## 5 Discussion
The RST local (at pixel scale) differential approach allows at overcoming this limitation for the permanent snow-covered areas that, showing all the time very high NDSI value, will not produce any anomalous ALICE values, also in presence of fresh snow. On the other hand, different bands combination, such as using the NIR channel in place of the SWIR one, could help to face this issue within the zones characterized by sporadic snow events. In any case, other test cases should be investigated in the next future to better assess the above-mentioned issues as well as to further confirm the quality of the results achieved in this work, considering other environmental scenarios as well as flooding events of a different nature and dynamics. Moreover, it should be stressed that a possible implementation of RST-FLOOD at global scale would require a consistent preliminary work, aimed first at collecting multiple-year time series homogeneous satellite records and then to their analyses in order to produce the reference fields. Finally, it is worth noting that the proposed approach can also be exported on other VIIRS bands able to detect flooded area due to the differential nature of RST scheme. Thermal Moderate bands (i.e., M15: 10.263-11.263 \(\upmu\)m; M16: 11.538-12.488 \(\upmu\)m) can be indeed used to identify inundated area by exploiting the difference in terms of thermal inertia (TI) that these areas should present respect with the unperturbed condition of the surrounding areas [63]. Water shows a higher TI than dry soil and consequently a less capability to change its temperature during the diurnal cycle. Therefore, those areas where soil water content increases because of the flood typically show a reduced diurnal temperature fluctuation compared to the dry conditions, with, in particular, a brightness temperature increase in night-time [64]. This physical property will allow also for a night-time flood detection that will enforce the potential of this sensor in providing near-real time and continuous information about flood dynamics. In this framework, we have implemented the same approach described in Section 3.3 to analyse the VIIRS M15 image acquired on 4 December 2013 at 00:45, producing the corresponding RST-FLOOD map (Figure 14), where the anomalous pixels in terms of increasing ALICE\({}_{\text{TIR}}\) values are reported. Although the lower spatial resolution of these data (750 m) than the Imagery ones, the result is quite impressive, because all the anomalous pixels are located along the rivers in good agreement with the flood hazard map (Figure 4) and they are almost well co-located to the areas already detected (see Figure 10). It is worth noting that ALICE\({}_{\text{TIR}}\) value higher than 4 were not found and that, in the night-time images of the following days, flooded pixels were detected at ALICE values less than 2, hence resulting no statistically significant. The low spatial resolution of the thermal data, as well as a lower signal intensity in correspondence of flooded areas than the one measured at VNIR wavelengths, is probably at the basis of such a behaviour. Figure 14: ALICE\({}_{\text{TIR}}\) map for the VIIRS data of 04/12/2013 at 00:45 GMT. Considering that such a VIIRS image has been acquired almost 12 hours before the first diurnal one considered in the previous analysis, the integration of the maps referred to night-time (Figure 14) and daytime (Figure 10) conditions, further extends the investigated temporal domain.