by Goutham Krishna
It’s been a few weeks since Chennai went through another November of Heavy downpour!
The average annual rainfall of Chennai city is about 10 times greater than the national average. North-East and South-West monsoons are the major contributing factors to the total rainfall of the city. The city is also blessed with 3 major rivers and numerous large and small water bodies which all point towards the high-water storage potential of the city.
But it’s ironic to find that a large section of the population in the city; mostly the underprivileged, suffers from water scarcity and drought in the summers. The major factor behind this paradox is the lack of water security and management mechanism followed in the city.
In this context, this article is a hypothetical attempt to quantify and understand the relation between annual rainfall and the annual water requirement of the city and to check whether any feasible mechanism is suitable to improve the water security of Chennai city.
The table given below visualizes annual rainfall received by Chennai city from the year 2015 to 2021 (For 2021, data till 3rd December is considered)
| Year | 2015 | 2016 | 2017 | 2018 | 2019 | 2020 | 2021 |
| Annual Rainfall (mm) | 2285.06 | 977.4 | 1447.2 | 933.34 | 1228.92 | 1726 | 1999.1 |
From definitions, any location with 1 mm rainfall recorded will receive 1 liter of rainwater per square meter. i.e. If a geographical area of 1 square Kilometre receives a steady rainfall of 1 mm, 100,00,00 liter of rainfall is precipitated there.
The geographical area of Chennai city is 426 square kilometers; That means 426*100,00,00 liter of rainwater will be received in the city if it receives rainfall of 1 mm.
Now, the total of annual rainwater received by the city can be easily calculated with the above data.
| Year | 2015 | 2016 | 2017 | 2018 | 2019 | 2020 | 2021 |
| Annual Rainfall (mm) | 2285.06 | 977.4 | 1447.2 | 933.34 | 1228.92 | 1726 | 1999.1 |
| Total rainwater In Litre | 97343,55,60,000 | 41637,24,00,000 | 61650,72,00,000 | 39764,28,84,000 | 53089,34,40,000 | 74,63,20,00,000 | 86361,12,00,000 |
An estimate of average rainwater received in Chennai on a daily basis can be inferred from the above data (by dividing total rainwater by 365).
| Year | 2015 | 2016 | 2017 | 2018 | 2019 | 2020 | 2021 |
| Annual Rainfall (mm) | 2285.06 | 977.4 | 1447.2 | 933.34 | 1228.92 | 1726 | 1999.1 |
| Total rainwater In Litre | 97343,55,60,000 | 41637,24,00,000 | 61650,72,00,000 | 39764,28,84,000 | 53089,34,40,000 | 74,63,20,00,000 | 86361,12,00,000 |
| Average rainwater per day in Litre | 266,69,46,739.7 | 114,07,46,301.4 | 168,90,60,821.9 | 108,94,32,558.9 | 145,45,02,575.3 | 204,28,27,397.3 | 236,60,58,082.2 |
Considering 150 litres as the average water requirement per head per day, we can derive into the following findings. (Population of Chennai is approximated to 1,00,00,000)
| Average rainwater per day in Litre | 266,69,46,739.7 | 114,07,46,301.4 | 168,90,60,821.9 | 108,94,32,558.9 | 145,45,02,575.3 | 204,28,27,397.3 | 236,60,58,082.2 |
| Maximum number of people can be benefited | 1,77,79,644.9 | 76,04,975.3 | 1,12,60,405.5 | 72,62,883.7 | 96,96,683.8 | 1,36,18,849.3 | 1,57,73,720.5 |
| % Of Chennai population | 178 | 76 | 113 | 73 | 97 | 136 | 158 |
From the above findings, it is crystal clear that proper conservation of rainwater alone can account for a good share of the water requirement of Chennai city. Hence actions to store and harvest maximum rainwater is an optimal solution to address the water scarcity faced by the city. Chennai is blessed with numerous water bodies, major canal systems, and three lakes which all can be managed properly for sustainable water conservation and management.
Hence an integrated framework, encompassing maximum storage of rainwater, proper conservation of water bodies, and sustainable consumption of water should be implemented which will definitely improve the water security statistics of the city for the long term.
Lakes of India 
The Cauvery water row that breaks out between 2 states at regular intervals could be avoided through efficient water management but somehow such issues are kept burning to gain political mileage.
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