*2.3. Approaches Used to Configure the Threshold Analysis*

In this study, different approaches were considered for deriving intensity–duration thresholds. The method of choosing rain gauges based on the nearest rain gauge is still one of the simplest and, although criticized, widely used methods [35]. To correlate each landslide to the nearest rain gauge, the district was divided into four zones, based on the spatial distribution of rain gauges, by means of the Thiessen polygons technique, as shown in Figure 6. The method is based on a proximal mapping in which the estimate of rainfall at any point is considered equal to the observation of the nearest sampling point in the area [47]. Based on the location of the landslides, the triggering rainfall was identified using the data from the rain gauge stations, using different approaches. Since the temporal resolution of available rainfall data is one day, multiple landslides within the same polygon that occurred on the same day were counted as a single landslide event for the definition of intensity–duration threshold. The first approach was based on a regional scale, where the nearest rain gauge was chosen as the reference gauge and data from all four rain gauges are merged to establish one single regional scale threshold (merged threshold).

**Figure 5.** Box and Whisker plot showing the variation of annual rainfall across the four rain gauge stations.

**Figure 6.** Location of rain gauges and definition of their area of influence based on nearest rain gauge.

Considering the vastness of area and geographical settings, the measurement from the nearest available rain gauge could underestimate the threshold as the localized convective storms might not be recorded at the rain gauge location [35]. Hence, another approach was adopted in which the rain gauge which records the maximum average intensity was chosen as the reference gauge, irrespective of the spatial distribution (Imax threshold).

In the third approach, the peak daily intensity observed from the beginning of landslide event was considered for the analysis instead of the average intensity (Peak I threshold). The reason for a deviation from the conventional average intensity approach is the possible avoidance of the underestimation of thresholds due to the low density of rain gauges.

Lastly, analyses were conducted separately on a local scale, so that four thresholds were derived separately for each polygon (R1, R2, R3 and R4 thresholds).

Separate intensity–duration thresholds were defined for each approach with different exceedance probabilities of 5%, 2.5%, 1% and 0.05% to find the best suited method.
