THE toll was not shocking by Indian standards: two dead, nearly 100 vehicles torched and some 400 “miscreants” arrested. Nor did the violence that erupted on September 12th in Bangalore, capital of the southern state of Karnataka, last long. Within 48 hours police, their numbers boosted to 15,000 by reinforcements sped from across the country, had lifted a curfew. The prospering city of 8.5m, which happens to be India’s high-tech Mecca, was soon back to its normal bustle.

But the issue that stoked the unrest is a perennial one: the division of the water of the Cauvery, an 800km- (500 mile-) long river that rises in Karnataka’s western highlands and supplies rich farmland in the south of the state, as well as thirsty Bangalore itself, before tumbling east into the even thirstier state of Tamil Nadu. The two states have long tussled over rights to the Cauvery. The spark for the riots was a ruling by India’s supreme court ordering Karnataka to open its reservoirs to relieve its downstream neighbour.

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In a country as crowded, rural and dependent on fickle rains as India, troubles over water are to be expected. This dispute dates to 1892, when the British rulers of what is now Tamil Nadu extracted a promise from Mysore, a princely state that included most of what is now Karnataka, not to build dams on the Cauvery without their permission. A further treaty in 1924 underlined the principle of dividing the waters, with farmers downstream, where the river flows through densely populated regions and a highly fertile delta, getting the lion’s share.

After independence, things got trickier. In 1974 Karnataka declared the 1924 treaty void. It had been imposed by imperialist Britain, the state argued, and farmers in Tamil Nadu had expanded land under irrigation at Karnataka’s expense. In the four decades since, the two states have continued to wrangle even as their populations have grown, irrigation has extended further and cities such as Bangalore have swollen beyond recognition.

As politicians seeking rural votes in both states have fulminated, India’s institutions have dithered. It took 17 years for a special tribunal to decide how to allocate the river’s water and after it did, in 2007 (recognising the 1892 and 1924 treaties and again granting Tamil Nadu a bigger share), another six years followed before India’s national government approved the deal. Even so, Karnataka has resisted applying the terms stipulated by the tribunal, including the creation of independent bodies to manage the river.

In times of good monsoon rains none of this has mattered much. But in years of shortage there has been trouble: in 1991 riots in Karnataka left 18 dead, most of them Tamil-speaking immigrants. In tit-for-tat moves, both states temporarily banned films in the “enemy” state’s language.

This year the monsoon has been good nearly everywhere, with the glaring exception of the Cauvery basin. There, rainfall from June to mid-September was 23% below normal. Thanks to prior shortfalls, the amount of water in reservoirs is now 47% below the average level of the past decade. The biggest reservoir on the Cauvery stands at an alarming 31% of capacity. It is this dam that supplies Bangalore’s drinking water, and it is this dwindling supply that Karnataka has been ordered to share.

The supreme court’s order does not just anger Karnataka’s farmers, or the urban thugs who attacked Tamil Nadu-registered vehicles. It worries water experts, who note a convergence of alarming trends. One is a longer-term decline in rainfall in the region. Another is the failure of government institutions to monitor water supply or demand properly: the supreme court’s ruling was based on guesswork, and even as the riot raged, a committee charged with delivering a more precise figure for how much water Karnataka owes its neighbour had to adjourn for lack of adequate data.

What statistics do show is that, with or without more water in the Cauvery, farmers in Tamil Nadu are sucking up too much of the stuff. Since the river-flow is unreliable, they have turned increasingly to pumps. Wells are growing steadily deeper as groundwater dries up, yet there has been no effort to persuade farmers to plant less thirsty crops. A third of the state’s farmland is rice paddy. The reason is simple: obsessed with the farming vote and guided by fears of famine that are absurdly outdated for a country that is now a big net exporter of food, the government subsidises inputs such as power and fertiliser and guarantees farmers a plum price. In other words, while more rain would certainly be nice, what the Cauvery basin really needs is better government.