Spatially distributed streamflow buffering by glaciers during recent droughts in Switzerland

During dry and hot years in the Swiss Alps, melt water from glaciers can moderate streamflow deficits caused by reduced precipitation and enhanced evapotranspiration rates. However, little is known about how glacier melt water contribution to streamflow varies sub-seasonally and in space, especially further downstream from glacierized catchments, where additional streamflow contributions are modulated primarily by rainfall and the biosphere (vegetation, soils). We study distributed catchment hydrology in Switzerland using a land surface model that constrains energy and mass fluxes using advanced physical representations of both cryospheric and biospheric processes at a 250 m spatial resolution. We simulate catchment runoff in Switzerland during the past 6 years, including two recent severe drought years (2018 and 2022), characterized by particularly warm summers and reduced precipitation. The model is forced with hourly observed meteorological data based on the weather station network SwissMetNet and the precipitation product RhiresD, and uses state-of-the-art land cover, soil characteristics, glacier area and debris thickness as initial conditions. The spatially explicit simulations allow, when temporally aggregated, to trace upstream contributions of individual water balance components for any downstream point in the catchment. We use the model to quantify the amount and timing of glacier melt and how it affects downstream runoff composition, especially during drought conditions, along the river network. We do this across regions from the Swiss Alps headwaters to the lowlands in a spatially continuous way. When comparing runoff composition during moderate summer months to periods of drought conditions in the Swiss Alps, our simulations show both an increase in intensity and downstream propagation of ice melt contribution to total runoff. During extreme drought periods, ice melt makes up >70% of streamflow (~doubling the contributions during more moderate periods) in some of the Alps headwater regions (>1500 m a.s.l.), and still exceeds 10% of streamflow contribution downstream of the pre-alpine region. Quantifying the timing and amount of glacier melt contributions to downstream water resources under recent drought conditions improves our understanding of potential cryosphere-biosphere interactions and their impacts under future extreme scenarios, when cryospheric runoff contributions may be reduced or completely lost.