Jökull - 01.12.1970, Blaðsíða 52

On the Recession of Vatnajökull
GUTTORMUR SIGBJARNARSON,
national energy authority, reykjavík, iceland
ABSTRACT
There is a certain discrepancy between pre-
cipitation maps and run-off maps of Iceland
now available in the way that the run-off seems
to have been greater than the precipitation.
An attempt is made to calculate to what degree
the glacier recession is responsible for this dis-
crepancy. Measurements of the recessio?i of
Breidamerkurjökull were gathered. During the
period 1894—1968 the glacier front retreated
up to 2300 m, exposing 52 km2 of deglaciated
area. During the same span of time Breida-
rnerkurjökull lost in volume 49 kmz, which
corresponds to an increase of 19 kl/sec. in the
average run-off.
ln this period the shrinkage of Vatnajökull
was approximately 270—350 km3 which
amounts to 8—10% of its total volume of ice.
With a comparable recession to that of the
years 1932—1968 it would take 500—600 years
to waste the whole of this ice mass away. Cal-
culated as annual increase in run-off the total
glacial recession in Iceland during the period
1931—1960 approximates to 7—8-109 m3. It
partly explains the above discrepancy but is far
from being a complete explanation.
INTRODUCTION
According to S. Rist (1956) the average annu-
al run-off of Iceland as a whole is about
1.7 • 1011 m3 corresponding to 1650 mm of
average annuai precipitation yield. A. B. Sigfús-
dóttir (1968) has compiled a new precipitation
map of Iceland based on data for the period
1931—1960 to which she has kindly given me
access. According to this map the average annu-
al precipitation has been 1540 mm for the
country as a whole, which reckoned as annual
run-off, is equal to 1.6- 1011 m3. No regard is
here given to evapotranspiration and seepage
50 JÖKULL 20. ÁR
direct to the ocean. This leaves us with a
certain discrepancy which will be discussed
more thoroughly later.
Rist’s run-off map is to a large extent based
on discharge measurements performed in the
years 1946—1955. According to the annual re-
ports of the Icelandic Meteorological Office
(1946—1955) both mean temperature and mean
precipitation for this 10 years period were
slightly below the average as compared to the
30 years 1931—1960. The above discrepancy
therefore cannot be accorded to any altera-
tion in climatic conditions. On the other hand
the discharge measurements include the in-
crease due to glacial recession which to some
degree offsets the difference. At the National
Energy Authority some research as to the effect
of the glacial wastage on the discharge of the
glacier rivers has been carried out. The work
was carred out by collecting all available re-
sults of measurements on a certain part of each
glacier. Simultaneously the lowering of the
glacier has been measured as far as possible.
The results of these measurements were then
used to calculate the total recession of this part
of the glacier both on its accumulation and
ablation areas. A corresponding recession was
then assumed for the appropriate glacier as a
whole. Surely this is only approximation, since
much more measurements are needed. Similar
calculations for Langjökull were carried out by
Sigbjarnarson (1967). The present calculations
for Vatnajökull are based on the recession mea-
surements for Breidamerkurjökull.
BREIDAMERKURJ ÖKULL
According to the maps of the Geodaetic In-
stitute, Copenhagen, the area of Breidamerkur-
jökull is 1183 km2. It covers about 14% of