# Sentinel-2 Reveals Record-Breaking Po River Shrinking Due to Severe Drought in 2022
## 3 Results
### Changes in Monthly Water Frequency  
MWF in the June-September period of 2022 was significantly lower than in other years on 71.4% of the analyzed area of the Po River (Table 2), whereas the remaining years show proportions lower than 20%. Figure 5 shows the significant reduction in the river width between July 2018 and July 2022 in the focus area of Calendasco (located near Piacenza, 340 km from the river mouth, 45 m a.s.l.). In 2022, the number of pixels with MWF equal to 100% is approximately one order of magnitude lower than in 2018, associated with a corresponding decrease in water-full pixels (>90% water content, Figure 5c). The anomaly against the 2016-2023 mean (Figure 5d) also indicates a reduction in the water content in July 2022 compared to the average of previous years, highlighting with a red color scale (i.e., negative PWC anomaly) the areas where the riverbed has narrowed as a result of the reduced river flow due to drought conditions. Figure 6 shows the DPM during the period May 2022-October 2022 in Pontelagoscuro (where the reference hydrometer is located, 83 km from the river mouth, 5 m a.s.l. ), highlighting the shrinking during the period May-July, followed by a widening of the riverbed as rainfall returned. The MWFA against the 2016-2023 mean was further evaluated across space and time. Figure 7 shows the spatio-temporal evolution of MWFA during the whole study period and across the ~500 km of the Po River analyzed in this study. Across the temporal dimension, the entire year 2022 results as a negative MWFA, while the spatial dimension shows areas with almost no morphological changes as an effect of artificial elements on channel forms and processes. The wide range of river morphologies along the Po course resulted in markedly different CVOWF (Figure 8), with values close to 10% in meander-rich areas and almost zero in confined single-thread channels. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Year** & **Number of 1 km Slices** & **Proportion of the Entire Study Area (\%)** \\ \hline
2016 & 89 & 18.5 \\
2017 & 66 & 13.7 \\
2018 & 27 & 5.5 \\
2019 & 21 & 4.3 \\
2020 & 8 & 1.7 \\
2021 & 32 & 6.7 \\
**2022** & **342** & **71.4** \\
2023 & 61 & 12.7 \\ \hline \hline \end{tabular} Values in bold refers to the analyzed drought year 2022.  
\end{table} 
Table 2: Results of \(t\)-test analysis on MWF for the period June-September. For each year in the period 2016-2023, average number of slices where the MWF of the given year was found to be lower than the average (\(p<0.05\)), along with the % over the whole study area, are reported. Figure 5: Calendasco focus area (green rectangle in the overview map). Monthly water frequencies (MWFs) observed in **(a)** July 2018 and **(b)** July 2022. **(c)** Comparison of distribution frequency of MWF in July 2018 and July 2022. **(d)** Monthly water frequency anomalies (MWFAs) in July 2022. Figure 6: Pontelagoscuro focus area (green rectangle in the overview map). Difference from the previous month (DPM) during the period May 2022–October 2022. **(a)** May 2022. **(b)** June 2022. **(c)** July 2022. **(d)** August 2022. **(e)** September 2022. **(f)** October 2022. The OWF along the Po River has the capability to represent the diversity of geomorphic units, showing braided patterns (Figure 8a), partially confined channel forms (Figure 8b), and longitudinal lateral bars (Figure 8c). The analysis of the MWF values highlights a higher variability for more dynamic river areas, characterized by larger riverbeds and the presence of longitudinal bars, also associated with higher CV\({}_{\text{OWF}}\) values. Figure 8: Spatial distribution of OWF for the three selected focus areas (green rectangles in the overview map, with corresponding sub-figure letter). (**a**) Braided patterns. (**b**) Partially confined channel forms. (**c**) Longitudinal lateral bars. (**d**) Coefficient of variation in monthly water frequency (CV\({}_{\text{OWF}}\), 10 km moving average) over the river course.