What is the water cycle?
The water cycle (or hydrological cycle) is the continuous movement of water around Earth. Water changes state between liquid, gas (water vapour) and solid (ice and snow), and moves between the oceans, atmosphere, land, living things and rocks underground. It is powered by the Sun’s energy, which evaporates water, and by gravity, which pulls rain, rivers and groundwater back down. The total amount of water stays essentially the same; it just keeps moving.
Where Earth’s water is
| Store | Share of all water |
|---|---|
| Oceans | about 96.5 % |
| Ice caps and glaciers | about 1.7 % |
| Groundwater | about 1.7 % |
| Lakes, rivers, soil | under 0.1 % |
| Atmosphere | about 0.001 % |
Less than 1 % of all water is liquid fresh water that people can easily use.
The processes
- Evaporation: the Sun heats water, which turns into invisible water vapour. Most comes from the oceans.
- Transpiration: plants release water vapour from their leaves. Together with evaporation this is called evapotranspiration.
- Condensation: rising vapour cools to its dew point and forms cloud droplets.
- Precipitation: droplets grow and fall as rain, snow, sleet or hail.
- Infiltration: water soaks into the soil and down into the rocks, becoming groundwater.
- Runoff: water flows over the surface into streams and rivers, and back to the sea.
- Melting and sublimation: snow and ice melt into water, or turn directly into vapour.
How the land changes the cycle
What happens to rain depends on where it lands. Forests intercept rain with their leaves, their roots open up the soil so water soaks in, and they return large amounts of vapour to the air by transpiration. Bare or compacted soil lets more water run off. In cities, roofs and roads are impermeable, so rain rushes into drains and rivers within minutes, raising the risk of flash floods and reducing the groundwater that keeps rivers flowing in dry weather. Mountains force moist air to rise, so the side facing the wind is much wetter than the sheltered “rain shadow” beyond.
How long water stays
Water passes quickly through some parts of the cycle and very slowly through others. A water molecule spends on average about 9 days in the atmosphere, weeks to months in rivers, decades to centuries in lakes, around 3,000 years in the oceans, and up to tens of thousands of years in deep groundwater or ice sheets.
The water cycle and climate
Evaporation takes in heat and condensation releases it, so the water cycle moves huge amounts of energy around the planet, warming high latitudes and powering storms. Warmer air can hold about 7 % more water vapour for each degree Celsius, so as the climate warms the water cycle intensifies: heavier downpours in some places and more severe droughts in others.
Key terms
- Water vapour
- Water as an invisible gas.
- Groundwater
- Water stored in the spaces in soil and rocks below the water table.
- Catchment
- The area of land that drains into a particular river.
Common misconceptions
- “Clouds are made of steam or vapour.” Clouds are liquid droplets or ice crystals; vapour is invisible.
- “Groundwater flows in underground rivers.” Mostly it seeps slowly through tiny gaps in soil and rock.
- “Water used up by people is gone forever.” It returns to the cycle, though it may become polluted or move somewhere less useful.
Key takeaways
- The Sun’s energy and gravity drive water around the cycle.
- Evaporation, condensation, precipitation, infiltration, runoff and transpiration move water between stores.
- Ground cover strongly affects how much rain soaks in versus runs off.
- Water is conserved: the same water is used again and again.
Model note: each drop stands for a large amount of water, and the rates are chosen to show the processes clearly rather than to match a real place.