Drought Monitoring Techniques for Famine Early Warning Systems in Africa - Monitoring and Predicting Agricultural Drought: A Global Study - page 257

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Line
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4.5
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Norm
PgEn

Fig ure 19.1 Start-of-season (SOS) map for 2001-2002 growing season in southern Africa.

At the end of the crop growth cycle, or up to a certain dekad in the cycle,
cumulative AET and cumulative WR are used to calculate WRSI. A case
of no deficit will result in a WRSI value of 100, which corresponds to no
reduction in yield related to water stress. A seasonal WRSI value < 50 is
regarded as a crop failure condition (Smith, 1992).
Yield reduction estimates based on WRSI contribute to food security
preparedness and planning. As a monitoring tool, the crop performance
indicator can be assessed at the end of every 10-day period during the
growing season. As an early warning tool, end-of-season crop performance
can be estimated by incorporating long-term average of climatological data
for the period from the current dekad to EOS. Due to the different growing
seasons, WRSI maps are generated and distributed on a region-by-region
basis (e.g., Sahel, southern Africa, GHA). At the end of each dekad, two
image products associated with the WRSI (the current WRSI and extended
WRSI) are produced and disseminated by FEWS NET.
The current WRSI map portrays WRSI values for a particular crop from
the onset of the growing season until the current dekad. It is based on actual
estimates of meteorological data to date. For example, if the cumulative
crop water requirement up to this dekad is 200 mm and only 180 mm
was supplied in the form of rainfall and available soil moisture, the crop
experienced a deficit of 20 mm during the period, and the WRSI value
will be (180/200)100 = 90%. This approach is slightly different from the
traditional FAO update, where the cumulative deficit-to-date is compared
to the seasonal crop water requirement instead of the requirement up
to the current period. The FEWS NET WRSI may increase, decrease, or

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