Quantifying the impacts of hydraulic infrastructure on tropical streamflows

Hydraulic infrastructures, such as reservoirs and water diversion channels, cause altered streamflow worldwide. Therefore, this study aimed to assess the coupled impacts of reservoir operations and water transfer on downstream streamflow over 42 years (1979-2020) for a tropical river in Vietnam, the Vu Gia Thu Bon (VGTB). We also quantified variations in the multi-sub-basin contributions to the water budget associated with hydraulic structure development. Therefore, a semi-distributed hydrological model, SWAT (Soil and Water Assessment Tool), was developed for the entire VGTB basin considering two plausible scenarios: with a dam and without a dam. In this study, reservoirs substantially affected the streamflow during the 2011-2020 period when 12 cascading hydropower dams were constructed in the Vu Gia sub-basins. The cascading reservoirs across the Vu Gia River reduced the annual average streamflow by 28.1% during this period, whereas their influence was augmented by 13.9% at reaches further downstream. In contrast, the local reservoir and flow diversions created on the Thu Bon River resulted in a 6.5% increase in streamflow. The upstream reservoir operation significantly increased streamflow at the midstream stations by 27.8% compared to the no-dam period. The streamflow decreased in the dry season by 5.6% in the Vu Gia sub-basins and increased by 61.7% in the Thu Bon sub-basins. However, the impacts decreased in the wet season by 41.3% due to the operation of reservoirs, in which Dak Mi 4 had the most significant influence. It was found that the water diverted to the Thu Bon River was governed and reduced by the cascading hydropower dams. Therefore, the operation of 11 reservoirs has partially compensated for the lost water in the Vu Gia sub-basins, to which the Dak Mi 4 plant has transferred 19.7 m(3)/s (14%). Our findings classify the impact of cascading dams and diversion structures and their interaction with climate change.

Automated summary

This study assessed the combined impacts of reservoir operations and water transfer on downstream streamflow in a tropical river in Vietnam. The results showed that the reservoirs significantly reduced the annual average streamflow in the Vu Gia sub-basins, while increasing it in the Thu Bon sub-basins. The impacts varied across different seasons and locations.