Measurements of methane flux densities from paddy fields by eddy accumulation method

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Eddy accumulation method, which had been proposed theoretically as a new micrometeorological method to directly measure the gas flux density from the sources, were applied to measure the biosphere/atmosphere flux density of methane from paddy fields.A new automatic gas sampler, which satisfies the theoretical requirements of this method, was developed. The vertical wind velocity w is measured with 5 Hz by the sonic anemometer and recorded on the data-logger. The velocity is used to determine the conditional sampling at a constant flow rate by opening one valve to a reservoir when w is positive and another valve to another reservoir when w is negative. The gas samples are collected for30 minutes in every 2 hours and average concentrations are measured by gas chromatography. Using these data, methane flux density f can be estimated as follows: f=bsigma_w(C_<up>-C_<down>)In order to check the validity of the measurement of the eddy accumulation method, the sensible he at flux densities estimated by the eddy correlation method, the Bowen ratio method, and the eddy accumulation method were compared. The result shows close agreement between two methods and suggests the reliability of the eddy accumulation method.Applying the eddy accumulation method with the automatic gas sampler, methane flux densities from a paddy field were measured in the south of Thailand. The daily courses of the flux densities changed between20 to 60 mg/m^2/hr. Also, they showed maxima and minima in the afternoon and after-midnight, respectively.