Present and future dynamics of salt intrusion in a mega-Delta under climatic and anthropogenic drivers

We show a novel approach to develop representative environmental pathways at scale of a mega-delta. By integrating climatic, anthropogenic, local, and regional drivers of exposure and vulnerability, this work provides a blueprint for similar studies in other deltas. Deltas, among the most vulnerable coastal bodies, in response to global climate change, require downscaled environmental pathways to develop effective adaptation policies. However, climate change adaptation planning in coastal and lowland areas cannot be independent of the local drivers of exposure and vulnerability. Very often, domestic, or regional anthropogenic processes play an equal or perhaps even larger roles in the observed trends. The Mekong Delta, the 3rd largest delta in the world, has been subject to various academic studies and experimental adaptation measures. This is, on one hand, due to its importance to domestic and regional livelihood of millions of people, and, on the other hand, due to its physical complications and vulnerabilities. However, very few of these studies or adaptation measures incorporated a comprehensive vision with respect to climate change. Saline Water Intrusion (SWI) has been the greatest challenge, stressing freshwater supply to the rice bowl of Vietnam. Previous projections of SWI, in response to climate change, have already been exceeded during the recent drought events. This is mainly because preceding studies did not integrate the wide range of physical drivers other than sea level rise, such as, amongst others, upstream discharge anomalies, land subsidence, and riverbed level changes. Building on the previous works, and incorporating the results of a 7-year research program, we have integrated the results of a 3D surface water baroclinic model at scale of the Mekong Delta, a 3D hydrogeological model of the delta, five different global and one regional climate model, and abundance of collected unique datasets, to project different scenarios of SWI. The approach integrates sea level rise, upstream discharge anomalies, spatio-temporal variation of extraction-induced land subsidence, and riverbed level changes in response to anthropogenic sediment starvation. The results provide a comprehensive image of environmental change in the Mekong Delta, paving way to develop dynamic adaptation pathways and strategies. Disentangling the impacts of various drivers of exposure and vulnerability provides invaluable insight to address priorities to develop effective adaptation strategies for the policy makers. Our research shows that depending on the policy development, in the first half of the century, the anthropogenic processes dominate environmental change in the delta, while in the second half of the century, climate change and accelerated sea level rise poses the greatest risks to the delta.