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 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.

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

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 Earth’s 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 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, 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

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.

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).

Hydrologic cycle

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

See also Estuary; Leaching

[Kenneth N. Brooks ]

RESOURCES

BOOKS

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

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

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

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

PERIODICALS

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

OTHER

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