In only 25 years, the Mekong Delta has been transformed from a stable area into a rapidly-subsiding delta. Using a new three-dimensional groundwater model of the entire delta, researchers from Utrecht University and Deltares have shown how much and where the land is subsiding due to groundwater extraction. The study shows that Vietnam is pumping itself down faster and faster. The rate of subsidence has now reached several centimetres a year, which is a lot higher than the global sea-level rise of a few millimetres a year. That is alarming news for the delta, which is only 1-2 metres elevated above sea level. Salinisation, higher floods risks and coastal erosion increasingly threaten the delta. Unless action is taken, this fertile area – which is home to 20 million people and which produces food for almost 200 million others – faces a major catastrophe.
Groundwater extraction is a major cause
The researchers have published Impacts of 25 years of groundwater extraction on subsidence in the Mekong delta, Vietnam, identifying groundwater extraction as a major cause of land subsidence in the Mekong Delta. The 3D groundwater model is based on geological and geotechnical data, and groundwater measurements. The results have been compared with satellite elevation measurements.
‘This is the first time a delta of this size has been modelled to calculate land subsidence,’ says Philip Minderhoud, a PhD candidate researching land subsidence. ‘Our study shows that the delta is sinking 1-3 cm per year and that this rate is increasing. In some places, we have even found a loss in elevation of 25-35 cm in the past 25 years. That shows how serious this insidious problem is: it is even worse than we expected.’ Awareness in Vietnam is gradually increasing of the fact that land subsidence has far-reaching consequences for life in the Mekong Delta. The extraction of groundwater for drinking water, agriculture, and fish and shrimp farms is largely responsible for the increase in subsidence.
Knowledge on land subsidence is vital to increase climate resilience
This research allows Vietnam to include land subsidence in its plans for making this economically important area climate-resilient. The next step will be to extend the model so that it can be used to make forecasts. ‘Groundwater extraction is not the only cause of land subsidence in this area,’ says Gilles Erkens, a land-subsidence expert at Deltares. ‘In addition to groundwater extraction from deeper layers, there are other causes, such as compression due to the pressure of loads and the lowering of the shallow water table’. In the future, the researchers hope to quantify the different causes of the subsidence observed at the delta surface.
Land subsidence is an insidious worldwide problem
Land subsidence is an insidious problem that is often noticed only when actual damage occurs. Other low-lying deltas such as those in Bangladesh, Indonesia, China and the United States can benefit from this study and the results. ‘This Mekong Delta study is an important first step in improving our understanding of land subsidence in deltas and developing viable solutions,’ says Esther Stouthamer, the land-subsidence researcher leading the Future Deltas interdisciplinary research programme at Utrecht University.
Future Delta focuses on the development and integration of knowledge for global sustainable delta management. Alongside flooding and salinisation, land subsidence is one of the main priorities of the programme.