The Mekong Delta in Viet Nam could be nearly fully submerged by the end of the century if urgent actions are not taken across the river basin. Continuing with business as usual could drown 90% of this agro-economic powerhouse that is home to nearly 20 million people – with immense local and global impacts.
Only concerted action by the six countries in the Mekong basin and better management of water and sediments within the delta could avoid such a devastating outcome, argues an interdisciplinary research team in an article published in Science.
Land subsidence and rapid hydropower development threaten the future
Most of 40,000 km2 delta is less than 2m above sea level and is thus prone to climate change induced sea level rise. On top of that, actions in the delta lead to land subsidence, such as over pumping of groundwater and unsustainable sand mining to construct expanding cities across Asia. Large parts of the delta are subsiding, with rates up to 5-6 cm a year. The impact is increasingly felt: more flooding and salinisation of groundwater and surface water.
"Land subsidence in a delta is a natural process, but in recent decades the delta has been sinking faster due to increasing human activities, such as excessive groundwater extraction. Large parts are subsiding 10-20 times faster than sea level is rising," according to Philip Minderhoud, Assistant Professor at Wageningen University & Research, and also affiliated with Padova University and Deltares. "In addition, we have recently discovered that the delta is much lower elevated above sea level than previously thought." This means that the area is extremely vulnerable. If current development continues, large parts could sink below sea level in the next ten to twenty years.
Lack of sediment
"It's hard to fathom that a landform the size of the Netherlands and with a comparable population might disappear by the end of the century," said lead author Professor Matt Kondolf from University of California, Berkeley. "Yet, like any river delta, the Mekong Delta can only exist if it receives a sufficient sediment supply from its upstream basin and water flows to spread that sediment across the delta surface, so that land is built at a rate that is equal or greater than global sea level rise." Also, countries in the basin develop hydropower dams as a source of renewable energy. Those dams trap sediment.
But not all the blame can be put to upstream actions and global climate change induced sea level rise. In the delta itself, high dikes have been built to control floods and thus enable high intensity agriculture. This also prevents the fertile sediment from being deposited on the rice fields.
Holistic approach is needed
However, the drowning of the delta is not a fait accompli. There are steps that can be taken to allow dynamic, natural processes to help prevent the delta from further sinking. The research team has identified measures that are feasible and have global precedents, and would significantly increase the lifetime of the delta. Implementing those measures will require participation from national governments and international actors as well as new actors, including the private sector and civil society. It is crucial to combine all efforts: large-scale in the upstream river basin, together with local actions within the delta focused on reducing human activities that speed up subsidence.
Minderhoud recently published another paper evaluating the effectiveness of the proposed sedimentation strategies in Communications Earth and Environment. "In this study we concluded that even in a situation where sediment is managed properly across the basin, there is not enough sediment to compensate for the current subsidence rates. This strengthens the message that there is not one 'silver bullet' to solve the problem. A holistic approach is needed."
