11. Canopy Fluxes
meters upwind and cover an area of 1000 to 10,000
km 2 (Raupach et al. 1992).
The budget method requires an estimate of the
CO 2 concentrations of the air that is entrained as
the CBL grows over the day. Tropospheric values
(C t ) may be estimated from oceanic sites. Also, if
there is a significant residual layer in the morning,
the concentration in this layer can be measured with
tethered balloon-borne instruments. The budget
method has been tested in a number of field experiments so far (Munley and Hipps, 1991; Denmead
et al. 1996; Levy et al. 1999), generally with encouraging results.
Again, as with aircraft platforms, the flux obtained by this boundary-layer budget approach is
unlikely to be exactly the same as that obtained by
tower-based systems because of the different source
areas for the two fluxes. The height of the CBL (for
inclusion in Equation 11.31) can be monitored by
a radiosonde system or modeled according to best
practice. Vertical velocity (W t ) can also be obtained
from radiosonde data if it is assumed that horizontal
divergence is constant with height (Gryning and
Batchvarova 1998) or from operational analysis
from major weather forecasting bureaux. Profiles
of CO2 concentration can be obtained either from
balloon-borne systems or from tall towers. The
method is sensitive to the value for C t and this is
best obtained with an aircraft-borne system.
Conclusions
Measuring canopy fluxes by micrometeorological
methods bridges the gap between plot- and leafscale process studies on the one hand and aircraftor space-borne platforms on the other. All such
techniques are complementary and are best deployed in combination with process-based models
of ecosystem functioning. Micrometeorological
flux measurements above the canopy can be made
routinely and can operate indefinitely given appropriate resources. They are a direct form of measurement, are nondestructive, and fluxes obtained
from them are the spatial average of several km 2
upwind from the measuring point. Careful implementation of these techniques in appropriate parts
of the landscape can mean that long-term canopy
measurements of carbon and water can be reported
to have an accuracy of more than 90 to 95%. The
177
proposed networks of flux stations based on eddy
covariance systems in particular, promise to deliver
routine and long-term measurements of the cycling
of energy, water, and carbon in a range of important
ecosystems (Baldocchi et al. 1996, Figure 11.7).
Such networks are essential to investigate and
monitor ecosystems as they undergo change in the
future.
References
Anderson, D.E.; Verma, S.B.; Clement, R.l.; Baldocchi,
D.D.; Matt, D.R. Turbulence spectra of CO2, water
vapor, temperature and velocity over a deciduous forest. Bound. Lay. Meteoroi. 38:81-99; 1986.
Auble, D.L.; Meyers, T.P. An open path, fast response
infrared-absorption gas analyzer for H20 and CO2,
Bound. Lay. Meteoroi. 59:243-256; 1992.
Aubinet, M.; Grelle, A; Ibrom, A.; Rannik, D.; Moncreiff, l; Foken, T.; Kowalski, G.S.; Martin, P.R.; Berbigier, P.; Bernhofer, Ch.; Clement R.; Elbers, l;
Wranier, A; Grunwald, T.; Morgenstern, K.; Pilegaard, K.; Rebmann, c.; Snijders, w.; Valentini, R.;
Vesala, T. Estimates of the annual net carbon and water
exchange of forests: the EUROFLUX methodology.
Adv. Ecoi. Res. 30:113-175; 2000.
Baldocchi, D.D.; Hicks, B.B.; Meyers, T.P. Measuring
biosphere-atmosphere exchanges of biologically related gases with micrometeorological methods. Ecology 69:1331-1340; 1988.
Baldocchi, D.D.; Valentini, R.; Running, S.; Oechel, w.;
Dahlman, R. Strategies for measuring and modelling
carbon dioxide and water vapour fluxes over terrestrial
ecosystems. Global Change BioI. 2:159-168; 1996.
Baldocchi, D.D.; Meyers, T. On using eco-physiological,
micrometeorological and biogeochemical theory to
evaluate carbon dioxide, water vapor and trace gas
fluxes over vegetation: A perspective. Agric. For. Meteoroi. 90:1-25; 1998.
Barr, A.G.; King, K.M.; Gillespie, T.1.; den Hartog, G.;
Neumann, H.H. A comparison of bowen ratio and
eddy correlation sensible and latent heat flux measurements above deciduous forest. Bound. Lay. Meteoroi. 71:21-41; 1994.
Beverland, I.1.; 6neill, D.; Scott, S.L.; Moncrieff, lB.
Design, construction and operation of flux measurement systems using the conditional sampling system.
Atmos. Env. 30:3209-3220; 1996.
Businger, lA Evaluation of the accuracy with which dry
deposition can be measured with current micrometeorological techniques. l. Clim. Appi. Meteoroi.
25:1100-1124; 1986.
Businger, l.A; Delaney, AC.; Chemical sensor resolution required for measuring surface fluxes by three
meters upwind and cover an area of 1000 to 10,000
km 2 (Raupach et al. 1992).
The budget method requires an estimate of the
CO 2 concentrations of the air that is entrained as
the CBL grows over the day. Tropospheric values
(C t ) may be estimated from oceanic sites. Also, if
there is a significant residual layer in the morning,
the concentration in this layer can be measured with
tethered balloon-borne instruments. The budget
method has been tested in a number of field experiments so far (Munley and Hipps, 1991; Denmead
et al. 1996; Levy et al. 1999), generally with encouraging results.
Again, as with aircraft platforms, the flux obtained by this boundary-layer budget approach is
unlikely to be exactly the same as that obtained by
tower-based systems because of the different source
areas for the two fluxes. The height of the CBL (for
inclusion in Equation 11.31) can be monitored by
a radiosonde system or modeled according to best
practice. Vertical velocity (W t ) can also be obtained
from radiosonde data if it is assumed that horizontal
divergence is constant with height (Gryning and
Batchvarova 1998) or from operational analysis
from major weather forecasting bureaux. Profiles
of CO2 concentration can be obtained either from
balloon-borne systems or from tall towers. The
method is sensitive to the value for C t and this is
best obtained with an aircraft-borne system.
Conclusions
Measuring canopy fluxes by micrometeorological
methods bridges the gap between plot- and leafscale process studies on the one hand and aircraftor space-borne platforms on the other. All such
techniques are complementary and are best deployed in combination with process-based models
of ecosystem functioning. Micrometeorological
flux measurements above the canopy can be made
routinely and can operate indefinitely given appropriate resources. They are a direct form of measurement, are nondestructive, and fluxes obtained
from them are the spatial average of several km 2
upwind from the measuring point. Careful implementation of these techniques in appropriate parts
of the landscape can mean that long-term canopy
measurements of carbon and water can be reported
to have an accuracy of more than 90 to 95%. The
177
proposed networks of flux stations based on eddy
covariance systems in particular, promise to deliver
routine and long-term measurements of the cycling
of energy, water, and carbon in a range of important
ecosystems (Baldocchi et al. 1996, Figure 11.7).
Such networks are essential to investigate and
monitor ecosystems as they undergo change in the
future.
References
Anderson, D.E.; Verma, S.B.; Clement, R.l.; Baldocchi,
D.D.; Matt, D.R. Turbulence spectra of CO2, water
vapor, temperature and velocity over a deciduous forest. Bound. Lay. Meteoroi. 38:81-99; 1986.
Auble, D.L.; Meyers, T.P. An open path, fast response
infrared-absorption gas analyzer for H20 and CO2,
Bound. Lay. Meteoroi. 59:243-256; 1992.
Aubinet, M.; Grelle, A; Ibrom, A.; Rannik, D.; Moncreiff, l; Foken, T.; Kowalski, G.S.; Martin, P.R.; Berbigier, P.; Bernhofer, Ch.; Clement R.; Elbers, l;
Wranier, A; Grunwald, T.; Morgenstern, K.; Pilegaard, K.; Rebmann, c.; Snijders, w.; Valentini, R.;
Vesala, T. Estimates of the annual net carbon and water
exchange of forests: the EUROFLUX methodology.
Adv. Ecoi. Res. 30:113-175; 2000.
Baldocchi, D.D.; Hicks, B.B.; Meyers, T.P. Measuring
biosphere-atmosphere exchanges of biologically related gases with micrometeorological methods. Ecology 69:1331-1340; 1988.
Baldocchi, D.D.; Valentini, R.; Running, S.; Oechel, w.;
Dahlman, R. Strategies for measuring and modelling
carbon dioxide and water vapour fluxes over terrestrial
ecosystems. Global Change BioI. 2:159-168; 1996.
Baldocchi, D.D.; Meyers, T. On using eco-physiological,
micrometeorological and biogeochemical theory to
evaluate carbon dioxide, water vapor and trace gas
fluxes over vegetation: A perspective. Agric. For. Meteoroi. 90:1-25; 1998.
Barr, A.G.; King, K.M.; Gillespie, T.1.; den Hartog, G.;
Neumann, H.H. A comparison of bowen ratio and
eddy correlation sensible and latent heat flux measurements above deciduous forest. Bound. Lay. Meteoroi. 71:21-41; 1994.
Beverland, I.1.; 6neill, D.; Scott, S.L.; Moncrieff, lB.
Design, construction and operation of flux measurement systems using the conditional sampling system.
Atmos. Env. 30:3209-3220; 1996.
Businger, lA Evaluation of the accuracy with which dry
deposition can be measured with current micrometeorological techniques. l. Clim. Appi. Meteoroi.
25:1100-1124; 1986.
Businger, l.A; Delaney, AC.; Chemical sensor resolution required for measuring surface fluxes by three