Weeks After Water Cuts, Why Did Mumbai Release 10 Days' Worth Of Drinking Water Into The Sea?

Just weeks after imposing water cuts as reservoir levels dipped, Mumbai pumped enough freshwater to meet nearly 10 days of the city's drinking water demand into the Arabian Sea. Between June 30 and July 7, nearly 4,290.5 crore litres of rainwater—enough to meet the city's drinking water needs for almost 10 days—was pumped into the Arabian Sea and nearby creeks after intense monsoon rainfall overwhelmed the city's drainage system.

The contrast raises an obvious question. How can a city that routinely faces water shortages afford to let freshwater flow into the sea? The answer lies in the way Mumbai's reservoirs and flood-control systems are designed. While the city depends almost entirely on monsoon-fed reservoirs for its drinking water, its drainage network is built to move excess rainwater out as quickly as possible during intense downpours. As rainfall becomes more erratic, balancing flood protection with water security is becoming increasingly difficult.

According to The Indian Express, Mumbai received 898 mm of rainfall between June 30 and July 7—more than the city's average rainfall for the entire month of July. The intense spell rapidly replenished reservoirs, flooded low-lying areas and forced the Brihanmumbai Municipal Corporation (BMC) to operate its pumping stations continuously. The episode has raised questions about whether Mumbai could have stored more of the rain it received, or whether releasing it was unavoidable.

Why was water released into the sea?

The water was not released because Mumbai had excess drinking water to spare. It was pumped out because the city needed to prevent flooding.

Mumbai's 19 operational pumping stations are designed to drain stormwater from roads and low-lying neighbourhoods during periods of heavy rainfall. Between June 30 and July 7, these stations discharged 4,290.5 crore litres of water into the Arabian Sea and surrounding creeks.

As The Indian Express reported, this volume is equivalent to the combined storage capacity of eight Powai Lakes and could have met Mumbai's daily water requirement for nearly 10 days.

The Irla pumping station in Vile Parle discharged the highest volume at 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 alone pumped out 1,914.26 crore litres—enough to meet Mumbai's water requirement for about four-and-a-half days.

The pumping stations perform a flood-control function rather than a water storage role. Since Mumbai is a low-lying coastal city surrounded by the Arabian Sea, rainwater must often be removed quickly to prevent roads, homes and transport networks from being inundated.

Even after the city's reservoirs approach full capacity, rainfall continues for several more weeks. Once reservoirs are full, additional inflows cannot be retained indefinitely and are released downstream.

How did Mumbai go from water cuts to surplus?

The shift was driven almost entirely by the monsoon.

Earlier this year, reservoir levels had fallen sharply after weak rainfall, prompting concerns over the city's water supply. Mumbai requires around 4,300 million litres of water every day but currently receives about 3,900 MLD, leaving a daily shortfall of roughly 400 MLD.

However, rainfall patterns changed dramatically within weeks.

The city recorded 898 mm of rain in just eight days between June 30 and July 7, exceeding the average rainfall Mumbai usually receives throughout July. Reservoirs filled rapidly as heavy rainfall continued across their catchment areas.

By July 31, the seven reservoirs supplying Mumbai were collectively 88.58 per cent full. Officials expect them to reach full capacity by the end of August, even though the southwest monsoon generally continues until mid-October. Once the reservoirs reach capacity, any additional inflow cannot be stored and is released downstream.

According to BMC officials, the civic body does not maintain records of how much water overflows from its reservoirs each year, although it estimates the volume is broadly comparable to each reservoir's own storage capacity.

The episode highlights how dependent Mumbai remains on the timing and intensity of the monsoon. A weak monsoon can force water cuts, while a few days of exceptionally heavy rainfall can quickly reverse the situation.

How do the city's reservoirs operate?

Despite being surrounded by seawater, Mumbai has no significant freshwater source of its own for drinking water.

Instead, it depends on seven reservoirs—Vihar, Tulsi, Tansa, Bhatsa, Upper Vaitarna, Middle Vaitarna and Modak Sagar—most of which are located in Thane and Nashik districts. The network has been built over more than 150 years. Vihar Lake, commissioned in 1860 inside Sanjay Gandhi National Park, is the oldest reservoir, while Middle Vaitarna, completed in 2012, is the newest.

Water collected during the monsoon is stored in these reservoirs, treated at the Bhandup filtration plant and then distributed across the city.

Three reservoirs in the Vaitarna system operate in sequence because they are located at different elevations along the 154-km Vaitarna River. Upper Vaitarna, which has a storage capacity of 2.27 lakh million litres, fills first. Its surplus then flows into Middle Vaitarna, which can store 1.73 lakh million litres, before finally reaching Modak Sagar, with a capacity of 1.28 lakh million litres.

As of July 31, Upper Vaitarna was 76.64 per cent full after receiving 2,611 mm of rainfall, while Middle Vaitarna stood at 89.83 per cent after 2,058 mm of rain. BMC data show that roughly every 34.06 mm of rainfall raises Upper Vaitarna's storage by one percentage point, equivalent to around 227 crore litres. At Middle Vaitarna, around 22.9 mm of rainfall increases storage by one percentage point, or about 173 crore litres.

According to The Indian Express, civic officials estimate that the volume overflowing from reservoirs every year is broadly comparable to each reservoir's own storage capacity, although no official records are maintained.

Could the excess water have been stored?

Experts say the answer is more complicated than it may appear.

Building enough infrastructure to store the overflow would require constructing additional reservoirs comparable in size to the existing ones, along with new transmission pipelines to carry the water to Mumbai.

BMC Additional Municipal Commissioner (Projects) Abhijit Bangar told The Indian Express that such projects would be financially unviable because rainfall varies significantly from year to year.

The costs are considerable even for much smaller storage facilities. The BMC recently constructed a three-crore-litre underground storage tank at Pramod Mahajan Park in Dadar at a cost of Rs 130 crore. Based on that scale, storing even 300 crore litres would cost at least Rs 2,000 crore, excluding long-term maintenance.

While civic officials argue storing reservoir overflow is neither practical nor financially viable, experts say the bigger opportunity lies elsewhere.

Hydrologists, however, argue that the focus should shift from building massive reservoirs to capturing rain where it falls.

Dr Rajendra Deshmukh, former professor at the National Environmental Engineering Research Institute (NEERI), has said rainwater harvesting structures, holding tanks, ponds and borewell recharge systems could retain a larger share of monsoon rainfall. Rainwater can be treated for drinking purposes, while untreated water can also be supplied for industrial use, where demand remains substantial.

What does this reveal about climate extremes?

Mumbai's experience highlights how climate variability is making water management more difficult.

The city is witnessing fewer evenly distributed rainy days and more short bursts of intense rainfall.

According to BMC data, the average intensity of "very heavy" rainfall has increased from 132 mm to 182 mm over the past six years. The past decade recorded 28 spells of very heavy rainfall lasting at least four hours, while between 2021 and 2024 Mumbai experienced 66 days with more than 100 mm of rainfall.

Such intense rainfall leaves little time for water to percolate into the ground, particularly in a city covered largely by concrete and asphalt. Instead, stormwater accumulates rapidly, increasing flood risk and forcing authorities to pump large volumes into the sea.

The result is a growing paradox: the same city can move from imposing water cuts to dealing with overflowing reservoirs within weeks.

Can Mumbai manage water more efficiently?

The BMC has already begun exploring ways to retain more rainwater without constructing entirely new reservoirs.

One ongoing project aims to capture an additional 200 MLD from Vihar Lake, the only reservoir within Mumbai's limits. Every monsoon, once Vihar reaches its storage capacity of 27,698 million litres, surplus water flows into the Mithi River, contributing to flooding in areas such as Santacruz, Kalina, Kurla and Ghatkopar.

To address this, the BMC is planning a Rs 98-crore pumping station that will divert surplus water into a holding tank before sending it to the nearby Bhandup filtration plant for treatment and distribution. Officials consider the project viable because of Vihar's proximity to the treatment facility, and expect it to become operational in 2027.

The civic body has also proposed around 800 bioswales—vegetated channels that capture, filter and absorb stormwater—across parks and flood-prone areas including Dadar, Matunga, Santacruz, Andheri and Vakola. The aim is to improve groundwater recharge while reducing urban flooding.

Although these measures will not eliminate the need for pumping stations or reservoir releases during exceptionally heavy rainfall, they could help Mumbai retain more water within the city instead of allowing it to flow into the sea.

The city's experience this monsoon shows that ensuring water security is no longer only about building bigger reservoirs. It is also about making better use of the rain that already falls on the city, while adapting infrastructure to cope with increasingly intense and unpredictable monsoon events.