Drought Monitoring Techniques

In southern Africa most of the techniques used for drought monitoring are heavily influenced by the meteorological definition of drought. The rainfall index, the vegetation condition index, and the water requirement satisfaction index are used to monitor the progress of the rainfall season and its impact on agriculture.

Rainfall 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.

Vegetation 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).

Cold Cloud Duration

The Southern Africa Development Community Regional Remote Sensing Unit, a component of the Regional Early Warning System for Food Security, 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 rainfall over a 10-day period. Detailed description of this technique of rainfall estimation is provided in chapters 19 and 32.

Water 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.

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