Question

how is increasing in population responsible for shortage of water

Answer 1

Increased population means more stress on natural resources

Answer 2

if population increases then people will need more water to drink, bath, etc and we can only us the water from lakes rivers ponds and underground water which are produced by rains. And vast deforestation leads to the shortage of clouds which produce us rain. And we cannot even use sea water as it contains a lot of salinity.