| Variable | Evaluation grade | |||
| Low | Moderate | High | Very high | |
| Historical rainfall (mm) | ≤600 | 600 and ≤1700 | 1700 and ≤2500 | ≥2500 |
| (0 - 0.25) | (0.26 - 0.5) | (0.51 - 0.75) | (0.76 - 1) | |