IWA Publishing

Table 2

Drought classification based on the frequency of occurrence (WMO 2012)

Drought category	SPEI value	Number of times in 100 years	Severity of event
Moderate dryness	−1 s/d −1.49	10	1 in 10 years
Severe dryness	−1.5 s/d −1.99	5	1 in 20 years
Extreme dryness	≤− 2	2.5	1 in 50 years

Drought category	SPEI value	Number of times in 100 years	Severity of event
Moderate dryness	−1 s/d −1.49	10	1 in 10 years
Severe dryness	−1.5 s/d −1.99	5	1 in 20 years
Extreme dryness	≤− 2	2.5	1 in 50 years