At 6.32 am on Wednesday, 693 kilometres above the Arabian Sea, a European radar satellite passed over Mumbai and saw what no ordinary camera could.
Sentinel-1 does not need daylight or a clear sky; it reads the ground straight through monsoon clouds. On this pass, it mapped about 48 square kilometres of new floodwater across the greater Mumbai area that had not been there when the satellite last crossed overhead, on June 26.
On the ground, the India Meteorological Department had upgraded its warning for the city to an orange alert as intense monsoon activity lashed Mumbai and the surrounding region on Wednesday. The financial capital woke to another spell of heavy rainfall, with thunderstorms, lightning, and gusty winds disrupting traffic and waterlogging several low-lying areas. At the Santacruz gauge, 167 millimetres fell in a single day.
The radar showed the water did not spread evenly. It pooled in the low-lying floodplains on the city’s edges — the Vasai-Virar belt to the north and the Ulhas river basin around Kalyan and Dombivli to the east — not in the dense island city, whose built-up ground radar struggles to read.
Mumbai’s Wednesday is not an outlier. It is the newest entry in a record that has been filling for a quarter-century. Since 2000, India has logged 1,54,446 distinct floods. And they do not fall evenly across the map.
Laid onto a 10-kilometre grid, the record lights up in the same belts again and again: the Ganga plains of Uttar Pradesh and Bihar, the Brahmaputra valley in the Northeast, and a scatter of bright points over the country’s biggest cities.
The pattern matters now because the country has just entered the season that causes it. In this 25-year record, 80 per cent of floods began during the south-west monsoon, from June to September. August, still a month away, is the single worst month.
The map works like a memory: it shows where the water has already gone, and where it is likely to go again as this year’s rains build.
PLACES THAT KEEP DROWNING
At the state level, the geography is close to what a hydrologist would predict. Uttar Pradesh leads with 19,838 reported floods, followed by Maharashtra (15,460) and Bihar (14,438). These are the floodplains of the Ganga and its tributaries and the flash-flood-prone Western Ghats. Tamil Nadu and Karnataka round out the top five.
Zoom to the sharpest single cells, though, and a different story appears: they are almost all big cities. The most flood-marked 10-kilometre square in the country sits above Hyderabad, with 290 separate floods in 25 years. Behind it come Guwahati (283), Bengaluru (248), Prayagraj (229), Kolkata (220), Chennai (211), and Delhi (209).
Mumbai, where this story began, sits just outside that list. Its most flood-marked square, in central Mumbai, logged 189 floods in the same 25 years. And the record keeps lighting up right across the metro, from the island city to the Vasai-Virar and Ulhas floodplains. The radar caught on Wednesday.
That points to two things at once. India’s metros really do flood, with choked drains and built-over lakes leaving heavy rain little room to go. But it is also a warning about the data itself: a flood in Bengaluru makes national news, while one in a remote district may never be written up at all.
Floods in India run on a seasonal clock. Through the dry months, the map goes quiet; from June, the count climbs steeply as the monsoon sweeps up the subcontinent, peaks in August, then eases through September. That is the window the country is in right now, with the busiest weeks still ahead.
WHAT THE MAP CAN AND CAN’T TELL YOU
This inventory draws on Groundsource, a database of floods compiled from news reports rather than sensors, so it records where floods were covered, not only where they happened. Densely populated, well-connected districts show up more brightly than remote ones. And because digital news coverage itself expanded over these 25 years, the rise in reported floods from year to year is not clean evidence that flooding itself increased.
Read the map, then, for two things it shows reliably: where floods concentrate and when in the year they strike. Both are signals no newsroom’s attention can invent.
The monsoon filling the Ganga and the Brahmaputra this July is the same one that the map has been recording for a quarter-century. As the rains gather over those valleys once more, and over the cities that flood every time it pours, the inventory reads less like a chart of the past than a map of the places already bracing for the next one.
– Ends
