Jökull - 01.06.2000, Blaðsíða 51

Mass balance and precipitation on the summit plateau of
Öræfajökull, SE-Iceland
Magnús T. Guðmundsson
Science Institute, University oflceland Hofsvallagata 53, 107 Reykjavík, Iceland
Abstract — Mecisurements of mass balance were made in 1993-1996 and 1997-1998 at 1820 m elevation
on the wide summit plateau of Örœfajökull, a large, partly-ice covered strato-volcano on the southeast coast
of Iceland. The average winter balance over the period was about 6000 mm and the annual net balance as
high as 7780 mm, about double the measured precipitation at the nearby meteorological station of Kvísker.
The estimated annual precipitation on the summit plateau of Örœfajökull ranges from 7450 to 7800 mm in the
period 1993-1998. Although sparse, the data suggest that in warm summers, ablation and rainfall may lead to
several hundred millimeters difference between the net annual mass balance and precipitation. In cool summers
the annual net balance is about equal to the annual precipitation. The estimated precipitation values are the
highest sofar obtained in Iceland.
INTRODU CTION
Öræfajökull is a strato-volcano on the southeast coast
of Iceland and its highest mountain. An ice cap co-
vers the upper part of the mountain and merges to the
north with the main part of the Vatnajökull ice cap.
The volcano has a wide summit ice-plateau, cover-
ing a caldera with a maximum ice thickness of 500
m (Björnsson, 1988). A few nunataks on the caldera
rims rise above the summit ice plateau but the caldera
rims to the south, west and the southeast are concealed
by the ice cover. One of the nunataks is Hvannadals-
hnúkur, the highest peak in Iceland. The flat summit
plateau is 5 km long (N-S) and 3 km wide (E-W) and
has an elevation of 1800-1850 m above sea level.
Precipitation in Iceland reaches a maximum in the
south and southeast parts (Eythorsson and Sigtryggs-
son, 1971; Sigfúsdóttir, 1975) with the meteorological
station at Kvísker (Figure 1), on the lowland under the
eastern slopes of Öræfajökull, having the highest mea-
sured precipitation. Thus, the setting of Öræfajökull
and its meteorological conditions make it an interest-
ing place to study extremes in precipitation. Moreo-
ver, knowledge of the mass balance of its upper parts
is important to studies of the behaviour of its outlet
glaciers, most of which reach the lowlands. With these
aims in mind, annual measurements of mass balance
started in 1993, with the support of members of the
Iceland Glaciological Society. The results of the work
during 1993- 1998 are presented here. The 1993-1996
measurements were reported in Guðmundsson (1995,
1998a, 1998b) and Björnsson et al. (1998).
METHODS
The measurement site (63°59.9’N, 16°39.2’W,
elevation 1820 m) is located in the centre of the flat
summit ice-plateau covering the Öræfajökull caldera.
The nunataks closest to the site are about 2 km away
and the distance to the plateau edges is nowhere
less than 1.5 km, minimizing potential bias due to
snowdrift, as often seen close to mountain slopes. The
site was visited once each summer, a snow core taken
with an engine driven drill and the density of the core
measured.
The thickness of the annual layer was determined
in a conventional way by defining the previous ye-
ar’s summer surface from changes in grain size and
texture of the core. In some cases the uncertainty in
the location of the summer surface was 30-50 cm.
JÖKULL No. 48 49