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According to the normal distribution curve of variable Z (Zhang et al. 1998), seven grades of the Z index have been established for the drought–flood index as shown in Table 1.

Table 1

The grades of drought–flood index

GradeDrought–flood indexDrought and flood type
Z> 1.6485 Extreme flood 
1.0364 < Z 1.6485 Serious flood 
0.5244 < Z 1.0364 Light flood 
−0.5244 < Z 0.5244 Normal 
−1.0364 < Z − 0.5244 Light drought 
−1.6485 < Z − 1.0364 Serious drought 
Z< − 1.6485 Extreme drought 
GradeDrought–flood indexDrought and flood type
Z> 1.6485 Extreme flood 
1.0364 < Z 1.6485 Serious flood 
0.5244 < Z 1.0364 Light flood 
−0.5244 < Z 0.5244 Normal 
−1.0364 < Z − 0.5244 Light drought 
−1.6485 < Z − 1.0364 Serious drought 
Z< − 1.6485 Extreme drought 

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