times per year. These inputs of water to the atmosphere are balanced by outputs through precipitation of rain and snow, which deposit 3.7×1014 tons of water to the surface of the oceans each year, and 1.1×1014 tons/yr to the land.

These data suggest that the continents receive inputs of water as precipitation that are 67% larger than what is lost by evaporation from the land. The difference, equivalent to 0.44×1014 tons/yr, is made up by 0.22×1014 tons/yr of runoff of water to the oceans through rivers, and another 0.22×1014tons/yr of subterranean runoff to the oceans.

The movements of water in the hydrologic cycle are driven by gradients of energy. Evaporation occurs in response to the availability of thermal energy and gradients of concentration of water vapor. The ultimate source of energy for most natural evaporation of water on Earth is solar electromagnetic radiation. Heating from within Earth's mantle and crust that results from radioactive decay supplies the other thermal energy requirements. Solar energy is absorbed by surfaces, increasing their heat content, and thereby providing a source of energy to drive evaporation. In contrast, surface and ground waters flow in response to gradients of gravitational potential. In other words, unless the flow is obstructed, water spontaneously courses downhill.

The hydrological cycle of a defined area of landscape is a balance between inputs of water with precipitation and upstream drainage, outputs as evaporation and drainage downstream or deep into the ground, and any internal storage that may occur because of imbalances of the inputs and outputs. Hydrological budgets of landscapes are often studied on the spatial scale of watersheds, or the area of terrain from which water flows into a stream, river, or lake.

The simplest watersheds are so-called headwater systems that do not receive any drainage from watersheds at higher altitude, so the only hydrologic input occurs as precipitation, mostly as rain and snow. However, at places where fog is a common occurrence, windy conditions can effectively drive tiny atmospheric droplets of water vapor into the forest canopy, and the direct deposition of cloud water can be important.

Vegetation can have an important influence on the rate of evaporation of water from watersheds. This hydrologic effect is especially notable for well-vegetated ecosystems such as forests , because an extensive surface area of foliage supports especially large rates of transpiration. Evapotranspiration refers to the combined rates of transpiration from foliage, and evaporation from non-living surfaces such as moist soil or surface waters. Because transpiration is such an efficient means of evaporation, evapotranspiration from any well vegetated landscape occurs at much larger rates than from any equivalent area of non-living surface.

In the absence of evapotranspiration an equivalent quantity of water must drain from the watershed as seepage to deep groundwater or as streamflow.

Forested watersheds in seasonal climates display large variations in their rates of evapotranspiration and streamflow. This effect can be illustrated by the seasonal patterns of hydrology for a forested watershed in eastern Canada. The input of water through precipitation is 58 in (146 cm) per year, but 18% of this arrives as snow, which tends to accumulate on the surface as a persistent snow pack. About 38% of the annual input is evaporated back to the atmosphere through evapo-transpiration, and 62% runs off as river flow. Although there is little seasonal variation in the input of water with precipitation, there are large seasonal differences in the rates of evapo-transpiration, runoff, and storage of groundwater in the watershed. Evapotranspiration occurs at its largest rates during the growing season and runoff is therefore relatively sparse during this period. In fact, in small watersheds in this region forest streams can literally dry up because so much of the precipitation input and soil water is utilized for evapotranspiration, mostly by trees. During the autumn, much of the precipitation input serves to recharge the depleted groundwater storage, and once this is accomplished stream flows increase again. Runoff then decreases during winter, because most of the precipitation inputs occur as snow, which accumulates on the ground surface because of the prevailing subfreezing temperatures. Runoff is largest during the early springtime when warming temperatures cause the snow pack to melt during a short period of time, resulting in a pronounced flush of stream and river flow.

Some aspects of the hydrologic cycle can be utilized by humans for a direct economic benefit. For example, the potential energy of water elevated above the surface of the oceans can be utilized for the generation of electricity . However, the development of hydroelectric resources generally causes large changes in hydrology. This is especially true of hydroelectric developments in relatively flat terrain, which require the construction of large storage reservoirs to retain seasonal high-water flows, so that electricity can be generated at times that suit the peaks of demand. These extensive storage reservoirs are essentially artificial lakes, sometimes covering enormous areas of tens of thousands of hectares. These types of hydroelectric developments cause great changes in river hydrology, especially by evening out the variations of flow, and sometimes by unpredictable spillage of water at times when the storage capacity of the reservoir is full. Both of these hydrologic influences have significant ecological effects, for example, on the habitat of salmon and other aquatic biota.

Where the terrain is suitable, hydroelectricity can be generated with relatively little modification to the timing and volumes of water flow. This is called run-of-the-river hydroelectricity, and its hydrologic effects are relatively small. The use of geologically warmed ground water to generate energy also has small hydrological effects, because the water is usually re-injecting back into the aquifer .

Human activities can influence the hydrologic cycle in many other ways. The volumes and timing of river flows can be greatly affected by channeling to decrease the impediments to flow, and by changing the character of the watershed by paving, compacting soils, and altering the nature of the vegetation. Risks of flooding can be increased by speeding the rate at which water is shed from the land, thereby increasing the magnitude of peak flows. Risks of flooding are also increased if erosion of soils from terrestrial parts of the watershed leads to siltation and the development of shallower river channels, which then fill up and spill over during high-flow periods. Massive increases in erosion are often associated with deforestation, especially when natural forests are converted into agriculture.

The quantities of water stored in hydrologic compartments can also be influenced by human activities. An important example of this effect is the mining of groundwater for use in agriculture, industry, or for municipal purposes. The best-known case of groundwater mining in North America concerns the enormous Ogallala aquifer of the southwestern United States, which has been drawn down mostly to obtain water for irrigation in agriculture. This aquifer is largely comprised of "fossil water" that was deposited during earlier, wetter climates, although there is some recharge capability through rain-fed groundwater flows from mountain ranges in the watershed of this underground reservoir.

Sometimes industrial activities lead to large emissions of water vapor into the atmosphere, producing a local hydrological influence through the development of low-altitude clouds and fogs. This effect is mostly associated with electric power plants that cool their process water using cooling towers.

A more substantial hydrologic influence on evapotranspiration is associated with large changes in the nature of vegetation over a substantial part of a watershed. This is especially important when mature forests are disturbed, for example, by wildfire, clear-cutting, or conversion into agriculture. Disturbance of forests disrupts the capacity of the landscape to sustain transpiration, because the amount of foliage is reduced. This leads to an increase in stream flow volumes, and sometimes to an increased height of the groundwater table. In general, the increase in stream flow after disturbance of a forest is roughly proportional to the fraction of the total foliage of the watershed that is removed (this is roughly proportional to the fraction of the watershed that is burned, or is clear-cut). The influence on transpiration and stream flow generally lasts until regeneration of the forest restores another canopy with a similar area of foliage, which generally occurs after about 510 years of recovery. However, there can be a longer-term change in hydrology if the ecological character of the watershed is changed, as occurs when a forest is converted to agriculture.

See also Alluvial systems; Aquifer; Artesian; Atmospheric composition and structure; Hydrogeology; Hydrologic cycle; Hydrostatic pressure; Hydrothermal processes; Stream capacity and competence; Stream piracy; Troposphere and tropopause; Wastewater treatment; Water pollution and biological purification; Water table; Water

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Hydrologic Cycle

Hydrologic Cycle

There are about 1,360 million cubic kilometers (326 million cubic miles) of water on Earth. Most of this water is stored in reservoirs such as the oceans, in glaciers and ice caps, underground, and in the atmosphere. In distinction to other planets in the solar system, sizeable amounts of water on the Earth can be found in solid, liquid, and vapor form.

The largest amount of the water on Earth, about 97 percent, is stored in the oceans. The next largest amount of water, about 2 percent, is stored as ice in glaciers and polar ice sheets. A little more than half of the remaining one percent of water is stored underground as groundwater . The remaining less than one-half percent of the water on Earth is stored in lakes, rivers, and wetlands , and as vapor in the atmosphere.

Despite the vast amount of water on Earth, most is too salty for human use. This includes all the water in the oceans and much of the water deep in the groundwater system. Most ice, although it is fresh water, is present in the more remote parts of the Earth and is not easily accessible to humans. Of water on Earth, only about one-third of one percent is fresh water that is usable by humans, and nearly all of that fresh water (97 percent) comes from groundwater.

The Movement of Water

Water does not remain locked up in the oceans, icecaps, groundwater systems, or the atmosphere. Instead, water is continually moving from one reservoir to another. This movement of water is called the hydrologic cycle. A simplified illustration of the hydrologic cycle is shown in the figure.

Although water in the hydrologic cycle is constantly in motion, it never leaves the Earth. The Earth is nearly a "closed system" like a terrarium. This means that the Earth neither gains nor loses much matter, including water. Although some matter, such as meteors from outer space, is captured by Earth, very little of Earth's matter escapes into outer space. This is certainly true of water. This means that the same water existing on Earth millions of years ago is still here today.

Precipitation.

Precipitation is the movement of water from the atmosphere to the Earth's surface. Precipitation in the form of rain, snow, sleet, or hail is the source of nearly all the fresh water in the hydrologic cycle. Precipitation falls everywhere on Earth, but its distribution is highly variable. Largely owing to their greater surface area, the oceans receive three times more precipitation than the continents. Furthermore, on the continents it might not rain for years in parts of vast deserts, such as the Sahara in Africa. At the other extreme, precipitation might exceed 800 centimeters (315 inches) per year in tropical rain forests.

Water Vapor.

Just as most precipitation falls on the oceans, most of the water that evaporates and returns to the atmosphere as water vapor is also from ocean surfaces. In fact, about 85 percent of the water that evaporates and returns to the atmosphere is from the oceans. The remaining 15 percent of water that moves to the atmosphere is from the continents. This includes evaporation from lakes, rivers, and soil and rock surfaces, and transpiration from plants. Evaporation from open water such as lakes and surface reservoirs does not vary much, but transpiration by plants can be very different; for example, the amount of water transpired by widely spaced desert plants is far less than the total amount of water transpired from dense forests.

Surface Water and Groundwater.

Precipitation that falls on the continents either runs over the surface of the Earth into streams, lakes, and wetlands, or soaks into the ground. Water that remains on the Earth's surface, such as streams, lakes, and wetlands, is called surface water. Water that soaks into the ground either is stored in the soil or recharges groundwater.

Streams are the surface water that moves water from the continents back to the oceans as part of the hydrologic cycle. The surface water that is in lakes and wetlands is generally ponded, but this water also is active in the hydrologic cycle because lakes and wetlands evaporate water to the atmosphere, and they receive water from and lose water to the groundwater system.

Some of the water that seeps into the ground becomes soil moisture water and some becomes groundwater. Water in soils usually does not move very far because it is transpired back to the atmosphere by plants. However, some of the water that seeps into the ground moves downward past the soil and recharges the groundwater system. Many people think that groundwater is like an underground lake, but it is really the water that is found in the pore spaces between the grains of rock and sediment that make up the structure of the Earth.

Unlike soil water, which does not move very far, groundwater moves in flow systems that can range in size from only a few meters in length to many hundreds of kilometers. These groundwater flow systems eventually discharge groundwater to surface-water bodies such as streams, lakes, and wet-lands. Groundwater also discharges directly to bays, estuaries , and oceans, but the amount is much less than the amount that discharges to streams, lakes, and wetlands.

Watershed Concept.

The hydrologic cycle is usually depicted on a global scale. However, the hydrologic cycle operates at many scales, from the hydrologic cycle of the Earth to the hydrologic cycle of a person's back yard. Generally, to use the small amount of the Earth's water that is suitable for humans (that is, only about one-third of one percent), people who manage water resources are most interested in the hydrologic cycle of watersheds.

A watershed, which is sometimes called a drainage basin, is an area of land where all the water that falls on it will drain to a body of surface water, such as a stream or lake. An example of a drainage basin is a hillside that has a small creek at the bottom of it. All the rain that falls on the hillside will eventually flow downhill over the land surface or through the ground into the creek. In this simple example, the hillside is the creek's watershed. In reality, the watershed would be all the land area bordered by other such hillsides or elevated terrain. The watershed is like a large bowl that collects water and delivers it to the watershed outlet, which commonly is a stream or river.

The Focus of Water Managers

Because fresh surface water and fresh groundwater are the only parts of the hydrologic cycle that can be used by humans, most interest in the hydrologic cycle by water managers is focused on these resources. Although it is important to know how much water is stored in groundwater, lakes, and wetlands, understanding the movement of water to, within, and from watersheds is far more important, and a far greater challenge. Indeed, most research in the hydrologic sciences is devoted to understanding movement of water, and the movement of chemicals and sediment transported by water in watersheds.

To assure adequate water resources for human use, water managers need to be able to measure the amounts of water that enter, pass through, and leave watersheds. This is a challenge because the relative magnitudes of the individual transfers in the hydrologic cycle can vary substantially. For example, in mountainous areas, precipitation is more difficult to measure high in the mountains compared to in the valleys. Mountain snowpack and the amount of meltwater it can deliver can vary widely, thereby affecting natural water budgets at lower elevations.*

As a second example, evaporation rates may differ greatly among an agricultural field, a nearby woodland, and a nearby wetland. Thirdly, the discharge of groundwater to surface water may vary in different parts of watersheds because different rock and sediment types may be present.

The hydrologic cycle is a basic concept that water managers need to keep in mind in their daily work. When the flow of water is manipulated to fulfill human needs, it is necessary to understand how these actions will affect the hydrologic cycle and, ultimately, the availability and quality of water to downstream users. Thorough understanding of the hydrologic cycle is absolutely necessary if maximum use of the water resources is to be achieved, while avoiding detrimental effects to wildlife and the environment as a whole.

The Livable Earth.

The hydrologic cycle is a very important and practical concept for maintaining a healthy and livable Earth. To a large extent, water shapes the Earth through erosion and deposition of sediments and minerals. Water also is fundamental to life on Earth, where water makes up a substantial part of living organisms, and those organisms need water for life. Therefore, managing water resources by thoroughly understanding the hydrologic cycle at scales ranging from the entire Earth to the smallest of watersheds is one of the greatest responsibilities of humans.

see also Drinking Water and Society; Earth: The Water Planet; Fresh Water, Physics and Chemistry of; Glaciers and Ice Sheets; Global Warming and the Hydrologic Cycle; Groundwater; Instream Water Issues; Precipitation and Clouds, Formation of; Precipitation, Global Distribution of; Stream Hydrology; Transboundary Water Treaties; Wetlands.

Thomas C. Winter

Bibliography

Crowder, J. N., and J. Cain. Water Matters, Vol. 3. Arlington, VA: National Science Teachers Association, 1999.

National Geographic Society. "Water: The Power, Promise, and Turmoil of North America's Fresh Water" National Geographic Special Edition, 184, no. 5A (1993).

Winter, Thomas C. et al. Ground Water and Surface WaterA Single Resource. U.S. Geological Survey Circular 1139 (1998).

Internet Resource

Follow a Drip through the Water Cycle. Water Sciences for Schools, U.S. Geological Survey. <http://ga.water.usgs.gov/edu/followdrip.html>.

* See "Global Warming and Glaciers" for a photograph of a snowpack measurement.

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Hydrologic Cycle

Hydrologic cycle

Hydrologic cycle is the phrase used to describe the continuous circulation of water as it falls from the atmosphere to Earth's surface in the form of precipitation, circulates over and through Earth's surface, then evaporates back to the atmosphere in the form of water vapor to begin the cycle again. The scientific field concerned with the hydrologic cycle, the physical and chemical properties of bodies of water, and the interaction between the waters and other parts of the environment is known as hydrology.

The total amount of water contained in the planet's oceans, lakes, rivers, ice caps, groundwater, and atmosphere is a fixed, global quantity. This amount is about 500 quintillion gallons (1,900 quintillion liters). Scientists believe this total amount has not changed in the last three billion years. Therefore, the hydrologic cycle is said to be constant throughout time.

Earth's water reservoirs and the water cycle

Oceans cover three-quarters of Earth's surface, but contain over 97 percent of all the water on the planet. About 2 percent of the remaining water is frozen in ice caps and glaciers. Less than 1 percent is found underground, in lakes, in rivers, in ponds, and in the atmosphere.

Solar energy causes natural evaporation of water on Earth. Of all the water that evaporates into the atmosphere as water vapor, 84 percent comes from oceans, while 16 percent comes from land. Once in the atmosphere, depending on variations in temperature, water vapor eventually condenses as rain or snow. Of this precipitation, 77 percent falls on oceans, while 23 percent falls on land.

Precipitation that falls on land can follow various paths. A portion runs off into streams and lakes, and another portion soaks into the soil, where it is available for use by plants. A third portion soaks below the root zone and continues moving slowly downward until it enters underground reservoirs of water called groundwater. Groundwater accumulates in aquifers (underground layers of sand, gravel, or spongy rock that collect water) bounded by watertight rock layers. This stored water, which may take several thousand years to accumulate, can be tapped by deep

water wells to provide freshwater. It is estimated that the groundwater is equal to 40 times the volume of all the freshwater on Earth's surface.

A plant pulls water from the surrounding soil through its roots and transports it to its stems and leaves. Solar heat on the leaves causes the plant to heat up. The plant naturally cools itself by a process called transpiration, whereby water is eliminated through pores in the leaves (called stomata) in the form of water vapor. This water vapor then moves up into the atmosphere.

Solar heat also causes the evaporation of water from ground surfaces and from lakes and rivers. The amount of evaporation from these areas is far less than that from the oceans, but the amount of evaporation is balanced as gravity forces water in rivers to flow downhill to empty into the oceans.

An inconsistent cycle

Although the hydrologic cycle is a constant phenomenon, it is not always evident in the same place year after year. If it occurred consistently in all locations, floods and droughts would not exist. However, each year some places on Earth experience more than average rainfall, while other places endure droughts.

[See also Evaporation; Water ]

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hydrologic cycle

hydrologic cycle Representation of the flow of water in various states through the terrestrial and atmospheric environments. Storage points (stages) involve groundwater and surface water, ice-caps, oceans, and the atmosphere. Exchanges between stages involve evaporation and transpiration from the Earth's surface, condensation to form clouds, and precipitation followed by runoff. See also RESIDENCE TIME.

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hydrologic cycle

hydrologic cycle The flow of water in various states through the terrestrial and atmospheric environments. Storage points (stages) involve groundwater and surface water, ice-caps, oceans, and the atmosphere. Exchanges between stages involve evaporation and transpiration from the Earth's surface, condensation to form clouds, and precipitation followed by run-off. See also residence time.

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Hydrologic Cycle

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): 12. Regional Office for Asia and the Pacific (RAPA), Food and Agricultural Organization of the United Nations. Bangkok.

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Hydrologic Cycle

Hydrologic Cycle

Major compartments and fluxes of the hydrologic cycle

Hydrologic cycle of a watershed

Influences of human activities on the hydrologic cycle

Resources

The hydrologic cycle is the continuous circulation of water through the environment, which can be thought of as a series of hydrologic compartments. The most important places in which water occurs are the ocean, glaciers, underground aquifers, surface waters, and the atmosphere. The total amount of water among all of these compartments is a fixed quantity. However, water moves readily among its various compartments through the processes of evaporation and transpiration (often combined and referred to as evapotranspiration), precipitation, and surface and subsurface flows. Each of these compartments receives inputs of water and has corresponding outputs, representing a flow-through system. If there are imbalances between inputs and outputs, there can be significant changes in the quantities stored locally or even globally. An example of a local change is the drought that can occur in soil after a long period without replenishment by precipitation. An example of a global change in hydrology is the increasing mass of continental ice that occurs during glacial epochs, an event that can remove so much water from the oceanic compartment that sea level can decline by more than 328 ft (100 m), exposing vast areas of continental shelf for the development of terrestrial ecosystems.

Major compartments and fluxes of the hydrologic cycle

By far the largest quantity of water occurs in the deep lithosphere, which contains an estimated 27 x 1018 tons (27-billion-billion tons) of water, or 94.7% of the global total. The next largest compartment is the oceans, which contain 1.5 x 1018 tons, or 5.2% of the total. Ice caps contain 0.019 x 1018 tons, equivalent to most of the remaining 0.1% of Earths water. Although present in comparatively small amounts, water in other compartments is important ecologically because it is present in places where biological processes occur. These include shallow groundwater (2.7 x 1014 tons), inland surface waters such as lakes and rivers [0.27 x 1014 ton], and the atmosphere [0.14 x 1014 tons]).

The smallest compartments of water also tend to have the shortest turnover times, because their inputs and outputs are relatively large in comparison with the mass of water contained in the compartment at any time. This is especially true of atmospheric water, which receives annual inputs equivalent to 4.8 x 1014 tons as evaporation from the oceans (4.1 x 1014 tons/yr) and terrestrial ecosystems (0.65 x 1014 tons/yr), and turns over about 34 times per year. These inputs of water to the atmosphere are balanced by outputs through precipitation of rain and snow, which deposit 3.7 x 1014 tons of water to the surface of the oceans each year, and 1.1 x 1014 tons/yr to the land.

These data suggest that the continents receive 67% more water as precipitation than is lost by evapotranspiration from the land. The difference, equivalent to 0.44 x 1014 tons/yr, is made up by 0.22 x 1014 tons/yr of water discharged to the oceans through rivers, and another 0.22 x 1014 tons/yr of subterranean runoff to the oceans.

The movement of water through the hydrologic cycle is driven by energy gradients. Evaporation occurs in response to the availability of thermal energy and water vapor concentration gradients. The ultimate source of energy for almost all natural evaporation of water on Earth is solar electromagnetic radiation. This solar energy is absorbed by surfaces, increasing their heat content, and thereby providing a source of energy to drive evaporation. In contrast, surface water and groundwater flow in response to gradients of potential energy.

Hydrologic cycle of a watershed

The hydrological cycle of a watershed is a balance between water added by precipitation and upstream drainage, and water removed by evapotranspiration, surface water flow, infiltration into the ground, and any internal storage that may occur because of imbalances of the inputs and outputs. Hydrological budgets of landscapes are often studied on the spatial scale of watersheds, which are areas in which water flows into a stream, river, or lake.

The simplest watersheds are headwater systems that do not receive any drainage from watersheds at higher altitude, so the only hydrologic input occurs mainly as precipitation. In places where fog is common, wind can drive droplets of water vapor into the forest canopy and the direct deposition of cloud water can also be important. This effect has been measured for a foggy conifer forest in New Hampshire, where fog water deposition was equivalent to 33 in (84 cm) per year, compared with 71 in (180 cm) per year of rain and snow.

Vegetation can have an important influence on the rate of evaporation of water from watersheds. This hydrologic effect is especially notable for well-vegetated ecosystems such as forests, because an extensive surface area of foliage supports large rates of transpiration. Evapotranspiration refers to the combined rates of transpiration from foliage, and evaporation from non-living surfaces such as moist soil or surface waters. Because transpiration is such an efficient means of evaporation, evapotranspiration from any well vegetated landscape occurs at much larger rates than from any equivalent area of nonliving surface.

In the absence of evapotranspiration an equivalent quantity of water would have to drain from the watershed as seepage to deep groundwater or as stream flow. Studies of forested watersheds in Nova Scotia found that evapotranspiration was equivalent to 15-29% of the hydrologic inputs with precipitation. Runoff through streams or rivers was estimated to account for the other 71-85% of the atmospheric inputs of water, because the relatively impervious bedrock in that region prevented significant drainage to deep ground water.

Forested watersheds in seasonal climates display large variations in their rates of evapotranspiration and stream flow. This effect can be illustrated by the seasonal patterns of hydrology for a forested watershed in eastern Canada. The input of water through precipitation is 58 in (146 cm) per year, but 18% of this arrives as snow, which tends to accumulate on the surface as a persistent snowpack. About 38% of the annual input is evaporated back to the atmosphere through evapotranspiration, and 62% runs off as river flow. Although there is little seasonal variation in the input of water with precipitation, there are large seasonal differences in the rates of evapotranspiration, runoff, and storage of groundwater in the watershed. Evapotranspiration occurs at its largest rates during the growing season of May to October, and runoff is therefore relatively sparse during this period. In small watersheds in this region, forest streams can become seasonally dry because so much of the precipitation and soil water is utilized for evapotranspiration, mostly by trees. During the autumn, much of the precipitation input serves to recharge the depleted groundwater storage, and once this is accomplished stream flows increase again. Runoff then decreases during winter, because most of the precipitation inputs occur as snow, which accumulates on the ground surface because of the prevailing sub-freezing temperatures. Runoff is largest during the early springtime, when warming temperatures cause the snowpack to melt during a short period of time, resulting in a pronounced flush of stream and river flow.

Influences of human activities on the hydrologic cycle

Some aspects of the hydrologic cycle can be utilized by humans for a direct economic benefit. For example, the potential energy of water elevated above the surface of the oceans can be used to generate electricity. Hydroelectric resource development, however, generally causes large changes in hydrology. This is especially true of hydroelectric developments in relatively flat terrain, which require the construction of large storage reservoirs to retain seasonal high-water flows, so that electricity can be generated at times that suit the peaks of demand. These extensive storage reservoirs are artificial lakes, sometimes covering tens of thousands of acres. These types of hydroelectric developments cause changes in river hydrology, especially by reducing variations of flow and sometimes by unpredictable spillage of water at times when the storage capacity of the reservoir is full. Both of these hydrologic influences have significant ecological effects, for example, on the habitat of salmon and other aquatic biota. In one unusual case, a large release of water from a reservoir in northern Quebec drowned 10,000 caribou that were trapped by the unexpected cascade of water during their migration.

Where the terrain is suitable, hydroelectricity can be generated with relatively little modification to the timing and volumes of water flow. This is called run-of-the-river hydroelectricity, and its hydrologic effects are relatively small. The use of geologically warmed ground water to generate energy also has small hydrological effects, because the water is usually reinjected into the aquifer.

Human activities can alter the hydrologic cycle in other ways. The volume and timing of river discharges can be affected by channeling to decrease the impediments to flow. Changing the character of the

KEY TERMS

Evapotranspiration The evaporation of water from a large area, including losses of water from foliage as transpiration, and evaporation from nonliving surfaces, including bodies of water.

Hydrology The study of the distribution, movement, and physical-chemical properties of water in Earths atmosphere, surface, and near-surface crust.

Precipitation The deposition from the atmosphere of rain, snow, fog droplets, or any other type of water.

Watershed The expanse of terrain from which water flows into a wetland, water body, or stream.

watershed by paving, compacting soils, and altering the nature of the vegetation generally increases the intensity of runoff after rainstorms. Risks of flooding can be increased by increasing the rate at which water is shed from the land, thereby increasing the magnitude of peak flows. Risks of flooding are also increased if erosion of soils from terrestrial parts of the watershed leads to siltation and the development of shallower river channels, which then fill up and spill over during high-flow periods. Massive increases in erosion are often associated with deforestation, especially when natural forests are converted into agriculture.

The quantities of water stored in hydrologic compartments can also be influenced by human activities. One example is the mining of groundwater for agriculture, industry, and municipal consumption. The Ogallala aquifer of the southwestern United States, which has been drawn down mostly to obtain water for irrigation in agriculture, contains so-called fossil water that accumulated in the aquifer during wetter times.

Sometimes industrial activities lead to large emissions of water vapor into the atmosphere, producing a local hydrological influence through the development of low-altitude clouds and fogs. This effect is mostly associated with electric power plants that cool their process water using cooling towers.

A more substantial hydrologic influence on evapotranspiration is associated with large changes in the vegetation within a watershed. This is especially important when mature forests are disturbed, for example, by wildfire, clear-cutting, or conversion into farms. Disturbance of forests disrupts the capacity of the landscape to sustain transpiration, because the amount of foliage is reduced. This leads to an increase in stream flow volumes, and sometimes to an increased height of the groundwater table. In general, the increase in stream flow after disturbance of a forest is roughly proportional to the fraction of the total foliage of the watershed that is removed by logging or burning. The influence on transpiration and stream flow generally lasts until regeneration of the forest restores another canopy with a similar area of foliage, which generally occurs after about 5 to 10 years. However, there can be a longer-term change in hydrology if the ecological character of the watershed is permanently changed, as occurs when a forest is converted to agriculture.

Resources

BOOKS

Ahrens, Donald C. Meteorology Today. Pacific Grove, CA: Brooks Cole, 2006.

Brustaert, W. Hydrology: An Introduction. Cambridge, United Kingdom: Cambridge University Press, 2005.

Chang, M. Forest Hydrology. 2nd ed. Boca Raton, FL: CRC Press, 2006.

Bill Freedman

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Hydrologic Cycle

Hydrologic Cycle

Introduction

The continuous circulation of water through regions of the global environment—including the ocean, glaciers, underground reservoirs of water (aquifers), and surface waters such as lakes, rivers, and streams—and through the atmosphere, is vital for life on Earth and is responsible for the planet's climate. This cycling of water is called the hydrologic cycle.

The total amount of water present on Earth as a solid, liquid, and gas is constant, but the water shifts between the various states in various environmental locations. For example, at different times a given molecule of water can be part of a frozen lake, water vapor in the atmosphere, or a raindrop falling back to the surface.

Although the total amount of water remains the same, it is not distributed evenly. Temperate regions north and south of the equator receive much precipitation, while other arid areas of Earth receive so little water that they are desert-like.

This disparity will increase due to global warming, according to the Intergovernmental Panel on Climate Change (IPCC). In its 2007 Assessment Report, the IPCC notes that increased runoff and less severe winters

may be affecting hydrological systems, particularly in the polar regions. Although the consensus among scientists is that the changing hydrologic cycle will influence climate, it is not clear whether the change will be slight or great.

Historical Background and Scientific Foundations

The cycling of water between Earth's surface and the surrounding environment has been occurring since the atmosphere developed nearly five million years ago. The amount of water that formed as Earth cooled is about the same as the volume present in 2007. The water has been endlessly cycling since the atmosphere formed.

Some of Earth's freshwater has not been part of the hydrological cycle, having become bound as frozen water in glacial or polar ice. Yet, this previously inaccessible water is now increasingly joining the global water cycle, as polar ice melts under the influence of the warming atmosphere.

The hydrologic cycle consists of five parts: condensation, precipitation, infiltration, runoff, and evaporation. Condensation is the process where water changes from a gas (water vapor) to a liquid due to cooling temperature. Clouds form due to condensation. Moisture released from clouds as precipitation falls to the surface and can percolate down into the soil (infiltration) or run along the surface of impermeable surfaces (runoff) to collect in stream, rivers, and larger bodies of water. Some of the water that has infiltrated into Earth will move underground and emerge at surface water sources as well. Water also remains underground where it becomes a source of moisture for vegetation and drinking water. Finally, the heat from the sun will cause evaporation of the surface water, changing it from a liquid to water vapor that rises in the atmosphere, where it cools and condenses, beginning another round of the hydrologic cycle.

The seemingly simple nature of the water cycle that can be depicted on paper is more complex in real life. Influences include the amount of vegetation present, nature of the surface (soil absorbs water better than rockier terrain), and atmospheric temperature. All these are influenced by human activities.

Impacts and Issues

The hydrologic cycle is vital to Earth's climate, and is being affected by the increasing temperature of the planet's atmosphere. Increased evaporation in equatorial areas increases the amount of water vapor in the atmosphere, which can help retain heat. The result in the twenty-first century may, according to some climate scientists, be a vicious circle, whereby global warming accelerates the hydrologic cycle, which then speeds up global warming.

Other scientists argue that this scenario fails to account for the complexity of the interactions that occur between Earth's surface and the atmosphere. Indeed, different computer models produce different predictions of the extent of global warming. However, climate models all agree that warming of the atmosphere will continue, and that the hydrologic cycle is involved.

Analysis of data from the past 150 years has established that human activities influence the hydrologic cycle. For example, the removal of large numbers of trees by deforestation to produce agricultural land or urban development reduces the amount of water that is transferred to the atmosphere. In its 2007 assessment report, the IPCC concluded that it is very likely that natural systems, including the hydrologic cycle, are being affected by human-mediated climate change.

Humans have used parts of the hydrologic cycle to create power for centuries. Two thousand years ago, waterfalls were already being used to move stone wheels that ground wheat into flour. Damming of rivers allows the use of water energy to generate electricity. These types of hydroelectric developments cause local changes in hydrology, especially by reducing variations of flow and sometimes by unpredictable spillage of water at times when the storage capacity of the reservoir is full.

Human activities may be having a much greater influence on the hydrologic cycle than local activities such as deforestation and urbanization. In polar areas, the freshwater content of surface and deeper water of the North Atlantic has been increasing since the mid-twentieth century with the increased warming of the atmosphere, according to the IPCC. At the same time, water loss from tropical surface waters has been increasing due to the increased evaporation caused by global warming. These are indications that the hydrologic cycle is changing.

WORDS TO KNOW

CLIMATE MODEL: A quantitative way of representing the interactions of the atmosphere, oceans, land surface, and ice. Models can range from relatively simple to quite comprehensive.

DEFORESTATION: Those practices or processes that result in the change of forested lands to non-forest uses. This is often cited as one of the major causes of the enhanced greenhouse effect for two reasons: 1) the burning or decomposition of the wood releases carbon dioxide; and 2) trees that once removed carbon dioxide from the atmosphere in the process of photosynthesis are no longer present and contributing to carbon storage.

HYDROLOGY: The science that deals with global water (both liquid and solid), its properties, circulation, and distribution.

PRECIPITATION: Moisture that falls from clouds. Although clouds appear to float in the sky, they are always falling, their water droplets slowly being pulled down by gravity. Because the water droplets are so small and light, it can take 21 days to fall 1,000 ft (305 m), and wind currents can easily interrupt their descent. Liquid water falls as rain or drizzle. All raindrops form around particles of salt or dust. (Some of this dust comes from tiny meteorites and even the tails of comets.) Water or ice droplets stick to these particles, then the drops attract more water and continue getting bigger until they are large enough to fall out of the cloud. Drizzle drops are smaller than raindrops. In many clouds, raindrops actually begin as tiny ice crystals that form when part or all of a cloud is below freezing. As the ice crystals fall inside the cloud, they may collide with water droplets that freeze onto them. The ice crystals continue to grow larger, until large enough to fall from the cloud. They pass through warm air, melt, and fall as raindrops.

WATER VAPOR: The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. Although humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation.

IN CONTEXT: IMPACTS ON WATER RESOURCES

According to the National Academy of Sciences: “Climate-driven changes in precipitation in certain regions could have significant impacts on water availability for agriculture, residential and industrial use, and recreation. Such regional impacts will be much more noticeable than projected changes in global average temperature.”

SOURCE: Staudt, Amanda, Nancy Huddleston, and Sandi Rudenstein. Understanding and Responding to Climate Change. National Academy of Sciences, 2006.

The effect of a changed global water cycle is not clear. Climate models have shown that the increased source of freshwater in more northern latitudes could affect ocean currents, in particular the Gulf Stream. In 2005, a paper published in the journal Nature reported that the northern flow of the warm Gulf Stream had decreased by 30% as compared to earlier surveys. The scientific consensus is that an altered Gulf Stream would affect the climates of eastern North America and the United Kingdom (the U.K. would become colder). But, the extent of the temperature change and whether the changed Gulf Stream flow was only a single incident or the beginning of a more persistent change that is related to a global climate change was unclear as of 2007.

See Also Agriculture: Vulnerability to Climate Change; Biogeochemical Cycle; General Circulation Model (GCM); Ocean Circulation and Currents.

BIBLIOGRAPHY

Books

DiMento, Joseph F. C., and Pamela M. Doughman. Climate Change: What It Means for Us, Our Children, and Our Grandchildren. Boston: MIT Press, 2007.

Gore, Al. An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. New York: Rodale Books, 2006.

Web Sites

“Climate and the Water Cycle.” National Center for Atmospheric Research. <http://www.ncar.ucar.edu/research/earth_system/watercycle.php> (accessed November 30, 2007).

“Common Misconceptions About Abrupt Climate Change.” Woods Hole Oceanographic Institution, August 30, 2007. <http://www.whoi.edu/page.do?pid=12455&tid=282&cid=10149#ocean_9> (accessed November 30, 2007).

Bill Freeman

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Hydrologic Cycle

Hydrologic Cycle

Introduction

The continuous circulation of water through regions of the global environment—including the ocean; glaciers; underground reservoirs of water (aquifers); and surface waters such as lakes, rivers, and streams—and through the atmosphere, is vital for life on Earth and is responsible for the planet’s climate. This cycling of water is called the hydrologic cycle.

The total amount of water present on Earth as solid, liquid, and gas is constant, but water shifts between these three states over time. For example, at different times a given molecule of water can be part of a frozen lake, water vapor in the atmosphere, in a living creature, or a raindrop or snowflake falling to the surface.

Moreover, Earth’s water is not evenly distributed. About 70% of Earth’s surface is ocean; some land areas receive regular precipitation, while others are deserts.

The unevenness and irregularity of precipitation patterns in some regions will likely increase due to global warming, according to the Intergovernmental Panel on Climate Change (IPCC). In its 2007 Assessment Report, the IPCC noted that increased runoff and less severe winters may be affecting hydrological systems, particularly in the polar regions.

Historical Background and Scientific Foundations

The cycling of water between Earth’s surface and the surrounding environment has been occurring since the atmosphere developed over four billion years ago. The amount of water on Earth has changed little from the planet’s formation to the present, and this water has been endlessly cycling over the surface since the atmosphere formed.

Some of Earth’s freshwater is temporarily removed from the hydrological cycle by becoming bound as frozen water in glacial and polar ice. During periods of warming climate, this bound-up water is melted and so returned to the hydrological cycle. We are presently in a period of climate warming caused by human activities. Increased warmth is causing accelerated melting of mountain glaciers and parts of the Greenland and Antarctic ice sheets, adding more water to the oceans and raising their levels. Shifts in evaporation and rainfall patterns are also occurring in many regions as a result of human-caused global climate change.

The hydrologic cycle consists of five processes: condensation, precipitation, infiltration, runoff, and evaporation. Condensation is the process where water changes from a gas (water vapor) to a liquid due to cooling temperature. Clouds form due to condensation. Moisture released from clouds as precipitation falls to the surface and can percolate down into the soil (infiltration) or run along the surface of impermeable surfaces (runoff) to collect in stream, rivers, and larger bodies of water. Some of the water that has infiltrated into Earth will move underground and emerge at surface water sources as well. Water also remains underground, where it can become a source of moisture for vegetation, enter rivers, lakes, or oceans, or be pumped to the surface for irrigation or drinking water. Finally, heat from the sun evaporates surface water, changing it from a liquid to water vapor that rises in the atmosphere, where it cools and condenses, beginning another round of the hydrologic cycle.

In reality, the cycle is more complex. Regional factors include the amount of vegetation present, the nature of the surface (soil absorbs water better than rockier terrain), weather, and climate (long-term average weather). All these factors are influenced by human activities.

WORDS TO KNOW

CLIMATE MODEL: A quantitative method of simulating the interactions of the atmosphere, oceans, land surface, and ice. Models can range from relatively simple to quite comprehensive.

DEFORESTATION: A reduction in the area of a forest resulting from human activity.

EVAPOTRANSPIRATION: The sum of evaporation and plant transpiration. Potential evapotranspiration is the amount of water that could be evaporated or transpired at a given temperature and humidity, if there was plenty of water available. Actual evapotranspiration can not be any greater than precipitation, and will usually be less because some water will run off into rivers and flow to the oceans.

HYDROLOGY: The study of the distribution, movement, and physical-chemical properties of water in Earth’s atmosphere, surface, and near-surface crust.

PRECIPITATION: Moisture that falls from clouds as a result of condensation in the atmosphere.

WATER VAPOR: The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. While humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation.

WATERSHED: The expanse of terrain from which water flows into a wetland, water body, or stream.

Impacts and Issues

The hydrologic cycle is vital to Earth’s climate, and is being affected by the increasing temperature of the planet’s atmosphere. Increased evaporation in equatorial areas increases the amount of water vapor in the atmosphere, which can help retain heat. The result in the twenty-first century may, according to some climate scientists, be a vicious circle, whereby global warming accelerates the hydrologic cycle, which then speeds up global warming.

Other scientists argue that this scenario fails to account for the complexity of the interactions that occur between Earth’s surface and the atmosphere. Indeed, different computer models produce different predictions of the extent of global warming. However, climate models all agree that warming of the atmosphere will continue, and that the hydrologic cycle is involved.

Data from the past 150 years show that human activities influence the hydrologic cycle in a variety of ways. For example, the removal of a large numbers of trees by deforestation to produce agricultural land or urban development reduces the amount of water that is transferred to the atmosphere. In its 2007 Assessment Report, the IPCC concluded that it is very likely that natural systems, including the hydrologic cycle, are being affected by human-mediated climate change.

Humans have used parts of the hydrologic cycle to create power for centuries. Two thousand years ago, waterfalls were already being used to power stone wheels that ground wheat into flour. Damming of rivers allows the use of water energy to generate electricity. These types of hydroelectric developments cause local changes

in hydrology, especially by reducing variations of flow and sometimes by unpredictable spillage of water at times when the storage capacity of the reservoir is full.

Human activities may be having a much greater influence on the hydrologic cycle than local activities such as deforestation and urbanization. In polar areas, the freshwater content of surface and deeper water of the North Atlantic has been increasing since the mid-twentieth century with the increased warming of the atmosphere, according to the IPCC. At the same time, water loss from tropical surface waters has been increasing due to the increased evaporation caused by global warming. These are indications that the hydrologic cycle is changing.

How much, and in what ways, the global water cycle is changing is not clear. Climate models have shown that increased freshwater input to the North Atlantic due to accelerated melting of the Greenland ice sheet may slow the meridional overturning circulation (MOC) of the oceans, which transports water in a global “conveyor belt” that strongly affects climate. In particular, strong slowing or stoppage of this circulation might lead to drastic cooling in Northern Europe. Studies of ancient climate data show that such changes have occurred in the past; however, scientists do not at present understand the affects of Greenland melting on the MOC well enough to say whether a major shift due to human-caused global warming is likely.

See Also Agricultural Practice Impacts; Climate Change; Global Warming; IPCC 2007 Report; Ocean Circulation and Currents

IN CONTEXT: OBSERVED IMPACTS OF CLIMATE CHANGE ON THE HYDROLOGIC CYCLE

According to the Intergovernmental Panel on Climate Change (IPCC): “Based on growing evidence, there is high confidence that the following effects on hydrological systems are occurring:”

  • “increased runoff and earlier spring peak discharge in many glacier- and snow-fed rivers;”
  • “warming of lakes and rivers in many regions, with effects on thermal structure and water quality.”

Source: Parry, M. L., et al. IPCC, 2007: Summary for Policymakers. Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

BIBLIOGRAPHY

Books

DiMento, Joseph F. C., and Pamela M. Doughman. Climate Change: What It Means for Us, Our Children, and Our Grandchildren. Boston: MIT Press, 2007.

Gore, Al. An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. New York: Rodale Books, 2006.

Periodicals

Worden, John, et al. “Importance of Rain Evaporation and Continental Convection in the Tropical Water Cycle.” Nature 445 (2007): 528–532.

Web Sites

National Center for Atmospheric Research. “Climate and the Water Cycle.” http://www.ncar.ucar.edu/research/earth_system/watercycle.php (accessed March 29, 2008).

Woods Hole Oceanographic Institution. “Common Misconceptions About Abrupt Climate Change.” http://www.whoi.edu/page.do?pid=12455&tid=282&cid=10149#ocean_9 (accessed March 29, 2008).

Bill Freeman

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Hydrologic Cycle

Hydrologic cycle

The hydrologic, or water , cycle is the continuous, interlinked circulation of water among its various compartments in the environment. Hydrologic budgets are analyses of the quantities of water stored, and the rates of transfer into and out of those various compartments.

The most important places in which water occurs are the ocean , glaciers , underground aquifers, surface waters, and the atmosphere. The total amount of water among all of these compartments is a fixed, global quantity. However, water moves readily among its various compartments through the processes of evaporation , precipitation , and surface and subsurface flows. Each of these compartments receives inputs of water and has corresponding outputs, representing a flow-through system. If there are imbalances between inputs and outputs, there can be significant changes in the quantities stored locally or even globally. An example of a local change is the drought that can occur in soil after a long period without replenishment by precipitation. An example of a global change in hydrology is the increasing mass of continental ice that occurs during glacial epochs, an event that can remove so much water from the oceanic compartment that sea level can decline by more than 328 ft (100 m), exposing vast areas of continental shelf for the development of terrestrial ecosystems.



Major compartments and fluxes of the hydrologic cycle

Estimates have been made of the quantities of water that are stored in various global compartments. By far the largest quantity of water occurs in the deep lithosphere , which contains an estimated 27 × 1018 tons (27-billion-billion tons) of water, or 94.7% of the global total. The next largest compartment is the oceans, which contain 1.5 × 1018 tons, or 5.2% of the total. Ice caps contain 0.019 × 1018 tons, equivalent to most of the remaining 0.1% of Earth's water. Although present in relatively small quantities compared to the above, water in other compartments is very important ecologically because it is present in places where biological processes occur. These include shallow groundwater (2.7 × 1014 tons), inland surface waters such as lakes and rivers [0.27 × 1014 ton], and the atmosphere [0.14 × 1014 tons]).

The smallest compartments of water also tend to have the shortest turnover times, because their inputs and outputs are relatively large in comparison with the mass of water that is contained. This is especially true of atmospheric water, which receives annual inputs equivalent to 4.8 × 1014 tons as evaporation from the oceans (4.1 × 1014 tons/yr) and terrestrial ecosystems (0.65 × 1014 tons/yr), and turns over about 34 times per year. These inputs of water to the atmosphere are balanced by outputs through precipitation of rain and snow, which deposit 3.7 × 1014 tons of water to the surface of the oceans each year, and 1.1 × 1014 tons/yr to the land.

These data suggest that the continents receive inputs of water as precipitation that are 67% larger than what is lost by evaporation from the land. The difference, equivalent to 0.44 × 1014 tons/yr, is made up by 0.22 × 1014 tons/yr of runoff of water to the oceans through rivers, and another 0.22 × 1014 tons/yr of subterranean runoff to the oceans.

The movements of water in the hydrologic cycle are driven by gradients of energy . Evaporation occurs in response to the availability of thermal energy and gradients of concentration of water vapor. The ultimate source of energy for virtually all natural evaporation of water on Earth is solar electromagnetic radiation . This solar energy is absorbed by surfaces, increasing their heat content, and thereby providing a source of energy to drive evaporation. In contrast, surface and ground waters flow in response to gradients of gravitational potential. In other words, unless the flow is obstructed, water spontaneously courses downhill.


Hydrologic cycle of a watershed

The hydrological cycle of a defined area of landscape is a balance between inputs of water with precipitation and upstream drainage, outputs as evaporation and drainage downstream or deep into the ground, and any internal storage that may occur because of imbalances of the inputs and outputs. Hydrological budgets of landscapes are often studied on the spatial scale of watersheds, or the area of terrain from which water flows into a stream, river, or lake .

The simplest watersheds are so-called headwater systems that do not receive any drainage from watersheds at higher altitude, so the only hydrologic input occurs as precipitation, mostly as rain and snow. However, at places where fog is a common occurrence, windy conditions can effectively drive tiny atmospheric droplets of water vapor into the forest canopy, and the direct deposition of cloud water can be important. This effect has been measured for a foggy conifer forest in New Hampshire, where fogwater deposition was equivalent to 33 in (84 cm) per year, compared with 71 in (180 cm) per year of hydrologic input as rain and snow.

Vegetation can have an important influence on the rate of evaporation of water from watersheds. This hydrologic effect is especially notable for well-vegetated ecosystems such as forests , because an extensive surface area of foliage supports especially large rates of transpiration . Evapotranspiration refers to the combined rates of transpiration from foliage, and evaporation from nonliving surfaces such as moist soil or surface waters. Because transpiration is such an efficient means of evaporation, evapotranspiration from any well vegetated landscape occurs at much larger rates than from any equivalent area of non-living surface.

In the absence of evapotranspiration an equivalent quantity of water would have to drain from the watershed as seepage to deep groundwater or as streamflow. Studies of forested watersheds in Nova Scotia found that evapotranspiration was equivalent to 15-29% of the hydrologic inputs with precipitation. Runoff through streams or rivers was estimated to account for the other 71-85% of the atmospheric inputs of water, because the relatively impervious bedrock in that region prevented significant drainage to deep ground water.

Forested watersheds in seasonal climates display large variations in their rates of evapotranspiration and streamflow. This effect can be illustrated by the seasonal patterns of hydrology for a forested watershed in eastern Canada. The input of water through precipitation is 58 in (146 cm) per year, but 18% of this arrives as snow, which tends to accumulate on the surface as a persistent snowpack. About 38% of the annual input is evaporated back to the atmosphere through evapotranspiration, and 62% runs off as river flow. Although there is little seasonal variation in the input of water with precipitation, there are large seasonal differences in the rates of evapotranspiration, runoff, and storage of groundwater in the watershed. Evapotranspiration occurs at its largest rates during the growing season of May to October, and runoff is therefore relatively sparse during this period. In fact, in small watersheds in this region forest streams can literally dry up because so much of the precipitation input and soil water is utilized for evapotranspiration, mostly by trees. During the autumn, much of the precipitation input serves to recharge the depleted groundwater storage, and once this is accomplished stream flows increase again. Runoff then decreases during winter, because most of the precipitation inputs occur as snow, which accumulates on the ground surface because of the prevailing sub-freezing temperatures. Runoff is largest during the early springtime, when warming temperatures cause the snowpack to melt during a short period of time, resulting in a pronounced flush of stream and river flow.


Influences of human activities on the hydrologic cycle

Some aspects of the hydrologic cycle can be utilized by humans for a direct economic benefit. For example, the potential energy of water elevated above the surface of the oceans can be utilized for the generation of electricity . However, the development of hydroelectric resources generally causes large changes in hydrology. This is especially true of hydroelectric developments in relatively flat terrain, which require the construction of large storage reservoirs to retain seasonal high-water flows, so that electricity can be generated at times that suit the peaks of demand. These extensive storage reservoirs are essentially artificial lakes, sometimes covering enormous areas of tens of thousands of hectares. These types of hydroelectric developments cause great changes in river hydrology, especially by evening out the variations of flow, and sometimes by unpredictable spillage of water at times when the storage capacity of the reservoir is full. Both of these hydrologic influences have significant ecological effects, for example, on the habitat of salmon and other aquatic biota. In one unusual case, a large water spillage from a reservoir in northern Quebec drowned 10,000 caribou that were trapped by the unexpected cascade of water during their migration .

Where the terrain is suitable, hydroelectricity can be generated with relatively little modification to the timing and volumes of water flow. This is called run-of-the-river hydroelectricity, and its hydrologic effects are relatively small. The use of geologically warmed ground water to generate energy also has small hydrological effects, because the water is usually re-injecting back into the aquifer .

Human activities can influence the hydrologic cycle in many other ways. The volumes and timing of river flows can be greatly affected by channeling to decrease the impediments to flow, and by changing the character of the watershed by paving, compacting soils, and altering the nature of the vegetation. Risks of flooding can be increased by speeding the rate at which water is shed from the land, thereby increasing the magnitude of peak flows. Risks of flooding are also increased if erosion of soils from terrestrial parts of the watershed leads to siltation and the development of shallower river channels, which then fill up and spill over during high-flow periods. Massive increases in erosion are often associated with deforestation , especially when natural forests are converted into agriculture.

The quantities of water stored in hydrologic compartments can also be influenced by human activities. An important example of this effect is the mining of groundwater for use in agriculture, industry, or for municipal purposes. The best known case of groundwater mining in North America concerns the enormous Ogallala aquifer of the southwestern United States, which has been drawn down mostly to obtain water for irrigation in agriculture. This aquifer is largely comprised of "fossil water" that was deposited during earlier, wetter climates, although there is some recharge capability through rain-fed groundwater flows from mountain ranges in the watershed of this underground reservoir.

Sometimes industrial activities lead to large emissions of water vapor into the atmosphere, producing a local hydrological influence through the development of low-altitude clouds and fogs. This effect is mostly associated with electric power plants that cool their process water using cooling towers.

A more substantial hydrologic influence on evapotranspiration is associated with large changes in the nature of vegetation over a substantial part of a watershed. This is especially important when mature forests are disturbed, for example, by wildfire , clear-cutting, or conversion into agriculture. Disturbance of forests disrupts the capacity of the landscape to sustain transpiration, because the amount of foliage is reduced. This leads to an increase in streamflow volumes, and sometimes to an increased height of the groundwater table. In general, the increase in streamflow after disturbance of a forest is roughly proportional to the fraction of the total foliage of the watershed that is removed (this is roughly proportional to the fraction of the watershed that is burned, or is clear-cut). The influence on transpiration and streamflow generally lasts until regeneration of the forest restores another canopy with a similar area of foliage, which generally occurs after about 5-10 years of recovery. However, there can be a longer-term change in hydrology if the ecological character of the watershed is changed, as occurs when a forest is converted to agriculture.


Resources

books

Freedman, B. Environmental Ecology. 2nd ed. San Diego: Academic Press, 1995.

Herschy, Reginald, and Rhodes Fairbridge, eds. Encyclopedia of Hydrology and Water Resources. Boston: Kluwer Academic Publishing, 1998.

Ricklefs, R.E. Ecology. 3rd ed. New York: Freeman, 1990.

periodicals

Berbery, Ernesto Hugo. "The Hydrologic Cycle of the La Plata Basin in South America." Journal of Hydrometeorology 3, no. 6 (2002): 630-645.

"Temperature And Rainfall Tables: July 2002." Journal of Meteorology 27, no. 273 (2002): 362.


Bill Freedman

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Evapotranspiration

—The evaporation of water from a large area, including losses of water from foliage as transpiration, and evaporation from non-living surfaces, including bodies of water.

Hydrology

—The study of the distribution, movement, and physical-chemical properties of water in Earth's atmosphere, surface, and near-surface crust.

Precipitation

—The deposition from the atmosphere of rain, snow, fog droplets, or any other type of water.

Watershed

—The expanse of terrain from which water flows into a wetland, waterbody, or stream.

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Hydrologic Cycle

Hydrologic Cycle

Water is in constant motion. Energy from the sun and the force of gravity drive the hydrologic cycle, which is the endless circulation of water between the land, oceans, and atmosphere (air surrounding Earth). Water also changes in form: from gas (water vapor), to liquid, to solid (ice). Rain and snow falling on the land runs off into streams and lakes, or soaks into soil and rocks. Streams and rivers carry water downhill to lakes and, ultimately, to the ocean. Heat energy from the Sun transforms liquid water at the surface of lakes and oceans and other bodies of water into water vapor. Water vapor in the atmosphere rises and forms clouds. Cooling within clouds causes water vapor to become liquid once again. Rain and snow fall and the cycle begins anew.

The water budget

Earth's water budget, the total amount of water on the planet, does not change over time. The hydrologic cycle is a closed system. Water is constantly moving and changing form, but it is neither created nor destroyed. With the exception of a very small amount of water added to the hydrologic system by volcanic eruptions and meteors from space, Earth's total water supply is constant. In fact, most of the water on Earth today has been recycling through the hydrologic system for billions of years. The same water that comes from a kitchen faucet today could have been drunk by a dinosaur 170 million years ago during the Jurassic Period. It could have been frozen in an ice sheet during the Pleistocene Epoch (a division of geologic time that lasted from 2 million to 10,000 years ago), and could have flowed through a canal in the Roman Empire two thousand years ago. It could have been snow in the Rocky Mountains last winter, flowed in a river to the city's municipal water intake, and out of the faucet this morning. Maybe it will return to the river via the sink drain and city sewage system, and then flow to the ocean.

Reservoirs

Within the hydrologic system, water resides in environments called reservoirs. Earth's largest reservoirs, the oceans, contain about 97% of the planet's total water. Ice, including sheets of ice on the North and South Poles and mountain glaciers (a large body of slow moving ice), and groundwater reservoirs called aquifers hold most of the remaining 3%. Reservoirs of readily useable fresh water—rivers, lakes, soil moisture, atmospheric water vapor, and water in living cells—account for only about 1% of the fresh water, and less than 0.02% of water on Earth.

If a bathtub filled with 100 gallons (379 liters) of water represented Earth's total water budget, three gallon (11 liter) jugs would hold all the fresh water, and the fresh water available for immediate use by humans would only fill a tablespoon. A microscope would be needed to see the droplet representing the water bound up in plants and animals.

Water processes

All of Earth's water molecules are in constant motion. (A molecule is the smallest particle of a substance that has the chemical characteristics of the substance. A water molecule, symbolized by H2O, is made up of two hydrogen atoms and an oxygen atom.) Processes move water from one reservoir to another and within reservoirs. Liquid water flows downhill and circulates within lakes and oceans. Clouds of water vapor, liquid droplets, and ice crystals (snow) move across the sky. Even molecules bound in glacial ice flow downhill.

Energy from the Sun and the downward pull of gravity ultimately drive all the processes within the hydrologic cycle. Water cycle processes include evaporation, condensation, convection, precipitation, freezing and melting, groundwater flow, and runoff.

  • Evaporation is the conversion of water from a liquid to a gas. Water moves from bodies of water and land to the atmosphere when heat from the Sun transforms liquid water to water vapor. Most (about 80%) of the water vapor in the atmosphere evaporates from the oceans, especially the tropical oceans near the equator. Transpiration is evaporation of water from the leaves and stems of plants. It contributes about 10% of the water vapor in the atmosphere, and evaporation from inland seas, lakes and rivers accounts for the remaining ten percent.
  • Condensation is the conversion of water from a gas to a liquid. As air containing molecules of water rises in the atmosphere, the air cools, and the motion of the water molecules slows. The slower-moving water molecules accumulate as water vapor in the rising air. Water vapor then forms droplets of liquid water that group together into clouds, and eventually can fall as rain.
  • Convection is the large-scale circulation of the atmosphere and oceans. Warm air or water rises and cool air or water sinks, creating currents (a steady flow in a dominant direction) that transport water around the globe. Convection causes winds that blow rain clouds over the continents, and ocean currents that transport heat, and affect global climate.
  • Precipitation is the transfer of water from the atmosphere to Earth's surface. Rain, snow, sleet, and hail are all types of precipitation. When condensed water droplets or ice crystals in a cloud become too large and heavy to remain aloft, they fall to the ground as precipitation. Amounts of precipitation vary greatly between locations. For example, the deserts of the American Southwest receive less than 1 inch (2.5 centimeters) of rain per year, while the summit of Mt. Waialeale on the Hawaiian island of Kauai receives more than 400 inches (1,016 centimeters) of rain per year. Heavy precipitation over a short amount of time can cause rivers and groundwater reservoirs to overflow and lead to flooding. Lack of normal levels of precipitation for an extended period of time causes the dried soil and reduced water supplies associated with drought.
  • Freezing and melting are the transformations between liquid and solid water. Most freezing occurs in the atmosphere where condensed water vapor forms ice crystals in clouds. Glaciers form in areas near the North and South Poles and in high mountains where more snow falls than melts each year and ice accumulates over many years. In many regions, melting snow and ice replenish river and groundwater flow, as in aquifers, every spring. During the cold winter months in some regions, the surfaces of lakes and rivers freeze. In polar regions, even the seawater and groundwater freeze.
  • Groundwater flow is the movement of liquid water through the pores (openings) in soils and cavities in rocks near Earth's surface. Surface water becomes groundwater by soaking into these tiny spaces, which were filled with air. Groundwater then percolates downward to the surface of the water table, the line where all the spaces are saturated (completely full) with water. Water below the water table flows toward areas of lower pressure where it can be released, such as springs or wells.
  • Runoff is the transfer of water from the land surface to the oceans via streams and lakes. (Lakes only hold runoff temporarily, and lake water eventually ends up in the ocean.) Runoff consists of precipitation that neither evaporates back into the atmosphere, nor infiltrates into groundwater. Groundwater discharge can also replenish runoff. Excess runoff leads to flooding.

Dynamic equilibrium and residence times

All water molecules are in motion, but the total volume of water in a particular reservoir stays relatively constant because of a phenomenon called dynamic equilibrium. The processes that remove water molecules from a reservoir are balanced by the processes that add water. To illustrate, imagine trying to maintain a constant volume of water in a bathtub with an open drain. When the faucet is adjusted to add water at the same rate as it is draining, the water level stays constant, and dynamic equilibrium is reached. In the same way, sea level stays constant because the amount of water evaporating into the atmosphere matches the amount of water entering from rivers and melting glaciers. Over geologic time (the time from the formation of Earth to the present), this balance changes and the sea level rises and falls.

The atmosphere transfers water from the ocean to the land, but it only holds a tiny portion (.001%) of Earth's total water. Water has a short residence time in the atmosphere. Almost as soon (usually a few hours) as it evaporates into the air, water vapor condenses and falls again as precipitation. Water molecules stay in some glaciers, oceans, and groundwater reservoirs for thousands of years, while others only spend a few days or weeks in a reservoir. To maintain dynamic equilibrium, water must leave the reservoir at the same rate that it enters. In reservoirs with very long residence times, a change in the rate of water that enters or leaves can quickly affect the reservoir volume. For example, the Ogallala groundwater reservoir in the U.S. Great Plains region is a sandstone (rock formed from the compaction of sand) layer that filled with water a thousand years ago when the climate was wetter. In modern times, ranchers in Texas, Oklahoma, Kansas, Nebraska, and other states are using up the stored groundwater by withdrawing it much more quickly than it replenishes in today's dryer climate.

The hydrologic cycle as a component of the Earth system

The hydrologic cycle is intertwined with the other cycles that make up the Earth system. Moving water chemically and physically erodes (wears away) the solid Earth. It transports sediments (fine soil and other particles) and deposits them in river floodplains (lands near rivers that disperse overflow), deltas (where a river enters a lake or ocean, and continental margins (edges of continents that are underwater). It sculpts the land surface and seafloor. Water carries dissolved minerals and nutrients that nourish freshwater and marine ecosystems. Water in the oceans and atmosphere regulates Earth's climate and weather, which makes the planet habitable for biological life. Water is the largest component of most biological organisms. Jellyfish are more than 90% water. If a person weighs 120 pounds (54 kilograms), about 72 pounds (33 kilograms) of his or her weight is water.

Water is Earth's most essential renewable resource. It is conserved within the Earth system and cannot be "used up." However, water is very scarce in some regions and overly abundant in others. Deserts, rainforests, canyons, droughts, and floods all result from the uneven distribution of water on Earth. Scientists have concluded that human activities such as damming rivers, polluting waters, transporting water to arid (dry) regions to grow crops, and contributing to global climate change can alter the hydrologic cycle and change the patterns of water distribution. Water is continuously recycled and is ultimately a renewable resource, but challenges remain to manage water resources as the human population grows.

Laurie Duncan, Ph.D.

For More Information

Books

Postel, Sandra, and Brian Richter. Rivers for Life: Managing Water for People and Nature. Washington, DC: Island Press, 2003.

Websites

"The Hydrologic Cycle: Online Meteorology Guide." WW2010 Department of Atmospheric Sciences. University of Illinois at Urbana-Champagne.http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hyd/home.rxml (accessed on August 10, 2004).

"Water Basics." Water Science for Schools. United States Geological Survey. http://ga.water.usgs.gov/edu/mwater.html (accessed on August 10, 2004).

"The Water Cycle." Kidzone: Fun Facts for Kids.http://www.kidzone.ws/water/ (accessed on August 10, 2004).

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