China’s decision to construct the world’s largest hydropower dam on the Yarlung Zangbo river in Tibet has intensified fears in New Delhi of a looming water crisis, with Indian officials warning the project could reduce dry-season flows into India by as much as 85%, Reuters reports.

“Delhi estimates the Chinese dam will allow Beijing to divert as much as 40bn cubic metres of water, or just over a third of what is received annually at a key border point,” Reuters reported, citing an Indian government analysis and four sources familiar with the matter. “The impact would be especially acute in the non-monsoon months, when temperatures rise, and lands become barren across swathes of India.”

India has responded by fast-tracking its own Upper Siang Multipurpose Storage Dam, which, if completed, would be the country’s largest such project. With a planned storage capacity of 14bn cubic metres, it could partially offset the effects of China’s project. “The Upper Siang project would alleviate that with its projected 14bcm of storage capacity, allowing India to release water during the dry season,” Reuters noted. “That could mean the major regional city of Guwahati… would see a reduction in supply of 11%, as opposed to 25% if the Indian dam isn’t built.”

Officials say the dam would also serve as a buffer against sudden surges of water, a risk if Beijing were to release torrents downstream. According to the Indian government’s analysis, “if the dam is at its minimum drawdown level…it would be able to fully absorb any excess water released from a breach in Chinese infrastructure.” Delhi is considering maintaining 30% of storage empty for that purpose.

China has rejected accusations that its project will harm downstream nations. A foreign ministry spokesperson told Reuters: “The hydropower projects have undergone rigorous scientific research on safety and environmental protection, and will not adversely impact the water resources, ecology, or geology of downstream countries.”

But local resistance poses a significant challenge for India’s countermeasure. In May, residents of Arunachal Pradesh damaged equipment and destroyed a bridge to block work on the Upper Siang site. At least 16 Adi villages could be lost to the reservoir, directly affecting 10,000 people, with community leaders claiming more than 100,000 will feel the wider impact. One resident told Reuters: “The cardamom, paddy, jackfruit and pear we grow on this land help educate our children and support our family. We will fight the dam to death.”

Even if India pushes ahead, delays remain inevitable. “Even if the Upper Siang dam gets the go-ahead, it could take a decade to build after breaking ground,” Reuters reported, “meaning the project would likely be completed after China’s project, which Beijing expects to start generating power by the early-to-mid 2030s.”

Meanwhile, experts caution that both sides are venturing into dangerous territory. “The dam is being built in a zone of high seismicity and in a zone that experiences extreme weather events,” said Sayanangshu Modak of the University of Arizona. “These kinds of extreme weather events trigger landslides, mudslides, glacial lake outburst flooding. So that raises concerns about dam safety... it’s a very legitimate concern and India should engage with China.”