“We provide the first rates and maps of land subsidence throughout the lower Mekong Delta, a flat landscape, largely within 2 m of sea level,” Steve Gorelick told environmentalresearchweb. “Flooding by both fresh river water and intrusion of seawater is already a widespread problem for many millions of people living in the Delta. In the future, as the land subsides and sea level rises, flooding impacts will only get worse.” Sea level in the waters off the Delta is rising by about 0.3 cm per year.

There are around one million wells in the Mekong Delta region. As water is extracted from the soil, the pore pressure decreases and the ground compacts. “In many regions, ranging from Venice, Italy to the San Joaquin Valley, California, such extraction has caused severe problems of land subsidence due to sediment compaction,” said Gorelick.

The groundwater level in the Mekong Delta has declined on average by around 0.3 m per year, Gorelick and colleagues Laura Erban and Howard Zebker found by analysing data from 79 monitoring wells at a total of 18 sites. They also constructed a hydrogeomechanical simulation model, which revealed that this drop in groundwater level has created land subsidence at an average rate of 1.6 cm/year, with some areas seeing severe local subsidence.

“By showing the relation between pumping and subsidence, we provide critical information for regional planning,” said Gorelick. “We show that the effect of land subsidence due to groundwater extraction greatly exceeds that of anticipated sea-level rise due to global climate change through mid-century. Of course, sea-level rise could become a greater hazard later in the century.”

Since there are no observations from the ground of land subsidence in the Delta, the researchers turned to satellite-based radar imagery to validate their simulation model. They employed interferometric synthetic aperture radar (InSAR) data from the L-band PALSAR instrument on board ALOS (the Advanced Land Observing Satellite).

Phase changes in the radar backscatter from the ground surface over the period of a year and three days, averaged over 2006–2010, revealed that the land had subsided 1–4 cm per year over thousands of square kilometres. The highest subsidence was along a southwest-northeast axis in the heavily pumped central Delta. Subsidence was less towards the Cambodian border, where groundwater extraction is minimal.

“Using the two approaches, computer simulation and satellite remote sensing, we confirmed that subsidence was indeed related to groundwater pumping,” said Gorelick. “Furthermore, spatial patterns and magnitudes of subsidence determined by each method were in agreement. A more subtle implication is our contribution to applying InSAR to a nearly flat and often flooded landscape. The Mekong Delta is exceedingly difficult to study by this method, due to the effects on radar reflection of a land surface with constantly changing flooding and cropping conditions.”

If pumping continues at current rates, ~0.88 m of land subsidence is expected by 2050, according to the team. At the same time, sea level is likely to rise by around 0.10 m, so parts of the Delta are likely to see around 1 m of additional inundation hazard by mid-century.

“Because current groundwater extraction in the lower Mekong Delta is inducing ~1–4 cm/yr land subsidence over large areas, future management efforts should seek to reduce pumping to halt the subsidence-induced flooding hazard,” said Gorelick. “We continue to work on questions of water supply and groundwater contamination in the Mekong Delta, and hope to pursue the use of InSAR in other regions, particularly those where ground-based land-surface elevation changes are not being monitored.”

Gorelick and colleagues reported their study in Environmental Research Letters (ERL).

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