FLOWS

For 4 of the 6 measuring sections considered (whose spatial placement and their characteristics are reported respectively in Figure 4 and in Figure 5), the annual volumes recorded in 2021 are higher than those of the previous year, with percentages that for 3 of the 4 sections exceed 20%, while the gap is smaller than the average values calculated on the decade of comparison except for the Pontelagoscuro section on the Po (Figure 1). For this section, in fact, the decrease in volumes compared to both 2020 and the deed of comparison is about 20%. These evaluations are found from the observation of the graph shown in Figure 2 in which the normalized value of the monthly average amount is represented, obtained from the ratio between the average monthly flows recorded in 2021 and those obtained by mediating the values of the previous decade, for which a continuous series of data is available. The most critical months were those from September to November, the latter month in which all sections recorded reductions of more than 35% of the average monthly rates compared to the decent.  This is in line with what is described in the indicator "hydrological shock" published in the Bank data indicators ISPRA 2022, in relation to the fact that at the end of 2021 different localized drought situations have affected Northern Italy as precursors of severe or extreme drought situations that have occurred, persistently, in 2022. In the period September-November 2021, in particular, at the section of San Giovanni alla Vena on the Arno the monthly flow values were not only systematically below the average values referring to the previous decade, but in 2 of the 3 months were less than half. Conditions of deficit of marked scope also occurred in March and concerned the sections of San Giovanni alla Vena sull'Arno and Ripafratta sul Serchio, while in terms of persistence of the conditions of scale deficit, in addition to the section on the Arno, are the sections of Ripetta sul Tevere and Pontelagoscuro on Po with those in which for at least 6 months the monthly flow rates of 2021 were less than 80% of the tense. For the Ripetta section the reduction compared to the previous decade is observed for 10 consecutive months, with the exceptions of the months of January and February, in which all the sections recorded monthly loads above the averages of the decade. In the last 3 months of the year, the section of Montegalda on the Bacchiglione is characterized by monthly flow rates permanently below 70% of the flow rates of the decade. The only section in which in 2021 there is an increase in volumes compared to the previous year and compared to the decade of comparison is that of Ripafratta on the Serchio for which the increase is 25% compared to 2020 and 13% compared to the decade. In most of the flow sizes sections considered, the most significant floods occurred in January and February, with the exception of the Boara Pisani section on the Adige that is more affected by the effect of the Nivale dissolution in the summer period (Figure 3).