A Multi-Century Meteo-Hydrological Analysis in the Italian Alps: Daily Streamflow (1862–2022) at Different Time Scales

In this paper, the second longest time series of daily hydrometric levels and streamflow for an Italian river, Adige, in the Italian Alps, also one of the longest worldwide and unpublished so far, is reconstructed and analyzed. Daily streamflow prior to 1923, when the official mean daily discharge was first published, is estimated based on daily water levels collected since January 1862, cross-section geometry, discharge, and surface velocity measurements at the hydrometric station of Trento. The main objective of this paper is the identification, attribution, and quantification of the impact of natural and anthropic factors on changes in streamflow in a mountain region with marked orographic and climatic gradients. The resulting 161-year-long time series, until December 2022, for this 9763 km2 catchment is firstly analyzed in search of trends and their statistical significance, spectral properties at different time scales and periods, changes in the monthly regime prior to and after the constructions of reservoirs. The observed -1.0 mm year-1 slope of the annual streamflow linear trendline is statistically significant and indicates a decline of -1.4% per decade of available streamflow in the river, similar to the one observed in nearby basins. The spectral analysis conducted with the wavelet transform indicates that a sudden change of spectral properties and trends of daily streamflow occurred inside the pre- and post-reservoir construction period and can be explained also as a result of a more environment-oriented legislation. A wavelet coherence spectrum between streamflow and teleconnection indices indicates the existence of a significant coherence with the Atlantic Multidecadal Oscillation only. The comparison with estimated actual reference evapotranspiration losses and temperature points out that the observed temperature increase is not sufficient to explain the observed hydrological losses, being precipitation almost constant over the observation period. The observed increase of 86 mm of hydrological losses over the last century is explained in terms of water withdrawals for agricultural, civil, and industrial needs (38 mm), enhanced evapotranspiration due to temperature increase (30 mm), expanded artificial lakes' surface (1 mm), the residual of 17 mm being attributed to land-use changes with afforestation.