The water cycle

Water is a key compound for life on Earth.

Water is important for:

  • maintaining habitats
  • allowing nutrients to flow between organisms and the environment
  • survival of organisms

Some organisms can survive in a dormant state without water for long periods of time, but all organisms will quickly or eventually die without it.

Water cycle. Precipitation falls on ground, drains into ocean via percolation. Evaporation from ocean and transpiration of surface water create water vapour. Condensation creates clouds.

Processes in the water cycle

The water cycle is easiest to understand in terms of its processes and what happens to the water in each stage.

ProcessWhat happens to water
EvaporationWater turns from a liquid to a gas when it evaporates. Energy from the Sun can evaporate water from all places on the Earth's surface such as puddles, ponds, lakes and oceans.
CondensationAfter evaporation, water can cool and convert from gas to liquid, often forming clouds.
TransportWater within clouds can be blown many miles by strong winds and so is transported to other areas.
PrecipitationPrecipitation occurs when rain, snow, hail and sleet fall from the sky.
Surface runoffMuch water will be absorbed into the ground after precipitation but if a large volume falls or the ground is already wet then some water can run along the surface of the ground.
InfiltrationThis occurs when water that has fallen as precipitation is absorbed into the ground. This can then be stored within underground rocks called aquifers.
Transpiration Plants need to maintain a constant stream of water to their leaves for photosynthesis, support and transport of minerals. They allow some water to evaporate as water vapour from their leaves so that more water is continuously 'pulled' to their leaves from the soil.
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