Coaching Centre
The water cycle
Water as a resource that cycles through the environment, the three states of water, evaporation, transpiration, condensation, precipitation, run-off, infiltration and groundwater, how long water stays in each store, and how understanding the water cycle shapes how we manage water.
The water in your drink bottle is billions of years old. Some of its molecules have probably fallen as snow on Antarctica, flowed down the Amazon, been breathed out by a dinosaur and floated as a cloud over the Pacific. The Earth doesn't make new water or lose old water; it keeps moving the same water around and around.
The water cycle
Water is the only common substance found naturally on Earth as a solid (ice and snow), a liquid (oceans, rivers, rain) and a gas (invisible water vapour). The water cycle is the continuous movement of water between the oceans, the air and the land, changing state as it goes. It is powered by energy from the Sun, and gravity pulls the water back down.
| Process | What happens | Change of state? |
|---|---|---|
| Evaporation | The Sun heats water in oceans, lakes, rivers and soil, and it becomes water vapour | Liquid → gas |
| Transpiration | Plants draw water up from their roots and lose it as vapour through tiny holes (stomata) in their leaves | Liquid → gas |
| Condensation | Rising vapour cools and turns into tiny droplets that form clouds | Gas → liquid |
| Precipitation | Droplets join until they are heavy enough to fall as rain, hail, sleet or snow | (may freeze) |
| Run-off | Water flows over the ground into creeks, rivers, dams and the sea | No |
| Infiltration | Water soaks into the soil and down into rock, becoming groundwater stored in layers called aquifers | No |
| Melting and freezing | Snow and ice store water, then release it as they melt | Solid ↔ liquid |
Animals add a little water vapour to the air when they breathe out (respiration) and sweat.
- Press "Next move" a few times and read the log. Which processes moved your molecule?
- Run 40 moves. Where did the molecule spend most of its time?
- Why is groundwater pumped from deep bores much slower to replace than water in a river?
How long does water stay?
Water moves through some parts of the cycle quickly and gets "stuck" in others for a very long time:
- Atmosphere: about 9 days
- Rivers: weeks to months; soil: one to two months
- Lakes: years to centuries
- Groundwater: from a few years to over 10 000 years
- Oceans: about 3000 years on average; Antarctic ice: up to hundreds of thousands of years
Using the water cycle to manage water
Understanding the water cycle has changed the way we store, use and protect water:
- Catchments: every drop of rain that runs off a catchment ends up in its river or dam. Sydney's drinking water catchments around Warragamba are protected from farming and housing so the run-off stays clean.
- Dams and reservoirs catch run-off to store water through dry times, but some is lost to evaporation, especially in hot inland areas.
- Rainwater tanks and stormwater harvesting catch precipitation before it runs off into drains.
- Groundwater must not be pumped out faster than infiltration refills it.
- Treating and recycling wastewater: sewage is cleaned and returned to rivers or the ocean, or recycled to water parks, golf courses and farms. Perth even pumps highly treated recycled water back into its aquifers to top up its drinking water supply.
- Desalination copies the cycle's evaporation and condensation (or uses filters) to turn seawater into fresh water.
Clearing forests also changes the cycle: fewer trees means less transpiration, more run-off and more erosion.
Test yourself
The energy that drives the water cycle comes mainly from
It heats water so it evaporates.
Water vapour turning into droplets in a cloud is
Gas → liquid as it cools.
Transpiration is water lost
Through stomata in leaves.
Water soaking into the ground to become groundwater is called
It fills spaces in soil and rock.
A water molecule usually stays in the atmosphere for about
Then it falls as precipitation.
Label the water cycle.
Follow a drop from the ocean back to the ocean.
Liquid water turning to vapour is . Rain, hail and snow are kinds of . Water flowing over the ground into rivers is . Water stored underground in aquifers is .
Explain the physical processes of the water cycle, and describe one way that knowledge of the water cycle has influenced how Sydney manages its water.
Energy from the Sun heats water in the oceans, rivers and soil, and it evaporates, changing from liquid to invisible water vapour; plants also release vapour from their leaves by transpiration. As the warm, moist air rises it cools, and the vapour condenses into tiny liquid droplets, forming clouds. The droplets join together until they are heavy enough to fall as precipitation (rain, hail or snow). On land, some water runs off the surface into creeks and rivers and flows back to the sea, and some infiltrates the soil and becomes groundwater. Because run-off from a catchment ends up in its dam, Sydney protects the catchment around Warragamba Dam from farming and housing, so the water that runs into the dam stays clean and needs less treatment.