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EXPLANATION AND USE OF EQUATIONS THAT CAN AID GLOBAL 
MONITORING .OF VEGETATION RESOURCES 
C. L. WIEGAND and M. SHIBAYAMA 
USDA-ARS, Remote Sensing Research Unit, 2413 E. Highway 83, Weslaco, TX, USA 
78596-8344 Pho: 512/968-5533 FAX: 512/565-6133 
National Institute of Agro-Environmental Sciences, Remote Sensing 
Laboratory, Tsukuba, Ibaraki 305, Japan Pho:0298-38-8159 Fax:0298-38-8199 
ABSTRACT 
Plants integrate seasonal growing conditions and express their responses through the 
size and duration of the canopies. Vegetation indices (VI) that can be calculated 
from handheld or boom-mounted ground, aircraft, or polar orbiting satellite spectral 
observations in the near-infrared and visible red portions of the spectrum are a 
good measure of the photosynthetic size of plant canopies. In this paper equations, 
that are collectively called spectral components analysis (SCA), are presented, 
explained, and discussed in terms of their use for monitoring vegetation conditions 
and production anywhere in the world. They are appropriate for guiding the 
interpretation of responses of specific indicator fields in LANDSAT TM and SPOT HRV 
data and for synoptic assessments using coarser resolution NOAA-AVHRR data. The 
approach is consistent with agronomic and physiological principles, and bridges 
between inferences based on spectral observations alone and traditional plant 
growth/yield models. We conclude that the approach offers the possibility of not 
only unifying interpretations but also strengthening their scientific basis. 
Key Words: Vegetation indices, Growth, Yield, Spectral components 
analysis, Environment, Drought, Stress responses 
INTRODUCTION 
Plants integrate the soil and aerial 
environments they are exposed to during 
the growing season and express their 
responses to those growing conditions 
through the size and persistence of 
their canopies (Wiegand and Richardson, 
1984). The canopies of range, forest 
and row crop plant communities are now 
periodically observed over the whole 
globe by sensors aboard the polar 
orbiting earth resources and weather 
satellites. Therefore, those 
observations must contain information 
on plant vigor and productivity. 
The calculation of spectral vegetation 
indices, such as the greenness (GVI), 
perpendicular (PVI), normalized 
difference (NDVI) and transformed soil 
adjusted (TSAVI) vegetation indices 
from reflectance factor or radiance 
observations in visible (400-740 nm) 
and near-infrared (750-1350 nm) 
wavebands (Rouse, et al., 1973; Kauth 
and Thomas, 1976; Richardson and 
Wiegand, 1977; Jackson, 1983; Baret et 
al., 1989), is now routine, and they 
are becoming recognized as measures of 
the photosynthetic size of the canopies 
(Wiegand et al. 1989; Wiegand and 
Richardson, 1990a). 
However, a unified theory for 
interpreting vegetation indices, called 
spectral components analysis, SCA 
(Wiegand and Richardson 1984, 1987, 
1990a; Wiegand et al. 1986, 1989) has 
only recently been developed. It 
provides a framework for interrelating 
a large number of variables including 
vegetation indices, VI; green leaf area 
index, L; photosynthetically active 
radiation absorbed, APAR (MJ/m 2 /day); 
yield (Y, g/m 2 ) of the salable plant 
part; total aboveground dry phytomass 
(DM, g/m 2 ), and évapotranspiration 
(ET, mm/day) or their daily cumulations 
that describe plant canopy and yield 
responses to environments and 
stresses. Photosynthetically active 
radiation absorbed by the canopy is 
defined by 
APAR = (IoMFPAR) [0] 
where Io is daily incident PAR 
(MJ/m 2 /day) and FPAR is the decimal 
fraction of it the canopies absorb. 
The purposes of this paper are to 
present, illustrate, and explain the 
SCA equations and to discuss their use 
for monitoring vegetation conditions 
and estimating production anywhere in 
the world. 
EQUATIONS AND ILLUSTRATIONS 
Fractional Light Absorption