Drought Monitoring Techniques for Famine Early Warning Systems in Africa - Monitoring and Predicting Agricultural Drought: A Global Study - page 255

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for estimating predisposition to drought. NDVI and RFE are the principal
tools for current seasonal monitoring.
The primary geospatial climate monitoring products used by FEWS
NET are derived from remote sensing data collected by meteorological
satellites (Hutchinson, 1991). NDVI images produced from Advanced Very
High Resolution Radiometer (AVHRR) imagery acquired by the National
Oceanic and Atmospheric Administration (NOAA) polar orbiters have the
longest history of use in the project (French et al., 1996). They are pre-
pared for FEWS NET by the Global Inventory Monitoring and Modeling
Studies research unit at the NASA Goddard Space Flight Center accord-
ing to techniques described by Los et al. (1994). Since the NDVI signal is
approximately linearly related to the area average photosynthetic capacity
of the plant canopy at a location (Tucker and Sellers, 1986), it is used as
an indirect measure of the condition of rain-fed crops. Chapter 5 provides
details on NDVI applications.
Exploitation of NDVI by FEWS NET for monitoring is simple and
straightforward. The image for the current 10-day period (or dekad) is used
to compute two difference images. The first is the difference between the
NDVI for the current dekad and that of the previous dekad. This reveals
areas that are greening up or drying down. The second difference is with
respect to the average NDVI for the 1982-2002 historical period. This
reveals areas of anomalous conditions relative to the long-term average.
The other operational geospatial climate product used by FEWS NET
is the RFE produced by NOAA's Climate Prediction Center. They are
also compiled on a dekadal basis, with each pixel's value representing an
estimate of the total millimeters of rainfall that have fallen at that location
during the 10-day period. Image differencing is applied to them in much
the same way that it is to the NDVI images. A difference with respect
to long-term average shows wet and dry rainfall anomalies. However,
since the time series of the RFE is short, a standard based on surface
fitting of station data with long records is also used (Hutchinson et al.,
1996).
Apart from the difference-image products, FEWS NET also produces
area-average time-series traces of NDVI and RFE for key crop-growing
regions. Time series traces as well as other operational monitoring products
(e.g., NDVI, RFE, daily rainfall estimates, soil water index anomaly) are
available on the FEWS NET Web sites ( www.fews.net; edcintl.cr.usgs.gov/
adds) and are described in Rowland (2001).

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W ater Requirement Satisfaction Index

The spatially explicit water requirement satisfaction index (WRSI) is an
indicator of crop performance based on the availability of water to the
crop during a growing season. Food and Agriculture Organization (FAO)
studies (Doorenbos and Pruitt, 1977) have shown that WRSI can be related