Monitoring Agricultural Drought in Southern Africa - Monitoring and Predicting Agricultural Drought: A Global Study - page 272

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R ainfall Index

Because of the difficulty and cost of monitoring parameters such as soil
moisture and the lack of data to estimate potential evapotranspiration, the
most widely used index for drought monitoring across southern Africa is
rainfall expressed as a percentage departure from the long-term average
for a given period. With this approach, cumulative seasonal rainfall is
monitored and reported on weekly, monthly, and seasonal time scales
during the rainfall season. A rainfall departure of 25% from the long-term
average for several consecutive weeks during the rainfall season is generally
classified as drought across much of southern Africa. For sub-Saharan
Africa, a positive relationship between national average maize yields and
rainfall has been shown for major production areas, suggesting that total
rainfall may be a simple but useful indicator of drought in drought-prone
areas.

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V egetation Condition Index

In southern Africa, weather data, often from a very sparse meteorological
network, are incomplete or not always available to allow accurate and
timely monitoring of droughts. Studies have been carried out for several
parts of the world that showed the potential of satellite data for drought
monitoring (van Dijk, 1985; Kogan, 1995). The vegetation condition index
(VCI) has been evaluated as an alternative index for drought detection and
monitoring in southern Africa (Unganai and Kogan, 1998; http://orbit-net
.nesdis.noaa.gov/crad/sat/surf/vci, chapter 6).

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C old Cloud Duration

The Southern Africa Development Community Regional Remote Sensing
Unit, a component of the Regional Early Warning System for Food Secu-
rity, uses cloud top temperatures to monitor the progression of the rainfall
season. The thermal infrared signal from the METEOSAT radiometer is
used to produce statistics on the occurrence of clouds associated with rain-
fall over a 10-day period. Detailed description of this technique of rainfall
estimation is provided in chapters 19 and 32.

W ater Requirement Satisfaction Index
Crop moisture stress on grain crops across southern Africa has largely
been monitored using the water requirement satisfaction index (WRSI).
The WRSI indicates the extent to which the water requirements of the crop
have been satisfied in a cumulative way at any stage of the crop-growing
season. Details of computing the WRSI are available in chapters 19 and 32.
The WRSI ranges from 0 to 100. An index below 50 indicates crop failure
and a value 97-100 is associated with good crop condition.