Introduction.


Land subsidence is a natural phenomenon in delta systems. Deltaic sediments are highly compressible and susceptible to significant natural compaction during deposition and subsequent soil formation. Enhanced land subsidence in deltas due to human activities is widely recognized (e.g. Syvitski et al., 2009; Giosan et al., 2014). Human use of land and groundwater resources can amplify natural subsidence processes or initiate new anthropogenic subsidence in different ways (Galloway et al., 2016). Firstly, subsidence can be enhanced by direct loading of the delta surface, both by natural material, such as water and sediment, and by anthropogenic artifacts, such as buildings and infrastructure. Secondly, drainage of wetlands to prepare for agricultural use leads to a lowering of the phreaticwater table, causing compaction and aeration of the subsoil. Consequent decomposition of organic material (oxidation) causes additional volume reduction (e.g. Van Asselen et al., 2009). Additionally, the extraction of groundwater fromdeeper aquifers (water-bearing sediment layers), to meet the increasing freshwater demands of rapidly urbanizing areas, agriculture and aquaculture, can trigger aquifer-system compaction (e.g. Galloway and Burbey, 2011; Gambolati and Teatini, 2015). In the VietnameseMekong delta evidence of widespread absolute subsidence was recently revealed by InSAR (Interferometric Synthetic Aperture Radar) (Erban et al., 2013, 2014). Minderhoud et al. (2017) demonstrated that a steady increase of groundwater use and excessive pumping over the past decades has dramatically accelerated subsidence in this area. Together, these land-use developments of the past decades in the Vietnamese Mekong delta have affected the natural environment, and introduced anthropogenic drivers, enhancing subsidence rates.