Thewater cycle, also known as thehydrologic cycleor theH2O cycle, describes the continuous movement of water on, above and below the surface of theEarth. The mass of water on Earth remains fairly constant over time but the partitioning of the water into the major reservoirs of ice,fresh water, saline water andatmospheric wateris variable depending on a wide range ofclimatic variables. The water moves from one reservoir to another, such as from river toocean, or from the ocean to the atmosphere, by the physical processes ofevaporation,condensation,precipitation,infiltration,runoff, and subsurface flow. In doing so, the water goes through different phases: liquid, solid (ice), and gas (vapor).The water cycle involves the exchange of energy, which leads totemperaturechanges. For instance, when water evaporates, it takes up energy from its surroundings and cools the environment. When it condenses, it releases energy and warms the environment. These heat exchanges influenceclimate.The evaporative phase of the cycle purifies water which then replenishes the land with freshwater. The flow of liquid water and ice transports minerals across the globe. It is also involved in reshaping the geological features of the Earth, through processes includingerosionandsedimentation. The water cycle is also essential for the maintenance of most life andecosystems on the planet.Description[edit]The sun, which drives the water cycle, heats water in oceans and seas. Water evaporates as water vapour into theair. Ice,rain and snow cansublimatedirectly into water vapour.Evapotranspirationis watertranspiredfrom plants and evaporated from the soil. Rising air currents take the vapour up into the atmosphere where cooler temperatures cause it to condense into clouds. Air currents move water vapour around the globe, cloud particles collide, grow, and fall out of the upper atmospheric layers asprecipitation. Some precipitation falls as snow or hail, sleet, and can accumulate as ice caps and glaciers, which can store frozen water for thousands of years. Most water falls back into the oceans or onto land as rain, where the water flows over the ground assurface runoff. A portion of runoff enters rivers in valleys in the landscape, with streamflow moving water towards the oceans. Runoff and water emerging from the ground (groundwater) may be stored as freshwater in lakes. Not all runoff flows into rivers, much of it soaks into the ground asinfiltration. Some water infiltrates deep into the ground and replenishesaquifers, which can store 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. In river valleys and flood-plains there is often continuous water exchange between surface water and ground water in thehyporheic zone. Over time, the water returns to the ocean, to continue the water cycle.Processes[edit]PrecipitationCondensed water vapor that falls to the Earth's surface . Most precipitation occurs asrain, but also includes snow,hail,fog drip,graupel, andsleet.[1]Approximately 505,000km3(121,000cumi) of water falls as precipitation each year, 398,000km3(95,000cumi) of it over the oceans.[2]The rain on land contains 107,000km3(26,000cumi) of water per year and a snowing only 1,000km3(240cumi).[3]78% of global precipitation occurs over the ocean.[4]Canopy interceptionThe precipitation that is intercepted by plant foliage, eventually evaporates back to the atmosphere rather than falling to the ground.SnowmeltThe runoff produced by melting snow.RunoffThe variety of ways by which water moves across the land. This includes both surface runoff andchannel runoff. As it flows, the water may seep into the ground, evaporate into the air, become stored in lakes or reservoirs, or be extracted for agricultural or other human uses.InfiltrationThe flow of water from the ground surface into the ground. Once infiltrated, the water becomessoil moistureor groundwater.[5]Subsurface flowThe flow of water underground, in thevadose zoneand aquifers. Subsurface water may return to the surface (e.g. 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 ofgravityor gravity induced pressures. Groundwater tends to move slowly, and is replenished slowly, so it can remain in aquifers for thousands of years.EvaporationThe 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 primarilysolar radiation. Evaporation often implicitly includestranspirationfromplants, though together they are specifically referred to asevapotranspiration. Total annual evapotranspiration amounts to approximately 505,000km3(121,000cumi) of water, 434,000km3(104,000cumi) of which evaporates from the oceans.[2]86% of global evaporation occurs over the ocean.[4]SublimationThe state change directly from solid water (snow or ice) to water vapor.[7]DepositionThis refers to changing of water vapor directly to ice.

AdvectionThe 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.[8]CondensationThe transformation of water vapor to liquid water droplets in the air, creatingcloudsand fog.[9]TranspirationThe release of water vapor from plants and soil into the air. Water vapor is a gas that cannot be seen.PercolationWater flows vertically through the soil and rocks under the influence ofgravityPlate tectonicsWater enters the mantle via subduction of oceanic crust. Water returns to the surface via volcanism.

Effects on climate[edit]The water cycle is powered from solar energy. 86% of the global evaporation occurs from the oceans, reducing their temperature byevaporative cooling.[19]Without the cooling, the effect of evaporation on the greenhouse effect would lead to a much higher surface temperature of 67C (153F), and a warmer planet.[citation needed]Aquiferdrawdownoroverdraftingand the pumping of fossil water increases the total amount of water in the hydrosphere, and has been postulated to be a contributor to sea-level rise.[20]Effects on biogeochemical cycling[edit]While the water cycle is itself abiogeochemical cycle,[21]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 oferodedsedimentandphosphorus[22]from land towaterbodies. Thesalinityof the oceans is derived from erosion and transport of dissolved salts from the land. Culturaleutrophicationof lakes is primarily due to phosphorus, applied in excess toagricultural fieldsinfertilizers, and then transported overland and down rivers. Both runoff and groundwater flow play significant roles in transporting nitrogen from the land to waterbodies.[23]Thedead zoneat the outlet of theMississippi Riveris a consequence ofnitratesfrom fertilizer being carried off agricultural fields and funnelled down theriver systemto theGulf of Mexico. Runoff also plays a part in thecarbon cycle, again through the transport of eroded rock and soil.[24]Evaporationis a type ofvaporizationof aliquidthat occurs from thesurfaceof a liquid into a gaseous phase that is not saturated with the evaporating substance. The other type of vaporization isboiling, which is characterized by bubbles of saturated vapor forming in the liquid phase. Steam produced in a boiler is another example of evaporation occurring in a saturated vapor phase. Evaporation that occurs directly from the solid phase below the melting point, as commonly observed with ice at or below freezing or moth crystals (napthalene or paradichlorobenzine), is calledsublimation.Condensation is the change of water from its gaseous form (water vapor) into liquid water. Condensation generally occurs in the atmosphere when warm air rises, cools and looses its capacity to hold water vapor. As a result, excess water vapor condenses to form cloud droplets. The upward motions that generate clouds can be produced byconvectionin unstable air,convergenceassociated with cyclones, lifting of air byfrontsand lifting over elevatedtopographysuch as mountains.Meltingis a physical process that results in thephase transitionof a substance from asolidto aliquid. Theinternal energyof a substance is increased, typically by the application of heat or pressure, resulting in a rise of its temperature to themelting point, at which the ordering of ionic or molecular entities in the solid breaks down to a less ordered state and the solid liquefies. An object that has melted completely ismolten. Substances in the molten state generally have reduced viscosity with elevated temperature; an exception to this maxim is the elementsulfur, whose viscosity increases to a point due topolymerizationand then decreases with higher temperatures in its molten state.[1]precipitationis any product of the condensation ofatmosphericwater vapourthat falls under gravity.[1]The main forms of precipitation includedrizzle,rain,sleet,snow,graupelandhail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapour, so that the water condenses and "precipitates".Evaporation:This is when warmth from the sun causes water from oceans, lakes, streams, ice and soils to rise into the air and turn into water vapour (gas). Water vapour droplets join together to make clouds!

Condensation:This is when water vapour in the air cools down and turns back into liquid water.

Precipitation:This is when water (in the form of rain, snow, hail or sleet) falls from clouds in the sky.

Collection:This is when water that falls from the clouds as rain, snow, hail or sleet, collects in the oceans, rivers, lakes, streams. Most will infiltrate (soak into) the ground and will collect as underground water.