The year-round availability of groundwater resources has played a vital role over the past few decades in providing water for drinking and irrigation in Bangladesh. This hidden natural resource has transformed a once famine-prone land into a prosperous and food-secure nation. We take groundwater for granted due to its general presence at relatively shallow depths. However, it is still under-appreciated, with limited representation in national water policies. As we move into uncharted territory as a consequence of climate change, it is crucial that groundwater responses to changes, not only in the magnitude, duration, and variability of monsoon rainfall but also our use of land and water, are better understood.
As a geologist with a specialisation in groundwater science, I would like to point the reader’s attention towards a Bangladeshi research paper titled “The Bengal Water Machine: Quantified freshwater capture in Bangladesh,” which was published in the prestigious Science journal in September this year.
The article – written by myself and my colleagues – describes how 16 million smallholder farmers in Bangladesh have collectively created a water machine beneath the Earth’s surface by incrementally increasing pumping of shallow groundwater during the dry season to irrigate rice paddies over the last three decades. We call this the Bengal Water Machine. The main concept came from an earlier paper published in Science in 1975, known as The Ganges Water Machine, where the authors suggested that pumping more groundwater using deliberately positioned wells beside the River Ganges could enhance groundwater recharge over time and reduce flood disaster. However, there was no targeted implementation of the Ganges Water Machine by any government or private organisations, and no field observations have yet been presented to demonstrate its operation.
Nearly half a century later, we show that such a water machine operates in Bangladesh but is more diverse, and is operated, unintentionally and unknowingly, by millions of smallholder farmers.
Where will depleting groundwater leave us?
The discovery of the Bengal Water Machine has only been possible because of our country’s long-term dedication to monitoring groundwater levels. Bangladesh has a vast network of 1,250 groundwater-level monitoring stations that is managed by the Bangladesh Water Development Board (BWDB). Our analysis of 465 long-term records revealed that about 35 percent of the observation boreholes that measure groundwater levels reflect the operation of the Bengal Water Machine. Approximately 25 percent show depletion of groundwater levels of varying magnitudes, and the remaining 40 percent of boreholes show relatively stable trends with seasonal oscillations.
Let me explain what the Bengal Water Machine is and how significant its operation has been for Bangladesh.
Intensive dry season (November to April) groundwater irrigation to produce Boro rice lowers groundwater levels, increasing the space available underground to store water in shallow sandy sediments known as aquifers. Under favourable hydrological conditions, the space created by pumping fills up during the monsoon season from the infiltration of rainfall and leakage of floodwaters, not only in major rivers but also smaller seasonal rivers, canals, ponds, and lakes (beels). Over time, the available storage space in aquifers has incrementally increased and enhanced the seasonal capture of freshwater during monsoon in Bangladesh.
Our calculations suggest that between 1988 and 2018, this freshwater capture in Bangladesh, which is in excess to natural, predevelopment groundwater recharge, amounts to between 75 and 90 cubic kilometres. This colossal amount of freshwater is more than twice the reservoir capacity of the Three Gorges Dam in China, and 12 times more than the volume of water in Kaptai Lake of Bangladesh. This capture of monsoon freshwater flown by aquifers has, where conditions are favourable for the operation of the Bengal Water Machine, sustained increased use of groundwater for irrigation by farmers in the dry season. The Bengal Water Machine has ultimately helped Bangladesh become self-sufficient in food grain production, despite substantial variations in annual rainfall and a slight overall decline in basin rainfall. Further, we contend, but have yet to confirm, that this gradual increase in freshwater capture has served to reduce the magnitude of flood discharges in places where the Bengal Water Machine operates. Future research will also seek to understand better why over short distances of a few kilometres or less we observe a variety of groundwater responses, including those reflecting the Bengal Water Machine and those showing groundwater depletion.
Our study argues that what we call the Bengal Water Machine may operate and could potentially be replicated in alluvial plains beyond Bangladesh, including other Asian mega-deltas such as the Ganges floodplains in India, and the Mekong and Red River Deltas in Vietnam, which are similarly vulnerable to the adverse impacts of climate change. As such, the machine could help to enhance global food security and the resilience of water and food supplies to climate extremes. How the Bengal Water Machine will respond to changes in monsoon rainfall intensity and duration remain unclear, but further research on the matter can help in supporting climate adaptation policy and practice.
The outcomes of our study highlight the importance and value of long-term hydrological monitoring data to assess the status and trends of a country’s groundwater resources, which will become ever more important under the amplification of climate extremes caused by climate change.
Our research also highlights limitations to the operation of the Bengal Water Machine in areas of the country where leakage of water during the monsoon season is insufficient to fully replenish the groundwater withdrawn during dry seasons. In those areas, pumping depletes groundwater resources, making these inaccessible to households reliant on shallow wells for drinking-water.
Further research is also needed to assess the suitability of locations for the operation of the Bengal Water Machine to maximise benefits to farmers and minimise the risks of groundwater depletion. Pilot projects will be needed to test the viability of operation of the Bengal Water Machine in suitable areas in other parts of Bangladesh.
I hope that the article in Science will serve to increase public awareness of the importance of continuously monitoring groundwater levels and quality in order to safeguard this precious natural resource for future generations.