A country that produces 10 per cent of the world’s crops is now the world’s largest consumer of groundwater, and aquifers are rapidly becoming depleted across much of India.

These are the findings of a new study in Science Advances, led by experts from the University of Michigan and co-authored by the University of Vermont. It finds that switching to canal irrigation will not fully compensate for the expected loss of groundwater in Indian agriculture.

India is the world’s second-largest producer of wheat and rice and is home to more than 600 million farmers. The country has achieved impressive gains in food production since the 1960s, due in part to an increased reliance on irrigation wells, which allowed Indian farmers to expand production into the mostly dry winter and summer seasons.

But, as reported in the University of Vermont’s’ newsbrief, those gains have come at a cost. The abstract of the study states that results suggest that, given current depletion trends, cropping intensity may decrease by 20% nationwide and by 68% in groundwater-depleted regions. Even if surface irrigation delivery is increased as a supply-side adaptation strategy, which is being widely promoted by the Indian government, cropping intensity will decrease, become more vulnerable to interannual rainfall variability, and become more spatially uneven. Groundwater and canal irrigation are not substitutable and additional adaptation strategies will be necessary to maintain current levels of production in the face of groundwater depletion.

Indian government officials have suggested that switching from groundwater-depleting wells to irrigation canals, which divert surface water from lakes and rivers, is one way to overcome projected shortfalls.

The study estimates that if Indian farmers lose all access to groundwater in overexploited regions – and if that irrigation water is not replaced with water from other sources – then winter cropped acreage could be reduced by up to 20% nationwide. However, that scenario seems unlikely, and was included in the study only as an upper-bound estimate. The expected losses would largely occur in northwest and central India.

It seems more likely that any future groundwater shortfalls would be at least partially offset by increases in canal irrigation. But even if all Indian regions currently using depleted groundwater switch to canal irrigation, winter cropped acreage could still decline by 7% nationwide and 24% in the most severely affected locations, according to the researchers.

The study analysed high-resolution satellite imagery and village-level census data and focused on winter cropped acreage. While nearly all Indian farmers plant crops during the monsoon to take advantage of seasonal rains, winter agriculture is mainly reliant on groundwater irrigation and now accounts for 44% of the country’s annual cropped acreage for food grains.

The study is the first to use high-resolution empirical data, including census data about the irrigation methods used in more than 500,000 Indian villages, to estimate the crop production losses that may occur when over exploited groundwater is lost.

The proliferation of deep (>100 feet) irrigation wells called tube wells since the 1960s has enabled Indian farmers to increase the number of seasons when crops are planted in a given year. This increase in “cropping intensity” is credited for much of the country’s food-production gains.

The team used satellite data to measure Indian winter cropped area, a key determinant of cropping intensity. They then linked the satellite data to census information about the three main types of irrigation infrastructure in India: shallow “dug wells,” deeper tube wells, and canals that divert surface water.

Linking the two datasets allowed researchers to determine the relative efficacy of each irrigation method. That, in turn, enabled them to estimate potential future acreage losses and the ability of canal expansion to fill the gap.

The researchers also found that increased distance from existing irrigation canals is strongly associated with decreased acreage planted with winter crops. In the future, a greater reliance on canals could increase inequities related to irrigation access.

In addition, the lakes and rivers that feed irrigation canals rise and fall in response to rainfall variability, unlike deep groundwater wells. So, a greater reliance on canal irrigation in the future would result in increased sensitivity to year-to-year precipitation fluctuations, as well as any long-term trends due to human-caused climate change.

While groundwater depletion is becoming a global threat to food security, and the extent of current and projected groundwater depletion are well documented, the potential impacts on food production remain poorly quantified.