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Proceedings of the Symposium on Global and Environmental Monitoring

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
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
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.
Fractional Light Absorption