Long term (1930-2018) climate and snow cover trend in Val D’Ossola valley, Western Alps.

Val d’Ossola is an alpine valley in the Western Alps, nesting some of the highest peaks of Italy (4609 m a.s.l.), and several snowfield and glaciers, providing freshwater in thaw season. Since the ’30s snow and ice melting were exploited largely for hydropower production, and climate stations were installed to monitor local climate and to help the assessment of production. Here, we exploited data from such stations, pursing statistical analysis of 2 dataset, i.e. i) Set1 with daily temperature T, precipitation P, and snow depth HS for 9 stations (1930-2018), and ii) Set2 with data of snow depth HS, and density ρS measured on a fortnight basis (1° February to 1° June) for 47 stations (2007-2021).

After preliminary data quality assessment, we pursued Mann Kendall (MK, bulk, and progressive) test and Linear Regression LR (with change point detection CP), to highlight in Set1 a positive/negative trend for temperature/snow depth.

The use of progressive MK and CP provided evidence of negative trends of HS (ca. -100 mm for annual peak), and snow cover duration DS (ca. -27 days per year) since the late ’80s. Spring/summer T is significantly correlated with retirement of a nearby Swiss glacier (Muttgletcher).

For Set2, with snow depth and density available, we found Snow Water Equivalent SWE = HS • ρs peaking nearby May 15th, more than one month later than the peak in snow depth HS (April 1st), displaying that decrease of HS in April is likely due to settling, rather than to mass loss by ablation. The seasonal peak of SWE is however delayed with altitude, namely by ca. 6 days later, every 100 m upward.

Our results match those from other studies in the Alpine area, and can be used as a benchmark for snow cover assessment under climate change, and study of seasonal water delivery in the Alps of Italy, and Europe