Water is the most abundant liquid in nature. It is a compound of hydrogen and oxygen. It occurs on land as seas, oceans, rivers, springs, wells, etc. It also occurs in the atmosphere as rain, water vapour, clouds, etc. Water is the essential constituent of animal and plant life. Without water, no life could exist on earth. All living things need water to survive. About 60% of the human body by mass is made of water.
Water is always on move, travelling a never-ending, cyclical journey between the earth and the sky. This journey is referred to as the water cycle or hydrological cycle. The water cycle describes the continuous movement of water on, above and below the surface of the earth. During its movement, water is continuously reused and recycled. It also changes its physical state or form (liquid, vapour, and ice) at various stages in the water cycle. Figure bellow is a diagrammatic representation of the water cycle. It shows how the water moves around the earth’s environment, changing its form through the process of evaporation, transpiration (loss of water from plants), condensation and precipitation (rainfall, snow, hail, fog, smog, etc.) Stages of the water cycle are described below:
(a) Heat from the sun causes water to evaporate from exposed water bodies such as oceans, seas, lakes, rivers dams, etc. This causes huge amounts of water vapour to float (laden) in the air. The vapour rises up. In the cooler upper parts of the atmosphere, the vapour cools and condenses to form tiny water droplets. The droplets form clouds.
(b) The clouds are drifted by wind. They cool further, and the droplets join to form larger drops of water which fall down as rain due to gravitation pull. On the other hand, if the air is very cold, they fall as hail, sleet or snow. The whole process is called precipitation.
(c) Some rain water soaks, and reappears as springs. Some flows over the ground as streams. The springs and streams feed rivers. The rivers flow to the ocean, sea or lake. The whole cycle starts again.