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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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