What Is the Water Cycle?

The water cycle is the endless journey of water around the planet. Powered by sunlight and gravity, water is constantly moving between the oceans, the air, the land, plants, animals and back again. The total amount of water on Earth never really changes (around 1.4 billion cubic kilometres), but the same water molecules cycle through every part of the planet over and over. The water you drink today has been around since Earth formed approx. 4.5 billion years ago, and has probably been part of dinosaurs, ancient seas and many other things on its long journey.

  • Stages4 mainEvaporation, condensation, precipitation, collection
  • Powered byThe SunPlus gravity to pull water back down
  • Total water on EarthApprox. 1.4 billion km³Same amount as 4.5 billion years ago
  • Water in atmosphereApprox. 13,000 km³Tiny fraction of the total
  • Time water spends in cloudsApprox. 9 daysOn average
  • Time in oceansApprox. 3,200 yearsAverage residence time

The four stages

  1. Evaporation: the Sun heats water in seas, lakes and rivers, turning it into invisible water vapour that rises into the air.
  2. Condensation: high in the cold air, water vapour turns back into tiny droplets and forms clouds.
  3. Precipitation: droplets in the clouds combine until they are heavy enough to fall back to Earth as rain, snow, sleet or hail.
  4. Collection: water gathers in oceans, lakes, rivers and underground. It also flows over the land as run-off or seeps down to become groundwater. Eventually it evaporates again and the cycle continues.

Where Earth's water lives

The same water gets shared between many different stores.

  • Oceans: approx. 97% of all water. Salty.
  • Ice caps and glaciers: approx. 2%. Frozen fresh water.
  • Groundwater: approx. 0.6%. Hidden underground in aquifers.
  • Lakes and rivers: approx. 0.01%. The fresh water we use most.
  • Atmosphere: approx. 0.001%. As water vapour and clouds.
  • Living things: trace amounts but constantly flowing through.

Despite the huge oceans, only a tiny fraction of Earth's water is fresh and accessible. That is why protecting rivers, lakes and groundwater matters so much.

How long water stays in each place

Different parts of the water cycle hold water for very different lengths of time, called the "residence time".

  • Atmosphere: approx. 9 days.
  • Rivers: weeks to months.
  • Lakes: approx. 50 to 100 years.
  • Groundwater: hundreds to thousands of years.
  • Oceans: approx. 3,200 years.
  • Ice caps: tens to hundreds of thousands of years.
Fact Every glass of water you have ever drunk has been recycled countless times over the last 4.5 billion years. The same molecules have probably been part of prehistoric oceans, ancient clouds, dinosaur blood, Roman aqueducts and ice age glaciers. Earth does not gain or lose much water (just tiny amounts to and from space), so the same water just keeps cycling forever.

Why the cycle matters

The water cycle is one of the most important processes on Earth. It:

  • Distributes fresh water across the planet, refilling rivers, lakes and underground stores.
  • Drives most of the world's weather.
  • Moves heat around: warm water evaporating from the tropics carries energy to higher latitudes where it condenses and releases the heat.
  • Erodes mountains, sculpts valleys and deposits soil.
  • Supports every living thing: no creature on Earth survives long without water.
Did you know? Trees are a huge part of the water cycle. A single mature oak tree can pump approx. 150 litres of water per day from the soil up through its trunk and out through its leaves into the air (a process called transpiration). The Amazon rainforest releases so much water through transpiration that it generates much of its own rainfall, creating a self-watering ecosystem.
Deeper dive: water from space and where it came from

Where did Earth's water come from in the first place? When the planet first formed approx. 4.5 billion years ago, it was a molten ball with no surface water at all. Any water that was present would have boiled off into space.

Scientists think most of Earth's water arrived later, between approx. 4.4 and 3.8 billion years ago, when the early Solar System was full of icy comets and asteroids smashing into the young planet. Many of those objects were rich in water ice, and over millions of years their impacts delivered enough water to fill the oceans.

How we know: comparing the chemistry of Earth's ocean water (especially the ratio of deuterium, a heavy form of hydrogen, to ordinary hydrogen) with comets and asteroids. Earth's water most closely matches certain rocky asteroids from the outer solar system, suggesting they were the main source. Some water may have been brought even earlier, mixed into the rocky material that formed Earth.

Either way, the water in your glass once travelled through the early Solar System on board a chunk of ice from beyond the orbit of Mars. The same atoms are still here, just constantly rearranged through the water cycle.

For each stage, see evaporation, condensation, precipitation, run-off and groundwater.