Collective groundwater pumping by millions of farmers in Bangladesh has created vast natural underground reservoirs that rival the world’s largest dams for water storage.

The reservoirs are sustaining irrigation that has transformed the previously famine-prone country into a food-secure nation, finds a new study from University College London (UCL).

Published in September 2022, the research explores the combined impact of 16 million smallholder farmers pumping shallow groundwater during the dry season to irrigate rice paddies in the Bengal Basin of Bangladesh between 1988 and 2018.

At approximately 23,000 square miles, the Bengal basin is the largest delta basin in the world and sits at the confluence of the Ganges and Brahmaputra rivers. Researchers analysed a million weekly groundwater-level observations from 465 wells across Bangladesh, taken between 1988 and 2018 from a network of 1,250 monitoring stations.

The study revealed that by lowering groundwater levels through dry season pumping, leakage from rivers, lakes and ponds replenishing groundwater was spurred during the subsequent monsoon. This capture of surface water not only allowed groundwater levels to recover but, in doing so, helped to reduce flooding.

Through the process, which the authors describe as the Bengal Water Machine, more than 75m3 of freshwater was captured over 30 years - a volume equivalent to the combined reservoir capacities of China’s Three Gorges Dam and the Hoover Dam in the US.

Co-lead author Dr Mohammad Shamsudduha, UCL Institute for Risk & Disaster Reduction, said: “Despite substantial variations in annual rainfall and an overall decline in basin rainfall, this scalable, decentralised form of freshwater capture has sustained irrigated food production since the 1990s. This novel intervention helps to address seasonal imbalances in rainfall by increasing the capture and storage of seasonal freshwater surpluses and mitigating the monsoonal flood risk without the use of dams.”

The study highlights the intervention as a sustainable alternative to conventional approaches to storage, including dams and reservoirs, which are challenging to build in densely populated plains.

Co-lead author, Professor Richard Taylor, UCL Geography, said: “Whilst previous estimates of the magnitude of freshwater captured have been hypothetical and based on modelled scenarios, this is the first study to quantify the groundwater volume based on observations, revealing its significant potential.

“Our analysis has profound implications for the expansion and optimisation of this vital, under-recognised engineering marvel that sustains irrigated food production within alluvial plains of the seasonally humid tropics.”

The full paper, which was published in the Science journal, says with more research and piloting, the solution has the potential to be replicated across other regions vulnerable to climate change and could help enhance global food security and resilience.