Rainwater Harvesting Scenario in Chennai

Chennai: The main sources of public water supply in the city are the three reservoirs - Poondi, Redhills and Cholavaram - with an aggregate storage capacity of 175 MCM. Even when the reservoirs are not full, they get inflows from intermittent rains, which is then drawn. On the other hand, losses due to evaporation from the reservoirs result in the effective availability being lower than the storage.

The other major resource is groundwater from the well fields in the Araniar-Kortaliyar basin and the southern coastal aquifer, and a large number of wells and tubewells spread all across the city.

Over-extraction of groundwater in the north western coastal belt resulted in a rapid ingress of seawater, which extended from 3 km inshore in 1969 to 7 km in 1983 and 9 km in 1987. Groundwater levels within the city also fell and brackish water began to appear even in localities which earlier had good quality groundwater sources.

source: http://www.rainwaterharvesting.org/Crisis/Urbanwater-scenario.htm