EXISTENCE OF WATER AND ITS NECESSITY
Water occupies a very large area of the Earth’s surface and is also found underground. Some amount of water exists in the form of water vapour in the atmosphere. Most of the water on Earth’s surface is found in seas and oceans and is saline. Fresh water is found frozen in the ice-caps at the two poles and on snow- covered mountains. The underground water and the water in rivers, lakes and ponds is also fresh. However, the availability of fresh water varies from place to place. Practically every summer, most places have to face a shortage of water. And in rural areas, where water supply systems have not been installed, people are forced to spend considerable amounts of time in fetching water from far - away sources.
Activity 14.7:
* Many municipal corporations are trying water -harvesting techniques to improve the availability of water.
* Find out what these techniques are and how they would increase the water that is available for use
But why is water so necessary? And do all organisms require water? All cellular processes take place in a water medium. All the reactions that take place within our body and within the cells occur between substances that are dissolved in water. Substances are also transported from one part of the body to the other in a dissolved form. Hence, organisms need to maintain the level of water within their bodies in order to stay alive. Terrestrial life-forms require fresh water for this because their bodies cannot tolerate or get rid of the high amounts of dissolved salts in saline water. Thus, water sources need to be easily accessible for animals and plants to survive on land.
Activity 14.8:
* Select a small area (say, 1 m2) near a water-body, it may be a river, stream, lake or pond. Count the number of different animals and plants in this area. Also, check the number of individuals of each type or species.
* Compare this with the number of individuals (both animals and plants) found in an area of the same size in a dry, rocky region.
* Is the variety of plant and animal life the same in both these areas?
Activity 14.9:
* Select and mark out a small area (about 1 m2) in some unused land in or near your school.
* As in the above activity, count the number of different animals and plants in this area and the number of individuals of each species.
* Remember to do this in the same place twice in a year, once during summer or the dry season and once after it has rained
After compiling the results of the above two activities, think if there is any relationship between the amount of available water and the number and variety of plants and animals that can live in a given area. If there is a relationship, where do you think you would find a greater variety and abundance of life – in a region that receives 5 cm of rainfall in a year or a region that receives 200 cm of rainfall in a year? Find the map showing rainfall patterns in the atlas and predict which States in India would have the maximum biodiversity and which would have the least. Can we think of any way of checking whether the prediction is correct?
The availability of water decides not only the number of individuals of each species that are able to survive in a particular area, but it also decides the diversity of life there. Of course, the availability of water is not the only factor that decides the sustainability of life in a region. Other factors like the temperature and nature of soil also matter. But water is one of the major resources which determine life on land.
Questions
1. Were the numbers similar both times?
2. In which season did you find more variety of plants and animals?
3. In which season did you find more number of individuals of each variety?
4. Why do organisms need water?
5. What is the major source of fresh water in the city/town/village where you live?
Source: This topic is taken from NCERT TEXTBOOK
WATER POLLUTION
Water dissolves the fertilisers and pesticides that we use on our farms. So some percentage of these substances are washed into the water bodies. Sewage from our towns and cities and the waste from factories are also dumped into rivers or lakes. Specific industries also use water for cooling in various operations and later return this hot water to water -bodies. Another manner in which the temperature of the water in rivers can be affected is when water is released from dams. The water inside the deep reservoir would be colder than the water at the surface which gets heated by the Sun.
All this can affect the life-forms that are found in these water bodies in various ways. It can encourage the growth of some life-forms and harm some other life-forms. This affects the balance between various organisms which had been established in that system.
So we use the term water-pollution to cover the following effects:
1. The addition of undesirable substances to water-bodies. These substances could be the fertilisers and pesticides used in farming or they could be poisonous substances, like mercury salts which are used by paper -industries. These could also be disease-causing organisms, like the bacteria which cause cholera.
2. The removal of desirable substances from water-bodies. Dissolved oxygen is used by the animals and plants that live in water. Any change that reduces the amount of this dissolved oxygen would adversely affect these aquatic organisms. Other nutrients could also be depleted from the water bodies.
3. A change in temperature. Aquatic organisms are used to a certain range of temperature in the water -body where they live, and a sudden marked change in this temperature would be dangerous for them or affect their breeding. The eggs and larvae of various animals are particularly susceptible to temperature changes.
Questions
1. Do you know of any activity which may be polluting this water source?
Source: This topic is taken from NCERT TEXTBOOK