Long-term spatial and temporal variation of CO₂ partial pressure in the Yellow River, China

Abstract. Carbon transport in river systems is an important component of the global carbon cycle. Most rivers of the world act as atmospheric CO₂ sources due to high riverine CO₂ partial pressure (pCO₂). By determining the pCO₂ from alkalinity and pH, we investigated its spatial and temporal variation in the Yellow River watershed using historical water chemistry records (1950s–1984) and recent sampling along the mainstem (2011–2012). Except the headwater region where the pCO₂ was lower than the atmospheric equilibrium (i.e. 380 μatm), river waters in the remaining watershed were supersaturated with CO₂. The average pCO₂ for the watershed was estimated at 2810 ± 1985 μatm, which is 7-fold the atmospheric equilibrium. As a result of severe soil erosion and dry climate, waters from the Loess Plateau in the middle reaches had higher pCO₂ than that from the upper and lower reaches. From a seasonal perspective, the pCO₂ varied from about 200 μatm to > 30 000 μatm with higher pCO₂ usually occurring in the dry season and lower pCO₂ in the wet season (at 73% of the sampling sites), suggesting the dilution effect of water. While the pCO₂ responded exponentially to total suspended solids (TSS) export when the TSS concentration was less than 100 kg m⁻³, it decreased slightly and remained stable if the TSS concentration exceeded 100 kg m⁻³. This stable pCO₂ is largely due to gully erosion that mobilizes subsoils characterized by low organic carbon for decomposition. In addition, human activities have changed the pCO₂ dynamics. Particularly, flow regulation by dams can diversely affect the temporal changes of pCO₂, depending on the physiochemical properties of the regulated waters and adopted operation scheme. Given the high pCO₂ in the Yellow River waters, large potential for CO₂ evasion is expected and warrants further investigation.