Hydrological processes in firn on Storglaciären, Sweden

Sammanfattning: Understanding hydrological processes in temperate glaciers is important for both scientific and practical issues. Modelling glacier response to climatic change and the use of water from glaciers for electric power generation are some topics that require detailed information on water storage and drainage in glaciers. However, most investigations on glacier hydrology have focused on the ablation area and our understanding of the hydrological system in the accumulation area is poor. The aim of this thesis is to investigate hydrological processes in the accumulation area of Storglaciären, Northern Sweden, and to quantify their influence on hydraulics and mass balance of the entire glacier. Specific goals are to investigate water drainage through, and water storage in, the firn layer; to analyse the hydraulic connection between accumulation and ablation area; and to asses the influence of refreezing water in the firn layer on glacier mass balance. Detailed field investigations were carried out on Storglaciären, including firn core analysis, pumping tests, dye tracing, measurements of water level and temperature in firn, proglacial discharge and meteorological observations. Results from these field measurements together with theoretical considerations and modelling form the methodological backbone of this study.A firn aquifer, ~5 m thick, forms above the firn-ice transition on Storglaciären at a depth of ~20 m. Observed water table variations in the firn aquifer follow surface water input with a delay of some days. Observations in crevasses indicate that water flows along inter-connected pores in the firn. Conduits with a diameter of some centimetres exist at the bottom of the firn layer. Crevasses in the firn area function as links between the firn aquifer and an efficient englacial drainage system. Water flow through the glacier does not follow the theoretical maximum potential gradient but is re-directed by crevasses towards the northern proglacial stream. The ratio between surface and bed slope at the down-glacier margin of overdeepenings is above the critical value at which subglcial channels may freeze due to energy loss when adapting to higher pressure melting point. Thus, flow paths in these areas are englacially rather than subglacially.Simultaneous injections of fluorescent dye tracers into the firn aquifer and at the firn surface are used to investigate water drainage from the accumulation area. Dispersion modelling of dye return revealed at least two different flow systems: a fast but highly braided, multi-branched arborescent system, and a slow and less braided, less effective, single-channel system. Dye-flux peaks correlate inversely with discharge in diurnal cycles, which may be explained by dye being stored in englacial cavities where high water pressure prevent draining due to a hydraulic barrier and release of dye during periods of low water pressure. During flood events a more effective drainage system is activated, causing dye and discharge peaks to be in phase.Internal accumulation is determined from irreducible water content (~7%) and temperature in firn. The winter cold wave penetrates 7-8 m into the firn layer. Mass balance measurements underestimate annual accumulation by ~0.05 m w.eq. (~4%) when neglecting internal accumulation in firn.