The Mekong delta: the threat of drought today, but that of disappearance tomorrow?

Land subsidence

The threat for this great delta covering 40,577 km² (the third largest in the world) is that of the rising sea level. The conclusions of the Rise and Fall  research programme carried out by the Dutch scientists of Utrecht University and the Deltares Institute are frightening: the delta is sinking quickly and could disappear this century, thus faster than expected. The average altitude of the delta’s soil is extremely low, only 0.8 metre above sea level – i.e. 2 metres lower than the previous estimations. With the sea level rising at a rate of 3 to 4 mm a year and the subsidence of several rural areas of the delta from 10 to 20 mm a year, the result is obvious: at this pace, more than half the Mekong delta, located in Vietnam will have disappeared below the sea from now to 2100 if nothing is done to reverse the situation, obliging the displacement of millions of people.

Overexploitation of underground water

This situation is due to a large number of factors linked to climate change, urbanisation and economic development, the extraction of sand and dam operations.

The study shed light on the impacts of overexploiting groundwater reserves, estimated at about 2.5 million m³ a day in 2015, in particular for agriculture and aquaculture. It has led to the accelerated and massive subsidence of land in the delta, with certain areas sinking at a rate of as much as 6 cm a year. The groundwater will potentially become progressively salty due to the infiltration of seawater in the delta.

The impact will be considerable for this major rice-growing region in terms of food security, environmental balance and living standards.

The researchers recommend the Vietnamese government to target the underlying causes linked to utilisation of groundwater and fine sand in order to limit the phenomenon. According to their projections, if no increase in the extraction of groundwater is measured, the percentage of land that will become submerged will decrease, with nearly 50% below sea level if extraction remains stable and only a third if the consumption of groundwater is reduced by 75%.

The impact of upstream dams

Land subsidence in the Mekong delta is exacerbated by the shortage of river sediments. Indeed, they are blocked upstream by a cascade of dams built on the Mekong, or extracted by sand merchants. More than 1,000 dams are already operational on the river and its tributaries and tens more are being built in China and Laos, which hopes to become the “the battery of Southeast Asia “.

These infrastructures contribute towards modifying the river’s hydraulic regime and fish life and migration cycles. The Laotian authorities commissioned the mega-dam and hydropower plant of Xayaburi in September. It is the 44^th dam on the main channel of the Mekong and the most powerful (1,285 MW of installed capacity). Xayaburi is a controversial project for the environment and a reason for conflict between Laos and its neighbours on the Lower Mekong, namely Cambodia and Vietnam, and its commissioning once again made it a target of criticism. NGOs linked the long drought that hit the region 300 km downstream to the initial tests conducted in June prior to its commissioning.

 Photos credits : MRC