The Light Environment of Plant Canopies
The sensible heat flux from the vegetation.soi1 system is closely related
to surface aerodynamic temperature by
where gHa is the aerodynamic conductance or canopy boundary-layer
conductance given by Eq. (14.9) and T, is the air temperature. The apparent simplicity of Eq. (15.34) is deceptive. Assuming the information is
available on a continental basis to estimate gHa, and this is no minor task
because vegetation height, cover, and wind speed are required (remote
sensing of NDVI may help here), three major challenges remain in trying
to use radiometric temperature to estimate sensible heat flux:
1. The radiometric temperature and aerodynamic temperature are not the
same and usually differ by 1 to 5" C.
2. The near-surface air temperature is not known on the same spatial scale
as radiometric temperature and can vary by 5" C or more depending
on the temperature of the underlying surface.
3. Atmospheric corrections and uncertainties in surface emissivity associated with satellite-borne surface radiometric temperatures have
uncertainties of 1 to 3" C.
Unfortunately, an uncertainty of 1" C in T, , - Ta can result in a 50 W
m-* uncertainty in Hc,; a reasonable estimate of a tolerable maximum
error. These challenges have not deterred scientists from searching for a
solution.
From this discussion a practical method for using satellite surface
temperature measurements should have at least three qualities:
1. Accommodate the difference between aerodynamic temperature and
radiometric temperature.
2. Not require a measurement of near-surface air temperature.
3. Rely more on differences of surface temperature over time or space
rather than absolute surface temperatures to minimize the influence of
atmospheric corrections and uncertainties in surface emissivity.
Anderson et al. (1997) have proposed such a method based on satellite
observations from the Geosynchronous Orbiting Environmental Satellite
(GOES), which is used primarily for observations of clouds and weather
forecasting, having a ground spatial resolution of 4 km. In addition to the
satellite temperature observations, they use ground measurements and
balloon measurements from the weather forecasting network, a continental vegetation classification map, and vegetation cover estimated with
NDVI as described in the previous section. Uncertainties in sensible and
latent heat of 30 to 50 W m-2 are achievable by this method. Practical methods for using satellite observations of surface temperature to
partition sensible and latent heat fluxes on a continental scale are most
challenging.
The sensible heat flux from the vegetation.soi1 system is closely related
to surface aerodynamic temperature by
where gHa is the aerodynamic conductance or canopy boundary-layer
conductance given by Eq. (14.9) and T, is the air temperature. The apparent simplicity of Eq. (15.34) is deceptive. Assuming the information is
available on a continental basis to estimate gHa, and this is no minor task
because vegetation height, cover, and wind speed are required (remote
sensing of NDVI may help here), three major challenges remain in trying
to use radiometric temperature to estimate sensible heat flux:
1. The radiometric temperature and aerodynamic temperature are not the
same and usually differ by 1 to 5" C.
2. The near-surface air temperature is not known on the same spatial scale
as radiometric temperature and can vary by 5" C or more depending
on the temperature of the underlying surface.
3. Atmospheric corrections and uncertainties in surface emissivity associated with satellite-borne surface radiometric temperatures have
uncertainties of 1 to 3" C.
Unfortunately, an uncertainty of 1" C in T, , - Ta can result in a 50 W
m-* uncertainty in Hc,; a reasonable estimate of a tolerable maximum
error. These challenges have not deterred scientists from searching for a
solution.
From this discussion a practical method for using satellite surface
temperature measurements should have at least three qualities:
1. Accommodate the difference between aerodynamic temperature and
radiometric temperature.
2. Not require a measurement of near-surface air temperature.
3. Rely more on differences of surface temperature over time or space
rather than absolute surface temperatures to minimize the influence of
atmospheric corrections and uncertainties in surface emissivity.
Anderson et al. (1997) have proposed such a method based on satellite
observations from the Geosynchronous Orbiting Environmental Satellite
(GOES), which is used primarily for observations of clouds and weather
forecasting, having a ground spatial resolution of 4 km. In addition to the
satellite temperature observations, they use ground measurements and
balloon measurements from the weather forecasting network, a continental vegetation classification map, and vegetation cover estimated with
NDVI as described in the previous section. Uncertainties in sensible and
latent heat of 30 to 50 W m-2 are achievable by this method. Practical methods for using satellite observations of surface temperature to
partition sensible and latent heat fluxes on a continental scale are most
challenging.