Water cycle

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The water cycle — technically known as the hydrologic cycle or hydrological cycle — is the continuous circulation of water within the Earth's hydrosphere, and is driven by solar radiation. This includes the atmosphere, land, surface water and groundwater. As water moves through the cycle, it changes state between liquid, solid, and gas phases. Water moves from compartment to compartment, such as from river to ocean, by the physical processes of evaporation, precipitation, infiltration, runoff, and subsurface flow.^[1]

Contents 1 Description 2 Reservoirs 3 Residence times 4 Changes over time 5 Effects on climate 6 Effects on biogeochemical cycling 7 See also 8 References 9 External links

Description

The water cycle is the continuous movement of water over, above, and beneath the Earth's surface.^[2] It is powered by solar energy, and because it is a cycle, there is no beginning or end. As water moves around in the hydrosphere, it changes state among liquid, vapour, and ice. The time taken for water to move from one place to another varies from seconds to thousands of years, and the amount of water stored in different parts of the hydrosphere ranges up to 1.37 billion km³, which is contained in the oceans. Despite continual movement within the hydrosphere, the total amount of water at any one time remains essentially constant.

Movement of water takes place by a variety of physical and biophysical processes. The two processes responsible for moving the greatest quantities of water are precipitation and evaporation, transporting 505,000 km³ of water each year. The flow of water along rivers transports an intermediate amount of water, and sublimation of ice directly to vapour transports relatively very little. The different processes are as follows.

• Precipitation is condensed water vapor that falls to the Earth's surface. Most precipitation occurs as rain, but also includes snow, hail, fog drip, graupel, and sleet.^[3] Approximately 505,000 km³ of water fall as precipitation each year, 398,000 km³ of it over the oceans.^[4]

• Canopy interception is the precipitation that is intercepted by plant foliage and eventually evaporates back to the atmosphere rather than falling to the ground.

• Snowmelt refers to the runoff produced by melting snow.

• Runoff includes the variety of ways by which water moves across the land. This includes both surface runoff and channel runoff. As it flows, the water may infiltrate into the ground, evaporate into the air, become stored in lakes or reservoirs, or be extracted for agricultural or other human uses.

• Infiltration is the flow of water from the ground surface into the ground. Once infiltrated, the water becomes soil moisture or groundwater.^[5]

• Subsurface Flow is the flow of water underground, in the vadose zone and aquifers. Subsurface water may return to the surface (eg. as a spring or by being pumped) or eventually seep into the oceans. Water returns to the land surface at lower elevation than where it infiltrated, under the force of gravity or gravity induced pressures. Groundwater tends to move slowly, and is replenished slowly, so it can remain in aquifers for thousands of years.

• Evaporation is the transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere.^[6] The source of energy for evaporation is primarily solar radiation. Evaporation often implicitly includes transpiration from plants, though together they are specifically referred to as evapotranspiration. Approximately 90% of atmospheric water comes from evaporation, while the remaining 10% is from transpiration.^{[citation needed]} Total annual evapotranspiration amounts to approximately 505,000 km³ of water, 434,000 km³ of which evaporates from the oceans.^[7]

• Sublimation is the state change directly from solid water (snow or ice) to water vapor.^[8]

• Advection is the movement of water — in solid, liquid, or vapour states — through the atmosphere. Without advection, water that evaporated over the oceans could not precipitate over land.^[9]

• Condensation is the transformation of water vapour to liquid water droplets in the air, producing clouds and fog.^[10]

Reservoirs

Volume of water stored in
the water cycle's reservoirs^[11]

Reservoir	Volume of water (106 km³)	Percent of total
Oceans	1370	97.25
Ice caps & glaciers	29	2.05
Groundwater	9.5	0.68
Lakes	0.125	0.01
Soil moisture	0.065	0.005
Atmosphere	0.013	0.001
Streams & rivers	0.0017	0.0001
Biosphere	0.0006	0.00004

In the context of the water cycle, a reservoir represents the water contained in different steps within the cycle. The largest reservoir is the collection of oceans, accounting for 97% of the Earth's water. The next largest quantity (2%) is stored in solid form in the ice caps and glaciers. The water contained within all living organisms represents the smallest reservoir.

The volume of water in the fresh water reservoirs, particularly those that are available for human use, are important water resources.^[12]

Residence times

Average reservoir residence times^[11]

Reservoir	Average residence time
Oceans	3,200 years
Glaciers	20 to 100 years
Seasonal snow cover	2 to 6 months
Soil moisture	1 to 2 months
Groundwater: shallow	100 to 200 years
Groundwater: deep	10,000 years
Lakes	50 to 100 years
Rivers	2 to 6 months
Atmosphere	9 days

The residence time of a reservoir within the hydrologic cycle is the average time a water molecule will spend in that reservoir (see the adjacent table). It is a measure of the average age of the water in that reservoir, though some water will spend much less time than average, and some much more.

Groundwater can spend over 10,000 years beneath Earth's surface before leaving. Particularly old groundwater is called fossil water. Water stored in the soil remains there very briefly, because it is spread thinly across the Earth, and is readily lost by evaporation, transpiration, stream flow, or groundwater recharge. After evaporating, water remains in the atmosphere for about 9 days before condensing and falling to the Earth as precipitation.

In hydrology, residence times can be estimated in two ways. The more common method relies on the principle of conservation of mass and assumes the amount of water in a given reservoir is roughly constant. With this method, residence times are estimated by dividing the volume of the reservoir by the rate by which water either enters or exits the reservoir. Conceptually, this is equivalent to timing how long it would take the reservoir to become filled from empty if no water were to leave (or how long it would take the reservoir to empty from full if no water were to enter).

An alternative method to estimate residence times, gaining in popularity particularly for dating groundwater, is the use of isotopic techniques. This is done in the subfield of isotope hydrology.

Changes over time

Over the past century the water cycle has become more intense,^[13] with the rates of evaporation and precipitation both increasing. This is an expected outcome of global warming, as higher temperatures increase the rate of evaporation due to warmer air's higher capacity for holding moisture.^[14]

The scientific consensus expressed in the 2007 Intergovernmental Panel on Climate Change (IPCC) Summary for Policymakers^[15] is for the water cycle to continue to intensify throughout the 21st century, though this does not mean that precipitation will increase in all regions. In subtropical land areas — places that are already relatively dry — precipitation is projected to decrease during the 21st century, increasing the probability of drought. The drying is projected to be strongest near the poleward margins of the subtropics (for example, the Mediterranean Basin, South Africa, southern Australia, and the Southwestern United States). Annual precipitation amounts are expected to increase in near-equatorial regions that tend to be wet in the present climate, and also at high latitudes. These large-scale patterns are present in nearly all of the climate model simulations conducted at several international research centers as part of the 4th Assessment of the IPCC.

Glacial retreat is also an example of a changing water cycle, where the supply of water to glaciers from precipitation cannot keep up with the loss of water from melting and sublimation. Glacial retreat since 1850 has been extensive.^[16]

Human activities that alter the water cycle include:

• agriculture
• alteration of the chemical composition of the atmosphere
• construction of dams
• deforestation and afforestation
• removal of groundwater from wells
• water abstraction from rivers
• urbanization

Effects on climate

The water cycle is powered from solar energy. 86% of the global evaporation occurs from the oceans, reducing their temperature by evaporative cooling. Without the cooling effect of evaporation the greenhouse effect would lead to a much higher surface temperature of 67 °C, and a warmer planet.^[17]

Most of the solar energy warms tropical seas. After evaporating, water vapour rises into the atmosphere and is carried by winds away from the tropics. Most of this vapour condenses as rain in the Intertropical convergence zone, also known as the ITCZ, releasing latent heat that warms the air. This in turn drives the atmospheric circulation.^{[citation needed]}

Effects on biogeochemical cycling

While the water cycle is itself a biogeochemical cycle,^[18] flow of water over and beneath the Earth is a key component of the cycling of other biogeochemicals. Runoff is responsible for almost all of the transport of eroded sediment and phosphorus^[19] from land to waterbodies. The salinity of the oceans is derived from erosion and transport of dissolved salts from the land. Cultural eutrophication of lakes is primarily due to phosphorus, applied in excess to agricultural fields in fertilizers, and then transported overland and down rivers. Both runoff and groundwater flow play significant roles in transporting nitrogen from the land to waterbodies.^[20] The dead zone at the outlet of the Mississippi River is a consequence of nitrates from fertilizer being carried off agricultural fields and funnelled down the river system to the Gulf of Mexico. Runoff also plays a part in the carbon cycle, again through the transport of eroded rock and soil.^[21]

See also

• Flood
• Drought

References

External links

• The Water Cycle, United States Geological Survey
• The water cycle, from Dr. Art's Guide to the Planet.
• Water cycle slideshow, 1 Mb Flash multilingual animation highlighting the often-overlooked evaporation from bare soil, from managingwholes.com.
• Will the wet get wetter and the dry drier? - Climate research summary from NOAA Geophysical Fluid Dynamics Laboratory including text, graphics and animations

Biogeochemical cycles
Carbon cycle - Hydrogen cycle - Nitrogen cycle
Oxygen cycle - Phosphorus cycle - Sulfur cycle - Water cycle