1. Introduction

The Mekong is one of the largest rivers in the world [ MRC, 2010 ]. It rises in Tibet, China, flows through five riparian countries over 4800 km, and draining a catchment area of 795,000 km2 [ MRC, 2005 ], before dividing into several branches in a delta and reaching the East Sea [ Figure 7.1 ]. With its flat and low-elevation delta plain [ Minderhoud et al., 2019 ], Viet Nam’s Mekong Delta (VMD) is highly sensitive to hydrological changes and global sea level rise, and therefore appears to be a “hotspot” of climate change impacts. It covers an area of about 4 million hectares, crossed by a complex network of rivers and canals [ Figure 7.2 ], and is currently home to 17 million inhabitants, i.e. about

[ Figure 7.1 ]

The Mekong River Basin and the Mekong Delta with its estuarine distribution network

[ Figure 7.2 ] The Mekong Delta’s flood zones and its complex canals network

Source: Thanh et al., 2020.

18% of Viet Nam’s population [ GSOVN, 2020 ]. Their livelihoods depend mainly on agricultural and aquaculture production. Often described as the “rice bowl” of Viet Nam (see Chapter 4), the VMD supplies more than half (~56%) of the country’s rice. production, 90% of which is exported [ GSOVN, 2020 ]. Hence, the region is crucial for both national and international food security. However, agriculture and aquaculture in the delta strongly depends on water quality and availability. Water-related issues, including floods [ MRC, 2009 ], droughts and saline water intrusions [ Nguyen et al., 2021 ], are already challenging livelihoods in the delta and are expected to worsen in future decades, due to climate change and anthropogenic pressures. Further understanding projected climate change and the environmental impact of human activities is thus crucial to set up mitigation and adaptation strategies.

Different studies have already highlighted the vulnerability of the VMD to climate change [ e.g. Van et al., 2012; Tri et al., 2013; Chapman

and Darby, 2016 ]. Several climate projections for the Mekong basin have been provided by global circulation models (GCM), and dynamical or statistical downscaling (see Chapter 1). Some of these projections have been used to assess the impacts of precipitation and temperature changes on the hydrological regime of the Mekong river, using hydrological modelling [ e.g. Kite, 2001; Yoshimura et al., 2009; Lauri et al., 2012; Hoang et al., 2015; Wang et al., 2016 ] . Compared to 1971–2000, total upstream flow at Kratie is projected to increase by 3–8% under climate scenario RCP4.5, and by -7% to +11% under the RCP8.5 scenario over the 2036–2065 period [ Hoang et al., 2015 ]. Extreme flow occurrences are also expected to increase.

On the other hand, human activities in the delta or upstream in the Mekong basin have been shown to have strong impacts on the hydrology and sedimentology of the delta, including changes in the river flow and sediment trapping by upstream dams [ Arias et al., 2014a; Kondolf et al., 2014; Eslami et al., 2021a ], excessive sand mining in the riverbeds [ Bravard et al., 2013; Eslami et al., 2019b; Jordan et al., 2019; Hackney et al., 2020 ], groundwater over-extraction [ Minderhoud et al.; 2017 ], fast-growing urbanization and surface drainage causing increased compaction of shallow sediments [ Minderhoud et al., 2018 ], and dyke-building to control natural flooding which impedes sedimentation [ Thanh et al., 2020 ]. These factors may exacerbate climate change impacts by causing land subsidence, for example, and enhance salinization, which may pose a greater threat in the short- to mid-terms [ Chapter 9 ].

The VMD has long historical experience of dealing with water- and climate-related factors that can threaten agricultural production (mainly floods, droughts and associated salinity intrusions, tropical cyclones and water pollution). A wide range of coping or adapting measures has been deployed at different levels. For instance, the provinces in the VMD have built and planned hydraulic structures to cope with floods and salinity intrusion [ Thanh et al., 2020 ], while the farmers may change their practices to adapt to these phenomena. However, the increasing impacts of climate change and anthropogenic activities challenge the sustainability of the different measures implemented so far.

As an introduction to the chapters focusing on the Mekong Delta in the present report, this chapter (i) presents the main geological and hydrological characteristics of the delta; (ii) analyses updated climate change projections for the delta; (iii) presents the anthropogenic pressures on the sedimentary regime and water resources; and (iv) reviews the measures currently being implemented to address water-related problems in the VMD. Some lessons learnt in adapting to climate-related problems will be presented, to contribute to future adaptation strategies. In addition, we also highlight opportunities for mitigating the anthropogenic pressures that have created undesired environmental changes in the Delta.