Gap-filling eddy covariance methane fluxes: Comparison of machine learning model predictions and uncertainties at FLUXNET-CH4 wetlands

Jeremy Irvin , Sharon Zhou , Gavin Mcnicol , Fred Lu , Vincent Liu , Etienne Fluet-Chouinard , Zutao Ouyang , Sara Helen Knox , Antje Lucas-Moffat , Carlo Trotta , Dario Papale , Domenico Vitale , Ivan Mammarella , Pavel Alekseychik , Mika Aurela , Anand Avati , Dennis Baldocchi , Sheel Bansal , Gil Bohrer , David Campbell , Jiquan Chen , Housen Chu , Higo Dalmagro , Kyle Delwiche , Ankur Desai , Eugenie Euskirchen , Sarah Feron , Mathias Goeckede , Martin Heimann , Manuel Helbig , Carole Helfter , Kyle Hemes , Takashi Hirano , Hiroki Iwata , Gerald Jurasinski , Aram Kalhori , Andrew Kondrich , Derrick Yf Lai , Annalea Lohila , Avni Malhotra , Lutz Merbold , Bhaskar Mitra , Andrew Ng , Mats Nilsson , Asko Noormets , Matthias Peichl , A. Camilo Rey-Sanchez , Andrew Richardson , Benjamin Rk Runkle , Karina Vr Schäfer , Oliver Sonnentag , Ellen Stuart-Haëntjens , Cove Sturtevant , Masahito Ueyama , Alex Valach , Rodrigo Vargas , George Vourlitis , Eric Ward , Guan Xhuan Wong , Donatella Zona , Ma. Carmelita R Alberto , David Billesbach , Gerardo Celis , Han Dolman , Thomas Friborg , Kathrin Fuchs , Sébastien Gogo (1, 2) , Mangaliso Gondwe , Jordan Goodrich , Pia Gottschalk , Lukas Hörtnagl , Adrien Jacotot (1, 2) , Franziska Koebsch , Kuno Kasak , Regine Maier , Timothy Morin , Eiko Nemitz , Walter Oechel , Patricia Oikawa , Keisuke Ono , Torsten Sachs , Ayaka Sakabe , Edward Schuur , Robert Shortt , Ryan Sullivan , Daphne Szutu , Eeva-Stiina Tuittila , Andrej Varlagin , Joeseph Verfaillie , Christian Wille , Lisamarie Windham-Myers , Benjamin Poulter , Robert Jackson

1 ISTO - Institut des Sciences de la Terre d'Orléans - UMR7327
2 Biogéosystèmes Continentaux - UMR7327

Time series of wetland methane fluxes measured by eddy covariance require gap-filling to estimate daily, seasonal, and annual emissions. Gap-filling methane fluxes is challenging because of high variability and complex responses to multiple drivers. To date, there is no widely established gap-filling standard for wetland methane fluxes, with regards both to the best model algorithms and predictors. This study synthesizes results of different gap-filling methods systematically applied at 17 wetland sites spanning boreal to tropical regions and including all major wetland classes and two rice paddies. Procedures are proposed for: 1) creating realistic artificial gap scenarios, 2) training and evaluating gap-filling models without overstating performance, and 3) predicting half-hourly methane fluxes and annual emissions with realistic uncertainty estimates. Performance is compared between a conventional method (marginal distribution sampling) and four machine learning algorithms. The conventional method achieved similar median performance as the machine learning models but was worse than the best machine learning models and relatively insensitive to predictor choices. Of the machine learning models, decision tree algorithms performed the best in cross-validation experiments, even with a baseline predictor set, and artificial neural networks showed comparable performance when using all predictors. Soil temperature was frequently the most important predictor whilst water table depth was important at sites with substantial water table fluctuations, highlighting the value of data on wetland soil conditions. Raw gap-filling uncertainties from the machine learning models were underestimated and we propose a method to calibrate uncertainties to observations. The python code for model development, evaluation, and uncertainty estimation is publicly available. This study outlines a modular and robust machine learning workflow and makes recommendations for, and evaluates an improved baseline of, methane gap-filling models that can be implemented in multi-site syntheses or standardized products from regional and global flux networks (e.g., FLUXNET).

Keywords: Machine learning; time series; imputation; gap-filling; methane; flux; wetlands

Domains: Sciences of the Universe [physics]; Continental interfaces, environment

Jeremy Irvin, Sharon Zhou, Gavin Mcnicol, Fred Lu, Vincent Liu, et al.. Gap-filling eddy covariance methane fluxes: Comparison of machine learning model predictions and uncertainties at FLUXNET-CH4 wetlands. Agricultural and Forest Meteorology, 2021, 308-309, pp.108528. ⟨10.1016/j.agrformet.2021.108528⟩. ⟨insu-03286676⟩