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The water cycle — technically known as the hydrologic 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.
Description The water cycle is the continuous movement of water over, above, and beneath the Earth's surface. It is powered by solar energy, and because it is a cycle, there is no beginning or end. As water moves around 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 thousand 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.
Reservoirs 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. Residence times 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, 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. 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. 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, and a warmer planet. 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. Effects on biogeochemical cycling While the water cycle is itself a biogeochemical cycle, 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 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. 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. See Also | ||||||||
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