652
2. METHODS
2.1. Simulation of the canopy reflectance data
The ABSAIL model was used to simulate radiometric data in the red (650nm), near infrared (850nm) and PAR
(400-700nm) wavebands. Three simple, two-layer chlorosis distributions with equal amounts of green and
chlorotic leaves were simulated. The first of these had an upper layer of green leaves and a lower of chlorotic
leaves, the second had a uniform mixture of green and chlorotic leaves in each layer, and the third an upper layer
of chlorotic leaves and a lower layer of green leaves. The optical properties of the green and chlorotic leaves
used in the simulations are shown in Table 1. The other variables required by ABSAIL to simulate canopy
reflectance are shown in Table 2. The model was run for all combinations of the parameters shown in Table 2
(7 soils, 9 ALA and 8 LAI) and for each of the three chlorosis distributions.
red (650nm)
nir (850nm)
PAR (400-700nm)
green leaf reflectance %
7.5
46.5
15.0
green leaf transmittance %
3.0
49.0
7.5
chlorotic leaf reflectance %
2.5
46.5
5.0
chlorotic leaf transmittance %
1.0
49.0
2.5
Table 1 Leaf optical properties used in the calculation of canopy reflectance.
soil reflectance % (650nm)
5, 10, 15, 20, 25, 30, 35
soil reflectance % (800nm)
10, 16, 22, 28, 34, 40, 46
leaf angle distribution
ellipsoidal
average leaf angle
30°, 35°, 40°, 45° ,50°, 55° ,60°, 65°, 70°
leaf area index
0.1, 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, 12.8
view zenith angle
nadir
solar zenith angle
45°
fraction of diffuse skylight
0.0
Table 2 Other variables used to calculate canopy reflectance.
NDVI and TSAVI were calculated from the red and near infrared reflectances. NDVI is defined as:
NDVI = (nir-r)/(nir+r)
and the definition of TSAVI given by Baret and Guyot (1991) is adopted here:
TSAVI = a(nir-ar-b)/(anir+r-ab+X(l+a 2 ))
The values of the parameters used in this study are typical of a wide range of soils: a=1.2, b=0.04, and X=0.08,
Note that the soil line concept is implicitly included in this index since a and b also define the soil line:
nir g = 1.20 x red g + 0.04
