Three years of eddy covariance measurements of a tropical forest in the Congo Basin.

The CongoFlux climate site in the Yangambi UNESCO biosphere reserve (04852.0"N, 243008.9"E) hosts the first Eddy Covariance (EC) flux tower in the central Congo Basin. The site, recently labeled as an ICOS associated ecosystem station, was built in 2020 to address the lack of observations of the tropical forest ecosystems in Central Africa. We aim to quantify the net ecosystem exchange (NEE) and the water use efficiency (WUE) of the tropical forest in the footprint of the tower. A set of meteorological and hydrological data is also recorded, including several climatic factors that could explain the seasonal patterns of NEE and WUE. However, processing EC data still remains a challenge in the tropics. Tall vegetation and frequent low turbulent conditions call for attention to the storage correction term. Multiple set ups on the site were used to study the best way to handle the nighttime buildup of CO2 in the canopy. The threshold for the friction velocity to differentiate low from wel mixed turbulent conditions, needs to be selected with care to minimize the influence of the frequent low turbulent conditions, while at the same time minimizing the amount of data that needs to be filtered out. Power cuts and data filtering result in many and sometimes large data gaps, which increase the importance of accurate gap filling techniques. An uncertainty assessment that looks into the influence of corrections and filtering steps and takes the effect of the gap filling procedure into account, is important to interpret the resulting NEE budget and to correctly use a final data set of half hourly NEE values. We here present three years of processed EC data together with the challenges of an EC station in the tropical forest of the Congo basin.