Jökull - 31.12.2001, Side 77

Mass balance of Vatnajökull (1991-2001) and
Langjökull (1996-2001), Iceland
Helgi Björnsson1, Finnur Pálsson1 and Hannes H. Haraldsson2
^Science Institute, University oflceland, Hagi Hofsvallagata 53, IS-107, Reykjavík, Iceland;
hb @ raunvis.hi. is; fp@raunvis. hi. is
2National Power Company, Háleitisbraut 68, IS-I03, Reykjavík, Iceland; hannes@lv.is
Abstract — The mass balance ofthe Vatnajökull ice cap has been monitored since 1991 and since 1996 on the
Langjökull ice cap. On Vatnajökull the mass balance was positive from 1991 to 1994, close to zero 1994-1995,
but negative since then. Vatnajökull has lost 0.6% ofthe total ice mass, 2.37m ofwater evenly spread over its
area during the entire 11 year survey period. The mass balancefor Langjökull was negative after 1996, in total
by 5.73 m, or 3% ofthe total ice mass. The mass balance ofthe two ice caps is directly correlated reflecting
low winter snow accumulation and high annual temperatures. However, larger variations are seen in the mean
specific winter balance of Langjökull than of Vatnajökull.
INTRODUCTION
Since 1991-92 annual mass balance measurements
have been carried out on Vatnajökull (8100km2) in
collaboration between the National Power Company
and the Science Institute, University of Iceland. The
spatial and temporal distribution of the mass bal-
ance of Vatnajökull during the years 1991-95 was
described in detail by Bjömsson et al. (1998) but
this paper summarizes results of the 11 year period.
Furthermore, we present outcome of similar work on
Langjökull (925 km2) from 1996 to 2001.
METHODS
The mass balance was calculated by a stratigraphic
method, measuring changes in thickness and density
relative to the summer surface. The methodology has
been described in detail by Björnsson et al. (1998).
Mass balance observations were carried out at 45-50
locations on the Vatnajökull ice cap and 22 locations
on the Langjökull ice cap (Figure 1). The survey sites
are situated along a number of selected flow lines,
which span the elevation range of these glaciers. The
mass balance values span the time interval between
given survey dates, which are not fixed annually. The
dates in the autumn are separated by approximately
one calendar year, which roughly coincides with the
hydrological year between October 1 and Septem-
ber 30. The surveys in the spring were carried out
between late April and mid-May. Digital maps of
the specific mass balance were constructed and volu-
mes of mass balance were obtained by integrating
the specific mass balance over the total area of the
glacier. Mean mass balance values were derived as
volumes divided by the glacier drainage area; which
were delineated using surface elevation maps drawing
the ice flow lines perpendicular to smoothed elevati-
on contours. Error limits for the area integrals of the
mass balance components are assigned less than 20%.
RESULTS
The mass balance measurements on Vatnajökull span
over 11 years (Table 1 and Figure 2). The annual net
mass balance was positive in 1991 to 1994, close to
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