What is the meaning of water cycle?

The different compartments in the hydrosphere are connected and water moves between them in what is known as the water cycle.

1 . The processes of the water cycle

The water cycle involves processes through which water moves between different compartments in the hydrosphere; in some cases, it entails a change of state.

Evapotranspiration

This term covers two processes:

• Evaporation: liquid water from the ocean and land surface moves into the atmosphere as vapour, as energy in the form of heat breaks the bonds that hold the water molecules together. This is the main process by which water moves into the atmosphere.
• Transpiration: water is converted into vapour by the action of living beings, especially plants. They contribute about 10% to the water that moves into the atmosphere.

Precipitation

Precipitation is any liquid or solid water that falls on the Earth's surface as a product of the condensation of water vapour. Approximately 90% of the water that evaporates from the oceans returns to the ocean as precipitation. The most common form of precipitation is rain, but it can also occur in the form of snow, hail, dew or frost.

Runoff

Runoff is the downhill flow of water over the land surface. It occurs when water from precipitation can no longer sufficiently rapidly infiltrate in the soil. We can differentiate two terms:

• Surface runoff or direct runoff flows over the ground surface and ends up in rivers.
• Groundwater or indirect runoff flows below the land surface.

Infiltration

The water that falls on the ground surface can soak into the soil and rocks, and become part of groundwater. Portions of it can infiltrate deeper, thus recharging groundwater aquifers, and portions can be discharged to the surface.

Groundwater discharge

Part of the water that seeps into the ground can seep back into surface-water bodies. Water may travel long distances, or remain for long periods as groundwater before returning to the surface, or seep into other bodies of water, such as streams or oceans.

2 . Water distribution

Some water cycle facts:

• In total there are 1,386 million cubic kilometres of water on the planet and about 96% of it is salt water.
• Of the existing freshwater, 68% is confined in glaciers and snow and 30% is in the ground.
• There are approximately 93,100 cubic kilometres of freshwater in surface waters.