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

• Main Authors: S. Li, R. Mao, Y. Ma, V. V. S. S. Sarma
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-02-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/16/681/2019/bg-16-681-2019.pdf
• ISSN: 1726-4170; 1726-4189

<p><span class="inline-formula">CO₂</span> 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 <span class="inline-formula">CO₂</span> flux rates and calculated <span class="inline-formula"><i>k</i></span> and partial pressure (<span class="inline-formula"><i>p</i>CO₂</span>) 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 <span class="inline-formula"><i>k</i>₆₀₀</span> (gas transfer velocity normalized to a Schmidt number of 600 (<span class="inline-formula"><i>k</i>₆₀₀</span>) at a temperature of 20&thinsp;<span class="inline-formula">^∘</span>C) value (<span class="inline-formula">48.4±53.2</span>&thinsp;cm&thinsp;h<span class="inline-formula">⁻¹</span>) showed large variability due to spatial variations in physical processes related to surface water turbulence. Our flux-derived <span class="inline-formula"><i>k</i></span> values using chambers were comparable with <span class="inline-formula"><i>k</i></span> values using the model derived from flow velocities based on a subset of data. Unlike in open waters, e.g., lakes, <span class="inline-formula"><i>k</i>₆₀₀</span> 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&thinsp;Tg&thinsp;<span class="inline-formula">CO₂</span> (1&thinsp;Tg&thinsp;<span class="inline-formula">=10¹²</span>&thinsp;g) during the monsoon period using varying approaches such as chambers, derived <span class="inline-formula"><i>k</i>₆₀₀</span> values and models. This study suggests that incorporating scale-appropriate <span class="inline-formula"><i>k</i></span> measurements into extensive <span class="inline-formula"><i>p</i>CO₂</span> investigations is required to refine basin-wide carbon budgets in subtropical streams and small rivers. We concluded that the simple parameterization of <span class="inline-formula"><i>k</i>₆₀₀</span> as a function of morphological characteristics is site specific for regions and watersheds and hence highly variable in rivers of the upper Yangtze. <span class="inline-formula"><i>k</i>₆₀₀</span> models should be developed for stream studies to evaluate the contribution of these regions to atmospheric <span class="inline-formula">CO₂</span>.</p>