Swiss glacier mass loss during the 2022 drought: persistent streamflow contributions amid declining melt water volumes

Abstract. The year 2022 was extremely warm and dry in Europe, resulting in a severe hydrological drought. In Switzerland, part of Europe’s water tower, streamflow in glacier-fed rivers could have been even more reduced if the situation had not led simultaneously to extreme glacier melt. Here we analyze the role of glaciers during the 2022 drought for 88 glacierized catchments by combining streamflow and meteorological observations with estimations of snow water equivalent, actual evapotranspiration and daily glacier storage changes. We also compared the year 2022 to earlier exceptionally warm and dry years (1921, 1928, 1947, 1998, 2003 and 2018) to assess if the ongoing glacier retreat has already caused a declining meltwater supply in such extreme conditions. Results show that 60–80 % of the total glacier melt in 2022 came from net mass loss (imbalanced melt). During summer, the increased melt could completely offset the precipitation and snowmelt deficits for catchments with around 15 % glacierization. Further downstream, the extra melt in summer alleviated water input deficits by up to 5 % at Basel (Rhine) and 70 % at Porte du Scex (Rhone). However, compared to other extreme years in the past, total melt volume has been noticeably declining due to a strong reduction in glacier area – despite more extreme melt rates per unit glacier area. In contrast, the relative contribution of glacier melt to streamflow stayed constant or even increased for some months, suggesting that glacier melt remains important during droughts. Comparing 2022 to 2003 –the most comparable recent extreme summer– shows a declining glacier meltwater supply for 55 % of the catchments during summer and 36 % during July, despite more intense melt, with the difference in summer/July reflecting the extremeness of the melt conditions, counterbalancing the reduction in glacier area. This declining meltwater supply raises concerns for future drought situations.