Interactions between nocturnal turbulent flux, storage and advection at an ‘ideal’ eucalypt woodland site

While the eddy covariance technique has become
an important technique for estimating long-term ecosystem
carbon balance, under certain conditions the measured turbulent
flux of CO2 at a given height above an ecosystem does
not represent the true surface flux. Profile systems have been
deployed to measure periodic storage of CO2 below the measurement
height, but have not been widely adopted. This is
most likely due to the additional expense and complexity and
possibly also the perception, given that net storage over intervals
exceeding 24 h is generally negligible, that these measurements
are not particularly important. In this study, we
used a 3-year record of net ecosystem exchange of CO2 and
simultaneous measurements of CO2 storage to ascertain the
relative contributions of turbulent CO2 flux, storage, and advection
(calculated as a residual quantity) to the nocturnal
CO2 balance and to quantify the effect of neglecting storage.
The conditions at the site are in relative terms highly
favourable for eddy covariance measurements, yet we found
a substantial contribution (40 %) of advection to nocturnal
turbulent flux underestimation. The most likely mechanism
for advection is cooling-induced drainage flows, the effects
of which were observed in the storage measurements. The
remaining 60% of flux underestimation was due to storage
of CO2.We also showed that substantial underestimation
of carbon uptake (approximately 80 gCm􀀀2 a􀀀1, or 25% of
annual carbon uptake) arose when standard methods (u filtering)
of nocturnal flux correction were implemented in the
absence of storage estimates. These biases were reduced to
approximately 40–45 gCm􀀀2 a􀀀1 when the filter was applied
over the entire diel period, but they were nonetheless large
relative to quantifiable uncertainties in the data. Neglect of
storage also distorted the relationships between the CO2 exchange
processes (respiration and photosynthesis) and their
key controls (light and temperature respectively). We conclude
that the addition of storage measurements to eddy covariance
sites with all but the lowest measurement heights
should be a high priority for the flux measurement community.