Which one is useful as a huge water reservoir?

Khadins,Bundhis,Ahars and Kattas are ancient structures that are examples for

Opposition to the construction of large dams is due to

It is important to make small check dams across the flooded gullies because they
$(i)$ hold water for irrigation
$(ii)$ hold water and prevent soil erosion
$(iii)$ recharge ground water
$(iv)$ hold water permanently

Compare the water harvesting system used in hilly/mountainous areas with those used in plains or plateau regions.

Suggest a few useful ways of utilizing waste water.