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;[[Runoff (hydrology)|Runoff]]: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.
;[[Runoff (hydrology)|Runoff]]: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 (hydrology)|Infiltration]]:The flow of water from the ground surface into the ground. Once infiltrated, the water becomes [[soil moisture]] or [[groundwater]].<ref>National Weather Service Northwest River Forecast Center. [http://www.nwrfc.noaa.gov/info/water_cycle/hydrology.cgi Hydrologic Cycle.] Retrieved on 2006-10-24.</ref>
;[[Infiltration (hydrology)|Infiltration]]:The flow of water from the ground surface into the ground. Once infiltrated, the water becomes [[soil moisture]] or [[groundwater]].<ref>National Weather Service Northwest River Forecast Center. [http://www.nwrfc.noaa.gov/info/water_cycle/hydrology.cgi Hydrologic Cycle.] Retrieved on 2006-10-24.</ref>
;[[Subsurface Flow (hydrology)|Subsurface Flow]]: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.
;[[Subsurface Flow (hydrology)|Subsurface Flow]]: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
;[[Evaporation]]:The transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere.<ref>Arctic Climatology and Meteorology. [http://nsidc.org/arcticmet/glossary/evaporation.html Evaporation.] Retrieved on 2006-10-24.</ref> The source of energy for evaporation is primarily [[solar radiation]]. Evaporation often implicitly includes '''[[transpiration]]''' from [[plant]]s, though together they are specifically referred to as '''[[evapotranspiration]]'''. Total annual evapotranspiration amounts to approximately {{convert|505000|km3|cumi|abbr=on}} of water, {{convert|434000|km3|cumi|abbr=on}} of which evaporates from the oceans.<ref name="The Water Cycle" />
;[[Evaporation]]:The transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere.<ref>Arctic Climatology and Meteorology. [http://nsidc.org/arcticmet/glossary/evaporation.html Evaporation.] Retrieved on 2006-10-24.</ref> The source of energy for evaporation is primarily [[solar radiation]]. Evaporation often as '''[[evapotranspiration]]'''. Total annual evapotranspiration amounts to approximately {{convert|505000|km3|cumi|abbr=on}} of water, {{convert|434000|km3|cumi|abbr=on}} of which evaporates from the oceans.<ref name="The Water Cycle" />
;[[Sublimation (chemistry)|Sublimation]]:The state change directly from solid water (snow or ice) to water vapor.<ref>Arctic Climatology and Meteorology. [http://nsidc.org/arcticmet/glossary/sublimation.html Sublimation.] Retrieved on 2006-10-24.</ref>
;[[Sublimation (chemistry)|Sublimation]]:The state change directly from solid water (snow or ice) to water vapor.<ref>Arctic Climatology and Meteorology. [http://nsidc.org/arcticmet/glossary/sublimation.html Sublimation.] Retrieved on 2006-10-24.</ref>
;[[Advection]]:The movement of water — in solid, liquid, or vapor states — through the atmosphere. Without advection, water that evaporated over the oceans could not precipitate over land.<ref>Arctic Climatology and Meteorology. [http://nsidc.org/arcticmet/glossary/advection.html Advection.] Retrieved on 2006-10-24.</ref>
;[[Advection]]:The movement of water — in solid, liquid, or vapor states — through the atmosphere. Without to liquid water droplets in the air, producing [[cloud]]s and [[fog]].<ref>Arctic Climatology and Meteorology. [http://nsidc.org/arcticmet/glossary/condensation.html Condensation.] Retrieved on 2006-10-24.</ref>
;[[Condensation]]:The transformation of water vapor to liquid water droplets in the air, producing [[cloud]]s and [[fog]].<ref>Arctic Climatology and Meteorology. [http://nsidc.org/arcticmet/glossary/condensation.html Condensation.] Retrieved on 2006-10-24.</ref>
;[[Transpiration]]:The release of water vapor from plants into the air. Water vapor is a gas that can not be seen.
;[[Transpiration]]:The release of water vapor from plants into the air. Water vapor is a gas that can not be seen.


Revision as of 21:36, 4 June 2009

The water cycle.

The water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth. Since the water cycle is truly a "cycle," there is no beginning or end. Water can change states among liquid, vapor, and ice at various places in the water cycle. Although the balance of water on Earth remains fairly constant over time, individual water molecules can come and go.

Description

The sun, which drives the water cycle, heats water in the oceans. Water evaporates as vapor into the air. Ice and snow can sublimate directly into water vapor. Evapotranspiration is water transpired from plants and evaporated from the soil. Rising air currents take the vapor up into the atmosphere where cooler temperatures cause it to condense into clouds. Air currents move clouds around the globe, cloud particles collide, grow, and fall out of the sky as precipitation. Some precipitation falls as snow and can accumulate as ice caps and glaciers, which can store frozen water for thousands of years. Snowpacks can thaw and melt, and the melted water flows over land as snowmelt. Most precipitation falls back into the oceans or onto land, where the precipitation flows over the ground as surface runoff. A portion of runoff enters rivers in valleys in the landscape, with streamflow moving water towards the oceans. Runoff and groundwater are stored as freshwater in lakes. Not all runoff flows into rivers. Much of it soaks into the ground as infiltration. Some water infiltrates deep into the ground and replenishes aquifers, which store huge amounts of freshwater for long periods of time. Some infiltration stays close to the land surface and can seep back into surface-water bodies (and the ocean) as groundwater discharge. Some groundwater finds openings in the land surface and comes out as freshwater springs. Over time, the water returns to the ocean, where our water cycle started.

Different Processes

Precipitation
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.[1] Approximately 505,000 km3 (121,000 cu mi) of water fall as precipitation each year, 398,000 km3 (95,000 cu mi) of it over the oceans.[2]
Canopy interception
The precipitation that is intercepted by plant foliage and eventually evaporates back to the atmosphere rather than falling to the ground.
Snowmelt
The runoff produced by melting snow.
Runoff
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
The flow of water from the ground surface into the ground. Once infiltrated, the water becomes soil moisture or groundwater.[3]
Subsurface Flow
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
Evaporation
The transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere.[4] The source of energy for evaporation is primarily solar radiation. Evaporation often as evapotranspiration. Total annual evapotranspiration amounts to approximately 505,000 km3 (121,000 cu mi) of water, 434,000 km3 (104,000 cu mi) of which evaporates from the oceans.[2]
Sublimation
The state change directly from solid water (snow or ice) to water vapor.[5]
Advection
The movement of water — in solid, liquid, or vapor states — through the atmosphere. Without to liquid water droplets in the air, producing clouds and fog.[6]
Transpiration
The release of water vapor from plants into the air. Water vapor is a gas that can not be seen.

Changes over time

The water cycle describes the processes that drive the movement of water throughout the hydrosphere. However, much more water is "in storage" for long periods of time than is actually moving through the cycle. The storehouses for the vast majority of all water on Earth are the oceans. It is estimated that of the 332,500,000 mi3 (1,386,000,000 km3) of the world's water supply, about 321,000,000 mi3 (1,338,000,000 km3) is stored in oceans, or about 95%. It is also estimated that the oceans supply about 90% of the evaporated water that goes into the water cycle.[7]

During colder climatic periods more ice caps and glaciers form, and enough of the global water supply accumulates as ice to lessen the amounts in other parts of the water cycle. The reverse is true during warm periods. During the last ice age glaciers covered almost one-third of Earth's land mass, with the result being that the oceans were about 400 ft (122 m) lower than today. During the last global "warm spell," about 125,000 years ago, the seas were about 18 ft (5.5 m) higher than they are now. About three million years ago the oceans could have been up to 165 ft (50 m) higher.[7]

The scientific consensus expressed in the 2007 Intergovernmental Panel on Climate Change (IPCC) Summary for Policymakers[8] 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.[9]

Human activities that alter the water cycle include:

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 (153 °F), and a warmer planet.[10]

Effects on biogeochemical cycling

While the water cycle is itself a biogeochemical cycle,[11] 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[12] 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.[13] 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.[14]

See also

Notes

  1. ^ Arctic Climatology and Meteorology. Precipitation. Retrieved on 2006-10-24.
  2. ^ a b Dr. Art's Guide to Planet Earth. The Water Cycle. Retrieved on 2006-10-24.
  3. ^ National Weather Service Northwest River Forecast Center. Hydrologic Cycle. Retrieved on 2006-10-24.
  4. ^ Arctic Climatology and Meteorology. Evaporation. Retrieved on 2006-10-24.
  5. ^ Arctic Climatology and Meteorology. Sublimation. Retrieved on 2006-10-24.
  6. ^ Arctic Climatology and Meteorology. Condensation. Retrieved on 2006-10-24.
  7. ^ a b http://ga.water.usgs.gov/edu/watercycleoceans.html USGS, The Water Cycle: Water Storage in Oceans - Retrieved on 2008-05-14
  8. ^ Intergovernmental Panel on Climate Change. Climate Change 2007: The Physical Science Basis, WG1 Summary for Policymakers
  9. ^ U.S. Geologic Survey. GLACIER RETREAT IN GLACIER NATIONAL PARK, MONTANA. Retrieved on 2006-10-24.
  10. ^ "Water Cycle - Science Mission Directorate". Retrieved 7 January 2009.
  11. ^ The Environmental Literacy Council. Biogeochemical Cycles. Retrieved on 2006-10-24.
  12. ^ The Environmental Literacy Council. Phosphorus Cycle. Retrieved on 2006-10-24.
  13. ^ Ohio State University Extension Fact Sheet. Nitrogen and the Hydrologic Cycle. Retrieved on 2006-10-24.
  14. ^ NASA's Earth Observatory. The Carbon Cycle. Retrieved on 2006-10-24.

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