*3.2. Thresholds Based on Antecedent Rainfall*

Intensity-duration thresholds consider only the immediate preceding rainfall event as a triggering factor of landslides. Landslides may occur as result of moisture content variation due to continuous precipitation also, which is difficult to monitor precisely. Thus a simple way is to study the effect of antecedent rainfall and define a threshold based on antecedent rainfall before the landslide event. Studies have been conducted across the globe, considering different antecedent periods ranging from 3 days to 120 days [2,4,42].

The data of 225 landslides over a period of nine years has been used for the analysis. Daily rainfall records at failure are compared with the antecedent rainfall of 3, 10, 20, 30, and 40 days before failure. The graph is plotted with antecedent rainfall (mm) and daily rainfall (mm) in x and y axes respectively. The diagonal line of the plot determines the scattering bias of the data (Figure 10).

**Figure 10.** Plot of daily rainfall vs. antecedent rainfall (3, 10, 20, 30, and 40 days).

A significant share of landslide events is biased towards the antecedent rainfall in all cases. Hence a threshold is defined for all individual time durations of antecedent rainfall considered in the study as shown in Figure 11a–e. In the first case, three days' antecedent rainfall was considered, 28% of the total events considered are shifted towards daily rainfall, and the remaining 163 landslides are biased towards three days' antecedent rainfall. For other cases, the biasness ratio to daily rainfall and antecedent rainfall was found to be 11:214 for 10 days', 6:219 for 20 days', 3:222 for 30 days', and 1:224 for 40 days' antecedent rainfall prior to the slide event. It is evident from the analysis that the biasness towards antecedent rainfall, which was 72% in case of 3 days' antecedent rainfall increased to 99.56% when the antecedent rainfall of 40 days was considered as shown in Figure 11f. The study can be refined if the temporal resolution of the rainfall data available is improved.

**Figure 11.** Plot between daily rainfall and antecedent rainfall before failure for (**a**) 3 days, (**b**) 10 days, (**c**) 20 days, (**d**) 30 days, (**e**) 40 days, and (**f**) Biasness towards antecedent rainfall.
