Irrigation may have helped the Indo-Gangetic plains (IGP) become India’s food basket, but it is making living miserable for people in this highly-irrigated region, according to a study published in Nature Geoscience on Monday evening.

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The study, carried out by researchers led by Vimal Mishra of the Indian Institute of Technology, Gandhinagar, showed that irrigation is enhancing humidity, which is in turn increasing ‘moist heat stress’ experienced by 46 million people living in IGP. Heat stress occurs when the human body cannot cool itself. Moist heat stress is seen to be potentially more dangerous as compared to dry heat stress, which happens when the ambient temperature is high.

While humans have an ability to withstand relatively high dry temperatures, this comes down significantly with humidity. Studies have shown that a wet-bulb temperature of 35°C marks humans’ upper physiological limit and there are serious health and productivity issues even at much lower temperatures.

“Moist heat stress is more strongly related with mortality than dry heat stress. It also associated with higher discomfort for people working outside,” Mishra told BusinessLine .

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From 31 million hectares in 1970, the country’s net irrigated area rose to 60 million hectares by 2007. And it has increased further since then. IGP is one of the most intensively irrigated regions in the world with more than 60-70 per cent of the area irrigated with surface or groundwater.

Climate change

Ongoing human-induced climate change has already intensified extreme temperature and is expected to get heightened in India. This could further exacerbate heat stress, especially among those working outdoors.

Cooling due to water evaporating from irrigation systems might partially counteract dry heat stress, although associated changes in humidity that could affect moist heat stress are poorly understood.

Mishra and his colleagues analysed the influence of irrigation on both dry and moist heat stress using a variety of in situ and satellite observations, together with numerical simulations.

They found that although irrigation causes land surface cooling, it also leads to substantially higher surface humidity by reducing the height of the lowest part of the atmosphere, known as the planetary boundary layer. As a result, irrigation mitigated dry heat stress, but enhanced moist heat stress.

The researchers concluded that recent intensification of irrigation practices in India has enhanced moist heat stress, and the associated risks to human health, across the local region and the neighbouring countries of Pakistan and Afghanistan. They also said that similar impacts from irrigation could be expected in other regions with similar semi-arid and monsoon climates.

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