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derhoud et al., 2017 ]. While there was little extraction-induced compaction in the 1990s, the rates of aquifer-system compaction increased steadily in the following decades to a delta-average subsidence rate of 11 mm/year, locally exceeding 25 mm/year in 2015 [ Figure 9.7 ]. The accelerating trend in subsidence suggested by the model was confirmed by recent InSAR observations, compared with earlier observations [ Figure 9.5 ]. Subsidence processes in deltas can be distinguished between the larger, regional scale — which causes general landscape lowering — and local events, often causing infrastructural damage and local differential subsidence. In the VMD, two main processes are currently generating high subsidence rates at the regional scale, leading to elevation loss of up to several cm per year: 1 ] natural compaction of shallow sediments, and 2 ] groundwater extraction-induced aquifer-system compaction. Natural shallow compaction rates are highest along the coastline where young fine-grained deposits are present — e.g. (former) mangrove
3. Salt intrusion 3.1 Salinity in deltas Salt intrusion in deltaic surface and groundwater systems is a natural process [ Bierkens and Wada, 2019; Mac Cready and Geyer, 2010; Jay and Dungan Smith, 1990; Monismith et al., 2002; Werner and Simmons, 2009 ]. While fresh-saline dynamics in the groundwater system follow timescales in the order of decades, the dynamics of salt intrusion in the surface
forests in the southern part of the delta — and may reach up to 20–30 mm/year, which can be amplified by anthropogenic land-use change and drainage. Aquifer-system compaction following excessive groundwater extraction has been accelerating in the last decades and may locally reach rates of ~30–40 mm/year, depending on local hydro-geological settings and extraction schemes. Other large-scale processes, such as tectonics, isostacy and hydrocarbon-reservoir compaction, generally cause much lower subsidence rates [ Frederick et al., 2019; Kooi et al., 1998 ], and there is no solid evidence suggesting the VMD is an exception to this. At local scale (e.g., in cities), loading by infrastructure and buildings is an additional significant driver of subsidence. Variable surface loading can cause differential subsidence at small spatial scales, and can cause damage to buildings and infrastructure [ de Wit et al., 2021 ]. Furthermore, other drivers, such as groundwater extraction and drainage, also tend to be larger in urban areas, causing additional subsidence. This explains the subsidence hotspots in Can Tho and other large cities in the VMD.
waters vary at timescales in the order of hours, influenced by numerous natural and anthropogenic drivers [ Box 9.3 ]. Among the exposures and vulnerabilities, increased salt intrusion is a major threat to livelihoods in deltas. Considering its impact on water supply, food, and job security, agri-/aqua-cultural activities and the ripple effects on the human geography of deltas and beyond, recovery from — or adaptation to — increased salinity can take generations. As in several other deltas worldwide [ Bucx et al., 2014; Echezuría et al., 2002; Gong and Shen, 2011; Hoep-
