5. Discussion and conclusions

et al., 2014; Minderhoud et al., 2020b) (Chapter 9 deals with delta elevation and land subsidence in greater detail).

In addition to causing land subsidence, groundwater pumping also increases groundwater salinization, as freshwater reserves are being replaced by salt water, causing additional loss of freshwater. As a result, fresh groundwater reserves are rapidly decreasing.

4.3 Elevation loss in the delta

Over the past decades, the factors controlling the natural dynamics of relative delta elevation have changed drastically in the VMD. This is the result of a combination of climatic and anthropogenic impacts. Increased flood control and sediment starvation have decreased the amount of sediments delivered to channels and then to flood plains [ Kondolf et al., 2014; Li et al., 2017 ], while global warming increases global sea level rise. Meanwhile, human activities in the delta have enhanced natural subsidence [ Zoccarato et al., 2018 ] and created additional human-induced subsidence [ Minderhoud et al., 2017, 2018 ] [ see Chapter 9 ]. Subsidence is occuring throughout the Delta, at rates of up to several centimetres per year, and up to 5 cm/ year in places [ Erban et al., 2014; Minderhoud et al., 2020a ]. And this subsidence is no longer counterbalanced by the deposition of new sediments. In consequence, the Delta is currently experiencing rapid elevation loss [ Minderhoud et al., 2020b ]. As subsidence and consequential relative sea level rise act as amplifiers of other processes — including flood exposure — erosion and salinization are occuring at increasing rates in the Delta [ Tamura et al., 2020 ].

5. Discussion and conclusions

The VMD has extensive experience in mitigating and adapting to environmental changes. Figure 7.15 presents the evolution of adaptation to environmental problems in agricultural production since 1986 after the introduction of the “doi moi” (renovation) policy.

Water management in the VMD has historically followed the “Dutch dyke” strategy. It includes structures of dykes that encircle settlement and rice fields [ Biggs et al., 2009 ]. In the “doi moi” policy, agriculture and aquaculture were intensified and grown rapidly to supply food for local inhabitants and Viet Nam. A dense canal network was developed [ Figure 7.2 ] in order to drain floods to the West Sea, and to clean acid sulphate soils. However, despite this strategy, the rice crops in the upper VMD were still damaged and destroyed by annual floods from August to October. The historic flood of 2000, in particular, caused enormous damage to infrastructure, residents’ properties and agricultural crops. After this hugely damaging flood, rice crops were protected by a dyke system that consists of both low dykes and high dykes [ Thanh et al., 2020 ]. The VMD provinces rarely constructed high dykes before 2000 [ Duong et al., 2016 ]. Since 2000, the upper VMD (An Giang and Dong Thap provinces) has built and planned high dykes to protect rice crops in the entire provincial area. Until 2009, the area protected by high dykes was about 1,222 km2, covering around 35% of the An Giang province area, and

[ Figure 7.15 ] Evolution of climate-related events and adaptation measures

Source: Duc et al., 2019.

this percentage increased to over 40% [ about 1,431 km2) in 2011. Dong Thap has a much lower coverage of about 30%, corresponding to an area of 990 km2 .

At first, the dyke system was positive for rice cropping. However it is now recognized that dykes also have significant negative effects (see Chapter 9). First, they prevent the floodplains from playing their natural buffer role against flooding, deflecting and aggravating the floodpeaks [ Triet et al., 2017; Tran et al., 2018; Thanh et al., 2020 ] Tran et al., 2018 found that flood peaks would be drastically increased if high dykes were built in all the VMD floodplains. Secondly, high dykes inhibit the enrichment of agricultural soils with sediments and nutrients in the VMD floodplains, decreasing soil fertility and enhancing elevation loss. Hence, benefits in some places for agriculture are made at the expense of other communities or other sectors. Moreover, dykes may create a false sense of security, leading to more buildings and activities in flood-prone areas and then high damage if dykes are overtopped or fail. For example, some high dykes were broken in the 2011 flood season: 49 people were killed, and over

one hundred houses were flooded [ Sikkema and van Halsema, 2019 ].

Recently, the VMD has seen increasing trends of salinity, and a higher frequency of extreme droughts and salinity intrusion during the dry seasons, with strong negative impacts on rice production in the coastal VMD. The observed trends are heavily linked to anthropogenic sediment starvation due to upstream trapping and sand mining, as well as the hydrological regime shift driven by hydropower operations [ Eslami et al., 2019b ]. About 45% of the Delta area, approximately 1.77 million ha, is affected by salinity intrusion during the dry seasons [ Vo, 2012 ]. In order to protect freshwater crops in this area, the central government continues to construct dykes and sluice gates to prevent saline water intrusions in coastal provinces, but this is a high-cost measure. Additionally, the operation of saltwater prevention structures may cause pollution of the surface water resource. For instance Phuong et al. (2017) found that chemical oxygen demand (COD) concentrations outside the Ba Lai sluice gate, which closes off one of the former nine river branches in Ben Tre province, are much higher than those inside the sluice gate.

Hard adaptative measures to the aforementioned environmental changes may be effective in terms of agricultural production, but are not sustainable from a social, economic or environmental point of view. Therefore, a holistic approach is required to deal with future environmental changes and projected climate change. Recently, Resolution 120/ NQ-CP 2017 on Sustainable and Climate Resilient Development of the Mekong Delta was released. This resolution considers 8 different issues: traffic, education, rivers, connectivity, wealth, talent, ageing, and gender. It is currently detailed by the Decision 1163/QD-TTg 2020 on approving the task of planning the Mekong Delta for the period 2021–2030, with a vision to 2050 issued by the Prime Minister. The national plan to adapt to climate change includes three groups of tasks and solutions:

uFirst, improving the effectiveness of climate change adaptation by strengthening the state’s management capacity on climate change, and promoting the integration of climate change adaptation into strategic and regulatory system plans.

uSecond, enhancing resilience and adaptive capacity for communities, economic sectors, and ecosystems through investment in adaptation actions, science, and technology to be prepared to adapt to climate change.

uThird, boosting disaster risk reduction and damage reduction, along with preparedness to respond to natural disasters and increased climate extremes due to climate change, such as via the establishment of regional coordination institutions and mechanisms for sustainable development in the Mekong Delta. Alongside this, promoting the training and development of human resources, scientific research, and technological development, as well as strengthening international cooperation and strategic communication to raise public awareness of the impacts of climate change.

Based on the guidance of Resolution 120, we propose an approach of participatory planning and integrated management for sustainable agriculture and aquaculture in the Mekong Delta. The approach includes the following steps :

uConduct detailed assessments of climate change impacts at local levels. Then propose appropriate transformation;

uUndertake assessment of the social, economic and environmental impacts of land use transformation;

uApply new/modern technologies on monitoring, data analysis and visualization to support stakeholders in preparing production, investment, and business planning in each sub-biophysical land unit.

uInform the state management agency about the production situations, challenges, and opportunities to bring supportive solutions to farmers and businesses.

uBuild integrated information systems that support stakeholder engagement in addressing production issues in the context of globalization and climate change.