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

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