The impact of irrigation development on regional groundwater resources in Bangladesh

•Groundwater use for irrigation in much (but not all) of Bangladesh may not be as unsustainable as is often supposed.•Current policy to reduce groundwater use may be less necessary or at least less urgent than supposed.•Our analysis is approximate and should be followed up with detailed studies. There is increasing concern over falling groundwater levels in some areas of Bangladesh, and there is undoubted overuse of groundwater in the Barind Tract in northwest Bangladesh and around Dhaka. However, the volumes of water availability and use, and hence the sustainability of use, are not well known. We developed monthly water balances for the main regions of Bangladesh to investigate historic trends in water use and availability and possible future trends under changed management to lessen groundwater use by using more surface water for irrigation. Our results show that for many areas the fall in pre-monsoon groundwater levels (at the regional average level) over the last few decades may be largely explained by the continual withdrawal of ever greater volumes of water with the three-fold increase in the area of irrigation. Thus, for many areas, if there were no further increase in the area irrigated by groundwater, the rate of decline in groundwater levels would likely reduce and levels could even attain a new equilibrium at a lower level, implying that current pumping rates could be maintained (subject to the lower groundwater levels being acceptable on environmental, economic and social grounds). Post-monsoon groundwater levels are largely influenced by yearly rainfall variability. Thus, groundwater use in some areas may not be as unsustainable as feared, and policies to reduce groundwater use in such areas may not be as necessary or urgent as thought. However, our analysis is approximate and detailed local studies are required to assess the sustainability of use.