How were traditional water-harvesting structures made? What is the main role of these structures?

Explanation

i] In level terrain, the water-harvesting structures are mainly crescent shaped with earthen embankment or low, straight, concrete-and-rubble .check dams. built across seasonally flooded gullies.
ii] Monsoon rains fill ponds behind the structures. Only the largest structures hold water year round whereas most dry up in six months or less after the monsoons.
iii] Their main purpose, hovwever, is not to hold surface water but to recharge the groundwater beneath. The advantages of water stored beneath the ground are many:
It does not evaporate, but spreads out to recharge wells and provides moisture for vegetation over a wide area
It does not provide breeding grounds for mosquitoes, like stagnant water collected in ponds or artificial lakes.
The groundwater is also relatively protected from contamination by human and animal waste