PhD Thesis, 2007 - University College Cork

Chapter 3
Materials and Methods
The sensible heat flux (H) is computed as:
H
≅ c w'T '
(3.6)
ρ a
p
where ρ a is the moist air density (kg m -3 ), c p is the specific heat capacity of moist
air (J kg -1 °C -1 ) and T ' the temperature fluctuations (°C).
For the present study, the CO 2 (Chapter 5, Appendix 1), the latent and the sensible
heat fluxes (Chapter 6) were measured and computed using equation 3.4, 3.5 and
3.6 respectively. The micrometeorological convention (used in this work) treats
fluxes from the atmosphere as negative and fluxes from the ecosystem as positive.
The measured CO 2 flux was considered to correspond to the net ecosystem
exchange (NEE), because the stored flux in the canopy was considered negligible
in this treeless ecosystem for most of the day (Lafleur et al., 2003).
3.4.2 Advantages and limitation of the Eddy Covariance technique
The EC system is today a widely used technique for the measurements of gas
fluxes between the atmosphere and natural ecosystems (Baldocchi, 2003), because
of its many advantages. The EC system allows whole ecosystem gas exchange
measurements 1) through direct measurements, 2) over large sampling areas, 3)
across a range of time scales, 4) without disturbing the studied ecosystem
(Baldocchi et al., 1988; Baldocchi, 2003). The main weaknesses of the EC
techniques lie in the underestimation of the ecosystem night fluxes, due generally
to low turbulence conditions (Baldocchi, 2003), and in the uncertainties about the
source location (footprint) of the materials flux (Sogachev & Lloyd, 2004) (see
paragraph 3.4.3).
In addition, the assumptions of stationarity and homogeneity should be fulfilled so
that the fluxes measured by the EC system are representative of the biosphereatmosphere
material exchange.
- Stationarity means that statistical properties of the flow do not vary in time.
This condition can not be realized in its strict sense because of the long-term
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