Eddy Covariance Measurements
Water (H2O), carbon dioxide (CO2) and
momentum fluxes were measured at the site with an Eddy Covariance (EC)
system deployed at the meteorological tower from 7 June to 10 September.
The EC system consisted of a 3D sonic anemometer (CSAT3; Campbell
Scientific Inc., Logan, Utah, USA) and an open-path infrared
CO2/H2O gas analyzer (IRGA) (LI-7500,
LI-COR Inc., Lincoln, Nebraska, USA) mounted 3 m above the surface. Raw
fluxes were sampled at a frequency of 10 Hz on a CR1000 datalogger
(Campbell Scientific, Logan, Utah, USA) and averaged over a half-hour.
All fluxes were corrected for density, sensor separation, time lag and
coordinate rotation (double coordinate rotation was used) following
common Fluxnet protocols (Webb, Pearman, & Leuning, 1980; Kaimal and
Finnigan, 1994; Leuning and Judd, 1996; Foken and Leclerc, 2004; Aubinet
et al., 2012; Burba et al., 2012). The resulting half-hour fluxes were
then processed in a custom R-Software script, which filtered each
half-hour to ensure that it had at least 80% of the high-frequency
records and that there was no potential for dew formation on the IRGA
lenses by either precipitation or by comparing the dew point temperature
to Ta. Additional filtering removed values that were
greater than ±3 standard deviations of a moving average, which consisted
of 10 half-hour neighbouring values; further filtering was completed to
check for physically improbable values. The Kljun, Calanca, Rotach, &
Schmid (2015) footprint analysis was used to constrain the measured
fluxes to be within 80% of the desired site boundaries, using their FFP
R-functions. Thereafter, fluxes with a corresponding friction velocity
(u*) of u* < 0.15ms-1 were removed from the
dataset. Following this, the filtered net ecosystem exchange (NEE) data
was gap-filled using the MDS method outlined in Reichstein et al.
(2005); and then partitioned into respiration (R) and gross primary
productivity (GPP) using the relationship between night-time respiration
and Ta (Reichstein et al., 2005). The filtered energy
fluxes were forced closed, with the additional energy partitioned based
on the Bowen ratio (ß) (Wilson et al., 2002). ß is the ratio of sensible
heat (Qh) to latent heat (Qe). For each
available half-hour, Qe was gap-filled by scaling
potential evapotranspiration, PET (Priestley and Taylor, 1972) to ET.
The scaler (α) was calculated from the ratio between ET and PET. For
periods where no α was available, the ratio was gap-filled using the MDS
method outlined in Reichstein et al. (2005), where Q*, VPD, and u* were
used as the gap-filling conditions; however, when this was not possible,
the seasonal mean α was used. Qh was then gap-filled by
solving the energy balance equation using the measured and gap-filled
Q*, Qe and Qg.