The Palmer Drought Index (PDSI) can be used to evaluate the severity of drought over time. It is most useful for quantifying the impact of a prolonged change in precipitation. The author was a meteorologist for the US Department of Commerce.
drought index =
= (SUM(monthly moisture anomaly indices over time period)) / ((0.309 * (number of months in time period)) + 2.691)
where:
• The monthly moisture anomaly index ("z") is derived by multiplying a monthly weighting factor (k value) and the monthly moisture departure (d value).
• I am not sure if this only works for periods up to 48 months (see Figure 1, page 20).
Interpretation:
• minimum score: < -4.00
• maximum score: > +4.00
Severity Index
Class of Conditions
<= -4.00
extreme drought
-3.00 to –3.99
severe drought
-2.00 to –2.99
moderate drought
-1.00 to –1.99
mild moderate
-0.50 to –0.99
incipient drought
-0.49 to +0.49
near normal
+0.50 to +0.99
incipient wet spell
+1.00 to +1.99
slightly wet
+2.00 to +2.99
moderately wet
+3.00 to +3.99
very wet
>= +4.00
extremely wet
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