Around the world, high temperatures typically increase demand for freshwater, particularly in the agricultural sector. Warming conditions are often associated with heat stress and more evapotranspiration in crops, thereby increasing the amount of water they need to develop and produce sustainable yields.
“Climate warming will lead to increased irrigation demands, which can lead to more groundwater pumping.”
In India, crop irrigation is already the primary driver of declining groundwater levels, and a new study published in Science Advances reports that rates of groundwater depletion could triple by midcentury.
In short, “climate warming will lead to increased irrigation demands, which can lead to more groundwater pumping,” explained Vimal Mishra, a civil engineer at the Indian Institute of Technology Gandhinagar who was not involved in the new study.
Shrinking Buffer
“Groundwater is considered a buffer under climate change because it provides water for reducing heat stress in crops,” said Nishan Bhattarai, a geographer at the University of Oklahoma and lead author of the study. “We wanted to understand this relationship between groundwater levels, crop water stress, and climate change.”
Bhattarai and his colleagues looked at how farmers have already increased irrigation in response to warming to forecast how they are likely to adapt as temperatures continue to rise in the future. The researchers consulted records of groundwater depth in wells across India, remote sensing data evaluating crop stress, and weather logs tracking precipitation and temperature.
Combining the three data sets, they found that groundwater levels shrank and crops became more stressed as temperatures rose. An exception was the summer monsoon season, when torrents of rainfall likely contributed to crop health. However, their data also showed that warming temperatures reduced groundwater recharge levels during both the monsoon and dry seasons because of higher rates of evapotranspiration.
Along with rising temperatures, scientists expect that climate change will strengthen the summer monsoon, Mishra said, and though increased rainfall could help mitigate some amount of groundwater depletion, the deeper aquifers tapped for irrigation take more time to refill than the shallower ones—used mostly for residential purposes and small-scale agriculture—that monsoon rains can replenish.
Using multiple climate models, the researchers projected future groundwater demand, factoring in rising temperatures and increased monsoon rainfall. They estimated that should 1°C of warming occur, the groundwater depletion rate in 2041–2080 would be roughly 36 centimeters per year across India—more than triple that of today.
Limits to Adaptation
Bhattarai and his colleagues stressed the need for policies and interventions to conserve groundwater in light of climate-driven challenges to water security.
“Groundwater irrigation, which has been our main way of adapting to rainfall variability, will not remain an option in many parts of India.”
“Groundwater irrigation, which has been our main way of adapting to rainfall variability, will not remain an option in many parts of India, particularly where groundwater overuse is already a problem,” said Aditi Mukherji, director at Climate Change Adaptation and Mitigation Impact Action Platform at CGIAR (formerly Consultative Group for International Agricultural Research). Her main takeaway from the study, she said, is that agriculture needs to reduce its reliance on groundwater irrigation by growing less water intensive crops.
Other climate experts, however, advised caution in interpreting the results.
Thiagarajan Jayaraman, a climate policy analyst at the M. S. Swaminathan Research Foundation in India, said, “There are many uncertainties, such as rainfall distribution in the future, as well as the role of nonclimatic factors in determining the current rate of groundwater depletion.”
Bhattarai acknowledged that he and his coauthors did not account for nonclimate factors such as changes in crop varieties or cropping patterns because fine-scale data for these variables were not available.
“There’s definitely a lot of uncertainty in the future, and we acknowledge there are limitations” to forecasting, he said.
“We need more work based on different approaches to better understand the role of climate change and human activities on groundwater storage in India,” Mishra said.
—Rishika Pardikar (@rishpardikar), Science Writer