Seasonal ice storage changes and meltwater generation at Murtèl rock glacier (Engadine, eastern Swiss Alps): estimates from measurements and energy budgets in the coarse blocky active layer

Abstract. Intact rock glaciers, a permafrost landform common in high-mountain regions, are often conceptualised as (frozen) water reserves. In a warming climate with slowly degrading permafrost, the large belowground ice volumes might suggest a buffering effect on summer streamflow that due to the climate insensitivity of rock glaciers only increases with rapidly receding glaciers. In this case study, we assess the role and functioning of the intact Murtèl rock glacier in the hydrological cycle of its small (30 ha) periglacial and unglacierised watershed located in the Upper Engadine (eastern Swiss Alps). Our unprecedentedly comprehensive hydro-meteorological measurements include belowground heat flux measurements in the 2–5 m thick coarse blocky active layer (AL), belowground stake measurements of the seasonal evolution of the ground-ice table, and discharge and isotopic signatures of the outflow at the rock-glacier front. The detailed active-layer energy and water/ice balance quantifies precipitation, evaporation, snowmelt, ground-ice melt, and catchment surface outflow. Our single-site, but detailed, case study resolves thermo-hydraulic processes in the coarse blocky AL that might enhance the snowmelt–groundwater connectivity in periglacial high-mountain watersheds underlain by discontinuous permafrost. A substantial part of the snowmelt refreezes in the cold AL (∼ 150–300 mm w.e. or ∼ 20 %–40 % of the snowpack), forming AL ice that is released during the thaw season at melt rates low enough for the meltwater flow to be routed through the permafrost aquitard to deeper sub-permafrost aquifers. Meltwater fluxes are low (1–4 mm w.e. d−1) but sustained throughout the entire thaw season (∼ 100 d) due to small ground heat fluxes and the dampening effect of the AL. The AL ice acts as a coupled thermo-hydrological buffer that (to some extent) protects the underlying ice-rich rock-glacier core by converting most of the ground heat flux to meltwater during the thaw season. Consequently, meltwater release from the old permafrost ice due to climate-induced permafrost degradation is currently ∼ 10 mm yr−1 or an order of magnitude smaller than the contribution of AL meltwater and not more than a few percent of the overall water/ice fluxes. In view of the widespread and long-lasting occurrence of climate-robust permafrost in high-mountain watersheds and the increasing importance of groundwater-sustained late-summer baseflow relative to vanishing glaciers and diminishing snowpacks, it is important to investigate mechanisms, flow paths, and efficiency of groundwater recharge in mountain permafrost terrain.