The water cycle
The water cycle, also known as the hydrologic cycle, is the process by which water travels from the Earth's surface to the atmosphere and then back to the ground again. The sun provides the energy for a continuous exchange of moisture between the oceans, the land and the atmosphere.
The Earth's water
Nearly all (about 97%) of the Earth's water is contained in the oceans. A smaller amount is locked away as ice sheets and glaciers. This leaves a very small amount which travels around in our water cycle, although it may not always seem this way on wet days.
Water enters the atmosphere as water vapour through evaporation, transpiration and sublimation. Water vapour high in the atmosphere forms into clouds through condensation. Water eventually returns to earth through precipitation as rain, snow, sleet and hail. When precipitation occurs over land, some water seeps into the ground as groundwater, a small amount is taken up by plants and animals, and the rest will return to rivers and streams as surface run-off to begin its journey back to the oceans.
Surface runoff is the precipitation that falls on land and flows downhill towards stream channels which join rivers and eventually reach the oceans. Only about one third of precipitation falling on land will return to rivers and oceans. The rest will be soaked into the soil as groundwater, evaporated or transpired.
Some of the water from precipitation will soak into the soil and rocks as groundwater. A varying proportion of groundwater stays in the shallow soil layer, and will move slowly towards streams and rivers. When groundwater soaks deeper into the soil it refills the underground aquifers, where it can stay for long periods of time or be used by humans through drilling wells into aquifers.
An aquifer is a layer of water soaked sand, soil, stone, silt or clay underground. Aquifers act as huge underground water storage systems which people all over the world rely on for fresh water.
Reversible change of state
A change that can be undone or reversed. Energy is required for a material to change state and whilst it may change in appearance, it will still remain the same material. Melting, freezing, boiling, evaporating and condensing are always reversible changes and can be reversed by heating or cooling.
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