I. The water reservoir is a pond in fig. (a) and undergroung water body in fig. (b) i.e., underground water body has more advantage over fig. (a) and water reservoir,
II. There are many advantages of water stored in the ground. Some of these are
(i) It does not evaporate.
(ii) It spreads out to recharge wells and provides moisture for crops over a wide area.
(iii) It does not promote breeding of mosquitoes (unlike stagnant water collected in ponds etc).
(iv) It is free from contamination by human and animal wastes.
(v) It is utilised for the benefit of local population.