A gigantic plan to link several of India’s rivers and divert vast volumes of water for irrigation could result in reduced Rainfall in already water-stressed regions, according to a paper1 published in Nature Communications last month. The water transfer could affect the climate systems driving the Indian monsoon and reduce September rainfall by as much as 12% in some of the country’s states, according to the study.

The paper is one of a string of independent research studies into the controversial plan. Some scientists have cautioned that too little is known about the environmental effects of the river engineering project for it to be implemented.

The plan, first suggested by the British during colonial rule and most recently refined in 2015-2016, is “probably the largest manipulation of India’s hydrology to ever be conceived”, says Jagdish Krishnaswamy, an eco-hydrologist at the Indian Institute for Human Settlements in Bengaluru.

The Indian water ministry plans to create a network of 15,000 kilometres of canals and thousands of reservoirs to transfer 174 billion cubic metres of water annually — roughly equivalent to the yearly water use of neighbouring Pakistan — from regions with abundant water to those that are in need of it. The study’s authors write that the goal of the project “is to keep the maximum possible water — which earlier used to reach oceans from river basins — on the land to meet the growing water demand of the country”.

Other studies have assessed the potential impacts of the project, including sediment deposition and the consequences for aquatic ecosystems, but this is the first to assess how the land and the atmosphere interact to affect the way in which water cycles between them.

Subimal Ghosh, one of the authors of the study and a climate scientist at the Indian Institute of Technology Bombay in Mumbai, describes the water cycle as involving interaction between atmospheric moisture, oceans, plants releasing moisture and climactic patterns. He says his team aimed to study “how a river basin in one region impacts atmospheric processes and therefore impacts other regions as well”.

“River interlinking plans may be useful but we need to have detailed assessments of climatic impacts,” explains Roxy Mathew Koll, a climate scientist at the Indian Institute of Tropical Meteorology in Pune, and another co-author of the study.

More crops, more water

A core aim of the river-linking plan is to increase the area under irrigation by 35 million hectares. More crops would lead to higher levels of moisture being released from their leaves in a process known as evapotranspiration. With more moisture in the air locally, temperatures would reduce, and rainfall patterns and cloud formation would change.

The team used computer modelling to examine the interplay between rainfall, humidity, soil moisture, temperature and wind speed across seven river basins for the monsoon months — June to September. The team did not model other months.

The study found that the effect of the land–atmosphere interaction is highest in September. “September is when crops are at maturity and evapotranspiration is high,” explains Koll. This resulted in a reduction in September rainfall in the states of Rajasthan, Gujarat, Odisha and Andhra Pradesh of between 6.4% and 12%. The researchers also found an increase in September precipitation of up to 12% in northeastern states Bihar and Jharkhand and up to 10% in the central areas of Maharashtra and neighbouring Telangana.

Reduced rainfall will translate to less flow in rivers in subsequent months, and this could exacerbate water stress in regions that are already arid, such as Rajasthan and Gujarat, the authors say.

These effects do not factor in the impact of river flow into the ocean, which can also affect monsoonal rainfall, they also say.

Nature asked India’s National Water Development Agency, which oversees the river-linking project, to comment on the study but did not receive a response.

Scientists have welcomed the analysis. The paper highlights new implications of the project, says Krishnaswamy. “River linking may considerably reduce or neutralize the claimed benefits of inter-linking.”

Rupa Kumar Kolli, a meteorologist at the International Monsoons Project Office at the Indian Institute of Tropical Meteorology in Pune describes the paper as “a very important contribution”. He says he hopes that the paper will prompt a more thorough analysis of the river-linking project before it can go ahead. “There is no going back once the project is implemented.”