When the rest of India is sizzling, due to a quirk of geography and climate, it is usually raining in May in Bengaluru. In fact, it rains as much in the month of May on average as it does in June and July, when the South-West monsoon has set in.
Here is some information on the rainfall pattern in Bengaluru as provided by the Indian Meteorological Department (see table).
At the same time, the water levels in the dams which feed the city are at their lowest, waiting for the June rains. One needs only to look here to see the state of water in our reservoirs - http://ksndmc.org/Uploads/RL.pdf. This time the IMD predicts a slightly delayed onset of the monsoon of 2019, making the wait a bit more tedious.
If one calculates the volume of rain falling on a rooftop of 100 square metres, in May alone 10,740 litres of rain would have fallen. If this rain were to be collected, it would dispense almost 90 litres per person per day, for a family of 4. Not bad for peak summer.
If the city as a whole were to be looked at, on the 1250 sq. km. which is the planning area of the city, the total rain falling would be the equivalent of 1,34,250 million litres or 4,475 million litres per day equivalent. If uncollected, most of it would evaporate or reach lakes in a polluted state to evaporate from there. Only about 3 to 8 % would naturally recharge, a slightly less number because the soil moisture is in deficit at this time and tends to absorb a lot of rain.
Putting in place a strategy to collect or recharge even 50% of this rainwater would make eminent sense. The rules are simple: from every clean rooftop, channel, filter and store the rain in sump tanks.
Where it is not possible to store – channel, filter and recharge the unconfined aquifer through recharge wells. These recharge wells can also be placed in or adjacent to stormwater drains. They need a small filter to make sure that no organic matter goes in. It is important also to ensure that no polluted water gets into these wells. Aquifers can thus be fed with rainwater to keep them full for later use.
It is not how much it rains that i s important as much as what we do with the rains. If we are water- and climate-smart we will collect and use every drop of rainwater wisely and well.
