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Abstract:In many mountain regions, large land areas with heterogeneous soils have become ice-free with the ongoing glacier retreat. On these recently formed proglacial fields, the melt of the remaining glaciers typically drives pronounced diurnal stream level fluctuations that propagate into the riparian zone. This behaviour was measured on the Damma glacier forefield in central Switzerland with stage recorders in the stream and groundwater monitoring wells along four transects. In spite of the large groundwater stage variations, radon measurements in the near-stream riparian zone indicate that there is little mixing between stream water and groundwater on daily time scales. At all four transects, including both losing and gaining reaches, the groundwater level fluctuations lagged the stream stage variations and were often damped with distance from the stream. Similar behaviours have been modelled using the diffusion equation in coastal regions influenced by tidal sea level variations. We thus tested the ability of such a model to predict groundwater level fluctuations in proglacial fields. The model reproduced several key features of the observed fluctuations at three of four locations, although discrepancies also arise due to non representative input data and model simplifications. Nevertheless, calibration of the model for the individual transects yielded realistic estimates of hydraulic diffusivities between the stream and groundwater monitoring wells. We conclude that studying diurnal groundwater fluctuations can provide important information about the subsurface hydrology of alpine watersheds dominated by glacier melt.

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Abstract. This study aims at understanding interactions between stream and aquifer in a glacierized alpine catchment. We specifically focused on a glacier forefield, for which continuous measurements of stream water electrical conductivity, discharge and depth to the water table were available over four consecutive years. Based on this dataset, we developed a two-component mixing model in which the groundwater component was modelled using measured groundwater levels. The aquifer actively contributing to stream flow was assumed to be a superposition of two linear storage units. Calibrating the model against measured total discharge yielded reliable sub-hourly estimates of discharge and insights into groundwater storage properties. We found that a near-surface aquifer with high hydraulic conductivity overlies a larger reservoir with longer response time. Analyzing the mass balance of infiltration into the groundwater reservoir against exfiltration into the stream provided results that were in line with previous findings at this catchment.

Abstract. In this study, we investigated the application and the transferability of the Soil Water and Assessment Tool (SWAT) in a partly glacierized Alpine catchment characterized by extreme climatic conditions and steep terrain. The model was initially calibrated for the 10 km2 watershed of the Damma glacier Critical Zone Observatory (CZO) in central Switzerland using monitoring data for the period of 2009–2011 and then was evaluated for 2012–2013 in the same area. Model performance was found to be satisfactory against both the Nash–Sutcliffe criterion (NS) and a benchmark efficiency (BE). The transferability of the model was assessed by using the parameters calibrated on the small watershed and applying the model to the approximately 100 km2 catchment that drains into the hydropower reservoir of the Göscheneralpsee and includes the Damma glacier CZO. Model results were compared to the reservoir inflow data from 1997 to 2010 and it was found that the model predicted successfully snowmelt timing and autumn recession but could not accurately capture the peak flow for certain years. Runoff was slightly overestimated from late May to June, when it is dominated by snowmelt. Finally, we investigated the response of the greater catchment to climate change using three different climate change scenarios, and the results were compared to those of a previous study, where two different hydrological models, PREVAH and ALPINE3D, were used. The methodology presented here, where SWAT is calibrated for a small watershed and then applied for a bigger area with similar climatic conditions and geographical characteristics, could work even under extreme conditions like ours. However, greater attention should be given to the differences between glacier melt and snowmelt dynamics. In conclusion, this assessment test on the transferability of SWAT on different scales gave valuable information about the strengths and weaknesses of the model when it was applied under conditions different to those under which it was calibrated.

Abstract:We implemented multiple independent field techniques to determine the direction and velocity of groundwater flow at a specific stream reach in a glacier forefield. Time-lapse experiments were conducted using two electrical resistivity tomography (ERT) lines installed in a cross pattern. A circular array of groundwater tubes was also installed to monitor groundwater flow via discrete salt injections. Both inter-borehole and ERT results confirmed this stream section as a losing reach and enabled quantification of the flow direction. Both techniques yielded advection velocities varying between 5.7 and 21.8 m/day. Estimates of groundwater flow direction and velocity indicated that groundwater infiltrates from the stream nearby and not from the adjacent lateral moraine. Groundwater age estimated from radon concentration measurements supported this hypothesis. Despite uncertainties inherent to each of the methods deployed, the combination of multiple field techniques allowed drawing consistent conclusions about local groundwater flow. We thus regard our multi-method approach as a reliable way to characterize the twodimensional groundwater flow at sites where more invasive groundwater investigation techniques are difficult to carry out and local heterogeneities can make single measurements unreliable.

Abstract. This study aims at understanding interactions between stream and aquifer in a glacierized alpine catchment. We specifically focused on a glacier forefield, for which continuous measurements of stream water electrical conductivity, discharge and depth to the water table were available over 4 consecutive years. Based on this data set, we developed a two-component mixing model in which the groundwater component was modelled using measured groundwater levels. The aquifer actively contributing to streamflow was assumed to be constituted of two linear storage units. Calibrating the model against measured total discharge yielded reliable sub-hourly estimates of discharge and insights into groundwater storage properties. Our conceptual model suggests that a near-surface aquifer with high hydraulic conductivity overlies a larger reservoir with longer response time.

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