Unit C2 - Water and the Water Cycle


Unit C2


Water is one of the most common substances on the planet. The sea is made of water; the land contains water in many forms; the air contains water vapour which makes clouds and rain. Our bodies are made mostly of water, and if we do not drink water or other liquids containing water every day, we can quickly die of thirst.

All living things depend on water, including plants, animals and even microbes. Where there is no water, there is only a barren desert of rock and sand.

Water has qualities which make it a good material to support life. It is liquid at the right temperatures for most things to live. It can absorb lots of heat, and thus helps to even out extremes of hot and cold. Many things will dissolve in water, so that water can wash things clean or wash them away, or let many things be mixed together. In this way, the many substances that make up life can come together in water. This quality can be useful, as when it permits water to bring materials to plant roots, but it can also be bad for the environment when water becomes a carrier of pollutants.

Most water on earth occurs in the sea. In the hundreds of millions of years that the sea has existed, many salts have dissolved and collected in it. There is too much salt in sea water for most land plants and animals to be able to use it. Life on the land needs fresh water without much salt, and this water comes through the water cycle.

The water cycle

The water cycle describes how water moves from the sea to the air to the land and back to the sea again. The energy from the sun heats the ocean surface causing evaporation, which is when some molecules of water escape from the water's surface and become water vapour in the air. The salts stay behind in the sea. The warmer the air is, the more water vapour it can hold. As the sun shines and the wind blows over the ocean, the air warms and becomes full of water vapour. Warm air tends to rise, or it may be pushed up by passing over an island or other land. As it rises it cools until it can no longer hold all the water vapour in it. The water vapour condenses to make tiny drops of water which we see as clouds. If these small drops get crowded and keep bumping into each other, they become larger and larger until they are heavy enough to start falling as rain.

Some rain falls back into the sea, but much of it falls on land. What happens to it there depends on the nature of the land and the environment. If the land is sloping and unprotected, the rain water runs quickly down hill into rivers or streams and back into the sea.

If the land is covered by plants and trees, the fall of the raindrops is softened by the leaves and branches. The plants, dead leaves and soil soak up the water like a sponge. Some water is taken up by the plant roots and evaporated through the leaves, and some evaporates directly from the soil, going back to the air. Some of the water runs off the land, but more slowly because of all the plants. It feeds the streams and rivers and keeps them flowing. More of the water has time to sink down into the ground where it becomes ground water that supplies springs and wells. The ground water also moves very slowly down hill until it too returns to the sea, completing the water cycle.

Thus the amount of water that stays on the land, and the speed with which the water cycle is completed, depend on the quality of the natural environment. The water behaves differently depending on the kind of land and its structure.

Surface and ground water on different land types

If the land is continental or of old volcanic origin, there may be layers of rock or clay in the ground through which the water cannot pass. The movement of the ground water can therefore be very complicated, with the water moving over or collecting between these layers as it feeds streams and springs. More porous land, such as young volcanic deposits, may have many tunnels, holes and layers of ash through which the water can seep away very quickly. Such land may not have any streams that flow except in heavy rains. If the land is made of limestone rock or sand, as on a coral island, there are many holes so the rain water sinks in very quickly, and may even float in a layer on top of sea water.

Water supplies

People may get their water at different points in the water cycle. Sometimes the rain is caught directly on roofs or other surfaces and stored in tanks or cisterns; it is important to store enough water to last through any period without rain. Where there are streams or rivers that flow all year round, the water may be taken directly from the stream as needed. If the stream stops flowing in the dry season, then a dam or reservoir will be needed to store enough water for the dry period.

The ground water may be an important part of a water supply, either through natural water holes or springs, or to wells dug down to the level of the ground water. Where wells provide water, they must be used with care. The capacity of a well depends on how much ground water there is, how rapidly it is replaced by new rain water sinking into the ground, and how quickly the water can move through the ground to replace what is taken from the well. If a well is near the sea, pumping out too much water may cause sea water to be drawn into the well. Once a well becomes salty, it is very difficult to get it to produce fresh water again; a new well may be needed somewhere else.

It is important to remember that the amount of new ground water and the flow of rivers and streams all depend on the proper protection and management of the environment at the surface. If the plant cover is changed or the land is developed, the water supply of the area may be affected. Many areas now have water shortages they did not have before because of damage to the environment in places where the rain water is caught. Remember also that ground water, like surface water, tends to flow towards the coast. Water supplies and wells should therefore be inland of anything that might pollute the water.

Each land area is different and has its own special characteristics affecting its water supplies, but the general principles described above should help you to understand why the water behaves as it does in any local situation. Water is one of the most important resources, and it must be managed carefully if it is to be protected.


Where does the rain come from?

Where does the water on your land go to?

What would happen to your land if the rain stopped?

What does the forest do for the water?

What kinds of water supplies do you have?

Is there enough water?

If not, why not?

What could be done to make more water available?

Has development created any problems for your water supply?

What needs to be done to protect your water supply?

Instructions for trainers in the use of this unit

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Last updated 15 November 2006