Jökull - 01.06.2000, Blaðsíða 54

Magnús T. Guðmundsson
mýri (Trausti Jónsson, personal comm., 1998). For
comparison with the meteorological stations it is assu-
med here that winter accumulation starts in Septem-
ber. This is reasonable for the highest parts of Vatna-
jökull, although 1 October may better deíine the start
of the balance year for the lower and medium elevati-
ons (Björnsson et al. 1998). The ratio of observed
mass balance at Öræfajökull and the precipitation at
Kvísker (from 1 September in the previous year until
the date of measurement at Öræfajökull) varies from
l. 9 to 2.4 with a mean of 2.12 (Table 1 and Figure 3).
PRECIPITATION ON ÖRÆFAJÖKULL
It is of some interest to consider how well the obser-
ved mass balance values correspond to precipitation
and use the data to estimate the annual precipitation
on the summit plateau of Öræfajökull. Mass balance
data fróm Vatnajökull (Björnsson et al. 1998) suggest
that in winter no mass should be lost by melting at
1800 m elevation. However, there are several factors
that may bias the measurements. Firstly, snowdrift
may cause systematic variations if a net transport
of snow occurs from the summit plateau onto the
slopes on either side. Secondly, summer ablation may
remove mass from the annual layer. Thirdly, some of
the summer precipitation may fall as rain and seep
through the annual layer. All the above factors would
cause the observed mass balance to be lower than the
true precipitation.
The significance of snowdrift at the observation
site has not been measured but considering that the
summit plateau is very flat and has width of a few ki-
lometers, the net removal of snow is probably small.
For comparison, Föhn (1980) found that snowdrift-
induced variations in snow depth were confined to 200
m wide region on either side of a mountain ridge crest
in the Alps.
At an elevation of 1800 m, the summer balance
is usually positive on Vatnajökull (Björnsson et al.,
1998). Some summer melting usually occurs at 1800
m, however, and occasionally precipitation may fall as
rain. Thus, it is to be expected that summer precipitati-
on is greater than summer balance. Loss of mass by
summer ablation and seepage through the annual layer
is unlikely in 1993 since the lower part of the core was
still frozen at the time of measurement. The same app-
lies to the June measurements (1995,1996 and 1998).
In 1994 some mass loss during summer may have
occurred. However, the ratio of mass balance at Ör-
æfajökull and the precipitation at Kvísker in 1994 is
about the same as for the other years in question, sug-
gesting that possible losses were comparatively small.
Moreover, some of the ablation and rainfall in summer
refreezes within the annual layer. This can be seen by
higher density of the summer/autumn cores which is
partly caused by increased thickness and number of
ice lenses compared with the spring (June) cores.
Data from 1996 indicate some mass loss by sum-
mer ablation and seepage of rain through the annual
layer. A stake was left in place in June, and a reading
taken on 1 October after an unusually warm Sept-
ember. The stake readings indicate a positive sum-
mer balance of 600 mm (water equivalent) while the
estimated summer precipitation for 1996 is 1280 mm
(Table 2).
In Table 2 an estimate is given for the precipitati-
on on the summit plateau of Öræfajökull between the
time of measurement of mass balance and the end of
the balance year (September lst). This period ranges
from 0 days (1993-1994) to 80 days (1994-1995 and
1995-1996). The Öræfajökull precipitation is assumed
to be 2.12 times the observed precipitation at Kvísker
for the same period. Thus, a minimum estimate of the
annual precipitation at Öræfajökull is obtained, rang-
ing from 7450 mm (1995-1996) to 7800 mm (1997-
1998), similar to the observed annual net balance in
1993-1994.
When compared with other data on precipitati-
on and glacier mass balance in Iceland, the values
obtained at Öræfajökull are higher than previously
reported. However, the values reported for Mýrdals-
jökull in south Iceland are of similar magnitude. Ey-
þórsson (1945) observed an annual layer thickness
of 785 cm at about 1300 m a.s.l. in early Aug-
ust 1944 when melting was well advanced, and Rist
(1957) measured a winter balance of 5800 mm at
1350 m a.s.l. in June 1955. These values are only slig-
htly lower than those presented here for Öræfajökull.
Thus, it is likely that precipitation at Mýrdalsjökull is
similar to that observed at Öræfajökull.
52 JÖKULL No. 48