Food and Agriculture Organization and Agricultural Droughts - Monitoring and Predicting Agricultural Drought: A Global Study - page 416

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the planting to account for moisture stored in the soil. From the planting
dekad, the crop water requirements are calculated as the potential evapo-
transpiration (PET) times the crop coefficient (KCR). Thus, the available
water amount is the difference between the crop water requirements and
the working rainfall. These amounts do not consider water stored by the
soil. The working rainfall amount reflects the effective water received by
the crop and is calculated through a ratio defined by the user on the basis
of the type of soil, slope, and so on. Normal rainfall is used in case of miss-
ing values. Surplus or deficit result from the water balancing and ranges
between the field capacity and the permanent wilting point, depending on
the root depth and the soil water-holding capacity. Finally, the WRSI in-
dicates the degree to which cumulative crop water requirements have been
met at the growth stage. The WSI represents, at any time of the growing
period, the ratio between the actual and the potential evapotranspiration
(FAO, 1996).
This WRSI model could be considered a combination of dynamic (wa-
ter balance) and statistical (calibration of yield function) approaches. In
fact, at harvest time, the sum of dekadal water deficit (or stress) suffered
by the crop can be used as a variable along with some other relevant vari-
ables to forecast crop yield by statistical regression. At present, it is difficult
to incorporate, into yield models, the variables derived from soil fertility,
technology (mechanization, fertilizer use), varietal differences, and farming
practices. It is a characteristic of the FAO approach that these important
parameters are considered, along with NDVI, at the next stage of develop-
ment of yield (“agmet”) models.
The yield function is valid for a crop and a group of stations in a homo-
geneous cropping area. The input data correspond to different geographical
units, from weather stations, to pixels (NDVI, CCD: 50 km 2 ), to adminis-
trative units. It is an important step in the forecasting method to convert the
data to comparable units (area averaging)—usually administrative areas
that are used by planners or decision-makers in the field of food security.

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A gricultural Drought Mitigation
FAO attaches particular importance to promoting the production and
consumption of drought-tolerant food crops in areas where rainfall is
uncertain. This in some cases involves redressing tendencies that were
widespread in agricultural development programs during the 1970s and
1980s. During that period, green revolution packages involving high-yield
hybrids or composites were promoted in pursuit of higher overall produc-
tion levels. This strategy often did not identify the risks inherent in adopt-
ing varieties that do not perform well under conditions of moisture stress.
Shifts in the cropping system may also entail a shift in food consumption
habits and raise the need for support to maintain nutritionally balanced
diets. Such support may be provided through nutrition education. Sup-
port could be furnished, for example, in the context of local-level planning
workshops for the formulation of drought mitigation plans.