The Water Cycle

Water in nature is continuously moving, even though in some cases it may move very slowly over millions of years, trapped in rocks deep below the earth’s surface. Not only is water continuously moving, it is also continuously changing its state between a liquid, a gas and a solid. This continual movement and changing of water is known as the water cycle and is illustrated in the diagram below.

The water cycle is indivisible and all parts of it are inter-related. Water is (generally speaking) neither made nor destroyed. Different parts of the cycle influence each other – rainfall affects the amount of water which is available to plants; the amount of water which plants use affects how much water seeps deep into the ground to form ground water; when ground water is pumped out of the ground, the level of underground water, or the ‘water table’ level, is affected; water table levels have an important influence on river flows.

Human activities also have an impact on the water cycle and, as development occurs, that impact increases. We are now better able to control the quantity of water available to us, using modern engineering methods, and we have a greater impact on the quality of water.

Before the introduction of high yielding, deep level ground water pumps, for example, we did not have much impact on ground water. Now, with modern technology, we are virtually able to destroy our resources and cause damage which will take thousands of years to repair. Therefore, as technology advances, so does our need for protection and regulation.

Different land uses also impact the water cycle. When a crop such as sugar cane or timber is planted, the runoff to rivers and streams will decrease and so will the rate of ground water recharge. This in turn will reduce the amount of water which is available for other uses. The development of an urban area can increase the runoff and risk of flooding. It is not possible, therefore, to manage water resources without having some influence and control over land-use practices.

Land management also affects water quality. Where poor practices are followed, topsoil is washed into rivers which silts up dams and increases the turbidity of the water. These are but two examples of how to certain use of land may affect the water cycle and therefore the interest of other water users in a catchment area.

Underground water needs special consideration. Because it is slow moving and invisible, it is difficult to understand and to predict. Some aquifers (underground formations in which water is stored) have low storage capacity while others have a higher capacity.

The idea of the “catchment” is important. It is a natural drainage area made up of interlinking systems of streams and tributaries flowing under the influence of gravity. All naturally occurring water in a catchment can, for practical management purposes, be treated as inter-related in terms of both quantity and quality. With the use of storage facilities such as dams and reservoirs, water can be regulated and controlled in a catchment to meet different demands. In most instances surface water catchment boundaries or watersheds catchments is sometimes complicated by the need to transfer water between catchments.

Different countries have different climates but all are characterised by variability and unpredictability. Droughts are followed by floods, rainfall fluctuates from year to year and from region to region. The storage of water to help balance the extremes becomes very important. The demand for water in a temperate climate with plenty of water will be different from that in a semi-arid climate.

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