The 2022–2023 snow drought in the Italian Alps doubled glacier contribution to summer streamflow

Abstract. We quantified the role of glaciers in mitigating snow-drought impacts on downstream streamflow during the severe 2022- 2023 event in the Italian Alps. In order to do so, we compared glacier-melt contribution to streamflow during these years with the 2011-2023 historical period in two catchments, Dora Baltea (Aosta Valley) and Adda (Lombardy). We employed spatially distributed estimates of glacier melt, Snow Water Equivalent (SWE), air temperature and total precipitation over glaciers from an operational cryospheric model (S3M Italy), and compared these estimates with downstream observations of streamflow at the closure sections of both catchments. Results showed a severe snow water equivalent deficit over glaciers across both catchments and both years (between ∼ -45 % at 4000 m in 2022 and ∼ -75 % at 2000 m a.s.l. during both years), which was largely driven by anomalous air temperatures and seasonal-precipitation patterns (up to +2–3 °C and -73 %, respectively). Air-temperature anomalies displayed a clear signature of orographic-dependent warming, with anomalies at 4000 m a.s.l. that were 1 to 1,5 °C higher than at 2000 m a.s.l.. Glacier contribution to streamflow doubled to tripled during these snow droughts in both catchments, a process that manifested itself through four mechanisms: an earlier-than-usual onset of the glacier melt season, an intensification of glacier melt contribution to streamflow, an earlier-than-usual seasonal peak in glacier melt contribution, and an extension of the glacier melt season. Still, glacier melt contribution to streamflow remained highly sensitive to short-term meteorological events, such as a sudden drop of temperatures, as well as early/late snowfalls. These results highlight the critical role of glacier melt in maintaining streamflow during severe droughts and emphasize the need to integrate glacier dynamics into water management strategies for alpine areas facing increasingly frequent and intense drought events.