Cumulative impacts of interbasin water transfer and salinity intrusion on water security in central Vietnam

Saltwater intrusion poses a significant threat to freshwater availability in downstream regions globally, leading to economic losses, compromised water supply, reduced agricultural productivity, social instability, and environmental degradation. This study assesses the extent and drivers of salinity intrusion in the downstream areas of the Vu Gia Thu Bon River basin, central Vietnam, with a particular focus on Da Nang City. The analysis considers the impacts of upstream dam operations and inter-basin water transfers on water supply and irrigation, encompassing both direct and indirect damages. Utilizing the Soil and Water Assessment Tool (SWAT) model, long-term field observations of water quantity and quality, and statistical methods, we investigated the combined effects of anthropogenic activities and natural factors on hydrological alterations, salinity intrusion, and environmental changes from headwaters to estuarine regions. Our findings reveal a significant decrease in flow discharge and water levels along the Vu Gia River, despite an observed increase in precipitation. Specifically, annual flow discharge decreased by 36.9 % at Thanh My and 16.5 % at Ai Nghia hydrological stations compared to pre-dam periods. Additionally, the annual water level at Ai Nghia station declined by 0.32 m, from 3.34 m to 3.10 m. These hydrological changes have led to frequent exceedances of salinity thresholds during the dry season, compromising freshwater availability for various uses. Rising sea levels further exacerbate water quality degradation, affecting parameters such as pH, dissolved oxygen (DO), chemical oxygen demand (COD), biological oxygen demand (BOD), and coliform levels, thereby impacting crop yields. The study underscores the necessity for integrated water resource management strategies, including adaptive reservoir operations and real-time salinity monitoring, to mitigate the adverse effects of salinity intrusion. The results serve as a foundation for developing decision-support tools aimed at controlling salinity intrusion in both the short and long term.