Hydrological response to meteorological drought in southern Piedmont

This study aims at characterizing the response of hydrological basins of southern Piedmont (Italy) to meteorological drought that, through the “propagation” process, can evolve in hydrological drought, causing several damages to the economic and social sectors. More specifically, in this work we detect the correlation between meteorological and hydrological drought on a basin scale for catchments of different size. We select three direct tributaries of the Po River in southern Piedmont: Varaita, Maira and Tanaro rivers, the latter being much wider than the first two. First, we quantify the severity of the meteorological and hydrological droughts as independent phenomena, through the computation of two standardized indexes for the three basins from the last years of the 1990s to 2022 (availability of data). To identify meteorological droughts, we exploit the Arpa Piemonte dataset NWOI, with a 0.125 resolution, containing daily precipitation, daily maximum and minimum temperature, and calculate the monthly Standardized Precipitation Evapotranspiration Index (SPEI) for 1, 3, 6, 9, 12 and 24 months aggregation periods. The SPEI's are then averaged on the areas of the three basins, achieving a single monthly value for each catchment and aggregation period. To identify hydrological droughts, we consider the gauging stations located at the closing section of each basin, and we use the Standardized Streamflow Index (SSI). To calculate SSI, the capability of five probability distributions to represent all monthly streamflow data is quantified and tested through goodness-of-fit tests. Among all tested distribution, the log-logistic is chosen since it shows a good fit for all the monthly series. Once the monthly series of SPEI and SSI is obtained for each catchment, a correlation analysis is set by using the Pearson coefficient, both on the whole period and on a monthly scale. The results show a high positive correlation for all the aggregation periods. Interestingly, and unexpectedly, the highest correlation is detected for 6 months and 24 months for the Varaita and Maira catchments and for 3 months aggregation period for the Tanaro basin, even though the correlation remains higher than 0.6 even for longer timescales. The hydrological response of the first two catchments is similar, as expected, since they are medium-high slope mountainous basins with fast response in the short time scale, showing unexpectedly a good memory for the response associated to the long-time scales. The Tanaro basin responds to short and medium time scale in autumn and winter, presenting a lower correlation in the long-time scale. The reasons for the difference in the timescales characterizing the propagation from meteorological to hydrological droughts should be investigated by expanding the analysis on more catchments characterized by different climatic and hydrological regimes.