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

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 nmol m−2 s−1. The HREA method also contained uncertainties associated with scalar dissimilarity of 1.26 nmol m−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 in forest ecosystems, especially in remote sites where frequent maintenance is not practical.