- About two-thirds of the Earth is covered with water, most of which is in oceans and seas.
- The water in the oceans and seas is saline- it contains many salts dissolved in it. It is not fit for drinking and other uses.
Loss of water by evaporation
- During the daytime, sunlight falls on the water in oceans, rivers, lakes and ponds.
- As a result, water from all these places continuously evaporates into vapour.
- However, the salts dissolved in the water are left behind.
Loss of water by transpiration
- Plants need water to grow.
- A part of this water is used to make food and another is retained in different parts of the plants.
- Remaining part of this water is released by the plants into air, as water vapour through the process of transpiration.
Formation of water by condensation
- Condensation plays an important role in bringing water back to the surface of the earth.
- As we go higher from the surface of the earth, it gets cooler. When the air moves up, it gets cooler and cooler.
- At sufficient heights, the air becomes so cool that the water vapour present in it condenses to form tiny drops of water called droplets.
- It is these tiny droplets that remain floating in air and appear to us as clouds.
- Many droplets of water come together to form larger sized drops of water.
- Some drops of water become so heavy that they begin to fall as rain.
Thus, water in the form of vapour goes into air by evaporation and transpiration, form clouds, and then comes back to the ground as rain, hail or snow.
- Most of the water that falls on the land as rain and snow sooner or later goes back to the oceans.
- The circulation of water in this manner is known as the water cycle.
Collecting rainwater and storing it for later use is called rainwater harvesting.
- There are two techniques of rainwater harvesting:
- Rooftop rainwater harvesting: the rainwater is collected from the rooftop to a storage tank, through pipes. This water may contain soil from the roof and need filtering before it is used. Instead of collecting rainwater in a tank, the pipes can go directly into a pit in the ground. This then seeps into the soil to recharge or refill the ground water.
- Allow water to go into the ground directly from the roadside drains that collect rainwater.
Read More: Chapter 13