A multi-century meteo-hydrological analysis for the Adda river basin (Central Alps). Part I: Gridded monthly precipitation (1800-2016) records

The 1800-2016 monthly precipitation record for the upper Adda river basin is presented. It is computed by applying the anomaly method to a quality-checked and homogenized observation database. The reconstruction accuracy and its evolution over the study period is evaluated at both station and grid-cell levels. The anomaly-based interpolation provides rather robust estimates even for the early years of sparse station coverage with basin precipitation reconstruction errors around 10%. The Theil-Sen trend analysis on the basin precipitation series shows significant (Mann-Kendall p value <.05) long-term tendencies of -3.8 +/- 1.9% and -9.3 +/- 3.8% century(-1) for annual and autumn precipitation, respectively, even though the annual trend is not significant by excluding the first decades from the evaluation. As the basin precipitation record is expected to be underestimated due to the rain-gauge snow undercatch, the monthly precipitation fields are subjected to a correction procedure which allows to derive the multiplicative correcting constant to be applied to the basin annual precipitation series. The comparison between 1845 and 2016 yearly corrected precipitation and runoff records highlights current annual water losses of about 400 mm while the annual runoff coefficients exhibit a long-term significant decrease of -6.4 +/- 1.0% century(-1). This change in the hydrological cycle is mostly to be ascribed to the strong long-term reduction in annual runoff values (-11.8 +/- 3.2% century(-1)) driven by increasing evapotranspiration due to both temperature increase and, likely, land-use changes.