The water cycle

Water is a key compound for life on Earth. All living organisms need water. Some can survive in a dormant state without it for long periods of time, but all organisms will quickly or eventually die without it. Water is also a habitat for many organisms in oceans, rivers and lakes.

Water acts as the solvent for chemical reactions in the cytoplasm of all living cells. It also helps to transport dissolved substances into and out of cells. Water is also a reactant or product of many biochemical processes. For example:

Water is found on Earth in all three states of matter:

  • as a solid in glaciers and ice sheets
  • as a liquid in oceans, rivers, lakes and aquifers
  • as a gas in the atmosphere

Water cycles through the environment via a number of processes (including freezing, melting, evaporating and condensing) which together make the water cycle.

Water cycles through the environment via a number of processes (including freezing, melting, evaporating and condensing) which together make the water cycle.The water cycle, showing the key stages of the process

The water cycle is easiest to understand in terms of its processes and what happens to the water in each of these. These are the key processes:

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 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 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.
TranspirationPlants need to maintain a constant steam of water to their leaves for transport and support. So they allow some water to evaporate as water vapour from their leaves to mean that more is continually ‘pulled’ to their leaves from the soil.