On the hydrological significance of rock glaciers: A case study from Murtèl rock glacier (Engadine, eastern Swiss Alps) using below-ground energy-flux measurements, ground-ice melt observations and hydrological measurements

Abstract. Intact rock glaciers, a permafrost landform common in high-mountain regions, are often conceptualized as (frozen) water reserves. In a warming climate with slowly degrading permafrost, the large below-ground ice volumes might suggest a buffering effect on summer streamflow that due to the climate resiliency of rock glaciers only increases with rapidly receding glaciers. In this case study, we assess the role and functioning of the active Murtèl rock glacier in the hydrological cycle of its small (17 ha) periglacial and unglacierized watershed located in the Upper Engadine (eastern Swiss Alps). Our unprecedentedly comprehensive hydro-meteorological measurements include below-ground heat flux measurements in the 3–5 m thick coarse-blocky active layer (AL), direct observations of the seasonal evolution of the ground-ice table, and discharge and isotopic signature of the outflow at the rock-glacier front. The detailed active-layer energy and water/ice balance quantifies precipitation, evaporation, snow melt, ground ice melt, and catchment surface outflow. Murtèl rock glacier stores and releases water and ice over three different time scales with varying magnitudes and residence times: (1) Liquid water storage on short-term (sub-monthly) scale is small in the permafrost-underlain coarse-debris catchment, as shown by the ‘flashy’ hydrograph during the thaw season with rapidly varying discharge and little sustained surface baseflow (<3 L min^-1) in the dry summer months. (2) Seasonal ground ice accumulation and melt in the coarse-blocky AL is substantial: Independent direct ground-ice observations and an AL energy budget suggests AL ice melt rates of 1−4 mm w.e. day^-1, amounting to 150−300 mm w.e. over the thaw season. In the comparatively cool–wet year 2021, ground ice melt represented ca. 13 % of the annual precipitation and outflow, but ca. 28 % in the hot–dry year 2022. The superimposed AL ice is sourced by refreezing snowmelt in spring (annually replenished), protracts the snowmelt into late summer (intermediate-term storage), and cannot increase the total yearly runoff. (3) Meltwater release from the ‘old’ permafrost ice due to climate-induced permafrost degradation is ≤50 mm yr^-1 or ~ 5−10 times smaller than the AL meltwater contribution and in the order of a few % of the overall water/ice fluxes (long-term storage). Our case study suggests that a hydrologically relevant ice turnover occurs in the active layer in addition to meltwater released from slow degradation of the ice-rich permafrost. The seasonal ice turnover in the AL acts as a coupled thermal and hydrological buffer that to some degree protects the underlying ice-rich rock-glacier core by converting the ground heat flux to meltwater during the thaw season. More measurements of the seasonal ground-ice turnover should tell how generalisable our single-site findings are.