Náttúrufræðingurinn - 1990, Blaðsíða 16

Hofsjökli horfnum gætu þó smáfannir
lifað af sum sumur og speglast í nýju
öskjuvatni.
ÞAKKIR
Landsvirkjun kostaði íssjármælingar á
Hofsjökli og úrvinnslu þeirra. Finnur Páls-
son vann að tölvuteikningu korta og Krist-
offer J. Kristiansen að útlitshönnun og
frágangi.
HEIMILDIR
Helgi Björnsson, 1977. Könnun á jöklum
með rafseaulbvlgjum. Náttúrufrœðine-
urinn, 47, 184-194.
Helgi Björnsson, 1987. Könnun jökla með
rafsegulbylgjum. Bls. 279-292 í í hlutar-
ins eðli (Ritstj. Þorsteinn I. Sigfússon).
Bókaútgáfa Menningarsjóðs. Reykjavík,
bls. 279-292.
Helgi Björnsson, 1988. Hydrology of ice
caps in volcanic regions. Vísindafélag Is-
lendinga, Rit 45. 139 bls.+ 21 kort í
öskju.
Markús Á. Einarsoon, 1976. Veðurfar á
íslandi. Iðunn, Reykjavík, 150 bls.
Sigurjón Rist, 1990. Vatns er þörf. Bóka-
útgáfa Menningarsjóðs. Reykjavík. 248
bls.
SUMMARY
Hofsjökull: topography,
storage and drainage
of ice and water
by
Helgi Björnsson
Science Institute
University of Iceland
Dunltaga 5
IS-107 REYKJAVÍK
lceland
In 1983 the surface and bedrock topo-
graphy of Hofsjökull (923 km2) was
mapped by radio echo soundings and pre-
cision barometric altimetry. The highest
part of the ice cap forms a circular plateau
at an elevation of about 1800 m. An ice
ridge strikes northeast from this centre
and ice flows southeast and northwest
from this main ice divide. The most prom-
inent landform beneath the ice cap is a
large circular volcano with a 200 km2 base
at 1000 m elevation. The mountain rises
up to rims at 1500-1600 m that surround a
650 m deep caldera. The elevation of the
caldera bottom is about 980 m. About 2/3
of the bedrock is above 1000 m and 1/9 of
it is above 1300 m. The total volume of ice
on Hofsjökull is 208 km3 and the average
thickness only 225 m. The maximum ice
thickness, about 750 m, is found in the
caldera. Hofsjökull is made up of 22 ice
catchment basins. The ice cap drains
meltwater into four main glacial river
systems. The water drainage basin of the
river Þjórsá is 433 km2 and contains 101
km3 of ice, of the Jökulkvísl 51 km2 and 8
km3, of the Blanda 226 km2 and 51 km3, of
the Vestari-Jökulsá 94 km2 and 22 km3,
and of the Austari-Jökulsá 116 km2 and 24
km3.
The total volume of the ice cap equals
200 times the annual net balance in the ac-
cumulation area (which is 550 km2). If the
whole ice cap were to melt over the next
200 years due to CO, induced warming,
mean runoff would increase by 30 m3/s.
This would be a 60% increase in the pre-
sent runoff from Hofsjökull, estimated to
be of the order of 50 m3/s. Assuming no
increase in accumulation, an increase in
air temperature by 2°C, would raise the
equilibrium line by the order of 300 m, to
a 1400-1500 m elevation, and Hofsjökull
would shrink to a small ice cap covering
the caldera. Il' the temperature increased
by 3°C the ice cap would disappear alto-
gether.
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