About 450 cubic kilometres of groundwater was lost in northern India during 2002-2021 and climate change will further accelerate its depletion in the years to come, according to a new study.

This is about 37 times the quantity of water the Indira Sagar dam -- India's largest reservoir -- can hold at full capacity, lead author Vimal Mishra, Vikram Sarabhai Chair Professor of Civil Engineering and Earth Sciences at IIT Gandhinagar, said.

Using on-site observations, satellite data and models, researchers found that across north India, rainfall in the monsoon season (June to September) has reduced by 8.5 per cent during 1951-2021. Winters in the region have become warmer by 0.3 degrees Celsius over the same period, they found.

The team, comprising researchers from the National Geophysical Research Institute (NGRI) in Hyderabad, said lesser rainfall during the monsoons and warming of winters will increase irrigation water demand and reduce groundwater recharge, further stressing the already depleting groundwater resource in north India.

While a drier monsoon leads to more reliance on groundwater to sustain crops during rainfall-deficit periods, warmer winters result in relatively drier soils, again requiring more irrigation -- something the researchers observed during the unusually warm winter of 2022, the fifth warmest for India since the India Meteorological Department started records in 1901.

"The accelerating trend of depleting groundwater is expected to continue as the planet warms because even though climate change causes more rainfall, most of it is projected to occur in the form of extreme events, which does not support groundwater replenishment," Mishra told PTI.

The shortage of rainfall in the monsoons followed by warming winters, both driven by climate change, is projected to cause a "substantial decline" by about 6-12 per cent in groundwater recharge. The study's manuscript, accepted for publishing in the journal Earth's Future, was shared exclusively with PTI.

"For groundwater to get recharged, we need low-intensity rainfall spread over more days," Mishra explained. Changes in groundwater levels are known to be largely dependent on rainfall received during summer monsoons and groundwater pumped out for irrigating crops during their respective growing seasons -- June to September for Kharif crops and December to March for Rabi.

The combined effect of intensified irrigation demands and reduced groundwater recharge in the future, therefore, can put more strain on an already fast-depleting resource, he said.

The findings challenge the optimistic perception that climate change-driven increase in rainfall will solve our water problems, the lead author of the study said.

In 2009, a monsoon drier by almost 20 per cent, followed by an unusual winter that was warmer by a degree, had "detrimental" effects on groundwater storage -- it had reduced by 10 per cent, the authors found.

Moisture lost from soil during winters also was found to have significantly increased over the past four decades, suggesting the potential role of warming and stepped-up demands in irrigation.

The authors have projected that under continued warming, monsoons drier by 10-15 per cent and winters warmer by 1-5 degrees Celsius will together spike irrigation water demands by 6-20 per cent.

A warming of 1-3 degrees Celsius across north India will also hamper groundwater replenishment considerably by 7-10 per cent, they said.

"The findings have policy implications as the water crisis witnessed during this year's heatwave highlights the need for cautious and judicious exploitation of the groundwater," Mishra said.

Groundwater, vital for food and water security in India, will become a more crucial resource in a warmer climate because of increased demands for irrigation and industry alike, the author said.

"This is because surface water storage, such as in reservoirs and dams, is insufficient to meet demands during summer, as seen in cities such as Delhi and Bengaluru. Not paying attention to the resource could pose water security challenges in the future," Mishra added.

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