1. Introduction

The Vietnamese Mekong Delta (VMD) is the third largest delta in the world, covering 48,900 km2. The Mekong River branches out into 7 main VMD distributary channels before draining to the sea. In addition to these natural river branches, a large network of canals of varying sizes have been built over centuries, enabling navigation and irrigation [ Chapter 7 ]. The sub-aerial delta plain of the VMD is one of the most fertile regions in the world, home to ~17 M people, and produces 50% of Viet Nam’s food supply. Over the past decades, rapid urbanization and intensified agri-/aquacultural production have put significant stress on the delta’s natural resources, such as sand and fresh ground/surface water. Life and livelihood in the delta are punctuated by the tropical monsoon, with a wet season from July to October and a dry season from December to May, driving large seasonal variations of the hydrology [ Chapter 7 ]. The average monthly discharge of the Mekong measured upstream (Kratie, Cambodia) ranges from 2 103 m3s-1 in the dry season to 36 103 m3s1 during the wet season. During the latter season, the strong fluvial discharge drives a flood pulse that inundates the floodplains and brings new sediments and nutrients to the delta. The fluvial forces limit the salt intrusion from the sea in the estuary channels to only a few kilometres, whereas during the dry season ocean forces drive up-channel salt intrusion over tens of kilometres.

[ Figure 9.1 ] Changes in the Mekong delta

Drivers (italic) and consequences (bold) of changes in the Mekong delta. Relative sea level rise is the combined effect of global sea level rise and land subsidence. Source: P. Minderhoud (2019).

This densely populated area, crucial for Viet Nam’s food security and international food exports, is under pressure from both climate change and anthropogenic stressors [ Figure 9.1 ]. As a result of its low average altitude of ~80 cm above sea level [ Minderhoud et al., 2019 ], the sub-aerial delta plain seems particularly vulnerable to the global mean sea level rise (SLR) induced by climate change (See DEM in Figure 9.2). Current global rates are ~3.3 mm/year and are projected to accelerate during the XXIst century [ Chapter 1 ] depending on the climate scenario. Large uncertainties remain as to the values which might be reached by the end of the century and beyond, but even in the most optimistic climate scenarios global mean sea level rise could exceed 40 cm in 2100, which could put the lowest areas of the delta at risk of permanent inundation [ Bamber et al., 2019a; Ministry of Natural Resources and Environment (MoNRE), 2016 ].

But global sea level rise is not the only driver of elevation change relative to the sea in the delta. Elevation change of the delta plain relative to local sea level depends on relative SLR, which arises from the cumulative effect of global sea level change and land vertical movements ( i.e. land subsidence). As in many deltas worldwide [ Nicholls et al., 2021 ], current subsidence rates in the VMD outpace global sea level rise by an order of magnitude [ Erban et al., 2014; Minderhoud et al., 2017, 2020 ]. Subsidence in a delta is a natural phenomenon driven by tectonic movements, isostatic adjustment, and natural compaction of unconsolidated sediments. Anthropogenic activities can enhance shallow natural compaction rates, for example by draining the surface water table for agricultural practices or by adding additional loading through infrastructure or buildings [ Minderhoud et al., 2018 ]. Furthermore, human activities can also trigger new subsidence processes, as shown in recent hydrogeological studies highlighting the role of excessive groundwater extraction driving aquifer-system compaction and leading to accelerating subsidence rates in the VMD [ Minderhoud et al., 2017, 2020 ]. Under natural conditions, the deposition of new sediments during floods can partially offset relative sea level rise by building up elevation. However, sediment starvation due to sediment trapping by upstream dams and flood control in the delta during the wet season deprives the floodplains of new sediments [ Chapter 7 ]. Also, current accelerated subsidence rates are an order of magnitude larger than what could be compensated by new sediments under pristine conditions [ Schmitt et al., 2017 ]. As a result, the natural mechanism for coping with relative sea level rise no longer functions in the VMD, due to anthropogenic impacts upstream and in the delta itself. In the next decades, relative SLR may dramatically increase coastal flooding, erosion, and salt intrusion, all of which are already disrupting day-to-day life in the delta.

Salt intrusion in surface waters, as the main indicator for land use, has raised growing concerns over the past decade since life in the delta strongly depends on water quality and availability. In the past five years, record saline water intrusion events (in 2016 & 2019) have already resulted in huge financial and crop losses, and in freshwater shortages. Figure 9.2 shows how salt intrusion has been consistently and gradually increasing at some of the observation stations in the VMD. Climate change-driven sea level rise has often been blamed for the increase in experienced saline water intrusion events. However, salinity intrusion in surface waters is the outcome of a complex interplay of riverine and oceanic