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.
The grades of drought–flood index
| Grade . | Drought–flood index . | Drought and flood type . |
|---|---|---|
| 1 | Z> 1.6485 | Extreme flood |
| 2 | 1.0364 < Z≤ 1.6485 | Serious flood |
| 3 | 0.5244 < Z≤ 1.0364 | Light flood |
| 4 | −0.5244 < Z≤ 0.5244 | Normal |
| 5 | −1.0364 < Z≤ − 0.5244 | Light drought |
| 6 | −1.6485 < Z≤ − 1.0364 | Serious drought |
| 7 | Z< − 1.6485 | Extreme drought |
| Grade . | Drought–flood index . | Drought and flood type . |
|---|---|---|
| 1 | Z> 1.6485 | Extreme flood |
| 2 | 1.0364 < Z≤ 1.6485 | Serious flood |
| 3 | 0.5244 < Z≤ 1.0364 | Light flood |
| 4 | −0.5244 < Z≤ 0.5244 | Normal |
| 5 | −1.0364 < Z≤ − 0.5244 | Light drought |
| 6 | −1.6485 < Z≤ − 1.0364 | Serious drought |
| 7 | Z< − 1.6485 | Extreme drought |