Geoscience Reference
In-Depth Information
Green vegetation
0.6
Dry vegetation
0.4
Soil
0.2
Figure 5.6
Typic a l
variation of spectral
reflectance with wavelength
for green vegetation, dry
(dead) vegetation, and soil.
0.0
0.5
1.0
1.5
2.0
2.5
Wavelength (
μ
m)
The value of the albedo also strongly depends on the altitude of the Sun and so
varies through the day.
The reflection coefficient for electromagnetic radiation incident on land sur-
faces in the solar waveband changes with the wavelength of the radiation and this
behavior is important both in the context of remote sensing and when building
numerical models of land surface exchanges. Figure 5.6 shows examples of the
variation in reflection coefficient with wavelength for fresh green vegetation, dry
(dead) vegetation, and soil. The reflection coefficient for soil changes with wave-
length, but in a much less dramatic way than it does for green leaves. The distinct
change in the reflection coefficient for plant leaves above and below about
0.72
m is associated with the absorption of
Photosynthetically Active Radiation
(PAR), i.e., that portion of incoming solar radiation that plants use to provide the
energy they need to carry out photosynthesis. In Fig. 5.6, for example, the ratio of
the reflection coefficient at 0.65 and 0.85
μ
m is about 1.2 for soil and about 1.5 for
dry (dead leaves) but is much greater for actively transpiring green leaves. Some
remote sensing systems measure the relative reflection coefficient at selected wave-
lengths above and below 0.72
μ
m and use this distinct difference in the ratio of the
measured reflection coefficients to diagnose the extent to which vegetation covers
the soil beneath. The difference in reflection coefficient for leaves above and below
0.72
μ
m is so distinct that some advanced models of land surface exchanges also
choose to recognize it in their computations and they separately model the absorp-
tion and reflection of visible light in wavebands below and above this wavelength.
μ
Maximum solar radiation at the top of atmosphere
As mentioned earlier, the flux of solar energy at all wavelengths incident on unit
area normal to solar beam at the outer edge of atmosphere when the Earth is at its
mean distance (one astronomical unit) is called the '
solar constant
',
S
o
. In fact the
