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Title
Mesures physiques et signatures en télédétection

THE USE OF HIGH SPECTRAL RESOLUTION BANDS FOR ESTIMATING
ABSORBED PHOTOSYNTHETICALLY ACTIVE RADIATION (A^J
x Moon S. Kim, 2 C. S. T. Daughtry, 3 E. W. Chappelle, 2 J. E. McMurtrey, and 1 C. L. Walthall
1 Laboratory for Global Remote Sensing Studies
Department of Geography, University of Maryland
College Park, MD 20770
2 Remote Sensing Research Laboratory
ARS, United States Department of Agriculture
Beltsville, MD 20775
3 Laboratory for Terrestrial Physics
NASA/Goddard Space Flight Center
Greenbelt, MD 20771
ABSTRACT
Most remote sensing estimations of vegetation variables such as leaf area index (LAI), absorbed photosynthetically
active radiation (A ), and phytomass are made using broad band sensors with a bandwidth of approximately 100
nm. However, higii resolution spectrometers are available and have not been fully exploited for the purpose of
improving estimates of vegetation variables. The study was directed to investigate the use of high spectral
resolution spectroscopy for remote sensing estimates of A in vegetation canopies in the presence of
nonphotosynthetic background materials such as soil and leaf utter. A high spectral resolution method defined
as the chlorophyll absorption ratio index (CARI) was developed for minimizing the effects of nonphotosynthetic
materials in the remote estimates of A . CARI utilizes three bands at 550, 670, and 700 nm with bandwidth of
10 nm. Simulated canopy reflectance of a range of leaf area index (LAI) were generated with the SAIL model
using measurements of 42 different soil types as canopy background. CARI obtained from the simulated canopy
reflectance was compared with these broad band vegetation indices (normalized difference vegetation index
(NDVI), soil adjusted vegetation index (SAVI), and simple ratio (SR)). CARI reduced the effect of
nonphotosynthetic background materials in the assessment of vegetation canopy A par more effectively than broad
band vegetation indices.
KEY WORDS: A r CARI, SAIL MODEL, NDVI, SAVI, SR, VEGETATION INDICES
1. INTRODUCTION
Current remote sensing estimates of vegetation variables such as green biomass, leaf area index (LAI), and A mr
are made with broad band sensors with bandwidths of approximately 100 nm. These broad band vegetation indices
have been shown to suffer from a sensitivity and the reflectance of nonphotosynthetic background materials
(Choudhury, 1987, Huete, 1989, Goward et al., 1992).
Although recent advances in technology have allowed the use of high resolution spectroscopy for remote
sensing, this technology has not been fully exploited for characterization of the atmosphere-plant-soil complex.
Only a few researchers, to this time, have worked with narrow spectral band reflectance as a means of eliminating
the effects of nonphotosynthetic background materials in vegetation canopy reflectance. The ratio analysis of
reflectance spectra (RARS) by Chappelle et al. (1992) showed that ratios of reflectance in narrow bands correlated
well with leaf pigment concentration, and they suggested that photosynthetic pigments may be remotely estimated
with high accuracy from narrow band reflectance. The use of second derivative spectra has shown promise for
reducing the effects of nonphotosynthetic background materials in vegetation canopies (Hall et al., 1990;
Demetriades-Shah et al., 1990).
Canopy reflectance is am integrated function of leaf optical properties, plant structure, background
reflectance, and solar illumination and view angles. Canopy reflectance models have provided tools to; 1) assess
the effect of different canopy characteristic on reflectance; 2) evaluate plant canopy reflectance under varying
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