Estimation of the antecedent rainfall period for mass movements in Taiwan

Antecedent rainfall plays an important role in rainfall-induced mass movements. However, it is difficult to define the appropriate period of antecedent rainfall for mass movement assessment. To solve this problem, this study provides a simple approach that combines calibrated antecedent rainfall (CAR) and 24-h rainfall for 283 mass movements that occurred in Taiwan from 2006 to 2013. The 24-h rainfall at the time of each mass movement was compared with the total cumulative rainfall for various periods preceding the event. The lowest correlation was found for the total cumulative rainfall from 15 to 20 days before a mass movement day. The 24-h rainfall was compared with the cumulative CAR values for various days of antecedent rainfall. The effect of cumulative CAR on mass movements increased from 22.0 to 39.7% when the number of days considered was increased from 3 to 30 days. However, the increase became gradual after 15-18 days. In addition, the critical antecedent rainfall conditions occurred within 18 days before mass movements in all cases. These results suggest that the antecedent rainfall of 15-18 days is useful for mass movement assessment in Taiwan. This study also established a critical antecedent rainfall threshold for mass movements in Taiwan that is useful for early warnings: I-a = 28.7 D-a(-1.24), where Ia is critical mean rainfall intensity during the antecedent rainfall period up to 18 days (mm/day) and D-a is the length of the antecedent rainfall period. According to the relationship between 24-h rainfall and the critical antecedent rainfall conditions, low antecedent rainfall intensity that continues for a long time leads to a gradual increase in soil moisture such that a small amount of 24-h rainfall can trigger mass movements. On the other hand, high antecedent rainfall intensity for a short time is not enough to increase soil moisture, and a large amount of 24-h rainfall is needed to flush surface materials and cause mass movements.