The water cycle

Water is vital for all living things. Our bodies are made up of at least 60% water.

Water on Earth is constantly moving. The water moves between rivers, lakes, oceans, the atmosphere and the land. It is recycled over and over again in a continuous system called the water cycle. You are still using the same water that the ancient Greeks and the Romans used. The Earth has been recycling water for more than four billion years.

What happens during the water cycle?

Water moves into the atmosphere

Energy from the Sun heats the Earth and the temperature of the water in the rivers, lakes and oceans increases. When this happens, some of the liquid water forms water vapour. This is called evaporation. This happens because some of the particles in the liquid water gain enough energy to break free from the forces holding them together and they change to a gas. Water can also evaporate into the atmosphere from plants; this is called transpiration.

Water in the atmosphere cools down

As the water vapour goes up into the atmosphere, it cools and changes back into little droplets of water in the air, forming clouds. This process is called condensation. It happens because the particles in the water vapour lose energy and cannot move so quickly. Air currents high in the atmosphere move the clouds around the world.

Water falls from clouds

When a lot of water has condensed, the water droplets in the clouds become too heavy for the air to hold them. The droplets fall back to Earth as rain. If the drops become colder they may form snow, hail or sleet. This process is called precipitation.

Water falls on the Earth

The precipitation that falls then collects in rivers and open water such as large lakes and the oceans.

How it is collected depends on where it lands. Some precipitation will fall directly onto the rivers, lakes and oceans and will evaporate, then the cycle starts again.

If the precipitation falls on plants it may evaporate from the leaves back to the atmosphere or trickle down to the ground. The plant roots in the ground may then take up some of this water.

Water in the ground

Some of the water from precipitation will soak into the soil and rocks as groundwater. Some of this water will stay in the shallow soil layer and will move towards streams and rivers. When groundwater soaks deeper into the soil, it refills underground stores.

In cold climates the precipitation may build up on land as snow, ice or glaciers. If the temperatures rise, this solid snow and ice will melt into liquid water, which soaks into the ground or flows into rivers or the ocean.

Some of the precipitation will soak into the soil and move through the ground until it reaches the rivers or the open water, large lakes and the oceans.

Water that reaches the surface of the land may flow directly across the ground into the rivers, lakes and oceans. This water is called surface run-off. When there is a lot of surface run-off, soil can be carried off the land and into the rivers. This can cause them to become silted up and blocked.