After heavy rain, Mumbai dumped 10 days’ of drinking water into sea. Why?

Visitors enjoy the view as Vihar lake, one of Mumbai’s potable water-supplying lakes, started overflowing last night, at Aarey in Mumbai on 08 July 2026. (Express photo by Sankhadeep Banerjee)

Those reservoirs will likely reach capacity by the end of August, while the monsoon typically runs until mid-October. Beyond that point, large volumes of surplus water will spill downstream from dams that can hold no more, even as the pumps keep sending stormwater into the sea.

The result is that year after year a city that lives almost entirely off rain-fed reservoirs lets a substantial share of its freshwater go uncaptured.

Mumbai’s water supply system

Despite being surrounded by the Arabian Sea on three sides, Mumbai draws all its potable water from seven reservoirs, most of them beyond its boundaries. The Vaitarna system, anchored by Upper Vaitarna in Nashik district, Middle Vaitarna and Modak Sagar, supplies the island city and western suburbs. The Bhatsa system caters to the eastern suburbs.

The network was built over 150 years. Vihar Lake, the oldest, came up in 1860 inside the Sanjay Gandhi National Park, which also holds Tulsi Lake. Tansa Dam, commissioned in 1892, Modak Sagar and Bhatsa Dam, commissioned in 1982, are all in Thane district. Middle Vaitarna, completed in 2012, is the newest.

Rain lashes parts of the western suburbs as IMD has issued a yellow alert for Mumbai until 24 June. (Express photo by Sankhadeep Banerje)

The reservoirs fill during the monsoon and sustain the city through the year, their water treated at the Bhandup filtration plant and piped across Mumbai.

The system still falls short every day. Mumbai needs 4,300 million litres daily. The BMC supplies 3,900 MLD. That 400 MLD gap is another reason experts say the city must make better use of the rain it receives.

Capturing surplus water: A challenge of scale

Why not simply hold on to the water? The answer, civic officials say, is scale.

The three reservoirs forming the backbone of the Vaitarna system sit at different elevations along the 154-km Vaitarna River. Upper Vaitarna can hold 2.27 lakh million litres, Middle Vaitarna 1.73 lakh million litres and Modak Sagar 1.28 lakh million litres.

“The Upper Vaitarna has a catchment of its own. Once it gets filled, the surplus water is caught in the catchment of Middle Vaitarna, and when these two dams get filled the surplus is caught in the catchment of Modak Sagar, which is at the lowest elevation,” said Abhijit Bangar, Additional Municipal Commissioner (Projects).

As of July 31, BMC data show, Upper Vaitarna was 76.64 per cent full after 2,611 mm of rain over its catchment this year. Roughly every 34.06 mm translates into a one percentage point rise in storage, about 227 crore litres, enough to meet nearly half of Mumbai’s daily demand.

Middle Vaitarna was 89.83 per cent full after 2,058 mm, with about 22.9 mm adding a percentage point, around 173 crore litres, or roughly 40 per cent of the daily requirement.

How much escapes once the dams fill? The BMC keeps no records, officials say, but estimates the annual overflow is broadly comparable to each reservoir’s own storage capacity.

“To store this additional water, we would have to build new dams of similar size, which is financially unviable. Rainfall also varies every year, and laying fresh transmission pipelines to transport this surplus water to Mumbai would be another enormous challenge,” Bangar said.

The economics bear him out. The BMC recently built a three-crore-litre underground tank at Pramod Mahajan Park in Dadar for Rs 130 crore. On that basis, storage for even 300 crore litres would cost at least Rs 2,000 crore, excluding long-term upkeep.

Rain lashes parts of the western suburbs as IMD has issued a yellow alert for Mumbai until 24 June. (Express photo by Sankhadeep Banerje)

The case for rainwater harvesting

While holding back reservoir surplus may be impractical, hydrologists argue capturing rain where it falls, within Mumbai, is far more feasible.

The case strengthens as the city’s rain turns fiercer. BMC data show the average intensity of “very heavy” rainfall in a 24-hour period, which the IMD defines as 115.6 mm to 204.4 mm, has risen from 132 mm to 182 mm in six years. The past decade saw 28 spells of very heavy rain lasting at least four hours, and between 2021 and 2024 Mumbai recorded 66 days with more than 100 mm of rain.

“The substantial increase in rainfall is resulting in a much larger volume of water. Instead of pumping this water into the sea, storing it through rainwater harvesting structures or recharging borewells could significantly strengthen the city’s water security,” said Dr Rajendra Deshmukh, hydrological expert and former professor at the National Environmental Engineering Research Institute (NEERI).

Of the 4,290.5 crore litres pumped out between June 30 and July 7, the Irla station at Vile Parle discharged the most, 783.14 crore litres, followed by Haji Ali with 595.43 crore litres and Love Grove in Worli with 535.69 crore litres. Together, these three stations sent out 1,914.26 crore litres, enough to meet Mumbai’s needs for about four-and-a-half days.

“Rainwater is neither impure nor unusable. If it is stored in holding tanks or natural ponds, it can be treated and made fit for drinking. Even untreated water can be used for industrial purposes, where demand is substantial,” Deshmukh said.

Beyond reservoirs: Capturing rain within the city

The BMC has begun acting on that logic. It has initiated a project to capture an additional 200 MLD from Vihar Lake, the only reservoir within the city’s limits. Every monsoon, once Vihar fills to its 27,698-million-litre capacity, surplus water is discharged into the Mithi River, contributing to flooding in Santacruz, Kalina, Kurla and Ghatkopar. A Rs 98-crore pumping station is now planned to capture it.

“The surplus water will be captured in a holding tank or sump pit and then conveyed to the Bhandup filtration plant. After treatment, it can be supplied for domestic and commercial use,” a civic official said. Officials call it viable given Bhandup’s proximity, and expect operations to begin in 2027.

Separately, the BMC has proposed 800 bioswales, vegetated depressions that capture, filter and absorb stormwater into the ground, in parks and open spaces, starting with chronic flooding hotspots including Dadar, Matunga, Santacruz, Andheri and Vakola.

“Slow rainfall allows maximum percolation, but intense rainfall does not because Mumbai’s surface has become increasingly impervious. Water that cannot percolate accumulates and causes flooding. Our objective is to increase the city’s percolation capacity,” Bangar said.

Eight days of rain, ten days of water

• June 30 – July 7 rainfall: 898 mm
• Water pumped into sea & creeks: 4,290.5 crore litres
• Combined storage of: 8 Powai Lakes
• Enough to meet: Nearly 10 days of Mumbai’s water requirement

Top three pumping stations

• Irla (Vile Parle): 783.14 crore litres
• Haji Ali: 595.43 crore litres
• Love Grove (Worli): 535.69 crore litres
• Total: 1,914.26 crore litres (about 4.5 days of Mumbai’s requirement) pumped out between June 30 – July 7