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| Sunday, 3 April 2005 |
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Junior Observer |  |
| News Business Features Editorial
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The constant movement of water
Although the balance of water on Earth remains fairly constant over time, individual water molecules can come and go in a hurry. The water in the mango that you ate yesterday, may have fallen as rain half-way around the world last year, or could have been used a 100 million years ago. Precipitation, evaporation and transpiration are all terms that sound familiar, yet may not mean much to you. They are all part of the water cycle, a complex process that not only gives us water to drink, but also weather patterns that help grow our crops. Evaporation Evaporation is the process by which a liquid, in this case water, changes from its liquid state to a gaseous state. Liquid water becomes water vapour. Although lower air pressure helps promote evaporation, temperature is the primary factor. For example, all the water in a pot left on a table will eventually evaporate. It may take several weeks. But, if that same pot of water is put on a stove and brought to a boiling temperature, the water will evaporate more quickly. During the water cycle, some of the water in the oceans and freshwater bodies, such as lakes and rivers, is warmed by the sun and evaporates. During the process of evaporation, impurities in the water are left behind. As a result, the water that goes into the atmosphere is cleaner than it was on Earth. Condensation Condensation is the opposite of evaporation. Condensation occurs when a gas changes into a liquid, when the temperature of the vapour decreases. When the water droplets formed from condensation are very small, they remain suspended in the atmosphere. Millions of droplets of such suspended water form clouds in the sky or fog at ground level. Water condenses into droplets only when there are small dust particles present, around which the droplet can form. Precipitation When the temperature and atmospheric pressure are right, the small droplets of water in clouds form larger droplets and precipitation occurs. The raindrops then fall to Earth. As a result of evaporation, condensation and precipitation, water travels from the surface of the Earth into the atmosphere, and returns to Earth again. Surface run-off Much of the water that returns to Earth as precipitation runs off the surface of the land, and flows downhill into streams, rivers, ponds and lakes. Small streams flow into larger streams, then into rivers, and eventually, the water flows into the ocean. Surface run-off is an important part of the water cycle because, through surface run-off, much of the water returns again to the oceans, where a great deal of evaporation occurs. Infiltration Infiltration is an important process where rain water soaks into the ground, through the soil and underlying rock layers. Some of this water ultimately returns to the surface through springs or in low spots downhill. Some of the water remains underground and is called groundwater. As the water infiltrates through the soil and rock layers, many of the impurities in the water are filtered out. This filtering process helps clean the water. Transpiration One final process is important in the water cycle. As plants absorb water from the soil, the water moves from the roots through the stems to the leaves. Once the water reaches the leaves, some of it evaporates from the leaves, adding to the amount of water vapour in the air. This process of evaporation through plant leaves is called transpiration. In large forests, an enormous amount of water will transpire through leaves. Water is constantly being cycled between the atmosphere, the ocean and land. This cycling is a very important process that helps sustain life on Earth. As the water evaporates, vapour rises and condenses into clouds. The clouds move over the land, and precipitation happens in the form of rain, ice or snow. The water fills streams and rivers, and eventually, flows back into the oceans where evaporation starts the process anew. The state of water (solid, liquid or gas) is determined mostly by temperature. Although water continuously changes states from solid to liquid to gas, the amount of water on Earth remains constant. Cloud formation Precipitation is one key to the water cycle. Rain comes from clouds, but where do clouds come from? Through the process of evaporation and transpiration, water moves into the atmosphere. Water vapour then joins dust particles to create clouds. Eventually, water returns to Earth as precipitation in the form of rain, snow, sleet and hail. All clouds contain water vapour. You rarely ever see clouds in the desert because there is very little water to evaporate and form clouds. Coastal regions can receive a lot of rain because they pull up moisture from surrounding waters. Cloud size is influenced by many complex factors, some of which have still not been properly explained. These include heat, seasons, mountain ranges, bodies of water, volcanic eruptions, and even global warming. |
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The water on Earth is always in movement. The water cycle, also known
as the hydrologic cycle, describes the continuous movement of water on,
above, and below the surface of the Earth. Since the water cycle is truly
a "cycle", there is no beginning or end. Water can change its state from
liquid to vapour or ice at various places in the water cycle, with these
processes happening in the blink of an eye or over millions of years. 
