Understanding the nuances of groundwater depletion

In the past decades, experts have often expressed their concern over the depleting groundwater levels in Bangladesh, especially while advocating for more use of surface water in agriculture and industry. Sometimes, they blamed the farmers for applying excessive water during irrigation. 

Their point of view is not wrong. Every year, Bangladesh pumps out approximately 32 cubic kilometres (km³) of groundwater. Of the withdrawn water, 90 percent is used for irrigation and the rest is consumed by households. No data is available for groundwater use by the industrial sector in Bangladesh.

Groundwater abstraction for irrigation also has become a global concern now. Although the melting of polar ice associated with contemporary global warming is understood to be the main cause of sea level rise, scientists have traced that groundwater depletion resulting from irrigation also contributes to the sea level rising. 

A recent study—published in Advancing Earth and Space Science Journal—shows a model estimate of water redistribution from aquifers to the sea that would result in a drift of Earth's rotational pole, about 78.48cm toward 64.16°E. 

Globally around 980 km³ of groundwater is abstracted annually while Bangladesh's groundwater consumption (32km³) is around 3% of that. 

Groundwater plays a very crucial role in achieving Bangladesh's food security as 80% of water for the country's irrigation is sourced from the underground. Due to seasonality (90% of rainfalls occur only during monsoon), Bangladesh's dependence on groundwater for irrigation cannot be altered in any easy way. 

So, how does Bangladesh control groundwater depletion without disturbing irrigation or risking food security?  

Bangladeshi scientist Dr Mohammad Shamsudduha, currently working at the Institute for Risk and Disaster Reduction under the University College London, believes that the idea of limiting groundwater abstraction for irrigation is unrealistic and it will drastically reduce food grain production in Bangladesh.

Last year in September, the prestigious journal Science published a study titled 'The Bengal Water Machine: Quantified freshwater capture in Bangladesh'. In that study, Shamsudduha and his teammates showed that the pumping up of groundwater by Bangladesh's farmers, especially in dry seasons, led to a massive capture of freshwater in the underground storage in the last three decades. 

"No doubt, Bangladesh is blessed with highly productive aquifers," said Shamsudduha. 

How is that possible? 

"We all agree that groundwater depletion in some parts of Bangladesh is a reality. However, if we go through the monthly time series data on mean annual groundwater levels, dry season groundwater levels and wet season groundwater levels, we can see that the year-on-year variability of freshwater capture (by aquifers) has increased dramatically. That is because additional storage was created after pumping." 

The research team terms this condition as the Bengal Water Machine. 

In Bangladesh, the mean annual groundwater recharge in the pre-development period (1971 to 1975 - before widespread Boro rice cultivation started) ) was estimated to be approximately 23 km3 per year. The mean groundwater recharge has increased to about 28 km3 per year in the recent period (2012 to 2016). This has been possible because of the operation of the Bengal Water Machine in about one-third of Bangladesh over the last few decades.

The idea actually originates from two visionary scientists, Roger Revelle and V Lakshminarayana, who published a paper called Ganges Water Machine back in 1975.

The River Ganges frequently floods riverine places in the monsoon period and affects agricultural production heavily. The scientists basically argued that if the farmers along the River Ganges installed their irrigation wells along the river banks, and pumped the grounds heavily for water, during monsoon the rising surface water would seep into the aquifers created by the pumping of the farmers earlier. 

This would be a win-win situation because it would address food security while at the same time lessen the impact of floods. 

Shamsudduha said, "Unfortunately, since the publication of the paper, neither Indian nor Bangladesh governments materialised the idea. But it actually happened unknowingly and unintentionally in Bangladesh. Around 16 million farmers in Bangladesh have been pumping groundwater since the 1980s and they have created an invisible Bengal Water Machine. We can call it a symbiosis or interaction between humans and nature that led to the operation of the Bengal Water Machine." 

The study calculates groundwater recharge for the pre-development and development periods. For example, in the north of Mymensingh district (Haluaghat Upazila) the mean annual recharge between 1975 and 1980 was 88 mm per year. Now the mean annual recharge (in the last five years) is 185 mm. So, the mean annual groundwater recharge has increased by about 100 mm over the years. 

However, the Bengal Water Machine does not work at the same capacity everywhere in Bangladesh and groundwater depletion has taken place in some places. For example, groundwater is depleting at higher rates (>1 m per year) in some locations within the greater Dhaka City, Gazipur and the High Barind Tract areas, because abstractions have exceeded recharge. 

Other constraints are unfavourable surface geology and substantial variability in groundwater pumping and monsoon rainfall. Therefore, the capture of freshwater is highly variable and in some cases (e.g., Dhaka), it may even be masked by heavy and continuous withdrawals of groundwater.

Although the area of irrigated agriculture has been pretty much constant over the last 30 years, the yield has dramatically increased and so has the number of groundwater pumps. Along with high-yielding varieties of rice and the use of fertilisers, the contribution of groundwater to food production has undeniably been the greatest towards the nation's food security, claimed Dr Shamsuddoha.

"Groundwater has been the vital fuel for food security and our policy should really be based on the new scientific evidence of the positive impact of the Bengal Water Machine," he said. 

The researcher made a couple of recommendations, including the enactment of a separate groundwater use policy. 

"First of all, we need to recognise freshwater capture or the operation of the Bengal Water Machine when we estimate the safe yield level of groundwater abstraction for our aquifers and we also need to promote the conjunctive use of surface water and groundwater wherever and whenever possible," he said.

Professor Kazi Matin Ahmed, a senior faculty at geology department in Dhaka University, said, "We have seasonal surface water scarcity. We cannot use river water throughout the year because of lack of seasonal availability and pollution issues. More importantly, most of our big rivers are transboundary. So, whenever we plan to use more surface water, we will face challenges like water-sharing disputes in the Ganges, the Teesta, and others."

"So, there is no alternative to making sustainable use of our groundwater resources. This means that the government needs to map the regions where safe yield of groundwater under current and projected future abstraction is possible. Then the other areas where recharge is limited; dry-season flow in rivers has substantially reduced and sustained groundwater depletion is evident - those areas can be declared as 'water stressed areas'," Matin said.

Sadiqur Rahman. Sketch: TBS