Bengaluru's Rainwater Goes to Waste Despite Mandatory Harvesting Rules

Wednesday's heavy rain left Bengaluru's roads flooded and drains overflowing, but the massive amount of water was a minor problem compared to the larger issue: it all went to waste. Hydrologists estimate the rain delivered 20 to 22 million cubic metres of water, yet only a fraction recharged the groundwater. The rest ran off through drains and sealed surfaces, as it almost always does.

Fifteen years after rainwater harvesting (RWH) became mandatory for new buildings, the city still loses far more water than it saves. Bengaluru has nearly 2.4 lakh RWH structures, according to the Bangalore Water Supply and Sewerage Board (BWSSB). However, 49,958 properties remain non-compliant. For BWSSB, this is not much of a problem because these properties generate around Rs 4.5 crore every month in penalties.

Weak Compliance and Paved Surfaces

RWH was made mandatory under the Karnataka Groundwater (Regulation and Control of Development and Management) Act, 2012, and Rules, 2013. New buildings had to provide 60 litres of storage per square metre of roof area and 30 litres per square metre of paved surface. Unpaved and garden areas were exempt. Experts say compliance alone means little if systems are poorly maintained or surrounding land stays heavily paved.

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Bengaluru's concretisation has crippled its ability to absorb rainfall. The Groundwater Resources 2025 report shows rainfall contributes 10.2 billion cubic metres (BCM) to Karnataka's annual recharge of 19.3 BCM, roughly 53 percent. Yet Bengaluru's two districts rank among the most stressed in the state. Bengaluru Urban's annual recharge stands at 34,265.4 hectare-metre, but extraction runs at 177.3 percent of the extractable resource, the highest in Karnataka. Bengaluru Rural's recharge is just 18,678.6 hectare-metre, with extraction at 146.8 percent.

Water conservation expert Vishwanath S says nearly 80 percent of the city is now paved. "Short, high-intensity rainfall leaves little time for percolation. Roads and buildings seal the ground and prevent infiltration," he said.

Rainwater Turns into Runoff

Hydrogeologist Devaraj puts it starkly: roughly 90 percent of Bengaluru's rainfall runs off the surface, and only 10 percent percolates. Despite receiving around 900 millimetres annually, enough to generate over 1.1 lakh litres per household, only a fraction is retained. KC Subhash Chandra, former member of the Karnataka Groundwater Authority's expert panel, said the city generates nearly 17,000 hectare-metres of runoff annually, enough to serve 1.7 million people if conserved. A 2012 study he cited found that plugging leakages alone could serve 9.5 million people, and that less than 3 percent of water from heavy rainfall events infiltrates the city's soil. Around 25 percent is lost through stormwater drains, often mixed with sewage.

Restoring lakes, separating sewage from stormwater, and expanding permeable surfaces are non-negotiable, experts say.

Borewell Dependence Deepens

The city now has an estimated 3.7 lakh active borewells, with drilling demand rising sharply in peripheral areas. Without stronger RWH enforcement and recharge planning, experts warn dependence on tanker water and deep borewells will only deepen.

Official Response

BWSSB chairman Ram Prasanth Manohar said a pilot in south Bengaluru using 60-micron filters has successfully improved harvested rainwater quality to Cauvery standards. The board plans to scale it up. "Penalties alone have not led to behavioural change," he said. A senior water resources department official said good rainfall last year improved groundwater levels and recharged tanks ahead of summer. Technical support to local bodies continues, though implementation rests with individual government agencies.