CHAPTER 9 I A QUESTION OF WATER I 421
Riverbed level incision (~15 cm/year) [ Binh et al., 2018; Eslami et al., 2019b; Vasilopoulos et al., 2020 ] is responsible for the increased salt intrusion length in the delta, while the anthropogenically-altered hydrological regime [ Chapter 7 ] caused by upstream hydropower operations is responsible for the frequently observed longer periods of salinity. Sediment starvation due to upstream entrapment of over 350 tributary and 14 mainstream dams, and large amounts of sand mining [ Figure 9.10 ] in Viet Nam, Cambodia, and Laos (several times larger than the total fluvial sediment
4. The delta’s future 4.1 Elevation loss The delta is losing elevation in relation to sea level due to the combination of climate change-induced sea level rise [ Chapter 1 ], subsidence resulting from both natural and anthropogenic processes, and reduced sediment deposition. As the delta has an extremely low elevation above the sea, relative sea level rise poses a major and urgent threat to the delta. Projections of elevation are important to assess future flood exposure, saline water intrusion and potential permanent inundation as parts of the delta fall below sea level. Although subsidence is related to a large number of drivers and processes, only large-scale regional processes need to be considered, since local processes — such as sinking of individual objects or buildings — do no contribute to general delta elevation loss. As described above, the two main subsidence processes are natural shallow compaction and extraction-induced aquifer-system com-
supply [ Bravard et al., 2013; Brunier et al., 2014; Eslami et al., 2019b; Jordan et al., 2019 ] have resulted in the observed riverbed level incisions that drive salinity further inland [ Eslami et al., 2021a ]. Furthermore, historically, Lake Tonle Sap provided freshwater to the delta until late January. But with the observed peak water level decline in the Mekong River, Lake Tonle Sap does not store water to its capacity, and often drains by mid-late December, resulting in the maximum dry season salinity increase taking place earlier than before, and the delta experiencing a longer period of salinity.
paction, with the latter being the dominant driver of present-day subsidence. The effect of potential future groundwater extraction and consequent subsidence was modelled using the aforementioned 3D model, following several mitigation and non-mitigation scenarios of groundwater extraction during the XXIst century [ Figure 9.11 ]. The amount of future extraction-induced subsidence will strongly depend on the amount of future groundwater extraction, with much lower subsidence rates associated with stronger reductions in extraction. It also became clear that in the coming decades, almost regardless of the level of mitigation, extraction-induced subsidence will outpace global sea level rise in the VMD. However, in the second half of the century, the mitigation scenarios show equal or lower rates of subsidence in comparison to gradually increasing sea level rise. For the VMD, this means that directing efforts on mitigating subsidence by reducing groundwater extractions is the most efficient way to reduce relative sea level rise in the near future.
