*4.2. Validation of Rainfall Threshold*

The significance of any landslide study is determined by the validation of the results obtained. One review of rainfall threshold studies [40] emphasized the importance of validation of rainfall thresholds for conducting landslide studies. The rainfall threshold validation was performed using rainfall and landslide data from 1 January to 31 December 2018. During this period, a total of 52 landslide events occurred, out of which 80% of landslides (41) happened during the monsoon season. The threshold equation for Zone 1 was RTH = 150 − 0.24R30ad, and its validation for the monsoon of 2018 is depicted in Figure 7. The threshold exceedance axis depicts the value of RTH with respect to R30AD value for each day, wherein the positive values indicates landslide occurrence. This figure shows that a heavy measure of rainfall occurred, exceeding the threshold. The abrupt increase in the magnitude of daily rainfall or constant rise in 30 day antecedent rainfall is shown by the rise in the threshold curve.

During the validation period, the threshold was exceeded nine times, out of which seven times landslides occurred. No landslides were reported on 2 June and 7 June, even though the threshold was exceeded. This observation can be attributed to the fact that a landslide event does not generally happen with the increase in threshold curve, and sometimes happens a couple of days later as the result of a difference in pore pressure because of changes in the measure of antecedent rainfall. From 1 October to 30 December, there was no threshold exceedance, and no landslides occurred during that period. Similar validations were carried out for Zone 2 and Zone 3 threshold equations using 2018 rainfall and landslide data. These results indicate that the threshold model performed well for landslide forecasting in 2018.

**Figure 7.** Validation of the threshold equation for Zone 1. The positive values on the Y axis represent exceedance and negative values denote non-exceedance of the threshold.
