The Ganga Basin’s aquifers are one of the largest reservoirs of groundwater in the world
Photo: iStock_ Groundwater storage levels have been declining by 2.6 centimetres per year in the Ganga basin, according to new estimates.
The impacts were more pronounced in Rajasthan, Haryana and Delhi, with average storage declines of roughly 14 cm year−1, 7.5 cm year−1 and 7.2 cm year−1, respectively.
The average storage decline in Uttar Pradesh, Bihar and West Bengal was estimated to be roughly 2cm year−1, 1 cm year−1 and 0.6 cm year−1, respectively. These estimates have been published in the journal Nature Scientific Reports.
The Ganga Basin’s aquifers are one of the largest reservoirs of groundwater in the world, according to the report.
“The Ganga basin is noted for many factors, one of which is the groundwater decline,” Indu J, associate professor at IIT-Bombay and one of the study’s authors, told Down To Earth.
Most previous studies to our knowledge, the author added, used either satellite or modelling data. But there are many uncertainties involved when only one approach is followed, as each has its pros and cons. “So, we use multiple lines of evidence to get a complete picture,” she noted.
Indu and researchers from CSIRO Land and Water, University of Bergen, and Indian Institute of Technology, Roorkee, used three different methods to study long-term groundwater storage across six states.
First, the team collected groundwater level data between 1996 and 2017 from the Central Groundwater Board. The average groundwater levels have been declining at a rate of 2.6 cm year−1 between 1996-2017, the analysis found.
West and southwest areas, including agriculturally intensive regions and urban areas like Delhi and Agra, took the biggest hit, the study showed.
The second method involved the analysis of satellite data from the Gravity Recovery and Climate Experiment (GRACE), which yielded an average loss of 1.7 cm year−1. Grace satellites, launched in 2002, assess Earth’s water reservoirs over land, ice and ocean.
For the last method, the team turned to a model to study groundwater dynamics and storage changes. The team calculated the volume of water entering and leaving the aquifer storage.
The difference between the two represented storage loss, which stood at roughly 3.2 cm year−1.
“In all three analyses, we got the same answer: groundwater in the Ganga basin is declining,” Indu said.
Abhijit Mukherjee, professor at the Indian Institute of Technology Kharagpur, pointed out that the study supports previous studies that have documented the decline. “We have arrived at reasonably similar outcomes in our studies as well,” he noted.
Delhi and Haryana, he added, have high groundwater abstraction rates, which explains the steep decline.
Rajasthan, whose groundwater reserves contribute about 90 per cent of the drinking water and 60 per cent of the irrigation, is showing an improvement in groundwater levels in the recent past, the expert said.
A recent Central Groundwater Board yearbook, which monitors groundwater levels four times a year, found that the water levels in 2021-2022 rose compared to the 2011-2020 average, barring the pre-monsoon period.
Further, the new study included western Delhi, Haryana and Rajasthan under the Ganga basin. “These regions come under the Indus basin,” Mukherjee pointed out.
The Brahmaputra basin, which was not a part of the study, shows more groundwater level reduction than the Ganga and Indus basins, the expert noted.
A 2019 study estimated groundwater depletion of greater than 5 cubic kilometres per year in Assam, which falls under the Brahmaputra basin.
Indu now hopes to extend their study to identify districts in the Ganga basin with the highest rates of storage losses.
They also plan to perform scenario analysis of impacts from climate and other stresses for basin planning and policy decisions for management.
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