Theory:

     I give this example and could give many others to emphasis that this common substance which we take for granted in our everyday life is the most potent and the most wonderful thing on the face of our earth. It has played a role of vast significance in shaping the course of the earth’s history and continues to play the leading role in the drama of life on the surface of our planet.

     There is nothing which adds so much to the beauty of the countryside as water, be it just a little stream trickling over the rocks or a little pond by the wayside, where the cattle quench their thirst of an evening. The rainfed tanks that are so common in South India – Alas! often so sadly neglected in their maintenance – are a cheering sight when they are full. They are, of course, shallow, but this is less evident since the water is silt-laden and throws the light back, and the bottom does not therefore show up. These tanks play a vital role in South Indian agriculture. Some of these tanks are surprisingly large and it is a beautiful sight to see the sun rise or set over one of them. Water in a landscape may be compared to the eyes in a human face. It reflects the mood of the hour, being bright and gay when the sun shines, turning to dark and gloomy when the sky is overcast.
Explanation:
 
The author says he had mentioned the example of Nile, and that he can state many more - only to reiterate the fact that we do not take "water" seriously. Since it is the most common substance found on Earth, it does not mean it is not essential. It is the most powerful and glorious thing on Earth. It has a huge role in defining our history and is continuously playing a critical part in our life on this planet.
 
Water, in any form, adds beauty to a place - it can be a small stream dripping down the rocks, or a small pond on the way where domestic animals satisfy their thirst during evenings. It makes the whole place beautiful.
 
sunrise+river.jpg
There is nothing which adds so much to the beauty of the countryside as water!
  
The concept of rain-fed tanks are common in South India, where tanks are built to collect rainwater and used for irrigation. It looks lovely when such tanks are full, and is a wonderful treat for the eyes. But the system has not been given proper care and maintenance. They have less depth, but it does not matter - the water is full of mineral sediments and it reflects the light in such a way that the river-bed part is not seen from outside. These tanks are essential for agriculture in South India. Some huge tanks are a beautiful sight, especially during sunrise and sunset time. The author compares the water in a land area to the eyes in a human face. He  explains its importance as it demonstrates the mood of the hour - when the sun is bright and shining, the water also sparkles bright; it becomes dull and dark when it is cloudy.
 
Meanings of difficult words:
  
No.
Words
Meanings
1
tricklingflow in drips, drizzle
2
quenchto satisfy thirst, desires
Reference:
State Council of Educational Research and Training (2019). Term-2 English Standard-9. Water - The Elixir of Life - Sir C.V. Raman (pp. 121-145). Published by the Tamil Nadu Textbook and Educational Services Corporation.