Theory:

We know that water is an inevitable constituent of all living beings on the earth. So here, let's begin with the words of Thiruvalluvar,
 
நீர்இன்று அமையாது உலகெனின் யார்யார்க்கும்
வான்இன்று அமையாது ஒழுக்கு.
 
plant rain.jpg
 
The above Thirukkural states, "if it is said that the duties of life cannot be discharged by any person without water, so without rain, there cannot be the flowing of water".
 
From the above explanation, it is clear that ancient Tamils had an idea about the hydrological cycle thousands of years ago. It is well known that each and every aspect of every living being on the earth is one or the other way connected to water.
 
All plants and animals must have water for their survival. Apart from drinking, water has many significant roles in domestic, agriculture, industrial purposes etc. Water is essential for carrying out almost all economic activities. So, water is an indispensable element without which life form on the earth is not possible.
rain drop.jpg
Have you ever thought about why all the water on the planet is not getting exhausted on usage? Or how it is getting replenished? The answer is the Hydrologic cycle. In this chapter, we are going to look into the details of how much water and in what forms it is present in the earth, what is a hydrologic cycle and how it is taking place.