Evaluating stream CO2 outgassing via Drifting and Anchored flux chambers in a controlled flume experiment
Résumé
Quantifying CO2 fluxes across air-water boundaries is challenging due to practical difficulties in the estimation of reach-scale standardized gas exchange velocities (k600) and water equilibrium concentrations. Whereas craft-made floating chambers supplied by internal CO2 sensors represent a promising technique to estimate CO2 fluxes in rivers, the existing literature lacks of rigorous comparisons among differently designed instruments and deployment techniques. Moreover, as of now the uncertainty of k600 estimates relying on chamber data has not been evaluated. Here, these issues were addressed analyzing the results carried out in the Summer of 2019 in the Lunzer:::Rinnen – Experimental Facility (Austria). About 100 runs were performed using two different chamber designs (namely, a Standard chamber and a FlexibleFoil chamber with an external floating system and a flexible sealing) and two diverse deployment modes (drifting and anchored). The runs were performed using various combinations of discharge (Q) and channel slope (i), leading to highly variable turbulent kinetic energy dissipation rates (2·10^-3 < ε < 9·10^-2 m^2/s^3). Estimates of k600 were in line with previous results (4 < k600 < 32 m/d), with a general increase of k600 for larger ε. In particular, the FlexibleFoil gave consistent k600 patterns in response to changes in i and/or Q. Moreover, Acoustic Doppler Velocimeter measurements indicated a limited increase of ε induced by the FlexibleFoil on the flow field (26% of increase in ε, leading to a 6% of increase in k600). The uncertainty in the estimate of k600 was then estimated using a Generalized Likelihood Uncertainty Estimation procedure. Overall, uncertainty in k600 was moderate to high, with enhanced uncertainty in high-energy setups. For the anchored mode, the standard deviations of k600 were between 1.6 and 8.2 m/d, whereas significantly higher values were obtained in drifting mode. Interestingly, for the Standard the uncertainty was larger (+20%) as compared to the FlexibleFoil. Our study suggests that a FlexibleFoil design and the anchored deployment might be useful techniques to enhance the robustness and the accuracy of CO2 measurements in low-order streams. Furthermore, the study demonstrates the value of analytical and numerical tools in the identification of accurate CI for k600.