Determination of the average period of CH₄ flux in a mixed plantation in Xiaolangdi Area of the Yellow River based on eddy covariance method

  Objective  The eddy covariance technique provides a useful tool to directly measure CH₄ exchange between the vegetation and atmosphere. The eddy covariance data normally need to be regulated based on the actual situation of the study area for its application. An inappropriate average period can lead to the inaccurate estimation on CH₄ flux. Therefore, some of the regulations were selected to obtain the suitable sampling frequency and average period.

  Method  In this paper, the impact of average period on CH₄ flux calculation had been analyzed in a mixed plantation in the Xiaolangdi Area of the Yellow River from July to August of 2016. CH₄ flux was calculated in different average periods (15−720 min). CH₄ flux obtained in different periods was analyzed, and the calculation of CH₄ flux in different average periods was compared with that of 30 min average period. The influences of average period on the calculation of CH₄ flux based on the original data (sampling frequency was 10 Hz) were examined. In addition, the Ogive function was applied to determine the low-frequency contribution to CH₄ flux.

  Result  The diurnal variations of CH₄ flux were different in the average period of 15, 60, 120, 240, 360 and 720 min. Large differences occurred at noon and small differences occurred in the morning or evening. When the average period was shorter than 120 min, CH₄ flux had same diurnal variation characteristics with 30 min value, and increased with the increase of the average period. The calculation of CH₄ flux was estimated significantly inaccurately when the average period was longer than 240 min. The Ogive function increased when the averaging period was less than 15 min, and the Ogive function became gradually stable when the period was 60 min.

  Conclusion  Different average periods have different influences in calculating CH₄ flux. In this study, the optimal average period should be 60 min during the long-term observation, whereas the average period should be 15 min to obtain accurate CH₄ flux at the daily time scale.