Water is always cycling around, through and above the Earth. This is called the natural water cycle – the continuous movement of water between the land, ocean, rivers and creeks and atmosphere.
As water moves through the cycle, it changes state from liquid (rainwater, seawater) to gas (water vapour) and back to liquid. Liquid can also freeze and become solid (ice or snow). This natural process removes some of the water’s impurities, constantly refilling Earth’s fresh water supplies – it is our planet’s way of recycling water.
The water cycle is essential for life on Earth, and has been happening for billions of years. In fact, the water we drink today could have been drunk by a dinosaur millions of years ago, though it has been cleaned many times since then.
Stages of the water cycle
Evaporation is the process of changing water from liquid to gas.
When the sun heats water in rivers, lakes or oceans, it provides enough energy to break the hydrogen bonds between water molecules. The individual molecules rise through the air into the atmosphere, in the form of water vapour or steam.
Only fresh water makes its way up to the clouds, as ocean water leaves behind salt, minerals and metals when it evaporates.
Condensation is the process of changing water from gas to liquid.
As water vapour rises, it becomes cooler and changes back into tiny liquid water droplets. These merge together to form clouds.
Precipitation is when rain, snow, sleet or hail falls from the sky.
When so much water has condensed that the air cannot support its weight, water falls from the clouds back to Earth. Depending on the air temperature, water can take a liquid form (rain), or a solid form (snow, sleet or hail).
Infiltration occurs when water falls back to Earth, where some of it soaks into the ground. It is then collected underground in layers of rock, sand or gravel called aquifers – this water is known as groundwater. Groundwater eventually seeps to the bottom of rivers, providing a steady flow of water even after the rain has stopped.
Water in the ground can also be absorbed by plant roots. This water travels up through the plant to its leaves, where some of it is used in the process of photosynthesis.
Run-off is when water does not soak into the ground, but flows across land instead. This water is called surface water, and collects in creeks which flow into larger rivers.
Transpiration when water evaporates from plants, mainly through their leaves. This gets water vapour back into the air.