The floods that tore through Nepal’s river towns this season were a reminder of something I have spent a working lifetime watching on the Ganga: water is as dangerous in its excess as in its absence. Glacial lakes gave way and pushed walls of water through settlements within hours, and no warning system built so far could have held that back. India follows these events out of more than neighbourly concern. The same rhythm — cloudburst, then drought — describes the daily reality of much of this country.

Tamil Nadu’s coast shows it most plainly. When the North-East Monsoon reaches the Bay of Bengal each year, the districts from Chennai down to Cuddalore take in more water than their drains can carry. Homes are lost, public property is damaged, families are moved to relief camps. Within a few months those same towns report falling groundwater, borewells sunk deeper, and overhead tanks standing empty. This flip between flood and drought is not a natural riddle. It is a failure of planning, and Chennai, to its credit, was among the first Indian cities to treat it as one.

The answer it reached for was desalination. The Minjur plant, commissioned in 2010 on the northern edge of Chennai, was India’s first large-scale seawater desalination facility. It pulls in roughly 237 million litres of seawater a day and returns 100 million litres of treated drinking water. Built for something in the range of Rs 500 to 600 crore, it settled an open question: the technology works at city scale under Indian conditions.

Israel came to this out of plain necessity. With little freshwater and no rain it could count on, it turned to the sea and built an industry around it. The Sorek A plant on the Mediterranean produces........

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