Explain the importance of the water cycle, including the processes…

The idea

The water cycle moves water between oceans, atmosphere and land, so it continually supplies fresh water to living organisms and to human communities. Teach the processes as a linked sequence: energy from the Sun drives evaporation from seas and lakes and transpiration from leaves; water vapour cools and condenses to form clouds; precipitation returns water to the surface; then runoff, infiltration and percolation carry it through soil and rock into rivers and aquifers. Its importance is that it recycles a finite resource, distributes fresh water and supports ecosystems and agriculture. In drought, rainfall is too low, so supplies fall short. Potable water must be safe to drink, with low dissolved salts and no harmful microbes. Where fresh water is scarce, desalination of sea water by distillation or reverse osmosis can produce potable water, but it is energy-expensive.

What earns the marks

• States that the Sun's energy drives evaporation from oceans and lakes and transpiration from plants, releasing water vapour into the atmosphere.
• Describes condensation of water vapour into clouds and precipitation returning water to the land and sea.
• Identifies runoff, infiltration and percolation as routes by which water reaches rivers and underground stores (aquifers).
• Explains that the cycle recycles and redistributes a finite supply of fresh water, supporting organisms, ecosystems and human use.
• Defines potable water as water that is safe to drink, with sufficiently low dissolved salts and no harmful microorganisms.
• Explains that in drought rainfall is insufficient, so alternative sources such as desalination of sea water are needed.
• Describes desalination by distillation or reverse osmosis and notes that it requires large amounts of energy.

Common mistakes and how to put them right

• Thinking the water cycle creates new water.

  Put it right by explaining that the same water is recycled and redistributed, not manufactured.

• Confusing evaporation with condensation.

  Put it right by stating that evaporation is liquid to vapour (driven by the Sun) and condensation is vapour to liquid droplets forming clouds.

• Believing any clear water is potable.

  Put it right by stating potable water must also be free of harmful microorganisms and have low enough dissolved salts.

• Assuming desalination is cheap and easy.

  Put it right by noting it is effective but energy-intensive and costly.

Exam tips

• Link each process to the next in a chain (evaporation → condensation → precipitation → runoff/infiltration) so the sequence is clear.
• When asked about drought, name a specific method (distillation or reverse osmosis) and give one advantage and one limitation.
• Use the precise term 'potable' and define it in terms of safety to drink, low dissolved salts and absence of harmful microbes.