I'd advise that Modi refrain from weaponizing water

India could limit the water to which Pakistan has access without breaking the terms of the Indus Water Treaty, which both countries signed in 1960.
 
The agreement entitles India to 20 per cent of the water that flows through the Indus river, enough to irrigate 1.3m acres. Currently, however, the country is using enough to irrigate just 800,000 acres, according to reports sourced to Indian officials.

But while increasing its water usage might be a convenient and legal way to take revenge on Pakistan, it would not necessarily be easy. India lacks the facilities to store the extra water, and would have to build more if it wanted to maximise its usage.

The above is from the (UK) Financial Times report today, India sparks boycott of South Asia summit in Pakistan.  The boycott of a November meeting of South Asian countries was proposed by India and swiftly joined by Bhutan, Bangladesh and Afghanistan.

The proposed boycott is in response to an attack on a military base in the Indian state of Jammu & Kashmir on September 18, near the town of Uri, that according to Wikipedia's article killed 19 Indian troops. It was the most deadly attack on Indian security forces in Kashmir in two decades. While no group has claimed responsibility there is evidence to suggest the attack was launched and overseen from Pakistan.

There's been tremendous outrage in India about the attack, in the manner of the final straw, and so Prime Minister Narendra Modi and his political party (BJP) have been under severe pressure to retaliate against Pakistan.

As a September 27 Wall Street Journal editorial
indicated, so far Modi has resisted calls for a military response. But he's been trying to find ways to retaliate that don't take the two nations to the brink of war. One of these ways has been the proposed boycott, which Quartz discusses in even greater detail than the FT report.

He's also considering a proposal to withhold water from Pakistan, as FT details. I find this confusing given India's famous water woes, which get a slight reprieve only during the monsoon season then settle back into crisis mode. So I don't understand why India had the water to spare in the first place. Nor do I understand why India would have to store that much 'extra' water to make use of it.

In any case it would be very wrong to retaliate against a civilian population for an attack on the military.

And it would be incredibly stupid to use water as a weapon against Pakistan because it sets a precedent that China could fashion in coming years into a very powerful weapon against India.

But as long as I've returned to the subject of India's water problems it's a good time for a brief review. From The Economist's May 14 explainer article, Why India has a water crisis:    

[...]

Using subsidised electricity, [Indian] farmers pump groundwater at will, drawing up more annually than China and America combined. A recent European Commission report counted more than 20m boreholes in India, up from tens of thousands in the 1960s.

The water table is falling on average by 0.3 meters and by as much as 4 meters in some places.

Water-starved regions often cultivate water-hungry crops like paddy, cotton and sugarcane.

Punjab in the north and Tamil Nadu and Karnataka in the south continue to squabble over the ownership of rivers.

The problem is not lack of adequate water, but its reckless overuse. China, with a larger population, uses 28% less fresh water than India.

An ambitious $165 billion water-diversion scheme for drought-prone regions is in the works. A total of 15,000 kilometres of artificial waterways are to link no fewer than 37 rivers. The rigged system is set to relocate 174 cubic kilometres of water, ostensibly enough to quench the thirst of 100 metropolises the size of Mumbai.

The Economist goes on to note that such schemes tend to miss the point:

In February, Narendra Modi, the prime minister, pledged to double farm incomes by 2022. But before grand projects and promises, small steps are needed. Rainwater harvesting, an age-old technique for capturing monsoon run-off, can provide the country with reliable water supplies throughout the year. Building check dams on riverbeds will improve groundwater levels. Farmers should be trained and encouraged to switch to drip irrigation.

And the government should set a better example as India awaits the rains: when a minister visited Latur last month, local officials wasted 10,000 litres of water scrubbing the helipad for his arrival.

Such wise advice is perpetually ignored by Indians -- and Pakistanis. And Iranians, and a host of other societies that still aren't serious about water conservation. [shrugging] What else to say?

As to the best way for India to respond to the Uri attack as it's been labeled --  well, let me think on it. A little music to think by......