Traditionally India has relied on surface storage and gravity flow to water crops but in recent years it has come to depend heavily on groundwater to irrigate crops and to cope with dry spells. Today the number of irrigation wells equipped with diesel or electric pumps in the country stands at more than 19 million, compared with just 150 000 in 1950.

This dependence is only likely to increase in the future as more and more rural farmers scavenge this water using small mechanical pumps and private tubewells. The practice has a large carbon footprint and groundwater pumping using electricity and diesel accounts for an estimated 16–25 million metric tonnes of carbon emissions or up to 6% of the total carbon dioxide produced by India.

The situation is difficult to turn around because groundwater is relatively easy to access, and will become even more attractive as the climate warms since no water is lost to evaporation underground. Groundwater is available on demand nearly everywhere in India but Shah says that, if the current trend continues, the country's food baskets will be seriously threatened in the next 10–15 years at most as this water begins to run out.

Central and state governments have tried to remedy the situation over the last 20 years by investing more than $20 billion on building new surface-irrigation systems and rehabilitating existing ones. Despite these efforts, the overall area served by such surface structures has actually declined by more than three million hectares, while the area exploiting groundwater is steadily rising. "As a result, Indian irrigation has transformed from being a centrally managed surface-irrigation regime (dating from colonial times) to an atomistically water-scavenging one involving millions of pump-owners who divert surface and groundwater at will," says Shah.

The situation is unique to India, the only country in the world where vast areas of hard rock aquifers spanning some two million square kilometres have been intensively developed to provide additional irrigation.

India needs to learn intelligently from the experience of countries such as Australia and the US that have succeeded in managing aquifer recharge, adds Shah. Maintaining groundwater levels throughout India at 3–8 metres below ground level should be one of the key objectives in India's water policy. "This would minimize the carbon footprint of the country's irrigation economy and maximize agrarian resilience to hydro-climatic change," he told environmentalresearchweb.

India's greenhouse gas emissions from water pumping could also be significantly reduced by aggressive and sustained programmes of aquifer storage and recovery, which would help mitigate climate change. "The recommendations I make in my paper are implementable provided the government and policy makers put their energy and resources behind the job," said Shah. "What is most needed to begin with is an empowered coordinating agency that can set out a clear vision and road map for action, bringing together hundreds of implementing agencies under one umbrella."

He now plans to investigate how effective alternative technologies are for aquifer storage and recovery. "I will also endeavour to take my ideas to relevant policy makers in India and argue for change," he said.

The study will appear in Environmental Research Letters.