Exploring Freshwater Regimes and Impact Factors in the Coastal Estuaries of the Vietnamese Mekong Delta

Freshwater resources make an essential contribution to the livelihoods of millions of local people in the coastal estuaries of the Vietnamese Mekong Delta (VMD). However, coastal freshwaters currently face numerous threats, not least (i) changing tidal dynamics due to sea level rise and (ii) changes in river regimes due to dam construction upstream. This research explores the evolution of freshwater regimes in these coastal estuaries. Using process diagrams, freshwater distributions are mapped and analyzed. Application of statistical methods provides insight into freshwater flow cycles and variations in water regimes upstream at various measurement points within the estuaries. A previously calibrated and validated hydraulic model is used to simulate drought-year scenarios and spatial changes in freshwaters over time. Findings indicate decreasing river discharges in the flood season, but increasing discharges in the dry season, due to the impacts of hydropower dams. In addition, the driest months are shifting earlier. From this data, we derive rules of thumb regarding freshwater distributions in the coastal estuaries of the VMD. These relate to (i) the boundary beyond which freshwater is always found; (ii) the boundary where freshwater appears daily; (iii) the start of the freshwater season; (iii) the boundary where freshwater appears until February and until April; (iv) the end of the flood season; and (v) the number of days without freshwater per year. The trends discerned will help local freshwater users and decision makers formulate forward-looking, flexible strategies for freshwater exploitation, while also providing avenues for further research.