Monthly storminess over the Po River Basin during the past millennium (800–2018 CE)

Abstract

Reconstructing the occurrence of diluvial storms over centennial to millennial time-scales allows for placing the emergence of modern damaging hydrological events in a longer perspective to facilitate a better understanding of their rate of return in the absence of significant anthropogenic climatic forcing. These extremes have implications for the risk of flooding in sub-regional river basins during both colder and warmer climate states. Here, we present the first homogeneous millennium-long (800-2018 CE) time-series of diluvial storms for the Po River Basin, northern Italy, which is also the longest such time-series of monthly data for the entire Europe. The monthly reconstruction of damaging hydrological events derives from several types of historical documentary sources and reveals 387 such events, allowing the construction of storm severity indices by transforming the information into a monthly, quantitative, record. A period of reduced diluvial storms occurred in the ninth and tenth centuries, followed by a stormier period culminating in the eleventh and thirteenth centuries. More complex patterns emerge in the fourteenth to sixteenth centuries, with generally wetter and stormier conditions than during other centuries. From the seventeenth century onwards the number of damaging hydrological events decreases, with a return in recent decades to conditions similar to those prior to the thirteenth century The flood frequency tended to increase for all seasons during periods of low solar irradiance, suggesting the presence of solar-induced circulation changes resembling the negative phases of the Atlantic Multidecadal Variability as a controlling atmospheric mechanism.

Historical predictability of rainfall erosivity: a reconstruction for monitoring extremes over Northern Italy (1500–2019)

“…We use these factors (amounts of precipitation and weather anomalies) to develop a parsimonious model for reconstructing annual rainfall erosivity over the period 1500-2019 CE, allowing us to capture a wide range of climate variability, and identifying landscape-stress changes. The results of this study complement, for the past five centuries (1500-2019 CE), the twelve century-long (800-2018 CE) reconstructions of extreme hydro-meteorological events across Italy 56 and in the Po River Basin 70 .…”

Reconstruction of erosivity density in northwest Italy since 1701

“…• Annual severity storm index sum (ASSIS): A categorical variable, the annual severity storm index sum (ASSIS), was derived from Diodato et al (2020c) to overcome the lack of historical information about rain intensity. ASSIS was developed based on several written sources, by transforming documentary information into a record set to 0 (normal event), 1 (stormy event), 2 (very stormy event), 3 (great stormy event) or 4 (extraordinary stormy event).…”

“…The original ASSIS time series of yearly values (Diodato et al 2020c) was smoothed by applying the low-pass Gaussian filtering technique described by Gjelten et al (2016). Weighting coefficients (w ij ) were applied to derive the Gaussian function, ASSIS(GF), for each year j:…”

Climate Patterns in the World’s Longest History of Storm-Erosivity: The Arno River Basin, Italy, 1000–2019 CE

“…These contrasting results underline that different analyses can change the perception of the type of hazard associated with hydrological processes when dissimilar metrics are used. Reconstructed rainfall erosivity in two Mediterranean fluvial basins, the Calore Basin in southern Italy (Diodato et al, 2008) and the Po Basin in northern Italy (Diodato et al, 2020c), showed similar increasing trends since the end of the LIA. If this rainfall regime continues, it could result in an increased erosive hazard affecting Mediterranean lands because erosive events occur in a more erratic way.…”

“…That exceptional event was the result of a more general regional-scale shift, which can be considered the main cause of the hydrological change that occurred during that period, also in other areas of northern Italy (Glur et al, 2013;Baldini and Bedeschi, 2018;Diodato et al, 2020c). Leonardo da Vinci himself, who knew the territory of Tuscany well, left many drawings with projects that were supposed to alleviate the effects of the increasingly dangerous floods and soil erosion (Pelucani.et al, 2017).…”