Viet Nam: New elevation model shows that the Mekong Delta is just eighty centimetres above sea level
Land subsidence and sea level rise threaten to submerge large parts of the Mekong Delta in the next century. In recent years, physical geographer Philip Minderhoud (Deltares / Utrecht University) has found that the delta is just eighty centimetres above the local sea level, about two metres lower than international researchers often thought on the basis of freely available satellite data.
That makes related problems like salinisation and flooding even more pressing for the 18 million inhabitants of the delta. Minderhoud’s findings were published today in the authoritative journal Nature Communications.
When he was investigating land subsidence in Vietnam, he quickly suspected that something was wrong. ‘I saw strange patterns on a frequently used satellite map: stripes transversing riverbeds. They couldn’t be contour lines because then they would have to have been in the direction of the water flow.’ The stripes had always been attributed to ‘noise’ in the satellite observations but Minderhoud found the same stripes in other maps. Flood risk maps established using those satellite observations did not correspond with the reality: according to the maps, other areas should have been flooded. The elevation data did not concur with his own observations either: ‘For example, I saw roads that were only slightly higher than water that was directly connected to the sea; according to the maps, they should have been much higher.’
Discrepancy between Western maps and Vietnamese elevation data
It was already known that elevation data from satellite measurements include a certain margin of error and that it is necessary to allow for an uncertainty of at least several metres. In the case of the Mekong Delta, the real culprit was the right calibration point. Elevation measurements assume a predetermined zero point. That is because the earth is not a perfect sphere and the water level is not the same everywhere. For example, due to the presence of large land masses, gravity pulls on the water harder in some places than in others. Philip Minderhoud: ‘You can only relate a height to this sea level if the calibration point concurs with the local sea level but that is far from being the case everywhere.’ Until recently, the only international elevation data available for the Mekong Delta came from satellites. On the basis of those data, the average elevation of the delta was thought to be about 2.6 to 3.3 metres. So the calibration point in the satellite elevation models is not the same as the local sea level.’
The Vietnamese knew there was a major discrepancy between Western maps and their own data but they decided not to give their detailed elevation data to international researchers because those data were considered to be a military secret.
New elevation model based on field measurements
Minderhoud developed a new elevation model and he was able to use elevation data from field measurements. Until recently, those data were available to the Vietnamese government only. Despite this openness, it was still difficult to interpret the data correctly because no records had been kept of when the measurements were taken. It was therefore uncertain whether the data represented the current situation. In addition, Minderhoud saw that the elevation data for the Mekong Delta were related to the mean level of the sea near Hanoi as the calibration point. That city is two thousand kilometres to the north and it was not clear whether another calibration point applies there and whether the Vietnamese surveyors had taken it into account.
Minderhoud’s research shows that elevation differences can vary considerably depending on the old interpretations of elevation and the new elevation model. Hậu Giang used to be classified as one of the highest provinces in the Mekong Delta with an average elevation of 3.4 metres but the latest data show that it is actually the lowest province at just 38 centimetres above sea level.
No definitive map
‘I haven’t produced the definitive elevation map of the Mekong Delta either,’ emphasises Minderhoud. ‘The aim of my study is to show what happens when you fail to think carefully about the calibration point, when countries withhold data or when scientific disciplines collaborate too little. That can sometimes have drastic consequences because institutions like the World Bank make investment decisions on the basis of these models. And this isn’t confined to the Mekong Delta: it may also be happening elsewhere in the world.’