Uncovering mechanisms of episodic methane sources observed by a very tall eddy covariance tower

The mechanistic process that lead to methane emissions from terrestrial and freshwater ecosystems are by their nature, episodic and spatially disjunct. As a consequence, evaluation of predictive ecosystem models of regional methane emissions with field campaigns from short eddy covariance towers or methane chambers is not reliable, since measurements focused on a particular known source of methane emission will be biased compared to regional estimates of magnitude, spatial scale, or frequency of these emissions. Last year, we added methane eddy covariance observations to a very tall (447m) eddy covariance tower, the Park Falls, Wisconsin WLEF Ameriflux site (US-PFa), located in the upper Midwest USA. The flux measurements at 122m reveal a variety of episodic methane emissions, especially in winter, that currently cannot be explained by other observed environmental variables. Here, we show how a convective boundary layer (CBL) flux footprint model, maps of wetland extent and type, and the aforementioned flux observations can help reveal the magnitude and scale of episodic methane emissions, and what their likely contribution is to the regional net terrestrial ecosystem methane source. These results provide a new benchmark for rigorously evaluating ecosystem models and satellite or inverse model-based observations of methane sources and sinks.