If irony alone could replenish reservoirs, Mumbai would have enough water to last several summers. Yet the city finds itself in a peculiar situation—while heavy rains have flooded roads and disrupted daily life, its drinking water reserves remain critically low.
On Wednesday morning, residents navigated waterlogged streets as intense rainfall inundated key areas across the city. The Andheri subway was submerged, and traffic moved slowly along the Western Express Highway after several neighbourhoods recorded more than 100 mm of rainfall within 24 hours, according to the India Meteorological Department (IMD).
At the same time, officials monitoring Mumbai’s water supply were focused on a different concern. As of June 29, the seven reservoirs that provide drinking water to the city held less than 7 per cent of their usable storage capacity.
This contrast highlights one of Mumbai’s most striking monsoon paradoxes: the city can appear overwhelmed by water while simultaneously facing concerns over water scarcity. The explanation lies in the fact that the rainwater flooding streets and the water that replenishes reservoirs do not necessarily follow the same path or accumulate at the same pace.
Although Mumbai experienced one of its first major monsoon spells this week, with widespread flooding in low-lying areas, the Brihanmumbai Municipal Corporation (BMC) reported just days earlier that the city’s reservoirs contained only 6.93 per cent of live storage, compared to 39.5 per cent during the same period last year.
Floodwater and Drinking Water Follow Different Paths
The rainwater seen accumulating on roads and in underpasses during heavy downpours is not the same water that immediately becomes part of Mumbai’s drinking water supply.
The city relies on seven reservoirs—Bhatsa, Upper Vaitarna, Modak Sagar, Tansa, Middle Vaitarna, Tulsi and Vihar—for its water needs. These lakes depend on rainfall across their catchment areas, where water gradually flows into the reservoirs over time.
Unlike urban flooding, which can occur within hours of intense rainfall, reservoir replenishment is a much slower process. Sustained rainfall over weeks and months is required before water storage levels rise significantly.
In densely built urban areas, heavy rain can quickly overwhelm drainage systems, especially when drains are clogged or unable to handle the volume of water. Reservoirs, however, respond gradually as water accumulates through continuous inflows during the monsoon season.
As a result, dramatic scenes of flooded roads can occur even while reservoir levels remain dangerously low.
Mumbai’s drinking water system depends on seven lakes spread across Mumbai, Thane and Nashik districts, making the city’s water supply dependent on rainfall patterns extending well beyond its own boundaries.
Why Mumbai Is Facing a Water Shortage
The current water scarcity is largely linked to the delayed arrival of the southwest monsoon.
Under normal conditions, the monsoon reaches Mumbai around June 10 after arriving on the Kerala coast earlier. This year, however, the rains arrived later than usual. During the delay, the city continued drawing nearly 4,000 million litres of water per day from its reservoirs while receiving significantly less replenishment than expected.
BMC data illustrates the decline. On June 17, reservoir storage stood at 10.01 per cent of usable capacity. By June 29, that figure had fallen to 6.93 per cent.
The seven reservoirs collectively supply approximately 4,000 million litres of drinking water daily to Mumbai and surrounding metropolitan areas.
Compared with last year, the situation is considerably more challenging. Around the same period in 2025, storage levels were nearly 39.5 per cent, largely because the monsoon had arrived earlier and reservoirs had begun filling sooner. In contrast, this year Mumbai entered the monsoon season with already depleted reserves.
Why Flooding Happens Faster Than Reservoirs Fill
A common question during periods of flooding is why the large volumes of rainwater visible across the city cannot immediately solve water shortages.
The answer lies in the nature of urban infrastructure. Much of Mumbai is covered by concrete, asphalt and buildings, which prevent water from naturally soaking into the ground. During heavy rainfall, water rapidly flows across surfaces into storm-water drains.
When rainfall exceeds the capacity of the drainage system—or when clogged drains, high tides or other factors hinder drainage—water quickly accumulates on roads and in low-lying areas.
This was evident in locations such as the Andheri subway and Wadala, where commuters encountered severe waterlogging.
Reservoirs, by contrast, operate on a much longer timeline. They rely on repeated rainfall events across entire catchment regions rather than a single day of heavy precipitation. Their storage levels rise gradually through sustained inflows accumulated over the monsoon season.
Furthermore, because the reservoirs are located across Thane, Palghar and Nashik districts, rainfall in Mumbai itself does not directly determine their water levels.
Rainfall distribution across these regions is often uneven. For example, during the 24 hours preceding the June 29 assessment, Tulsi and Vihar lakes in northern Mumbai received 179 mm and 110 mm of rain respectively, while Modak Sagar in Thane district recorded only 38 mm.
This variation demonstrates why a single day of heavy rain in Mumbai cannot dramatically improve the city’s overall water reserves.
Reservoirs Still Need Weeks of Rain
Although the monsoon has finally become active, significant improvements in reservoir storage are unlikely to occur immediately.
The seven-lake system continues supplying around 4,000 million litres of potable water every day. Until inflows consistently exceed daily withdrawals, storage levels will remain under pressure.
This situation prompted the BMC to impose a 10 per cent water cut in May and later tighten restrictions on industrial and commercial consumption.
In essence, while the delayed monsoon may have finally arrived and flooded many parts of Mumbai, the real challenge lies in ensuring that weeks of sustained rainfall replenish the city’s reservoir network. Until that happens, Mumbai may continue facing drinking water concerns even as roads, streets and subways remain submerged by rainwater.
