When water falls back to earth as precipitation, it may fall back in the oceans, lakes or rivers or it may end up on land.

When it ends up on land, it will either soak into the earth and become part of the “ground water” that plants and animals use to drink, or it may run over the soil and collect in the oceans, lakes or rivers.

Sometimes it will fall in very cold areas of the world as snow. This snow can stay frozen for a long time until the sun melts it and it runs down into the rivers and on to the sea. Nearly half of all precipitation flows back across the land to seas and oceans. The water has now arrived back where it started and the whole process starts again.

The world's total water supply has a volume of about 13,700 million cubic kilometers. However most of this is salt water (97%) in the oceans and the remaining 3% is fresh water.