5. Main conclusions and policy implications

tal pathways in the second half of the century. The effect of land subsidence is more local, and especially impacts the coastal provinces that are already experiencing salinity. Assuming a stable water-/coastline, its impact on the aggregate areas affected by salinity is expected to be limited. Note that the decelerated all-drivers increase in salinity between 2040–2050 is mainly due to the assumption that riverbed levels will stabilize beyond 2040 (see Eslami et al. 2021b for further detail).

This report integrates the effects of climatic and anthropogenic drivers of change within the Mekong Delta. It addresses in detail the past, present and future dynamics of two main trends: 1 ] elevation loss (land subsidence + sea level rise) and 2 ] increased salt intrusion in the surface water system.

Under existing rates of sea level rise (~3 mm/ year) and climate scenarios, current rates of domestic anthropogenic groundwater extraction-induced land subsidence (~10–40 mm/ year, spatially varying) exceed the impact of global climate change. Depending on the policy development and enforcement, as well as the sea level rise acceleration rates towards second half of the century, a large part of the delta’s sub-aerial surface may descend permanently below mean sea level.

Furthermore, the effect of current climate change impact on increased salt intrusion has been shown to be limited. In the first half of the century, the increased river discharge during the dry season can partially counterbalance salt intrusion, especially in the estuarine channels. However, in coastal provinces distant from the river, salinity will still increase (0.1 PPT/year). While land subsidence marginally impacts salt intrusion in the surface water, sediment starvation — driven by upstream hydropower development and sand mining (within and beyond the delta), resulting in riverbed level incisions — has been the biggest driver of increased salt intrusion over the past decade, and will perhaps continue in the first half of the century. By our best estimates, we conclude that in the next three decades, environmental change in the delta will be dominated by human-induced activities and the exploitation of natural resources (sand and water). However, depending on the state of human activities in the next two decades, from the mid-century on global climate change and [potentially] accelerated sea level rise may become the main agents of change. Therefore, adaptation and mitigation policies in the coming decade are fundamental to the fate of the delta’s livelihood and the cost of adaptation to climate change.

5.1 Main conclusions

uThe VMD is subject to various drivers of change, of which the anthropogenic drivers — namely hydropower dams, sand mining, land-use change, and groundwater extraction — pose the greatest threats in the first half of the century, while climate change

effects such as fluvial discharge anomalies and higher frequency of extreme events are expected to become larger threats in the second half of the century.

uThe VMD has an extremely low-lying delta plain, with an average elevation of about ~0.8 m above local mean sea level; ~30% of the delta plain will fall below sea level with a relative sea level rise of 50 cm. DEMs derived from satellite data tend to overestimate the true elevation of the VMD, which previously led to underestimation of the delta’s exposure to sea level rise.

uLand subsidence in the VMD has accelerated over the past two decades due to human activities. Current rates of subsidence exceed global sea level rise by an order of magnitude, ranging from 10 mm/year to locally more than 50 mm/year. Although partly caused by natural compaction, much of the accelerating subsidence rates results from groundwater extraction, which has increased 5-fold over the last 20 years.

uLand subsidence together with global sealevel rise determine relative, or experienced, sea level rise. In the coming decades, future relative sea level rise will predominantly be determined by the amount of subsidence, which for a large part depends on human-controlled groundwater extraction. Extraction-induced subsidence alone has the potential to cause large parts of the delta to fall below sea level within coming decades. Should the rate of extraction remain at present-day levels, the average cumulative subsidence could be larger than 80 cm in 2100. Assuming sedimentation will counterbalance natural compaction and combined with a mid-range absolute sea level rise projection, groundwater extraction-induced subsidence can cause the majority of the delta to fall below sea level by the end of this century.

uWhile land is subsiding (~1 cm/year) and sea level is rising (~3 mm/year), tidal amplitudes in the delta (especially within the Tien River) have been increasing at alarming rates of 1–2 cm/year. This, together with higher subsidence rates in urban areas (~2–4 cm/year), has a considerable impact on exacerbated city flooding. Tidal amplification (increase in both vertical tidal water level and horizontal tidal discharge), driven by riverbed incisions (10–20 cm/year), is also expected to have significant impact on riverbank erosion, which remains to be studied in further depth.

uSalt intrusion is a growing concern, and salinization has been an increasing trend over the past two decades (0.2–0.5 PSU/year in the coastal areas). Among other drivers such as sea level rise, discharge anomalies and land subsidence, the main driver of the increase in observed salt intrusion over the past 20 years has been riverbed level changes (in rates of 10–15 cm/year) caused by sediment starvation from upstream dams and excessive sand mining.

uRelative sea level rise will most probably cause only moderate increases in the salt balance in the coastal provinces during the first half of the century. Climate change-driven variations have limited impact on salt intrusion (5–6% increase in salinity affected areas). Anthropogenic extraction-induced land subsidence can further increase salinity in the coastal provinces and add an additional 2–6% to areas affected by salinity. Anthropogenic riverbed level incision remains the main driver of salinization of the delta and can impact an additional 10–25% of land in the delta. The

only exception to this conclusion is the case of extreme acceleration of sea level rise (e.g., 50–60 cm by 2050). In the second half of the century, relative sea level rise may become the dominant driver of salt intrusion.

5.2 Policy implications

u Elevation as a natural asset: Elevation of the delta, whether sub-aerial (land) or subaqueous (submerged riverbeds), should be considered as a valuable natural asset/ resource to be protected rather than exploited. The lower the delta’s land and riverbeds, the more vulnerable it is to natural hazards. While “Resolution 120” restricts groundwater and sand exploitation, the scientific evidence demonstrates a much shorter window (than the projected government plans) to implement the changes required to control natural resource exploitation.

u Mitigating subsidence through groundwater management strategies: The rate of relative sea level rise in coming decades will largely be determined by the effectiveness of mitigating human-induced subsidence, mainly by reducing groundwater extraction. This requires finding alternative fresh water sources to meet demand. In addition, smart extraction strategies — e.g., relocating extraction to areas that are of higher elevation or less sensitive to subsidence — may moderate the impact. Managed aquifer recharge (MAR) and aquifer storage and recovery (ASR) strategies — i.e. storing harvested rainwater or river water underground during wet season and recovering it during dry season, — may also help to speed up the recovery of water levels in the aquifers, thereby mitigating further extraction-induced subsidence. u Controlling sand mining: Sediment starvation is one of the greatest threats to the livelihood of the delta. Riverbed, bank and coastal erosion and, consequently, saline water intrusion and partial elevation loss are driven by sediment starvation. Sand mining in the extraction volumes currently practised is a major contributor to sediment starvation. Its harmful effects are not only local, but also ripple upstream and downstream (in the tidal system). Enforced regulations seem to be an inevitable choice that needs to be implemented in the short term, for otherwise the irreversible effects of this practice will force policy makers to bring it to an abrupt halt. While halting all sand mining practices may be difficult at short notice, a search for recycling technologies and alternative sources of sand/construction material must be vigorously pursued to minimize the waste and assure efficient use of construction materials.

u Reinstating flooding in the delta: While uncontrolled flooding in cities should be avoided, controlled flooding in the floodplains can be significantly beneficial. As previous priorities of the delta were to maximize food production, a shifting paradigm towards sustainability is required. While empoldering and diking were popular in the late 1990s and early 2000s, the paradigm has to smoothly shift to reinstate natural and controlled flooding of the floodplains. Flooding helps the deltaic systems in at least four ways: (a) transport of new sediment to the delta surface to build elevation, (b) it is a major source of nutrients to the floodplains, (c) it creates ecological succession zones with high biodiversity during seasonal flooding, as well as fish spawning habitats, and (d) while within the VMD, city flooding is driven by a combination of land subsidence [ Minderhoud et al., 2017 ] and tidal amplification [ Eslami et al., 2019b ], inundated flood

plains form buffer zones that lower flood pulse water levels, thereby reducing undesired flood risk in cities.

uAim for multi-processes and integrated solutions in delta systems and at the river basin level rather than individual problems. The physical geography of the Mekong River Basin and the Vietnamese Mekong Delta is a highly interconnected one, which calls for multi-disciplinary, multi-sectorial and integrated adaptation/mitigation strategies. Failure to acknowledge this and adopting only incremental measures to remedy isolated problems will likely shift the challenges from one aspect to another. For example, addressing riverbank erosion in isolation from its main drivers, or independently of riverbed and coastal erosion, will likely worsen the other two (note that further riverbed erosion will significantly exacerbate saline water intrusion). Similarly, large-scale development of sea dikes will cut off the marine sediment supply to the coastal hinterland that is highly dependent on this supply to compensate for the high natural compaction rates.

u Active diplomacy towards a joint regime of trans-boundary river resource management (discharge, water level and sediment). Trans-boundary natural resources of the Mekong River are managed in isolation. While there is some regulation on the amount of freshwater discharge, there is limited regulation on sediment and water level management. These latter two parameters are equally important in river/delta management, as lack of sediment has been the culprit for much of the unfavourable environmental change and water level anomalies that have modified the hydrological regime, and thus the duration of the dry season. This active diplomacy could be through empowering the Mekong River Commission (MRC) as a true and strong moderator in managing the trans-boundary resources of the Mekong River.

u In light of the above, perhaps one of the first potential solutions to dry season salt intrusion would be to form an alliance with Cambodia, which is equally dependent on the ecosystem of Lake Tonle Sap, to restore the lake’s water levels so that it can act as a retention area during the dry season for the Mekong Delta.

u Transformation of farming practices to efficiently manage available surface freshwater resources. Analysis shows that [ Eslami et al., 2021b ] a 25% reduction in surface water demand can help reduce ~100 103 ha of areas affected by salt intrusion up to the year 2050. Therefore, efficient use of surface water in irrigation planning — as one of a raft of adaptation/mitigation measures — could make a significant contribution to reducing the impact of salt intrusion.

u While promoting any kind of sedimentation (organic/inorganic), mixed models of sustainable rice and aquaculture, or shrimp-mangrove farming could significantly benefit the food, physical and ecological resilience of the delta and its population. By promoting flooding of the farms, bringing suspended sediment to the fields during the wet season, and shifting to brackish aquaculture in times of limited freshwater — while adapting to the more saline condition — the VMD can restore sedimentation in the delta at the individual farm level. Such sustainable farming should, however, be implemented without aggravating environmental quality. Conjunctive use of rain and surface water, as it has long been practiced in the coastal provinces of the VMD [ Özdemir et al., 2011; Thuy et al., 2019 ], can

reduce groundwater extraction for domestic consumption.

u Adaptation measures such as mixed farming practices may have challenges in terms of water quality. However, these measures also normally function best at a large scale, where farmers’ solidarity helps maintain water quality in the delta at sustainable levels by reducing the use of toxic chemicals and encourages sustainable community farming.