Monitoring Agricultural Drought: The Case of Kenya - Monitoring and Predicting Agricultural Drought: A Global Study - page 241

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[241

Line
——
1.5
——
Long
PgEn

[241

ranked in ascending or descending order. Statistical techniques are used to
cluster the current observation within a group of similar past occurrences.
The methods that have been used to derive such clusters range from highly
sophisticated models like the Palmer drought index (PDI; Palmer, 1965;
WMO, 2000) to simple indices such as the tertiles, quartiles, deciles, and
percentiles. Most of the drought indices require long-term series of histori-
cal data. The choice of an index depends on the availability and extensive-
ness of data. For example, the use of percentiles requires about 100 years
of data.

Palmer Drought Index The PDI for any individual month ( X i ), may be

expressed as:

p i −

ˆ p

12

PE

R / P L ( 0 . 31 τ +

x i = [18.1]

+ − 2 . 69 )

i = 1

where ˆ P is climatically appropriate water balance for the existing condition