Regional Mountain Conference

Mountain regions are undergoing rapid changes in temperature and precipitation regimes, with direct consequences for seasonal snowpack and water availability. This study investigates recent snowpack dynamics and snow-drought behaviour in Valle d’Aosta (Western Italian Alps) using daily observations from the regional monitoring network. We analyse snow depth (HS) time series from 2007–2024 across a quality-controlled subset of stations and compute a suite of seasonal and monthly indices describing HS and the derived snowfall (HN).

For each station we compute the snow water equivalent (SWE) from HS with the semi-empirical model ΔSNOW (Winkler et al., 2020). We leverage these data to identify snow drought events with two complementary approaches. First, we apply the Standardised Snow Water Equivalent Index (SSWEI), a standardized, non-parametric index derived from snow water equivalent (SWE). Second, we propose a new persistence-based Snow Drought Index (SDI) that classifies drought severity based on the the fraction of days within a month when water stored in the snowpack was below a threshold. SDI is computed as the percentage of days in a given month when SWE fell below the 25th percentile of that month’s climatological distribution (following the threshold approach of Marshall et al., 2019). Months with 50–75% of days below threshold are classified as moderate drought, while values >75% indicate severe drought.

Results show an overall decline in snow depth, with the strongest decreases in December–February and indications of a delayed timing of maximum snow depth. Snow-drought conditions cluster in the early 2020s (notably 2020/21–2022/23). SSWEI and SDI identify consistent drought periods and broadly similar classifications. However, SDI retains information on event persistence; this feature highlights the occurrence of drought spells exceeding 75 consecutive days in the most recent period.