Combining eddy-covariance and chamber measurements to determine the methane budget from a small, heterogeneous urban wetland park

Abstract:

Methane (CH₄) emissions from wetlands have large uncertainties. They are often small in magnitude and originate from landscapes with highly heterogeneous land cover patterns posing challenges to determining their green-house gas (GHG) budget. We combined two CH₄ flux measurement approaches to overcome these issues and find the overall GHG budget of a small, heterogeneous, urban wetland park. Intermittent point measurements of fluxes from chambers provided information about the heterogeneity of fluxes, while eddy-covariance flux measurements provided information about the temporal dynamics of the fluxes. Both were combined to a scaled ‘fixed frame’ time series, correcting for temporal variability in the spatial coverage of the tower footprint.

The macrophyte-vegetated (VEG) and open-water (OW) areas followed similar cycles and emitted similar levels of CH₄ throughout the year. VEG exhibited a stronger late-summer emission peak than did OW, possibly due to CH₄ transport through mature vegetation. Normalizing the tower data to a fixed frame allowed us to determine the overall CH₄ budget of each patch type. OW was the strongest in terms of emissions per unit area, but the larger VEG area contributed the greatest total CH₄ emission. Using a scaling approach for carbon (CO₂) uptake we calculated the net total GHG contribution of this urban wetland park. Because chambers are not feasible over large vegetation or over extended time periods, the net CO₂ budget over the same fixed-frame was approximated over a range and cannot be accurately measured directly over all the components of the park landscape. Overall the newly constructed wetland park acted as a sink for GHG over the last 3 years.