Under ground water sources get stressed by Urbanisation. It happens through two mechanisms.
Firstly, as city population increases rapidly, municipal piped supply is not able to keep pace. Building new storage and pumping capacity takes long time and lot of money. People resort to the quick and easy solution of drilling a hole in the ground next to their house and putting a pump inside. In Ranchi, for example, one or two drilling rigs can be found working all the time in all the neighbourhoods, particularly during the summer months. And they are overbooked.
Secondly, when new houses, shops, schools, offices and roads are build, the capacity of the land to absorb rain water is reduced by about 75%. A flat ground surface like grassland or cultivated land has a run-off co-efficient of 0.00 to 0.3, whereas, the road surface or roof top or concrete pavement has a coefficient of 0.8. So instead of 80-85% rainwater absorption on a non developed plot, a developed plot absorbs only about 19% of rain water ( allowing for evaporation loss also).
This double squeeze, in which we draw more from the underground water source, even as we block 75% of what was being added to it earlier, causes the drying up of wells and lowering of water tables.
