# Sentinel-2 Reveals Record-Breaking Po River Shrinking Due to Severe Drought in 2022
## 4 Discussion
### Applicability of the Proposed Methodology: Po River Shrinking in Summer 2022  
The proposed methodology is a cost-effective tool, relatively easy to use also by non-specialists, and fully reproducible. Being based on normalized band reflectance differences, this method is easily applicable to other sensors and allows the synergic use of multiple sensors operating at different spatial and temporal scales. Po River shrinking during the record-breaking summer 2022 drought was analyzed using all the available Sentinel-2 MSI optical multispectral satellite acquisitions over the period 2016-2023. The analysis was based on the calculation of the PWC through the application of the fuzzy transform function, a mathematically well-founded soft computing method [53], on the normalized difference water index (NDWI), calculated from satellite imagery at 10 m spatial resolution. Spatio-temporal variations were analyzed through MWF maps, which were used to calculate the DPM and anomalies from the reference climatology (MWFA) (shown here as a time series over the period 2016-2021), showing the seasonal and interannual variation in the width of the riverbed. This approach enabled the quantitative assessment of drought impact as indicated by the reduction in the PWC compared to the previous year. Additionally, the difference in water frequency from the preceding month facilitated the identification of anomalies crucial for monitoring short-term dynamics and planning timely interventions. The main variations were evident especially in more dynamic river sections, characterized by braided patterns or longitudinal lateral bars, as the two selected focus areas, i.e., Calendasco and Pontelagoscuro (see Figures 5 and 6). Moreover, the OWF calculated over the period 2016-2023 was used to calculate CVOWF, which provided a summary of the temporal changes and river dynamics during the investigated years. The CVOWF values range between more than 10% in meander-rich areas and almost zero in single-thread channels (Figure 8). The results demonstrate a notable reduction in the riverbed area in 2022, particularly during the late spring and summer months. This is evident both by comparing the MWF of July in the years 2018 and 2022, and by comparing July 2022 with the climatological average of the study years. The statistical significance of the slices' average number where the MWF was found to be lower than the average demonstrates how effective MWF is in revealing the river shrinking. Further developments of the methodology based on the classification of additional variables, such as vegetation and sediments, could facilitate the interpretation of riverine data, potentially enabling the prediction of riverbed changes. Integration with in situ data and measurements has improved the interpretation of the results obtained through satellite image processing. Water discharge records across the years facilitated the reconstruction of the historical trend of Po River discharge (1961-2021), enabling a comparative analysis with the 2022 flow. In particular, the application of the SDI allowed for the quantification of the hydrological drought severity, while through the discharge CV, it was possible to identify the months of greatest water stress (late spring-summer), evidencing that the observed river shrinkage correlates with record-low water discharges in June and July 2022. The integration of diverse analytical approaches was crucial in addressing the limitations of employing individual methods, thereby facilitating a more precise comprehension of the hydrological drought dynamics and their repercussions on riverine systems. While this study is primarily focused on the Po River 2022 summer drought, the satellite observations facilitate not only the monitoring of climate change-related phenomena but also the observation of the geomorphological processes of the river.