Abstract
Abstract. The Red River (Vietnam) is representative of a south-east Asian river system, strongly affected by climate and human activities. This study aims to quantify the spatial and seasonal variability of CO2 partial pressure and CO2 emissions of the lower Red River system. Water quality monitoring and riverine pCO2 measurements were carried out for 24 h at five stations distributed along the lower Red River system during the dry and the wet seasons. The riverine pCO2 was supersaturated relative to the atmospheric equilibrium (400 ppm), averaging about 1589±43 ppm and resulting in a water–air CO2 flux of 530.3±16.9 mmol m−2 d−1 for the lower Red River. pCO2 and CO2 outgassing rates were characterized by significant spatial variation along this system, with the highest values measured at Hoa Binh station, located downstream of the Hoa Binh Dam, on the Da River. Seasonal pCO2 and CO2 outgassing rate variations were also observed, with higher values measured during the wet season at almost all sites. The higher river discharges, enhanced external inputs of organic matter from watersheds and direct inputs of CO2 from soils or wetland were responsible for higher pCO2 and CO2 outgassing rates. The difference in pCO2 between the daytime and the night-time was not significant, suggesting weak photosynthesis processes in the water column of the Red River due to its high sediment load.
Highlights
Natural hydrological processes and biogeochemistry of many rivers in the world have suffered from the influences of climate change and human activity in their drainage basins
The riverine water was supersaturated with CO2 in contrast to the atmospheric equilibrium (400 ppm), with pressure of CO2 (pCO2) values averaging about 1589 ± 43 ppm, resulting in a water–air CO2 flux of 550.3 ± 16 mmol m−2 d−1 from the lower Red River system
The pCO2 from the water surface of the lower Red River network was characterized by significant spatial variation, being highest at the Hoa Binh Dam downstream and in the main stem at Hanoi station
Summary
Natural hydrological processes and biogeochemistry of many rivers in the world have suffered from the influences of climate change and human activity in their drainage basins. Riverine carbon fluxes and outgassing are important parts of the carbon exchange among terrestrial, oceanic and atmospheric environments. Raymond et al (2013) estimated a global evasion rate of 2.1 Pg C yr−1 from inland waters and that global hotspots in stream and rivers which occupy only 20 % of the global land surface represented 70 % of the emission. They emphasised that further studies are needed for identifying the mechanisms controlling CO2 evasion on a global scale
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