Modification of a wavelet-based method for detecting ebullitive methane fluxes in eddy-covariance observations: application at two rice fields

Authors: RICHARDSON, W. - University Of Arkansas; Reba, Michele; RUNKLE, B. - University Of Arkansas

Submitted to: Boundary Layer Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/21/2022
Publication Date: 4/30/2022
Citation: Richardson, W.P., Reba, M.L., Runkle, B.R. 2022. Modification of a wavelet-based method for detecting ebullitive methane fluxes in eddy-covariance observations: application at two rice fields. Boundary Layer Meteorology. 184:71-111. https://doi.org/10.1007/s10546-022-00703-y.
DOI: https://doi.org/10.1007/s10546-022-00703-y

Interpretive Summary: Ebullition, the sporadic release of gas bubbles, is an important pathway of methane emission from rice fields and other wetlands but is challenging to measure because of its high variability. In this work, we applied an empirical method that separates ebullition from the total methane emission measured by the eddy covariance method from two rice fields. We found that ebullition accounted for about 10% of the total methane emission across the growing season at both fields, became more important as the crop developed, and was most strongly related to wind speed, microbial activity, and sensible heat exchange. This work should benefit scientists and engineers working to quantify global methane emissions and improve their representation in land surface models, as well as government officials developing plans for climate action.

Technical Abstract: Ebullition, the release of gas bubbles, is an important pathway of methane emission in many ecosystems, yet its high spatiotemporal variability makes it challenging to quantify. In this work, a methane flux partitioning method based on scalar similarity in the wavelet domain was applied to eddy covariance data collected at two flooded rice fields. Inspection of initial results showed that several modifications were needed for robust ebullition detection. With these modifications, our objectives were to compare the original and modified methods, conduct a sensitivity analysis of the program’s empirical parameters, characterize the importance of ebullition in rice across growth stages, and identify the primary drivers of ebullition. The modified method’s ebullitive fluxes were significantly lower and showed lower variance than those from the original method. Furthermore, the two methods produced distinct patterns of diel variation. While partitioning estimates showed nontrivial sensitivity to the program parameters, this sensitivity was lower in magnitude than random error in the ebullitive flux estimates. Ebullitive fluxes made up 10% of the total flux on average, with ebullition increasing in importance as plants developed. Ebullitive fluxes were best predicted by wind speed (negative effect), ecosystem respiration (positive effect), and sensible heat flux (positive effect), suggesting an indirect effect of plant-mediated transport, a link with temperature and methane production, and a potential effect of water column turnover, respectively. In addition to validating the method with independent ebullition observations, we recommend its application at more natural and managed wetlands to improve understanding of this highly variable transport pathway.