Íslenskar landbúnaðarrannsóknir - 01.03.1980, Page 22

20 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
radiance asymptote occurred at ~300
g/m2 total dry biomass. In addition, the
red radiance asymptote was reached at
much lower levels of total dry biomass
than the IR, IR/red ratio, IR-red differ-
ence, and VI asymptotes (Figures 3 and
4).
The IR radiance aslo approached a
radiance asymptote but this occurred at
much higher amounts of total dry biomass
than the red radiance asymptote (Figures
3b and 4b). The IR radiance asymptote
occurred at ~600 + g/m2 total dry
biomass. This is consistent with other
work on asymptotic properties of spectral
reflectance or radiance from grass
canopies where the senior author has
found that two to three times die arnount
of biomass can be estimated in the photo-
graphic infrared region (0.74 to 0.9 tim)
than can in the red region (0.60 to 0.70
íim). The biological basis for the different
asymptotic spectral responses is of use in
explaining the results of the data analysis
and will be reviewed briefly.
The red and IR asymptotic spectral
radiances in a grass canopy context are
related to the penetration of incident light
within ihe canopy. When the radiance in
the red or IR has asymptoted, the addition
of more vegetation per unit area effects no
change in the canopy radiance because
the incident light is incapable of addi-
tional canopy penetration and escape to
the measuring instrument. The red
radiance asymptotes at much lower levels
of total dry biomass than the IR radiance
because of the absorption of red light by
chlorophyll present in the green vegeta-
tion. The greater the chlorophyll density,
the lower the red radiance and hence the
inverse relationship seen in Figures 3a and
4a.
The situation in the near infrared re-
gion is quite different, however. Little or
no absorption occurs in this waveiength
region, and because of the eff-ct of reilec-
tance enhancement by the various ieaf
layers, high levels of reflectance are possi-
ble (Tucker 1977). The transparency of
leaves in this wavelength region results in
a gradual increase in radiance or reflec-
tance as the biomass (multiple leaf layers)
increases (Figures 3b and 4b).
The red and IR radiances were meas-
ured as radiances in /iW/cm-2 units. The
radiance, in this case a spectral radiance,
was the product of the spectral reflectance
and the solar spectral irradiance. Because
the spectral irradiance va"ies greatlv with
clouds, timeofday, timeofyear, and other
weather conditions, the spectral radiance
can also vary greatly. To compensate for
this possible (and frequent) variation, a
ratio approach was used to normalize the
red and IR radiances. This allows for the
uncorrected use of these spectral meas-
urements. (Note: An alternative would be
to carry around a 20 X 20 cm reference
panel lo adjust each reading to the refer-
ence panel.)
Transgenerated variables
In an attempt to avoid a cumbersome and
lime consuming calibration procedure,
various transgenerationsofthe red and IR
radiances were used. It has been found
that the IR/red ratio, the IR-red dilfer-
ence, and the vegetation index are some-
what similar (Figures 3 and 4) and that
these three transgenerated variables ef-
fectiveiy normalize varying solar irradi-
tional conditions under many cir-
cumstances. (Figure 5).