observation conditions; and 3) explore relationships between biophysical properties (i.e., LAI, A ) and canopy
reflectance (Suits, 1972; Kimes et al., 1984; Verhoef, 1984).
Photosynthetically active radiation (PAR) absorbed by plant pigments is the energy source for
photosynthesis. The pigments making the greatest contribution to light absorption and photosynthesis are
chlorophyll a, chlorophyll b, and carotenoids (Fig 1). Both chlorophylls a and b have absorption maxima in the
640 nm-690 nm region and in the 440 nm-470 nm region. Also /9 -carotene has an absorption maximum at 470 nm
region and minimum absorption beyond 530 nm. Both chlorophylls and /9 -carotene have minimal absorbance at
550 nm region and beyond 700 nm. One of the canopy reflectance characteristics that directly governs the amount
of light absorption and reflectance
is the quantity of these
photosynthetic pigments in the
plant leaves. Also, the amount of
exposed nonphotosynthetic
materials including soil, leaf litter,
and woody biomass such as twigs,
stems, and trunks have been shown
to be important factors in canopy
reflectance.
The main objective of this
research is to investigate the use of
narrow reflectance bands in remote
estimates of canopy A . The
research concentrateci toward
reducing the variability in the A
estimates due to the presents of
diverse nonphotosynthetic
materials. This technique utilized
narrow spectral bands with
bandwidth of approximately 10 nm.
For evaluating the proposed
narrow band method, the SAIL
(scattering from arbitrarily inclined
leaves; Verhoef, 1984) canopy
model was used; 1) for simulating
vegetation canopy reflectance as a mixture of green biomass and background material components; and 2) to
derive v4 par This model has been used successfully in similar applications (Goward et al., 1992, Hall et al., 1990).
Wavelength (nm)
Figure 1
Absorbance of pure plant pigments (Absorbance = log
(Iq/I), Iq = incident, I = transmitted light)
2. DEVELOPMENT OF NARROW BAND METHOD
2.1 Leaf level Reflectance
TABLE 1. Correlations (r 2 ) between soybean leaf reflectance (n = 50) bands. Reflectance measurements
were acquired using LI-COR 1800 spectroradiometer and integrating sphere.
Wave (nm)
550
670
690
700
710
750
800
550
1.000
0.786
0.930
0.992
0.981
0.000
0.062
670
0.786
1.000
0.914
0.802
0.710
0.005
0.083
690
0.930
0.914
1.000
0.955
0.871
0.001
0.075
700
0.992
0.802
0.955
1.000
0.973
0.000
0.059
710
0.981
0.710
0.871
0.973
1.000
0.008
0.034
750
0.000
0.005
0.001
0.000
0.008
1.000
0.903
800
0.062
0.083
0.075
0.059
0.034
0.903
1.000
