The Globe and the Water Cycle

It is hotter nearer the equator because the equator receives more direct and intense sunlight than the poles. This means that at the equator much more evaporation of water will occur. Much of the evaporated water comes from the lush vegetation of the equator. The water that is taken up by the plant roots evaporates out from their leaves. Plants use and give out enormous amounts of water. A tropical tree, such as a eucalyptus, may take up hundreds litres of water per day. Nearly the same quantity of water will evaporate through its leaves. This is why it is so humid and it rains so much in rain forests.

As some of these clouds move away from the equator, they lose more and more of their water as rain. The result is that as this air moves away, it becomes drier and drier. On either side of the equator are found the sub-equatorial regions which contain deserts, such as the Sahara. These are desert regions partly because the air moving over them is hot and dry, so there is very little rain. In turn, this means that very few plants can grow there.

Humans cannot drink salty water so they must use sources of fresh water. Less than 1% of all the water in the hydrosphere is fresh water in liquid form. Humans take water from lakes, reservoirs and rivers for things such as drinking, cleaning, cooling machines in factories, and watering crops. All the water that humans use ends up back in the hydrosphere. In industrialized countries, most cites and towns have sewage systems that clean the water before it goes back into the river and lakes.

Johnny's Fact File No.45 In 1832 there was a cholera epidemic in Paris because people drank from water which was not clean.

In developing countries, the water cleaning systems are not always present, so dirty water goes back into the hydrosphere without being treated. Luckily there are micro-organisms living in the water that can naturally clean it. If the amount of dirty water put back is too great, however, then the river or lake will become polluted. If this polluted water is to be consumed by humans again, this can lead to diseases.

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© Paul Billiet, Shirley Burchill, Alan Damon and Deborah James 2009