Agriculture Reference
In-Depth Information
CHAPTER SIX
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[79],
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Norm
PgEn
Operational polar-orbiting environmental satellites launched in the early
1960s were designed for daily weather monitoring around the world. In
the early years, they were mostly applied for cloud monitoring and for
advancing skills in satellite data applications. The new era was opened
with the series of TIROS-N launched in 1978, which has continued un-
til present. These satellites have such instruments as the advanced very
high resolution radiometer (AVHRR) and the TIROS operational verti-
cal sounder (TOVS), which included a microwave sounding unit (MSU), a
stratospheric sounding unit (SSU), and high-resolution infrared radiation
sounder/2 (HIRS/2). These instruments helped weather forecasters improve
their skills. AVHRR instruments were also useful for observing and moni-
toring earth surface. Specific advances were achieved in understanding veg-
etation distribution. Since the late 1980s, experience gained in interpreting
vegetation conditions from satellite images has helped develop new appli-
cations for detecting phenomenon such as drought and its impacts on agri-
culture. The objective of this chapter is to introduce AVHRR indices that
have been useful for detecting most unusual droughts in the world during
1990-2000, a decade identified by the United Nations as the International
Decade for Natural Disasters Reduction.
[79],
A
VHRR-Based Vegetation Indices
Radiances measured by the AVHRR instrument onboard National Oceanic
Atmospheric Administration (NOAA) polar-orbiting satellites can be used
to monitor drought conditions because of their sensitivity to changes in leaf
chlorophyll, moisture content, and thermal conditions (Gates, 1970; My-
ers, 1970). Over the last 20 years, these radiances were converted into in-
dices that were used as proxies for estimating various vegetation conditions
79
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