Monitoring Drought Using Coarse-Resolution Polar-Orbiting Satellite Data - Monitoring and Predicting Agricultural Drought: A Global Study - page 75

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in semiarid areas. Increases in atmospheric water vapor have the tendency
to reduce near-infrared reflectance and therefore decrease NDVI values,
resulting in an underestimation of primary production. Aerosols from fires
and volcanoes may further attenuate NDVI values in various ways depend-
ing on the underlying surface. At the regional scale, other factors such as
vegetation type, soil type, and topographic variations may alter NDVI val-
ues and may have to be taken into consideration for improved crop yield
estimates and assessments. In general the NDVI data presented here are
more than adequate for monitoring vegetation conditions over fairly large
areas, in the order of 100 km 2 . The data, however, remain inadequate for
monitoring and evaluating the status of crops at the field or individual plot
level. Most of the regions where these data are used comprise complex
mixtures of cropland, natural vegetation, and nonvegetated areas. This
therefore adds complexity to the interpretation of AVHRR-NDVI data at
local scales.
Alternative vegetation indices such as SAVI, MSAVI, PVI, GVI, and
other advanced satellite systems (such as SPOT VGT, SeaWiFS, and Terra/
Aqua-MODIS instruments) may offer improvements in drought impact
studies on crop yield assessments, but they have not yet been validated
or implemented globally, other than in specific case studies at regional
scales. The 4

[75],

Line
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4.6
——
Long
PgEn

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years of global MODIS EVI data (2000-2003) that are now
available may still be insufficient for constructing a baseline or reference
database from which one could measure anomalies in drought conditions.
Effective translation coefficients would be needed to bridge any improved
indices with the NDVI, as well as to extend the various indices across
different sensors and platforms. Despite the shortcomings, the NDVI data
set has become a principal layer of information for drought monitoring at
regional or global scales.

[75],

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