Jökull - 01.10.1998, Blaðsíða 37

Mass balance of westem and northem Vatnajökull, Iceland,
1991-1995
Helgi Björnsson1, Finnur Pálsson', Magnús T. Guðmundsson1 and Hannes H. Haraldsson2
'Science Institute, University of Iceland, Dunhaga 3, IS-107 Reykjavík, Iceland
2National Power Company, Háaleitisbraut 58, IS-105 Reykjavík, Iceland
Abstract - During the periodfrom 1991 to 1995 glacier mass balance has been monitored on the
western and northern outlets ofVatnajökull which altogether comprise about halfofthe total area
ofthe ice cap (4000 km2) and extendfrom an elevation of2000 m down to 600 m. In the central
areas of the ice cap, the winter balance has typically been about 2.5 m ofwater equivalent but the
summer balance has variedfrom +0.5 to -0.5 m, and hence the net mass balance has variedfrom
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 bal-
ance was positivefor all the northern outlets in thefirst 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 zerofor one outlet (Dyngjujökull).
For a year ofzero net balance, the equilibrium line is estimated to be about 1100 mfor the south-
western outlets but 1200-1300 m for the northwestern and northern outlets; the accumulation
area is typically about 60% ofthe total glacier area and the specific runojf, corresponding to the
summer balance, about 60 l s'' km2 averaged over the entire glacier and the whole year. During
the years ofthe most positive mass balance this contribution from the summer balance dropped
down to 30 l s'1 km 2. Precipitation on the glaciers during summer may add 10-20 l s1 km 2 to the
specific runoff.
INTRODU CTION
Vatnajökull is the largest ice cap outside the Polar
regions, 8100 km2 in area. It is therefore a most im-
portant ice mass in glaciological, hydrological and
climatological respects (Fig. 1). A wealth of informa-
tion is available about this ice cap including data on
the retreat and advance of its termini over centuries,
and information about floods from ice dammed lakes
and subglacial volcanic eruptions. More recently data
have been collected on its geometry by radio echo
sounding, ice flow, mass balance and metorological
conditions in a collaboration between the National
Power Company and the Science Institute, University
of Iceland (Björnsson, 1979, 1988; Björnsson and
others, 1992, 1995a,b). This paper summarises data
on the mass balance from 1991 to 1995.
The purpose of the mass balance measurements
has been to describe the geographical distribution of
the components of the mass balance, to evaluate rep-
resentative mean values of the components and their
variation from year to year, and to relate them to me-
teorological conditions and climatic variability. The
results will be used in studies of changes in the glacier
volume, estimates of meltwater contribution to glacial
rivers, mass balance modeling, evaluation of altitudi-
nal and regional variations of mass balance in re-
sponse to climatic variations, and to assess the hy-
drometeorological and dynamic response of the ice
cap to climate change.
JOKULL, No. 45, 1998
35