Internal accumulation in firn and its significance for the mass balance of Storglaciaren, Sweden

The discussion on global change has led to increased interest in glacier mass balance since glaciers can be used as climatic indicators. To meet the need for high-quality mass-balance data requires critical examination of traditional mass-balance methods and their possible errors. One issue regarding mass-balance measurements that has received little attention is internal accumulation. Our study shows that internal accumulation in the firn layer of Storglaciaren, Sweden, significantly affects the mass balance of the glacier. This occurs because the winter cold wave penetrates below the previous year's summer surface and into underlying firn. We estimated internal accumulation from measurements of temperature and water content in firn. The depth of the 0degreesC isotherm correlated with snow depth and air temperature, so that low snow depth and low air temperature separately cause a deeper 0degreesC isotherm. We determined irreducible gravimetric water content in firn to 2-3%, which corresponds to an irreducible water saturation of 6-8%. Our value for firn is relatively high compared with that for snow, probably due to trapped water in isolated firn pores. Refreezing of percolating meltwater in spring accounted for similar to30% of annual internal accumulation. The remaining 70% was due to refreezing of retained capillary water in firn pores during winter. Disregarding internal accumulation would lead to underestimation of annual net mass balance by 0.04-0.06 in w.e., corresponding to 3-5% of annual accumulation of the entire glacier in an average year. Hence, internal accumulation potentially becomes a source for systematic error if not accounted in mass-balance measurements.