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Jökull No. 45, 1998, 35-58

Mass balance of western and northern Vatnajökull, Iceland, 1991-1995

H. Björnsson1, F. Pálsson1, M. T. Gudmundsson1 and H. H. Haraldsson2

1 Science Institute, University of Iceland, Dunhaga 3, IS-107 Reykjavík, Iceland
2 National Power Company, Háleitisbraut 58, IS-105, Reykjavík, Iceland.



During the period from 1991 to 1995, glacier mass balance has been monitored on the western and northern outlets of Vatnajökull (8100 km2; Fig. 1) which altogether comprise about half of the total area of the ice cap (4000 km2) and extend from an elevation of 2000 m down to 600 m. In the central areas of the ice cap, the winter balance has typically been about 2.5 m of water equivalent but the summer balance has varied from +0.5 to -0.5 m, and hence the net mass balance has varied from 2 to 3 m. At the glacier termini of 700-800 m elevation, the summer balance was typically about -5 m, and the winter balance 1.5 m on the western outlets and 0.5 m on the northern ones. The mean specific winter balance of the glaciers was fairly constant over this period but the summer balance, and hence the annual net balance, decreased year by year. The specific annual net balance was positive for all the northern outlets in the first three years due to cold spells and snowfall during the summers but slightly negative for the western ones in the third year. In the last year (1994-95) the mass balance was in general negative but close to zero for one outlet (Dyngjujökull, Fig. 2).
For a year of zero net balance, the equilibrium line is estimated to be about 1100 m for the southwestern outlets but 1200-1300 m for the northwestern and northern outlets; the accumulation area is typically about 60% of the total glacier area and the specific runoff, corresponding to the summer balance, about 60 l s-1 km-2 averaged over the entire glacier and the whole year. During the years of the most positive mass balance this contribution from the summer balance dropped down to 30l s-1 km-2. Precipitation on the glaciers during summer may add 10-20 l s-1 km-2 to the specific discharge.