# Sentinel-2 Reveals Record-Breaking Po River Shrinking Due to Severe Drought in 2022
## 3 Results  
### River Discharge  
According to the data provided by the hydrological station in Pontelagoscuro, Po River's mean flow in July 2022 was approximately 160.6 m\({}^{3}\)/s, significantly lower than the 2016-2021 average of 750.8 m\({}^{3}\)/s [49]. The July 2022 drought was part of a broader trend of declining river flows, with a particularly pronounced summer decline.  
The combined analysis of SDI and CV for the year 2022 demonstrates the intensity and variability of the hydrological drought conditions observed throughout the year in the entire Po River basin. Figure 3 shows how the SDI values remained negative throughout the year, with the lowest values observed between June and July, indicating continuous moderate to severe drought conditions from February to September. On an annual basis, the comparison shows that 2022 has the lowest SDI value (\(-0.94\)) along the entire study period (1961-2023), followed by 2007 (\(-0.66\)). While considering individual months of 2022, the SDI values range between \(-0.67\) in December (mild drought) and \(-1.6\) in June (severe drought). In addition, CV, represented by the size of the bubbles in Figure 3, was generally higher in spring and summer (April to August), with a maximum in June, corresponding to the most severe SDI anomalies. This indicates increased variability in hydrological conditions, particularly river discharge, during the peak drought months. In contrast, there was a continuous increase in SDI values towards the end of the year (September-December), accompanied by a decline in CV, which points to an overall stabilization of river discharge variability.  
Figure 4 shows the SDI values calculated for the study period 1961-2023, referring to the single month and the entire hydrological year. Negative values indicate dry periods, ranging from 0 to \(-2\) as mild to extreme drought. In addition to 2022 and 2007, already mentioned, the years characterized by the lowest SDI annual values are 1990 (\(-0.60\)), 2005 (\(-0.55\)), 2006 (\(-0.56\)), 2017 (\(-0.62\)), and 2023 (\(-0.52\)), indicating that the frequency of drought conditions has increased over the last decades. Notably, the year 2022 has the lowest SDI, resulting in negative values during all the months and a minimum in June. A similar situation can be observed for other years (e.g., 2005 or 2017), but in these  
Figure 3: Monthly SDI and CV (the latter represented by bubble size) calculated for Po River discharge at the Pontelagoscuro hydrological station for the year 2022. See Table 1 for SDI values classification (drought).  
cases, the mean monthly discharge is always higher than the corresponding discharge of 2022 (excluding November 2017). On the contrary, during the year 2023, characterized by monthly discharge values even lower than 2022 from January to April, the discharge notably increased over the last months of the year (above all in November, with discharge higher than 2000 m\({}^{3}\)/s).  
Figure 4: Streamflow Drought Index (SDI) values calculated for the Po River discharge in the period 1961–2023. Discharge data refer to the Pontelagosucro hydrometer (see Figure 1). The blue gradient colors represent positive SDI values; the red gradient colors represent negative SDI values.  
Evidence suggests that the hydrological drought experienced in 2022 was not merely a sporadic occurrence confined to the summer months; rather, it is indicative of a more extensive and persistent decline in river discharge, reaching its peak during the June-July window. Compared to 2022, past years with a similar trend of flow rate and low SDI annual values, like 1990 or 2017, are characterized by higher monthly discharge values for almost all the months.