11. Canopy Fluxes
mospheric properties is increased. Measurements
over smooth surfaces, such as water or ice, are
difficult using any of the techniques mentioned.
Increasingly, eddy covariance-based systems are
being integrated into global observing networks
both to monitor carbon and water exchange on the
global scale and to validate remotely sensed products (Running et al. 1999).
Aerodynamic Method
The vertical exchange of atmospheric entities such
as momentum, temperature, water vapor, and CO 2
by turbulent transport is driven by and is proportional to their vertical concentration gradients. We
can describe the transport process in a flux-gradient
form which defines the constant of proportionality
K known as an eddy diffusivity. In generic form,
the flux density (F x' the amount of that entity transported vertically through unit area in unit time) of
any scalar (X) is:
(11.4)
The eddy diffusivities for scalars such as temperature, water vapor, and CO2 have been the subject
of much experimentation over several decades as
their relative magnitudes depend on both surface
and atmospheric features. In practice, the diffusivities are related to the eddy diffusivity for momentum, which can be established from profiles of wind
speed. Wind profiles are used to calculate the friction velocity, a measure of the degree of atmospheric mixing and from which Kx = ku*(z - d)
is calculated (Thorn 1975). The gradients of the
scalar in question need to be precisely determined
and obtaining accurate gradients over rough vegetation can be difficult because of the substantial degree of atmospheric mixing over such canopies
(McNeil and Shuttleworth 1975). One of the concerns about the aerodynamic technique is that a
number of empirical corrections are required to account for changes in atmospheric stability (which
influences the shape of wind profiles). A number of
semiempirical formulae have been extensively investigated and are well accepted (e.g., Webb 1970;
Paulson 1970). The aerodynamic technique has
been used extensively in the past to obtain fluxes
of sensible and latent heat over forests (e.g., Thorn
167
1975; Lindroth 1984) and extended to other trace
gas species over different surface types (e.g.,
Fowler and Unsworth 1979; Sutton et al. 1993). It
is not possible to use the aerodynamic technique in
the roughness sublayer or within the canopy as the
variation in sources and sinks for heat, water vapor,
and CO 2 invalidate the underlying assumptions in
the method (Raupach and Legg, 1984).
Energy BalancelBowen Ratio
The energy balance at the surface is
Rn - G - S = H + LE
(11.5)
Where Rn is net radiation absorbed by the vegetation, G is soil heat flux, and S is heat stored in the
vegetation. The ratio of sensible heat (ll) to latent
heat (LE) flux is known as the Bowen ratio (p) and
by writing the fluxes in their flux-gradient form, an
equation can be found that permits either flux to be
found from measurements of the gradient of temperature (T) and humidity (e), irrespective of atmospheric stability:
f3
dT
1/y -
de
(11.6)
where y is the psychrometric constant (Monteith
and Unsworth 1990). Once f3 has been obtained,
substituting for LE and H in Equation 11.5 we get
R - G - S
H = _n _ _ -::--;-1 + f3 1
(11.7)
and
R - G - S
LE = _n _ _ _ _
1 + f3
(11.8)
This method can also be used to measure fluxes of
other gases or pollutants by rewriting a more generalized form of the flux -gradient equation by combining H and LE as before to yield
dT e
Rn - G - S = Kpc -
P dz
(11.9)
where K is an eddy diffusivity assumed equal for
all entities (other than momentum), c p is the specific
heat at constant pressure, p is air density and Te is
the equivalent temperature, (T + ely) (Monteith
and Unsworth 1990). If all pollutants and gases
share this value of K then the flux density for
mospheric properties is increased. Measurements
over smooth surfaces, such as water or ice, are
difficult using any of the techniques mentioned.
Increasingly, eddy covariance-based systems are
being integrated into global observing networks
both to monitor carbon and water exchange on the
global scale and to validate remotely sensed products (Running et al. 1999).
Aerodynamic Method
The vertical exchange of atmospheric entities such
as momentum, temperature, water vapor, and CO 2
by turbulent transport is driven by and is proportional to their vertical concentration gradients. We
can describe the transport process in a flux-gradient
form which defines the constant of proportionality
K known as an eddy diffusivity. In generic form,
the flux density (F x' the amount of that entity transported vertically through unit area in unit time) of
any scalar (X) is:
(11.4)
The eddy diffusivities for scalars such as temperature, water vapor, and CO2 have been the subject
of much experimentation over several decades as
their relative magnitudes depend on both surface
and atmospheric features. In practice, the diffusivities are related to the eddy diffusivity for momentum, which can be established from profiles of wind
speed. Wind profiles are used to calculate the friction velocity, a measure of the degree of atmospheric mixing and from which Kx = ku*(z - d)
is calculated (Thorn 1975). The gradients of the
scalar in question need to be precisely determined
and obtaining accurate gradients over rough vegetation can be difficult because of the substantial degree of atmospheric mixing over such canopies
(McNeil and Shuttleworth 1975). One of the concerns about the aerodynamic technique is that a
number of empirical corrections are required to account for changes in atmospheric stability (which
influences the shape of wind profiles). A number of
semiempirical formulae have been extensively investigated and are well accepted (e.g., Webb 1970;
Paulson 1970). The aerodynamic technique has
been used extensively in the past to obtain fluxes
of sensible and latent heat over forests (e.g., Thorn
167
1975; Lindroth 1984) and extended to other trace
gas species over different surface types (e.g.,
Fowler and Unsworth 1979; Sutton et al. 1993). It
is not possible to use the aerodynamic technique in
the roughness sublayer or within the canopy as the
variation in sources and sinks for heat, water vapor,
and CO 2 invalidate the underlying assumptions in
the method (Raupach and Legg, 1984).
Energy BalancelBowen Ratio
The energy balance at the surface is
Rn - G - S = H + LE
(11.5)
Where Rn is net radiation absorbed by the vegetation, G is soil heat flux, and S is heat stored in the
vegetation. The ratio of sensible heat (ll) to latent
heat (LE) flux is known as the Bowen ratio (p) and
by writing the fluxes in their flux-gradient form, an
equation can be found that permits either flux to be
found from measurements of the gradient of temperature (T) and humidity (e), irrespective of atmospheric stability:
f3
dT
1/y -
de
(11.6)
where y is the psychrometric constant (Monteith
and Unsworth 1990). Once f3 has been obtained,
substituting for LE and H in Equation 11.5 we get
R - G - S
H = _n _ _ -::--;-1 + f3 1
(11.7)
and
R - G - S
LE = _n _ _ _ _
1 + f3
(11.8)
This method can also be used to measure fluxes of
other gases or pollutants by rewriting a more generalized form of the flux -gradient equation by combining H and LE as before to yield
dT e
Rn - G - S = Kpc -
P dz
(11.9)
where K is an eddy diffusivity assumed equal for
all entities (other than momentum), c p is the specific
heat at constant pressure, p is air density and Te is
the equivalent temperature, (T + ely) (Monteith
and Unsworth 1990). If all pollutants and gases
share this value of K then the flux density for