X-BASE: the first terrestrial carbon and water flux products from an extended data-driven scaling framework, FLUXCOM-X
Authors: Nelson Jacob A. , Walther Sophia , Gans Fabian , Kraft Basil , Weber Ulrich , Novick Kimberly , Buchmann Nina , Migliavacca Mirco , Wohlfahrt Georg , Šigut Ladislav , Ibrom Andreas , Papale Dario , Göckede Mathias , Duveiller Gregory , Knohl Alexander , Hörtnagl Lukas , Scott Russell L. , Zhang Weijie , Hamdi Zayd Mahmoud , Reichstein Markus , Aranda-Barranco Sergio , Ardö Jonas , Op de Beeck Maarten , Billdesbach Dave , Bowling David , Bracho Rosvel , Brümmer Christian , Camps-Valls Gustau , Chen Shiping , Cleverly Jamie Rose , Desai Ankur , Dong Gang , El-Madany Tarek S. , Euskirchen Eugenie Susanne , Feigenwinter Iris , Galvagno Marta , Gerosa Giacomo , Gielen Bert , Goded Ignacio , Goslee Sarah , Gough Christopher Michael , Heinesch Bernard , Ichii Kazuhito , Jackowicz-Korczynski Marcin Antoni , Klosterhalfen Anne , Knox Sara , Kobayashi Hideki , Kohonen Kukka-Maaria , Korkiakoski Mika , Mammarella Ivan , Mana Gharun , Marzuoli Riccardo , Matamala Roser , Metzger Stefan , Montagnani Leonardo , Nicolini Giacomo , O'Halloran Thomas , Ourcival Jean-Marc , Peichl Matthias , Pendall Elise , Ruiz Reverter Borja , Roland Marilyn , Sabbatini Simone , Sachs Torsten , Schmidt Marius , Schwalm Christopher R. , Shekhar Ankit , Silberstein Richard , Silveira Maria Lucia , Spano Donatella , Tagesson Torbern , Tramontana Gianluca , Trotta Carlo , Turco Fabio , Vesala Timo , Vincke Caroline , Vitale Domenico , Vivoni Enrique R. , Wang Yi , Woodgate William , Yepez Enrico A. , Zhang Junhui , Zona Donatella , Jung Martin 
Published: February 2024

Abstract. Mapping in-situ eddy covariance measurements of terrestrial land-atmosphere fluxes to the globe is a key method for diagnosing the Earth system from a data-driven perspective. We describe the first global products (called X-BASE) from a newly implemented up-scaling framework, FLUXCOM-X. The X-BASE products comprise of estimates of CO2 net ecosystem exchange (NEE), gross primary productivity (GPP) as well as evapotranspiration (ET) and, for the first time, a novel fully data-driven global transpiration product (ETT), at high spatial (0.05°) and temporal (hourly) resolution. X-BASE estimates the global NEE at -5.75 ± 0.33 Pg C ⋅ yr-1 for the period 2001–2020, showing a much higher consistency with independent atmospheric carbon cycle constraints compared to the previous versions of FLUXCOM. The improvement of global NEE was likely only possible thanks to the international effort to increase the precision and consistency of eddy covariance collection and processing pipelines, as well as to the extension of the measurements to more site-years resulting in a wider coverage of bio-climatic conditions. However, X-BASE global net ecosystem exchange shows a very low inter-annual variability, which is common to state-of-the-art data-driven flux products and remains a scientific challenge. With 125 ± 2.1 Pg C ⋅ yr-1 for the same period, X-BASE GPP is slightly higher than previous FLUXCOM estimates, mostly in temperate and boreal areas. X-BASE evapotranspiration amounts to 74.7x10³ ± 0.9x10³ km3 globally for the years 2001–2020, but exceeds precipitation in many dry areas likely indicating overestimation in these regions. On average 57 % of evapotranspiration are estimated to be transpiration, in good agreement with isotope-based approaches, but higher than estimates from many land surface models. Despite considerable improvements to the previous up-scaling products, many further opportunities for development exist. Pathways of exploration include methodological choices in the selection and processing of eddy-covariance and satellite observations, their ingestion into the framework, and the configuration of machine learning methods. For this, the new FLUXCOM-X framework was specifically designed to have the necessary flexibility to experiment, diagnose, and converge to more accurate global flux estimates.