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.

Explanation of the Water Cycle.

The water cycle is the process that all water takes. It includes precipitation which is the falling of water in any form to earth, infiltration which is the process in which water is absorbed into the soil (it may also flow off the surface called surface run off)evaporation or transpiration which is either when water is heated and turns into water vapour or when plants use the water and give it off as water vapour, condensation which is when the water vapour cools and forms clouds. This process is then repeated over and over again.

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.

• Evaporation is the transfer of water from bodies of surface water into the atmosphere. This transfer entails a change in the physical nature of water from liquid to gaseous phases. The source of energy is primarily solar radiation. Evaporation is closely related to transpiration from plants, as well as, to a lesser degree, perspiration from land mammals and marsupials. Thus, this transfer is sometimes referred to as evapotranspiration. 90% of atmospheric water comes from evaporation, while the remaining 10% is from transpiration.

• Condensation is the transformation of water vapor to liquid water droplets in the air, producing clouds and fog.

• Advection is the movement of water—in solid, liquid, or vapour states—through the atmosphere. Without advection, water that evaporated over the oceans could not precipitate over land.

• Precipitation is water vapor that has condensed into clouds and falls to the Earth's surface. This mostly occurs as rainfall, but also includes snow, hail, fog drip, graupel, and sleet.

• Runoff includes 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.

Less fundamental processes involved in the water cycle are:

• Sublimation is the state change directly from solid water (snow or ice) to water vapor.

• Canopy interception is the precipitation that is intercepted by plant foliage and eventually evaporates back to the atmosphere rather than falling to the ground. The amount of water that it intercepted depends on the duration of the storm, the windspeed and temperature, and the amount of foliage present.

• Infiltration is the flow of water on the ground surface into the ground. The speed of infiltration depends on how moist the ground already is, and on its infiltration capacity. Having infiltrated, water comprises soil moisture within the vadose zone, or groundwater in an aquifer.

• Snowmelt refers to the runoff produced by melting snow.

• Subsurface flow is 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.

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:

$\backslash mathrm\{Residence\}\; \backslash mbox\{\; \}\; \backslash mathrm\{time\}\; =\backslash begin\{matrix\}\; \backslash frac\{\backslash mathrm\{Volume\}\; \backslash mbox\{\; \}\; \backslash mathrm\{of\}\; \backslash mbox\{\; \}\; \backslash mathrm\{reservoir\}\}\{\backslash mathrm\{Rate\}\; \backslash mbox\{\; \}\; \backslash mathrm\{water\}\; \backslash mbox\{\; \}\; \backslash mathrm\{is\}\; \backslash mbox\{\; \}\; \backslash mathrm\{added\}\; \backslash mbox\{\; \}\; \backslash mathrm\{to\}\; \backslash mbox\{\; \}\; \backslash mathrm\{reservoir\}\}\; \backslash end\{matrix\}$

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.

$\backslash mbox\{Residence\; time\; \}|\backslash mbox\{\; ocean\}\; =\; \backslash frac\{1370\; \backslash times\; 10^6\; \backslash mbox\{\; km\}^3\}\{(0.398\; +\; 0.036)\; \backslash times\; 10^6\; \backslash mbox\{\; km\}^3/\backslash mbox\{year\}\}\; =\; 3200\; \backslash mbox\{\; years\}$

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^*.

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^*, 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:

• agriculture
• alteration of the chemical composition of the atmosphere
• construction of dams
• deforestation and afforestation
• removal of groundwater from wells
• water abstraction from rivers
• urbanization

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

• NOAA site
• The Water Cycle, United States Geological Survey
• Hydrologic Cycle, from the Geotechnical, Rock and Water Resources Library, University of Arizona
• The water cycle, from Dr. Art's Guide to the Planet.

Hydrology | Water | Forms of water | Environmental science | Soil physics

Cicle hidrològic | Koloběh vody | Wasserkreislauf | Κύκλος του νερού | Ciclo hidrológico | Cycle de l'eau | Ciclo hidrolóxico | Ciclo dell'acqua | מחזור המים | Waterkringloop | Cykl hydrologiczny | Water cycle | Daur cai | Veden kiertokulku | Vattnets kretslopp | วัฏจักรของน้ำ