Multiscale Climatology of Daily Runoff and Monthly Temperature and Precipitation Since the Mid 19^th Century in Two Alpine Rivers

Daily runoff data of the Adda River and of the Adige river were recovered by digitizing old yearbooks for the 1845-2016 and 1862-2011 period respectively. Monthly areal precipitation, temperature and evapotranspiration indexes were estimated for the same period at the spatial scale of the two catchments. A statistical analysis applying the Mann-Kendall, Spearman and Theil-Sen trend tests shows a decline, in the long term, of runoff and a more significant one of precipitation. To analyse changes occurred at different time scales Exploratory Data Analysis running trends point out two severe droughts in the 1940s and 2005s which were confirmed only in part by the wavelet transform applied to the two daily runoff series. The Fourier power spectrum of the 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. More relevant 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. 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 changes observed in the hydrological cycle and resulting water resources management practices in the Alps.