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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. Movement of water within the water cycle is the subject of the field of Hydrology . Movement of water within the water cycle There is no definable start or finish to the water cycle. Water molecules move continuously among different compartments, or reservoirs, of the Earth's hydrosphere, by different physical processes. Water evaporates from the oceans, forms clouds, which precipitate and the water falls back to Earth. However, water does not necessarily cycle through each compartment in order. Before reaching the ocean, water may have evaporated, condensed, precipitated, and become runoff multiple times. The physical processes The major physical processes involved in the water cycle are the evaporation of water from the oceans and land, the transport of water in the atmosphere, condensation, precipitation over the oceans and land, and the flow of water from land to the oceans.
Less fundamental processes involved in the water cycle are:
Conservation of mass The total amount, or mass, of water in the water cycle remains essentially constant, as does the amount of water in each reservoir of the water cycle. This means that rate of water added to one reservoir must equal, on average over time, the rate of water leaving the same reservoir. The adjacent table contains the amount of water that falls as precipitation or rises as evaporation, for both the land and oceans. The runoff and groundwater discharge from the land to the oceans is also included. From the law of the conservation of mass, whatever water moves into a reservoir, on average, the same volume must leave. For example, 107 thousand cubic km (107 × 10³ Km³ ) of water falls on land each year as precipitation. This is equal to the sum of the evaporation (71 × 10³ km³/year) and runoff (36 × 10³ km³/year) of water from the land. Water that cycles between the land and the atmosphere in a fixed area is referred to as Moisture Recycling . 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 is the average time a water molecule will spend in a reservoir. 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. (''See the adjacent table for residence times for the other reservoirs.'') Residence times can be estimated in two ways. The more common method relies on Conservation Of Mass , and may be expressed by the following equation: An alternative method, gaining in popularity particularly for dating groundwater, is the use of isotopic techniques. This is done in the subfield of Isotope Hydrology . Example: Calculating the residence time of the oceans As an example of how the residence time is calculated, consider the oceans. The volume of the oceans is roughly 1,370 km³. Precipitation over the oceans is about 0.398 km³/year and the flow of water to the oceans from rivers and groundwater is about 0.036 km³/year. By dividing the total volume of the oceans by the rate of water added (in units of volume over time) we obtain the residence time of 3,200 years—the average time it takes a water molecule that reaches an ocean to evaporate. Climate regulation 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 Degrees C , and a warmer planet {Link without Title} . 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 ITCZ , releasing latent heat that warms the air. This in turn drives the Atmospheric Circulation . Changes in the water cycle Over the past century the water cycle has become more intense {Link without Title} , 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. 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:
Biogeochemical cycles The water cycle is Biogeochemical Cycle . Other notable cycles are the Carbon Cycle and Nitrogen Cycle . As water flows over and beneath the Earth it picks up and transports soil and other sediment, mineral salt and other dissolved chemicals, and pollutants. The oceans are saline because of the movement of mineral salt from the land by the runoff of water, but which remains in the oceans as water evaporates. External links
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