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The water cycle, describes the continuous movement of water on, above and below the surface of the Earth.

The water cycle is the cycle water goes through on Earth. It makes the rain, clouds, and most of our weather.

• First, water on the earth and in the sea is evaporated by the heat from the Sun.
• Excess water from plants is also absorbed into the atmosphere, this process is called transpiration.
• Then, water collects as water vapor in the sky. This makes clouds.
• Next, the water in the clouds gets cold. This makes it become liquid again.
• Then, the water falls from the sky as rain, snow, sleet, or hail which is called precipitation.
• The water then collects into lakes, oceans, or aquifers. From there, it evaporates again and continues the cycle.

The water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above and below the surface of the Earth. Since the water cycle is truly a "cycle," there is no beginning or end. Water can change states among liquid, vapor, and ice at various places in the water cycle. Although the balance of water on Earth remains fairly constant over time, individual water molecules can come and go.

The sun, which drives the water cycle, heats water in oceans and seas. Water evaporates as water vapor into the air. Ice and snow can sublimate directly into water vapor. Evapotranspiration is water transpired from plants and evaporated from the soil. Rising air currents take the vapor up into the atmosphere where cooler temperatures cause it to condense into clouds. Air currents move water vapor around the globe, cloud particles collide, grow, and fall out of the sky as precipitation. Some precipitation falls as snow or hail, and can accumulate as ice caps and glaciers, which can store frozen water for thousands of years. Snowpacks can thaw and melt, and the melted water flows over land as snowmelt. Most water falls back into the oceans or onto land as rain, where the water flows over the ground as surface runoff. A portion of runoff enters rivers in valleys in the landscape, with streamflow moving water towards the oceans. Runoff and groundwater are stored as freshwater in lakes. Not all runoff flows into rivers, much of it soaks into the ground as infiltration. Some water infiltrates deep into the ground and replenishes aquifers, which 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. Over time, the water returns to the ocean, where our water cycle started.

Different Processes

• Precipitation: Condensed water vapor that falls to the Earth's surface. Most precipitation occurs as rain, but also includes snow, hail, fog drip, graupel, and sleet. Approximately 505,000 km3 (121,000 cu mi) of water fall as precipitation each year, 398,000 km3 (95,000 cu mi) of it over the oceans.
• Canopy interception: The precipitation that is intercepted by plant foliage and eventually evaporates back to the atmosphere rather than falling to the ground.
• Snowmelt: The runoff produced by melting snow.
• Runoff: 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.
• Infiltration: The flow of water from the ground surface into the ground. Once infiltrated, the water becomes soil moisture or groundwater.
• Subsurface Flow: The flow of water underground, in the vadose zone and 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 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.
• Evaporation: The transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere. The source of energy for evaporation is primarily solar radiation. Evaporation often implicitly includes transpiration from plants, though together they are specifically referred to as evapotranspiration. Total annual evapotranspiration amounts to approximately 505,000 km3 (121,000 cu mi) of water, 434,000 km3 (104,000 cu mi) of which evaporates from the oceans.
• Sublimation: The state change directly from solid water (snow or ice) to water vapor.
• Advection: The 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.
• Condensation: The transformation of water vapor to liquid water droplets in the air, producing clouds and fog.
• Transpiration: The release of water vapor from plants into the air. Water vapor is a gas that cannot be seen.

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 1,370×10⁶ km³. Precipitation over the oceans is about 0.398×10⁶ km³/year and the flow of water to the oceans from rivers and groundwater is about 0.036×10⁶ 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.

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

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, the effect of evaporation on the greenhouse effect would lead to a much higher surface temperature of 67 °C (153 °F), and a warmer planet.

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.

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.

Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)