Monitoring Drought Using Coarse-Resolution Polar-Orbiting Satellite Data - Monitoring and Predicting Agricultural Drought: A Global Study - page 60

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(EOS) Terra and Aqua ( http://aqua.gsfc.nasa.gov/) platforms, launched in
December 1999 and May 2002, respectively. The MODIS instruments
provide global coverage in 36 spectral bands with a spatial resolution rang-
ing from 250 m to 1 km. A number of spectral bands and band com-
binations on this instrument are invaluable to drought, vegetation, and
climate studies. Bands 1 and 2 provide 250-m resolution imagery in the
red and near-infrared (NIR) regions, and bands 3-7 provide 500-m resolu-
tion imagery in the visible, NIR, and SWIR regions. This includes bands 5
(1230-1250 nm) and 6 (1628-1652 nm). In these bands, leaf water content
influences the canopy reflectance response. The remaining bands are at 1
km spatial resolution and include measurements in the thermal infrared as
well as optical data useful for ocean, atmosphere, and cryosphere applica-
tions. This sensor therefore fills the spatial resolution data gap between
the Landsat and SPOT satellite series and the coarser NOAA-AVHRR
data. Already a number of valuable land products have been assembled
and are publicly available for use through various distribution centers
( http://edcdaac.usgs.gov/modis/dataprod.html). The products include the
traditional normalized difference vegetation index (NDVI) data, as well as
the enhanced vegetation index (EVI) data, which offer improvements over
the NDVI by minimizing saturation problems at high biomass conditions
and by reducing atmosphere and soil background noise (Huete et al., 2002;
Justice et al., 2002).

[60],

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-0.1
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Norm
PgEn

Re mote Sensing of Vegetation
Remote sensing of vegetation is accomplished by using the strong coupling
between reflected visible and NIR radiation with the physiological condi-
tion of leaves and their density. Photosynthesis generally occurs through
CO 2 gas exchange between hydrated chloroplast cells that are in direct
contact with the intercellular air spaces within the leaves (figure 5.2; Gates
et al., 1965).
Chlorophyll is a strong absorber of visible energy, and the interaction
of near-infrared wavelengths with vegetation provide important clues as
to the structures of plant leaves. A consequence of the internal structure of
leaves is that visible and near-infrared solar radiation passing through the
leaves are deflected and scattered due to the refractive index, differences
between hydrated cells ( η

[60],

1.0 µ m), and
the irregular pattern of cell facets in the mesophyll leaf. These scattering
effects increase the effective path length of radiation as it passes through the
leaves, resulting in increased absorption of visible light by plant pigments
and liquid water. In contrast, there is negligible absorption at NIR wave-
lengths, which results in an enhanced spectral reflectance response (figure
5.3; Tucker and Sellers, 1986).
In the natural world, plant leaves are organized into canopies of varying
structures and leaf orientations. The spectral reflectance and absorptance
of the leaf elements are modified by the canopy structure and the amount

∼ 1.3 µ m) and air spaces ( η =