Deventer M Julian, Griffis Timothy J, Roman D Tyler, Kolka Randall K, Wood Jeffrey D, Erickson Matt, Baker John M, Millet Dylan B
University of Minnesota - Dept. Soil, Water & Climate, United States.
US Forest Service - Northern Research Station Grand Rapids, United States.
Agric For Meteorol. 2019 Nov 15;278. doi: 10.1016/j.agrformet.2019.107638. Epub 2019 Jul 26.
Wetlands represent the dominant natural source of methane (CH) to the atmosphere. Thus, substantial effort has been spent examining the CH budgets of global wetlands continuous ecosystem-scale measurements using the eddy covariance (EC) technique. Robust error characterization for such measurements, however, remains a major challenge. Here, we quantify systematic, random and gap-filling errors and the resulting uncertainty in CH fluxes using a 3.5 year time series of simultaneous open- and closed path CH flux measurements over a sub-boreal wetland. After correcting for high- and low frequency flux attenuation, the magnitude of systematic frequency response errors were negligible relative to other uncertainties. Based on three different random flux error estimations, we found that errors of the CH flux measurement systems were smaller in magnitude than errors associated with the turbulent transport and flux footprint heterogeneity. Errors on individual half-hourly CH fluxes were typically 6%-41%, but not normally distributed (leptokurtic), and thus need to be appropriately characterized when fluxes are compared to chamber-derived or modeled CH fluxes. Integrated annual fluxes were only moderately sensitive to gap-filling, based on an evaluation of 4 different methods. Calculated budgets agreed on average to within 7% (≤ 1.5 g - CH m yr). Marginal distribution sampling using open source code was among the best-performing of all the evaluated gap-filling approaches and it is therefore recommended given its transparency and reproducibility. Overall, estimates of annual CH emissions for both EC systems were in excellent agreement (within 0.6 g - CH m yr) and averaged 18 g - CH m yr. Total uncertainties on the annual fluxes were larger than the uncertainty of the flux measurement systems and estimated between 7-17%. Identifying trends and differences among sites or site years requires that the observed variability exceeds these uncertainties.
湿地是大气中甲烷（CH）的主要自然来源。因此，人们花费了大量精力，利用涡度相关（EC）技术对全球湿地的CH收支进行连续的生态系统尺度测量。然而，对此类测量进行可靠的误差表征仍然是一项重大挑战。在此，我们使用在亚寒带湿地进行的3.5年同步开路和闭路CH通量测量时间序列，对系统误差、随机误差和填补间隙误差以及由此产生的CH通量不确定性进行了量化。在对高频和低频通量衰减进行校正后，系统频率响应误差的幅度相对于其他不确定性可以忽略不计。基于三种不同的随机通量误差估计，我们发现CH通量测量系统的误差在幅度上小于与湍流输送和通量足迹异质性相关的误差。单个半小时CH通量的误差通常为6%-41%，但不是正态分布（尖峰态），因此在将通量与静态箱法得出的或模型化的CH通量进行比较时，需要对其进行适当表征。基于对4种不同方法的评估，综合年通量对填补间隙的敏感性仅为中等。计算得出的收支平均一致率在7%以内（≤1.5 g - CH m yr）。使用开源代码的边际分布抽样是所有评估的填补间隙方法中表现最好的方法之一，鉴于其透明度和可重复性，因此推荐使用。总体而言，两个EC系统的年CH排放量估计值非常一致（在0.6 g - CH m yr以内），平均为18 g - CH m yr。年通量的总不确定性大于通量测量系统的不确定性，估计在7%-17%之间。识别不同地点或地点年份之间的趋势和差异需要观测到的变异性超过这些不确定性。
