Water Cycle

The water cycle is the way water moves endlessly between the oceans, the air, the land and back. Powered by sunlight and gravity, it is one of the most important processes on the planet. The cycle keeps fresh water flowing into rivers and lakes, makes the clouds and rain, and replenishes the underground water that almost every plant depends on. The amount of water on Earth never really changes, but the same water molecules cycle endlessly through every part of the planet, including living things.

  • % of Earth's waterApprox. 97% salty3% fresh, mostly locked in ice
  • % of fresh water available<1%In rivers, lakes and shallow groundwater
  • Water cycle stages4 mainEvaporation, condensation, precipitation, collection
  • Time a drop spends in the airApprox. 9 daysBefore falling back to Earth
  • Time in iceUp to 10,000 yearsIn polar ice caps and glaciers
  • Powered byThe SunPlus Earth's gravity

The four main stages

  1. Evaporation: the Sun heats water in oceans, lakes and rivers, turning it into invisible water vapour that rises into the air. Plants also release water vapour through their leaves (a process called transpiration).
  2. Condensation: as the water vapour rises and cools, it changes back into tiny droplets and forms clouds.
  3. Precipitation: when the droplets in a cloud combine to become large enough, they fall back to Earth as rain, snow, sleet or hail.
  4. Collection: the fallen water gathers in oceans, lakes and rivers, soaks into the ground to become groundwater, or is taken up by plants. From there, it eventually evaporates again and the cycle continues.

How long water stays in each place

Different parts of the water cycle hold water for very different lengths of time. Scientists call this the "residence time".

  • In the atmosphere: approx. 9 days.
  • In rivers: approx. 2 weeks to 6 months.
  • In lakes: approx. 50 to 100 years.
  • In oceans: approx. 3,200 years.
  • In groundwater: hundreds to thousands of years.
  • In ice caps and glaciers: hundreds of thousands of years.
Fact Almost every molecule of water on Earth has been here since the planet first formed approx. 4.5 billion years ago. The water you drink today has been recycled endlessly: through ancient oceans, prehistoric clouds, the bodies of dinosaurs and the rivers of every civilisation in history. The same water molecules just keep cycling around forever.

Pick a topic below to explore further.

What Is the Water Cycle?The endless journey water takes around the planet: from sea to sky to land and back again.
EvaporationWhen the Sun heats liquid water and turns it into invisible water vapour that rises into the sky.
CondensationWhen water vapour in the sky cools and turns back into tiny droplets, forming clouds.
PrecipitationWhen water droplets in clouds get heavy and fall back to the ground as rain, snow, sleet, or hail.
GroundwaterWater that soaks into the soil and rocks underground. Most of our drinking water starts here.
Run-offWater that flows over the surface of the land into rivers and back to the sea, completing the cycle.