Food and Agriculture Organization and Agricultural Droughts - Monitoring and Predicting Agricultural Drought: A Global Study - page 415

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density of the meteorological stations reporting to the GTS contributes
to the uncertainty. Recent approaches now combine METEOSAT with
GTS data and data from microwave images or sounding instruments, the
most popular microwave data being Special Sensor Microwave Imager.
Although improvements are still needed before remote sensing can provide
the quantitative data required for crop yield models, the remote sensing
data remain an essential, yet partial, component for monitoring food sup-
ply and demand to improve world food security.
Over the years, as the use of satellite-derived information became more
and more integrated in the operations of the above programs, there was
a growing demand for more and better data, which could only be par-
tially met. The ARTEMIS area coverage was extended first to South and
Central America with NOAA GAC-derived NDVI in cooperation with
NASA's Goddard Space Flight Center and, later, through cooperation with
the Japanese Meteorological Agency. Monsoon monitoring over Asia was
also explored, based on Geostationary Meteorological Satellite data. Al-
though this was certainly an improvement, many areas of special interest
to GIEWS, such as the Commonwealth of Independent States and North
Korea, were still not covered.
In southern Africa, FAO has assisted many SADC countries in rehabili-
tating the agricultural system after a devastating drought. In such circum-
stances, FAO makes arrangements for assessing the essential agricultural
inputs needed to restore production in the affected countries. FAO also
makes an appeal for financial assistance to implement emergency relief,
short-term rehabilitation, and preparedness interventions to the interna-
tional donor community.

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FA O Crop Water Requirement Satisfaction Index Model

The FAO crop water requirement satisfaction index (WRSI) has been used
extensively, especially in Africa, for crop monitoring for food security. The
model can detect the onset of agricultural drought, which is indicated by
the crop stress. The model has been used to effectively monitor agricultural
drought in many parts of the world.
The WRSI determines a cumulative water balance for each period of
10 days (1 dekad) from planting to maturity. The cycle of each crop is
subdivided into successive dekads. For each dekad, using a water-balance
approach involving rainfall, evaporation, crop water requirements, and soil
water-holding capacity, the cumulative water available (either surplus or
deficit) at the beginning of each dekad can be computed (FAO, 1996).
Basically, the water balance is the difference between the effective
amounts of rainfall received by the crop and the amounts of water lost by
the crop and soil due to evaporation, transpiration, and deep infiltration.
The amount of water held by the soil and available to the crop is also taken
into account. In practice, the water balance is computed using a bookkeep-
ing approach. The computation is done dekad by dekad and begins before