Jökull - 01.12.1985, Blaðsíða 109

The winter balance in Grímsvötn, Vatnajökull, 1950—1985
HELGI BJÖRNSSON
Science Institute, University of Iceland., Dunhaga 3, 107 Reykjavík
Snow pit studies and shallow corings have been car-
ried out since 1951 in the Grímsvötn area, central
Vatnajökull, in the expeditions of the Iceland Gla-
ciological Society. Table 1 and Fig. 1 summarize the
available data for snow depths down to the summer
surface and the winter balance calculated from snow
density measurements. The data from 1951 to 1968 have
been collected from various reports in Jökull but data
since 1970 have not been published before. The
measurements have been done at three sites in the
Grímsvötn area:
A: On the ice cover of Grímsvötn at 64°25'N and
17°20'W. The elevation varies between 1350 and 1440
m (see Björnsson and Kristmannsdóttir 1984).
B: The lowest point of the pass east of the Grímsvötn
lake at 64°25', 17°12'W, about 2.5 km NE of E-Svía-
hnúkur. Elevation about 1540 m a. s. 1.
C: Northeast of the lake at 64°26' 06"N, 17°09' 23"W
about 6 km NE of E-Svíahnúkur. Elevation about 1615
m a. s. 1.
Only limited data are available for comparing results
from the three sites. Data from sites A and C may
reflect the general decrease in precipitation towards
north on the ice cap or the accumulation of drifting
snow inside the Grímsvötn caldera depression. How-
ever, accumulation at the site B, east of the lake, may
be higher than at site A. Sudies of the general atmos-
pheric circulation for every year since 1950 may reveal a
regional distribution in the present data (correlation
with a single meteorlogical station is not high, see
footnote). If, however, all the observations in Table I
were given equal weight the average winter balance in
the Grímsvötn area would be estimated 2450 ± 250 mm.
The winter balance in the Grímsvötn area represents
the total precipitation for the nine months from Septem-
ber to the end of May. The total precipitation for these
nine months is for most meteorological stations about
80% of the annual precipitation (concluded by the
author on the basis of precipitation data from the
Meteorological Office). Hence, for the period 1950—
1985 we estimate the average annual precipitation in the
Grímsvötn area to have been 3100 ± 300 mm.
Footnote: The winter balance at Grímsvötn has been
compared with the total precipitation at Fagurhólsmýri
Fig. 1. The winter balance in Grímsvötn, Vatnajökull,
1951—55. Observation sites: A: on the ice cover
(64°25' N, 17°20' W), B: east of the lake (64°24' N,
17°12'°, 1540 m a.s.l.), C: northeast of the lake
(64°26' 06" N, 17°09' 23", 1615 m a.s.l.).
1. mynd. Vetrarafkoma í Grímsvötnum. Mœlistaðir: A:
á miðri íshellunni, B: í skarðinu austan við Vötnin
(64°25' 1V, 1T12' W, 1540 m. y. s., C: við mastur
norðaustan við Grímsvötn (64°26' 06" N, 1T09' 23"
W, 1615 m y. s.).
JÖKULL 35. ÁR 107