## 4 Discussion
### Comparison with Automated Mapping Methods
Histograms of the detection results of Table 2 are presented in Figure 17. The highest success ratio is achieved by the methods of [44; 46] with almost comparable results, as expected due to the similarities in the calculated change detection data (Sentinel-2 10 m) and parameters (time-series of normalized difference vegetation index). The worst percentage was achieved by ALDI [47], probably due to the lower resolution data used (Landsat multispectral bands with 30 m resolution instead of 10 m for Sentinel-2). SAR backscatter change [45] was close to the first two method results, with the difference attributed to the coarser pixel resolution of Sentinel-1 SAR imagery (20 \(\times\) 4 m, translated into >15 m ground \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Method** & **Positive** & **Negative** & **Success \%** & **Threshold Value** \\ \hline rdNDVI [46] & 872 & 800 & **52.15** & \(-\)0.1\(<\) \\ HazMapper [44] & 911 & 786 & **53.68** & \(-\)10\(<\) \\ ALDI [47] & 659 & 1037 & **38.85** & \(>\)0.025 \\ SAR backscatter change by [45] & 837 & 851 & **49.58** & \(-\)1 \(<\) 1 ratio \(>\) 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Positive and negative detection of mapped landslides by the different change detection methods. Figure 15: Change detection results from HazMapper [44; 48] utilizing multi-temporal Sentinel-2 imagery. Red colors show negative rdNDVI changes that correspond to land cover changes due to landslides and debris flows. Results were in par with manual mapping in the area of high-density landslides southeast of Plastira Lake (**a**), while in most areas change detection was more successful in depicting the major debris flows such as the ones upstream of Mouzakiotikos River (**b**). resolution when geometrically corrected to terrain) and the challenging oblique scanning of SAR satellites that can lead to terrain shadows. In conclusion, rapid manual mapping of landslides for Ianos outperformed automated mapping methods based on Google Earth Engine time-series change detection of satellite imagery. The most prominent factor was the small size of the majority of landslides, which was barely close to the ground resolution threshold of 10 m for Sentinel-2, making it harder to identify by automated change detection algorithms. A large number of false positives were visually identified in the various change detection results, a factor that could negatively affect early response and mapping (Figures S1 and S2). Debris flow and large slides were identified in most automated mapping results (Figure S1), while  
Figure 16: Debris flow and landslides along Megalos River in Filakti/Pezoula area (**a**). 3D view of the Megalos River watershed with debris flows along the main river course and its exit to Lake Plastira (red colors of ALDI change detection index values). Flooding, severe sedimentation and material transportation from upstream was mapped at its exit (photo (**b**), view towards east with location marked in (**a**)) and near Pezoula. Road bridges were destroyed in both locations, with positions labeled in (**a**). Figure 17: Histograms of pixel values by automated mapping methods corresponding to mapped landslides; (**a**) [46], (**b**) HazMapper by [44], (**c**) ALDI [47] and (**d**) SAR backscatter change by [45]. Yellow bars mark negative detections and blue bars positive detection, based on set threshold values for each method. smaller and shallow landslides were difficult to positively identify due to variations in image reflectance and terrain shadows. Surface manifestation of small shallow landslides kept being modified in the days and weeks following the landslide event, due to erosion and vegetation regrowth, a factor that affects the signal in the long time-series used by automated change detection methods. It is worth mentioning that a number of landslides occurred in the winter months following the lanos landslide event. These landslides would be included in most automated change detection results due to the larger time frame examined.