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A MDLTISPECTRAL CANOPY REFLECTANCE MODEL 
A. KUUSK 
Institute of Astrophysics and Atmospheric Physics 
Estonian Academy of Sciences, EE2444 Toravere (Estonia) 
ABSTRACT: 
The leaf optical model PROSPECT by Jacquemoud and Baret (1990), the soil 
reflectance spectrum representation with basis functions by Price (1990), and the 
skylight ratio spectral representation by McCartney (1978) have been integrated 
into Kuusk's (1993) fast canopy reflectance model. The resulting new multispectral 
canopy reflectance model describes the directional reflectance of a homogeneous 
vegetation canopy over 400 to 2500 nm with high spectral resolution. The number of 
input parameters of the model does not depend on the number of the spectral bands 
used. The model has high computational efficiency, and thus it can rather easily 
be inverted to determine the vegetation parameters from remote optical measure 
ments. The model is tested with the corn reflectance data of Ranson et al. (1985). 
KEY WORDS: Canopy Reflectance Spectrum, Directional Reflectance, Model 
1. BACKGROUND 
Several models have been developed to describe the reflection of optical radiation 
from vegetation canopies, for reviews see Myneni and Ross (1991), Myneni et al. 
(1989). The need for models with few input parameters and sufficient precision 
still stimulates the development of new models (Pinty et al., 1990; Verstraete et 
al., 1990; Bégué, 1992; Jacquemoud, 1993; Kuusk, 1993) to be applied in inversion 
problems. From measurements of the angular distribution of spectral reflectance, 
agronomic variables of a canopy can be determined with the help of canopy 
reflectance models (Goel and Strebel, 1983; Kuusk, 1991a). At the same time, it is 
rather complicated to apply such models for the inversion of satellite data: it is 
difficult to measure the angular distribution of canopy reflectance at satellite 
level, and very accurate atmospheric correction is needed. It is much easier to 
measure nadir reflectance spectra at satellite level. However, the increasing of 
the number of spectral channels is not a solution to the inversion problem, since 
the number of input parameters to canopy reflectance models increases correspon 
dingly. A possible solution is a model, which can describe simultaneously the 
directional reflectance of a canopy over the whole optical region. The first 
attempt to develop such a model has been made in the recent paper by Jacquemoud 
0993). In its PR0SPECT+SAIL model the wavelength-dependent optical parameters of 
the SAIL model are given with the PROSPECT model (leaf parameters) and the 
SOILSPECT model (soil parameters), the fraction of diffuse radiation SKYL is 
considered constant. 
The general outline of the present paper is rather similar: different 
submodels are applied to determine the wavelength-dependent input parameters of a 
base model. The fast canopy reflectance (FCR) model of Kuusk (1993) has been chosen 
for the base model. The FCR model is the successor to Nilson and Kuusk's (1989) and 
'»erhoef's (1984) models. It has only 8 input parameters and very high computer- 
efficiency. The FCR model describes well the angular distribution of the spectral 
directional reflectance, considering the specular reflection on leaf surfaces and 
the hot-spot effect. This model has three groups of input parameters: (1) optical