Author(s): Roberto Ranzi, Eleni Michailidi, Massimo Tomirotti, Emanuele Eccel, Michele Brunetti, Maurizio Maugeri, Alice Crespi
Keywords: Trend detection; Adda River; Adige river; Wavelet transforms; Long-term records;
Abstract: Daily runoff data of the Adda and of the Adige river basins, in northern Italy, were recovered by digitizing old yearbooks for the 1845-2016 and 1862-2011 period, respectively. Monthly areal precipitation, temperature and potential evapotranspiration were estimated for the same period at the spatial scale of the two catchments. A statistical analysis applying the Mann-Kendall and Theil-Sen trend tests shows, for the common 1862-2011 observation period, a small decline of precipitation and a more significant one of runoff. To analyse changes occurred at different time scales a wavelet spectrum points out one drought in the 2005s which is more severe than that of the 1940s. The Fourier power spectrum of the daily runoff data shows a signal of higher energy corresponding to a period between 11 and 13 years, close to the sunspots cycle period, but the coherence of solar energy and runoff in terms of wavelet co-spectrum results not to be significant. The co-spectrum of monthly runoff and the North Atlantic Oscillation, instead, exhibits a slightly significant coherence for the 11-13 years scale. More relevant than the precipitation decrease is the decline of runoff, which can be explained only in part as a result of increased evapotranspiration losses due to the temperature increase, estimated by monthly indexes. Therefore, other anthropic factors need to be considered, as enhanced water needs for irrigation or land use changes, to explain the increased water losses. The meteorological and hydrological data sets collected are useful to explain long term variability and better assess the impact of climate change and other forcing factors on the hydrological cycle and resulting water resources management practices in the Alps.