Salinity intrusion dynamics in the Vietnamese Mekong Delta: Combined effects of upstream development, climate change, and coastal engineering

Study region: Vietnamese Mekong Delta (VMD) in Southern Vietnam Study focus: This study assesses and predicts spatial and temporal salinity intrusion dynamics under changing inflows, climate scenarios, and sea-level rise. Using the MIKE 11 hydrodynamic model, calibrated with 2016–2020 salinity data, dry-season (February–May) intrusion was simulated across nine scenarios: a historical 1998 baseline (SCE1), 2020 present conditions (SCE2), projected sea-level rise (SCE3) under RCP4.5 and RCP8.5 (SCE4–5), and a future combined upstream-development package that integrates full hydropower operations with moderate or high irrigation expansion (SCE6–9). Three additional scenarios (SCE7-G1–G3) tested the effects of salinity-regulating sluice gates. New hydrological insights for the region: Compared to the historical baseline SCE1 and the present-day scenario SCE2, projected scenarios exhibit higher salinity, stronger penetration, and earlier arrival times due to upstream alterations. The most severe regional degradation occurs when climate-driven sea-level rise directly coincides with intensive upstream water diversions, with the combined drivers under RCP4.5 paradoxically generating a larger area enclosed by the 2 PSU interpolated river-salinity surface than the higher-emission RCP8.5 due to lower projected dry-season inflows. Furthermore, the simulations expose an important hydrodynamic trade-off in coastal engineering, while localized sluice gates successfully mitigate salinity on a local scale, restricting these natural channels alters tidal distribution and can inadvertently intensify saltwater encroachment in adjacent, unfiltered river branches.