Drought Monitoring Techniques for Famine Early Warning Systems in Africa - Monitoring and Predicting Agricultural Drought: A Global Study - page 259

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Line
——
0.8
——
Long
PgEn

Fig ure 19.2 Maize WRSI at the end of January 2002 during the 2001-2002 growing season.

pe riod, from third dekad of June to third dekad of July; second period,
fr om first to the third dekad of August; and third period, from first to third
de kad of September. Similar periods are defined for the Sahelo-Sudanian,
Su danian, and Sudano-Guinean zones.

[259

D elineation of Base-Unit Polygons NDVI data for the period from 1982
to 1995 were used to define five agroclimatological zones in the Sahel de-
pe nding on the timing of green-up. For example, zones are defined as “very
ea rly” green-up in the southernmost regions of the Sahel, all the way to
“l ate” green-up in the northern regions of the Sahel. The agroclimatolog-
ic al zones have been merged with conveniently small administrative units
of the Sahel, resulting in an agroclimatic-administrative unit map that con-
sis ts of polygons referred to as “base units.” This step is necessary because
th e subdivision into agroclimatological zones does not follow administra-
tiv e borders, and some administrative units are large enough to straddle
tw o or more agroclimatic zones. The base units are used to extract spatial
st atistics (e.g., average) for NDVI and RFE.

C omputation of RNCD Indices Data used for the computation of indices
ar e extracted from the images for all dekads during the growing season
us ing the base units. After data are extracted for each dekad and each
po lygon, a dekadal index (DI) is calculated according to

DI = ( x − ¯x )/σ [19.4]

where x is either NDVI or RFE for a given polygon for the current season,
x is the historical average, and σ is standard deviation.