View source for Hydrologic cycle

[[Category: Done 2015-09-06]]
<onlyinclude>The '''hydrologic cycle''', also known as the '''water cycle''' is a way of describing the [[material flow]] of water throughout the [[Earth]]. This series of steps describes how [[water]] moves across the Earth and changes form. These specific steps result in the circulation of water between [[ocean]]s, the [[atmosphere]], and the land. The water cycle involves natural phenomena that include [[precipitation]] such as [[rain]] and [[snow]], drainage from rivers, and the return of water to the atmosphere through [[evaporation]] and [[transpiration]].</onlyinclude><ref name=usgs>USGS Water School. (August 31, 2015). ''The Water Cycle Summary'' [Online]. Available: http://water.usgs.gov/edu/watercyclesummary.html</ref> This natural cycling of water through a variety of different locations on the Earth means that through the history of the Earth, the amount of water has been relatively constant. Little has been added or lost over the years, and this water has been continuously in motion.<ref name="RE1">Environment Canada. (August 20, 2015). ''The Hydrologic Cycle'' [Online]. Available: https://www.ec.gc.ca/eau-water/default.asp?lang=En&n=23CEC266-1</ref>

[[File:Watercyclesummary.jpg|400px|framed|center|Figure 1. A diagram showing the major components of the water cycle.<ref>Wikimedia Commons. (August 20, 2015). ''Water Cycle Summary'' [Online]. Available: https://upload.wikimedia.org/wikipedia/commons/1/19/Watercyclesummary.jpg</ref>]]

The cycle has two primary components: '''[[water storage|storage]]''' and '''movement'''. Storage is where water in the [[system]] "rests" when it is not being actively moved. For example, water enters a lake through some movement such as precipitation. After it enters the lake, it stays there for some period of time in its storage phase. Eventually, the water moves back out through some movement, such as evaporation into the atmosphere. This continuous cycling is the hydrologic cycle.<ref name="RE1"/>

It is important to note that water requires energy to change states - from [[solid]] to [[liquid]] is called the [[enthalpy of fusion]], and from liquid to gas is called the [[enthalpy of vaporization]]. Since nature follows the [[law of conservation of energy]], the [[energy]] to fuel this cycle must come from somewhere. In the hydrologic cycle, this energy comes from the [[Sun]].

==Response to global temperatures==
The hydrologic cycle largely depends on the [[solar energy to the Earth]], because hotter [[temperature]]s result in more evaporation, precipitation and higher humidity. With the concerning emissions of [[greenhouse gas]]es, [[global warming]] will likely play a large effect on this important cycle. Since a warming Earth means a moister atmosphere, the hydrologic cycle will become more intense; precipitation and river flows are expected to increase by 10% to 40% in certain tropical locations, while decreasing up to 30% in dry locations.<ref name=wolf2>R. Wolfson, "Consequences of global climate change" in ''Energy, Environment, and Climate'', 2nd ed., New York, NY: W.W. Norton & Company, 2012, ch. 15, sec. 3, pp. 398-399</ref> This means that floods will become more likely in areas with more precipitation, while droughts will affect the drier places. Other [[weather|extreme weather ]]events are also very likely to occur, such as hurricanes and typhoons.<ref name=wolf2/> 

==Storage==
:: [[water storage|''main article'']]
The storage of water in the hydrologic cycle is vastly important on the Earth. Water can be stored in three main places: the atmosphere, on the surface of the Earth, and underground.<ref name="RE3">NOAA. (August 20, 2015). ''Description of the Hydrologic Cycle'' [Online]. Available: http://www.nwrfc.noaa.gov/info/water_cycle/hydrology.cgi</ref> Specifically these water storage areas are known as [[reservoir]]s and include oceans, glacier ice, groundwater, lakes, soil moisture, living organisms, the atmosphere, and rivers.<ref name=geo>Stephen Marshak. ''Earth: Portrait of a Planet'', 3rd ed. New York, NY, U.S.A:W.W. Norton & Company, 2008</ref> Collectively, all water storage areas make up the [[hydrosphere]]. Most water on earth is found in oceans and seas, then in glaciers and groundwater. ~97% of the world's water is stored in the oceans as saltwater.<ref name=geo/> Because the overwhelming majority of water is stored here, this can be seen as the start and end point of the cycle. Water that starts here evaporates up into the atmosphere, where the majority of it falls back into the ocean as rain, or a much smaller amount falls onto land.

Water moves from reservoir to reservoir through a variety of different transportation mechanisms, but this water can stay in storage for varying lengths of time. The movement water from place to place can be fairly erratic, but there tends to be trends for how long certain types of reservoirs maintain their water for. The average length of time that water stays in any reservoir in the hydrologic cycle is known as the [[residence time]]. Water the water in the atmosphere is renewed every 15 days, while soil moisture lasts a couple of months. Lakes replenish their water every 50 to 100 years, while groundwater can last anywhere from 100 to 10 000 years. Finally, ice caps hold water the longest with residence times of up to 200 000 years.<ref name=geo/>

The type of storage that occurs on the land surface and under the ground largely depend on the geologic features related to the types of soil and the types of rocks present at the storage locations.

==Movements==
There are numerous different ways that water moves across the planet in the water cycle. All of these different types of movement are important in maintaining water levels in certain locations around the globe.<ref name="RE1"/> 

===Evaporation===
Here, water is heated by [[solar radiation]] from the Sun. Water [[molecule]]s on the surface of oceans, rivers, lakes, ponds, and other bodies of water become energized by this. When sufficiently energized, the water molecules are able to break free from the forces binding them together and they evaporate, rising into the atmosphere as [[water vapour]].<ref name="RE1"/> 97.5% of the earth's water is contained in the ocean, so a large amount of water enters the atmosphere through evaporation at the oceans surface.<ref name="RE2">Igor A. Shiklomanov, State Hydrological Institute (SHI, St. Petersburg) and United Nations Educational, Scientific and Cultural Organisation (UNESCO, Paris), 1999.</ref>
===Transpiration===
This is the process of water vapour being emitted by plant leaves.<ref name="RE1"/> 
===Sublimation===
When water moves directly from a solid to a gaseous state without ever entering the liquid state. This process allows water in glaciers to enter the atmosphere directly.
===Condensation===
When water vapour rises, it cools slightly and condenses. Generally, the water condenses on dust particles in the [[air]] and becomes liquid. Sometimes the water skips the liquid phase and turns directly into a solid - in the form of ice, hail, or snow. In the liquid form the particles collect and form clouds.<ref name="RE1"/>
===Precipitation===
:: [[precipitation|''main article'']]
Water falls from the sky in numerous different forms of precipitation including rain, snow, and hail. This precipitation comes from clouds, which are free to move around the world and are pushed by air currents. This allows water to move around the globe easily.<ref name="RE1"/>
===Runoff===
Rain or snowmelt can move water over land and form small creeks or collect in ditches. Runoff is the visible flow of water in rivers or creeks as water, previously stored in a basin, drains.<ref name="RE1"/>
===Percolation===
When precipitation falls on the ground, some of it moves downwards through cracks, joints, and pores in the soil until it reaches the water table. At this point it becomes groundwater. This is a slow process, which is why more water flows back to the ocean through surface runoff than groundwater discharge.<ref name="RE1"/>
===Groundwater Flow===
Groundwater is water that is held in cracks and pore spaces below ground. This water can be tapped by wells or could move below ground. The process by which groundwater exits the ground is known as groundwater discharge. This can either be direct discharge into oceans, or indirect discharge, where water stored in groundwater is discharged into a river baseflow, and then travels to the ocean as surface runoff.<ref name="RE1"/>

==References==
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