We report on methane (CH4) concentrations and efflux densities that were measured over a drained and grazed, peatland pasture in the Sacramento-San Joaquin River Delta of California over a three year period. The site was ideal for micrometeorological flux measurements due to its very flat topography, its exposure to vigorous winds and its extended fetch along the predominant wind direction. Nevertheless, the interpretation of methane fluxes with eddy covariance proved to be extremely complicated by a number of geographical, biophysical, biogeochemical and site management factors.
Initial inspection of the data revealed unexpected results—methane concentrations and efflux densities were greatest during the night rather than during the day. To explain this odd diurnal behavior in methane efflux densities and concentration, we tested two hypotheses. The prime hypothesis presupposed that the stable stratification of the nocturnal boundary layer elongated the flux footprint and enabled the flux tower to sense wetter fields at the western edge of the pasture, flooded drainage ditches, and/or a tidal marsh upwind of the pasture—these land forms emitted methane at rates 10–100 times greater than the drained portion of the peatland. And, this methane was emitted into a shallower volume of the atmosphere due to the collapse of the nocturnal boundary layer, causing methane concentrations to rise faster. The alternative hypothesis attributed the higher nocturnal methane fluxes to cattle, as they may have congregated near the tower at night.
We investigated these hypotheses with: (1) a series of micrometeorological field measurements at companion sites upwind and downwind of the pasture; (2) a series of chamber-based flux measurements on the representative land classes; (3) through the lens of a one-dimensional planetary boundary layer (pbl), box model; and (4) via inspection of digital camera images for the presence or absence of cattle. Together, these pieces of data suggest that elevated methane fluxes and concentrations at night were due to the combined correlation between: (1) the collapse of the nocturnal boundary layer; (2) the elongation of the flux and concentration footprints; and (3) the preferential sampling of an elevated methane source, be it the cattle, wet proportions of the field or some combination. On the other hand, our flux measurements were not perturbed by methane emanating from the tidal marsh that was several kilometers upwind of the peatland pasture site.