Question

how traditional water harvesting structures were made? Also draw a suitable diagram to explain these harvesting system. What is the main role of these harvesting system?

Answer 1

The traditional water harvesting structures were made with the underground water tanks and a rain water pipe to collect all the water from the rain that can later be used either to water the crops or for daily uses. 
From these tanks the water can be purified and reused when ever required 

Answer 2

Traditional waters systems are used to store rain water so that it can be used to farming etc. The rain water is stored in a tank where it is purified and reused.  

Some of the rainwater harvesting system includes: Small pits, lakes, dams, reservoirs and rooftop water collecting units.