The Central Groundwater Board has brought out an information booklet on the groundwater situation of Bengaluru (Urban) District. The booklet is downloadable and is on the web here http://cgwb.gov.in/District_Profile/karnataka/2012/BANGALORE_URBAN-2012.pdf ).
Though dated to the year 2012 it has some very good information leading to help understand the groundwater situation of Bengaluru and its potential. As per the booklet the total area of the district is 2,190 sq. km. and the rainfall is given as 1,049 mm.
The total annual replenishable groundwater is given as 11,723 hectare metres. If a quick calculation is made of the total annual rainfall and the rainwater that replenishes groundwater it will be seen that only 5.35 % of the rainfall actually reaches groundwater. As the city urbanises and concretises, this percentage will become even less.
ADVERTISEMENT
The 11,723 hectare metres translates to 1,17,230 million litres which means that if we draw the annual replenishable groundwater we should only be taking out 321 million litres daily. One estimate is that we pump out 400 million litres to 800 million litres per day in the city alone.
There is therefore a fall in the water table, especially in the city periphery, and alarm bells ring out warning that Bengaluru can run out of groundwater by 2020.
What is to be done about groundwater then? A massive programme of recharge needs to be taken up. Lakes are one way of recharging groundwater but a freshly desilted lake can recharge about 20 litres per square metre per day but a lake not desilted for 3 years can almost reach zero recharge.
ADVERTISEMENT
The rainwater harvesting policy adopted for Bengaluru insists on all properties with more than 120 sq. m. plot area harvesting rain either by storing or by recharging through a shallow recharge pit.
The minimum depth of the recharge structure is given as 3 metres. Recharge wells can be made upto 6 metres and where groundwater is available in shallow depths even deeper to access the groundwater.
Placing rainwater from rooftops which are kept clean, filtered into these recharge wells, will ensure that the recharge volume goes up from the 5.35% to more than 50%. Geology and soil profile permitting, a million recharge wells spread over the 2,190 sq. km. will be a good idea. If distributed equally, each well will have a catchment of 2,190 sq. m. and with a rainfall of 900 mm and a coefficient of collection of recharges being even 50, %, the potential for each well will be almost a million litres to recharge.
Deep borewell recharge too is being undertaken and in combination with shallow aquifer recharge the potential will only increase.
Storage in the aquifers permitting, we should be able to draw 1,400 million litres per day without depleting the groundwater table since 50 % of rainfall recharge means 3,000 million litres daily equivalent. There are many traditional well diggers from the Vaddar community who have the skill to dig these recharge wells. They will get a livelihood and the city will get water security.
Recharging
Globally the talk is on sponge cities, that hold on to and recharge rainwater. We can do that for Bengaluru by a combination of lake revival and recharge wells.
Already several communities, apartments, institutions and individual homes have recharged wells with success. This needs to become a people’s movement aided by the government with technical expertise and a better understanding of the shallow and deeper aquifers.
The monsoon of 2019 will arrive in a month. The preparation time is now. A million wells, a hundred lakes and a million trees for the city could be a goal. As the emperor Ashoka carved on his rock edict at Dhauli, “Plant trees, dig wells” and that was 2000 years ago. The culture of the well, the culture of lakes and the culture of forests was and is water wisdom.
(The author can be reached at zenrainman@gmail.com)