High-precision measurements of the methane flux over a larch forest based on a hyperbolic relaxed eddy accumulation method using a laser spectrometer

Authors: Ueyama M., Takai Y., Takahashi Y., Ide R., Hamotani K., Kosugi Y., Takahashi K., Saigusa N.
Journal: Agricultural and Forest Meteorology, 178-179 (2013) 183-193
DOI: -
Abstract: The precision of current micrometeorological techniques has restricted their applications in measuring small methane (CH₄) fluxes in forest ecosystems. In this study, we continuously measured CH₄ fluxes using a state of the art laser-based gas analyzer and a hyperbolic relaxed eddy accumulation (HREA) method. The precision of the half-hourly fluxes was estimated at 2.93 nmolm⁻² s⁻¹. The HREA method also contained uncertainties associated with scalar dissimilarity of 1.26 nmolm⁻² s⁻¹. The precision was higher than those obtained in previous eddy covariance studies, because the HREA system did not require density fluctuation and high-frequency attenuation corrections. The observed CH₄ uptake decreased with increases in soil water content during the summer months. The estimated annual methane sink was 673 mg CH₄ m⁻² yr⁻¹ with uncertainties of ±231 mg CH₄ m⁻² yr⁻¹. This annual methane sink increased to 868 mg CH₄ m⁻² yr⁻¹ with the application of friction velocity (u*) filtering. The uncertainties in gap-filling, storage correction, and scalar dissimilarity were less important than the uncertainties in the u* filtering. Our technique using the HREA method is a suitable tool for measuring small CH₄ fluxes inforest ecosystems, especially in remote sites where frequent maintenance is not practical.