Gas transfer velocities of CO2 in subtropical monsoonal climate streams and small rivers

Abstract. CO2 outgassing from rivers is a critical component for evaluating riverine carbon cycle, but it is poorly quantified largely due to limited measurements and modeling of gas transfer velocity in subtropical streams and rivers. We measured CO2 flux rates and calculated k and partial pressure (pCO2) in 60 river networks of the Three Gorges Reservoir (TGR) region, a typical area in the upper Yangtze River with monsoonal climate and mountainous terrain. The determined k600 (gas transfer velocity normalized to a Schmidt number of 600 (k600) at a temperature of 20 ∘C) value (48.4±53.2 cm h−1) showed large variability due to spatial variations in physical processes related to surface water turbulence. Our flux-derived k values using chambers were comparable with k values using the model derived from flow velocities based on a subset of data. Unlike in open waters, e.g., lakes, k600 is more pertinent to flow velocity and water depth in the studied river systems. Our results show that TGR river networks emitted approx. 0.69 to 0.71 Tg CO2 (1 Tg =1012 g) during the monsoon period using varying approaches such as chambers, derived k600 values and models. This study suggests that incorporating scale-appropriate k measurements into extensive pCO2 investigations is required to refine basin-wide carbon budgets in subtropical streams and small rivers. We concluded that the simple parameterization of k600 as a function of morphological characteristics is site specific for regions and watersheds and hence highly variable in rivers of the upper Yangtze. k600 models should be developed for stream studies to evaluate the contribution of these regions to atmospheric CO2.