Non-local Impacts on Eddy-Covariance Air–Lake $$\hbox {CO}_2$$ Fluxes

Abstract

Inland freshwater bodies form the largest natural source of carbon to the atmosphere. To study this contribution to the atmospheric carbon cycle, eddy-covariance flux measurements at lake sites have become increasingly popular. The eddy-covariance method is derived for solely local processes from the surface (lake). Non-local processes, such as entrainment or advection, would add erroneous contributions to the eddy-covariance flux estimations. Here, we use four years of eddy-covariance measurements of carbon dioxide from Lake Erken, a freshwater lake in mid-Sweden. When the lake is covered with ice, unexpected lake fluxes were still observed. A statistical approach using only surface-layer data reveals that non-local processes produce these erroneous fluxes. The occurrence and strength of non-local processes depend on a combination of wind speed and distance between the instrumented tower and upwind shore (fetch), which we here define as the time over water. The greater the wind speed and the shorter the fetch, the higher the contribution of non-local processes to the eddy-covariance fluxes. A correction approach for the measured scalar fluxes due to the non-local processes is proposed and also applied to open-water time periods. The gas transfer velocity determined from the corrected fluxes is close to commonly used wind-speed based parametrizations.

New insights into diel to interannual variation in carbon dioxide emissions from lakes and reservoirs

“…This introduced an additional problem with CO2 advection from catchments and flux contamination. While well-selected tower locations minimize the advection term to <3% of CO2 flux (Morin et al, 2018), the towers located in the middle of the lake can also be affected by CO2 advection, particularly small lakes surrounded by forest (Esters et al, 2021;Kenny et al, 2017). The contribution of advected air to annual CO2 lake budgets in this project is unknown and might be substantial.…”

Atmospheric CO2 Exchange of a Small Mountain Lake: Limitations of Eddy Covariance and Boundary Layer Modeling Methods in Complex Terrain

“…For terrestrial ecosystems, stable, low wind conditions are known to be challenging, concerning especially night-time flux quantification (Aubinet, 2008). For lakes, large influence from non-local processes has also been observed for high wind speeds (Esters et al, 2020). Yet overall, we could not determine any generalized condition under which the unrealistic flux values occurred.…”

Eddy Covariance Data Reveal That a Small Freshwater Reservoir Emits a Substantial Amount of Carbon Dioxide and Methane

“…While strict filtering processes were enacted to limit non‐local fluxes (i.e., filtering fluxes when the along‐wind distance providing 90% of the cumulative contribution was outside the reservoir), we are unable to completely rule out potential non‐local processes (e.g., land‐lake interactions) which occur outside the footprint and are entrained or advected into the EC footprint area (Esters et al., 2021; Vesala et al., 2006, 2012; Figure S2 in Supporting Information ). These processes may be particularly important in small freshwaters located in mountainous regions (Scholz et al., 2021).…”