Many major river deltas in the world are subsiding and consequently become increasingly vulnerable to flooding and storm surges, salinization and permanent inundation. For the Mekong Delta, annual subsidence rates up to several centimetres have been reported. Excessive groundwater extraction is suggested as the main driver. As groundwater levels drop, subsidence is induced through aquifer compaction. Over the past 25 years, groundwater exploitation has increased dramatically, transforming the delta from an almost undisturbed hydrogeological state to a situation with increasing aquifer depletion. Yet the exact contribution of groundwater exploitation to subsidence in the Mekong delta has remained unknown. In this study we deployed a delta-wide modelling approach, comprising a 3D hydrogeological model with an integrated subsidence module. This provides a quantitative spatially-explicit assessment of groundwater extraction-induced subsidence for the entire Mekong delta since the start of widespread overexploitation of the groundwater reserves. We find that subsidence related to groundwater extraction has gradually increased in the past decades with highest sinking rates at present. During the past 25 years, the delta sank on average ∼18 cm as a consequence of groundwater withdrawal. Current average subsidence rates due to groundwater extraction in our best estimate model amount to 1.1 cm yr−1, with areas subsiding over 2.5 cm yr−1, outpacing global sea level rise almost by an order of magnitude. Given the increasing trends in groundwater demand in the delta, the current rates are likely to increase in the near future.

中文翻译：

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越南湄公河三角洲 25 年地下水开采对沉降的影响

世界上许多主要的河流三角洲正在下沉，因此越来越容易受到洪水和风暴潮、盐碱化和永久性淹没的影响。据报道，湄公河三角洲的年沉降率高达几厘米。建议过度开采地下水作为主要驱动因素。随着地下水位下降，通过含水层压实引起沉降。在过去的 25 年中，地下水开采量急剧增加，将三角洲从几乎不受干扰的水文地质状态转变为含水层日益枯竭的情况。然而，地下水开采对湄公河三角洲沉降的确切贡献仍然未知。在这项研究中，我们部署了一种三角洲范围的建模方法，包括具有集成沉降模块的 3D 水文地质模型。这为整个湄公河三角洲自地下水储量开始普遍过度开采以来的地下水开采引起的沉降提供了定量的空间明确评估。我们发现与地下水抽取相关的沉降在过去几十年中逐渐增加，目前下沉率最高。在过去的 25 年中，由于地下水抽取，三角洲平均下沉约 18 厘米。在我们的最佳估计模型中，由于地下水抽取导致的当前平均沉降率为 1.1 cm yr-1，区域沉降超过 2.5 cm yr-1，几乎超过全球海平面上升一个数量级。鉴于三角洲地下水需求的增长趋势，目前的水率在不久的将来可能会增加。