The subsiding Mekong Delta; 3D numerical simulation on the impact of groundwater exploitation

The Vietnamese Mekong Delta, the third largest delta in the world, is facing land subsidence rates of 1–4.7 cm/yr-1 (Erban et al., 2014). These relatively high subsidence rates are largely attributed to groundwater extraction, which has drastically increased over the past decades due to growing domestic, agricultural and industrial demands. Hydraulic heads in the aquifers drop on average 0.3–0.7 m/yr-1, causing aquifer-system compaction. With over 50% of the delta surface elevated less than 1 meter above sea level, this poses a real threat to its inhabitants, enhancing flood risk and salinization in river channels.
To determine the contribution of groundwater extraction induced subsidence to total subsidence in the Mekong delta, we built a 3D numerical groundwater flow model. This model simulates hydraulic heads over the past 25 years based on the groundwater exploitation history. Subsequently, we calculate corresponding aquifer-system compaction using a coupled land subsidence module (SUB-CR), which includes a direct, elastic component and a secondary, viscous component (i.e. creep). Where InSAR is limited to the build-up environment, our approach enables modelling groundwater extraction induced subsidence in both the urban and rural part of the Mekong Delta.
Even though, not all measured subsidence is explained by the model, the calculated spatial subsidence patterns largely correlate with the measurements. This identifies groundwater extraction as a major subsidence driver, yet also other drivers seem to contribute to the total subsidence balance. Furthermore we found that delta-wide subsidence initiated around two decades ago. Since then subsidence accelerated, with present rates exceeding previous measured values.