Agriculture Reference
In-Depth Information
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[74],
Figure 5.12 The variation in beef and veal production in Botswana during 1981-2000.
Line
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-0.2
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Long
PgEn
Nemani et al., 1993) and the water deficit index (WDI; Moran et al., 1994)
use thermal and vegetation index measurements of vegetated surfaces to es-
timate the soil moisture status through the rate of evaporative water loss.
The SMI is based on the slope of the AVHRR-NDVI and maximum
temperature values. Gillies et al. (1997) later applied inversion techniques
to improve the estimates of available soil moisture from VI-T s scatterplots.
These concepts, however, need to be incorporated into vegetation indices
to be suitable for global monitoring of droughts.
[74],
C onclusions
Since the 1980s, NDVI time series data derived from the series of polar-
orbiting NOAA-AVHHR instruments have established a baseline (or refer-
ence) for monitoring global terrestrial vegetation. Early applications were
primarily aimed at validating the utility of the NDVI data by comparing
field measurements to satellite measurements of vegetation characteristics.
Perhaps one of the most critical findings from these early field studies was
that in semiarid areas, NDVI was linearly related to rainfall and thus could
be used as an indicator of past rainfall or vegetation conditions. Field stud-
ies of primary production in conjunction with analysis of satellite vege-
tation indices indicated a strong linear relationship between NDVI and
seasonal primary production, particularly over semiarid and arid environ-
ments. These are areas that are subject to lowest rainfall leading to droughts
that result in food shortages and famine. Although AVHRR data are at
coarse resolution, crop yield estimates can be satisfactorily made through
comparisons of the time series data with historical production statistics
at national and regional levels and through comparisons between current
crop-growing conditions with those of previous years or their historical
mean.
However, there are some problems associated with using the data set
 
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