Addressing effects of environment on eddy-covariance flux estimates at a temperate sedge-grass marsh

Boundary-Layer Meteorology. 2023. Vol. 186. P. 217–250.

Sergey N. Kivalov, Dušek J., Czerný R., Jocher G., Pavelka M., Fitzjarrald D., Dařenová E., Šigut L., Kowalska N.

We examined the net-ecosystem-exchange (NEE)-based annual carbon-balance estimates
obtained from eddy-covariance (EC) measurements at an unmanaged sedge-grass marsh
ecosystem (Tˇreboˇn, Czech Republic, 49°1′ N, 14°46′ E), seeking methods to improve the EC
measurements in inhomogeneous environment. The data filtering procedure was developed
using three thresholds: (a) a stationarity test; (b) a stability u∗-threshold; and (c) a high relative
humidity RH-threshold. This procedure was tested in 2014, a year without significant floods
and drought events led to a stable water table, reducing the effect of soil respiration on
the EC measurements. Estimates of annual carbon-balance were reduced from 182 to 234
± 12 gC m−2 year−1 for the initial data to 39–44 ± 8 gC m−2 year−1 after the R H ≤
95% filtering and to 24–26 ± 7 gC m−2 year−1 after the further u∗ ≥ 0.1 m s−1 filtering.
Applying the precipitation/fog threshold reduced this balance to 10–12 ± 7 gC m−2 year−1,
closer to carbon neutrality. Up to 9.5% of this identified shift occurred during apparent
nocturnal downslope katabatic drainage flows or plume descent coming from the nearby
town of Tˇreboˇn. High-RH conditions account for up to 27% of this shift. Moreover, both
conditions together account for an additional 67% of the identified carbon-balance change.
Removing these non-ecosystem-related processes brings EC measurements closer to values
of an unmanaged-ecosystem productivity, providing a better NEE-based estimate for the net
ecosystem production. The presented procedure is applicable to EC measurements conducted
at different wetlands or terrestrial ecosystems with similar conditions.

Keywords: carbon balance, Eddy covariance, Wetland, Urban surroundings, Katabatic drainage flow, High relative humidity