Jökull - 01.12.1967, Síða 41

that the Tungufljót is to a greater degree de-
pendent on the glacier regime than the other
rivers there, especially il it is kept in mincl
that the average elevation of the glacier in the
Tungufljót drainage area is about 1000 m while
the avrerage elevation for Langjökull as a whole
is about 1100 m (Bauer 1955).
Possible errors in the measurements
The preceding calculations of the recession
of Langjökull are chiefly based on the available
maps. The results of the calculations are there-
fore primarily dependent on their correctness.
In some details the maps can be expected to
be inaccurate. There are especially two factors,
which can cause serious errors.
1. The contour lines on the glaciers are often
inexact because in locating them very few
measured points can be relied on. This can
cause considerable deviations on the glacier
cap itself, but to a far less degree on the
outlet glaciers.
2. The glacier margin is often rnore or less
covered with debris, which makes their
location on the maps unprecise, especially
on those drawn from aerial photographs.
I have met with some evidence of this.
The author is well aware of the fact that
these imperfections greatly weaken the results
obtained by these calculations, but with regarcl
to the surveying methods the mapping errors
can be expected to neutralize each other to a
great degree. The iinal outcome should there-
fore not be very far from giving the true values.
Other Glaciers
In order to investigate more thoroughly the
efí'ect of the glacier recession on the discharge
of Icelandic rivers and for comparison with the
above mentioned results a compilation of data
from miscellaneous sources concerning other
glaciers and outlet glaciers than Langjökull
alone has been initiated. It is my hope that
this work will be carried on. The following
preliminary results were obtained by these in-
vestigations.
1. Hofsjökull 1938—1945. Its recession has
been found by the same method as with Haga-
fellsjökull Eystri. The result is that below the
1100 m elevation the glacier lias decreased in
volume for 4315 X 10° m3, which corresponds
to 17.6 m3/sec increase in drainage and about
56 cm in average thinning per year. On the
other hand sorne accumulation was measured
in the elevation between 1100 and 1400 m, but
shrinkage above that. If the increase is includ-
ed the net recession of the glacier proved to
be 2000 x 10° m3, which corresponds to 8.1
m3/sec increase in discharge and about 26 cm
in average thinning per year. The area of the
glacier decreased from 986 km2 to 945 km2 or
about 4% during these seven years.
The most part of the Thjórsá drainage area
on Hofsjökull was measured separately and
there the trend proved to be almost the same
as for the glacier as a whole. The glacier area
had decreased from 424 km2 to 400 km2 or
5.7%. The recession of the glacier below the
1100 contour line was 1557 X 108 m3, which
corresponds to 6.5 m3/sec increase in discharge.
About 510 km2, or about 25%, of the drainage
area of Thjórsá above Nordlingaalda is covered
by glacier. According to this the ice recession has
increased its discharge for about 8 m3/sec dur-
ing this period, which is about 8% of its total
discharge. If on the other hand the accumula-
tion measured in the higher elevation was in-
cluded in these calculations the net recession
of this part of the glacier was only about
140 x 10° m3, which would give less than 1%
increase in the discharge of Thjórsá above
Nordlingaalda.
No statement can be made at this point as
to wliich of these two conflicting results is
more correct, but according to what has already
been discussed errors in mapping are much
more likely to occur upon the ice caps than
on the outlet glaciers. It is also noteworthy
that above 1400 meters, where some nunataks,
which can be used as fixed points in survey-
ing, are to be found, sorne ice recession was
measured. Probably the measurements of the
outlet glaciers are the most reliable, although
the possibility of some accumulation between
1100 and 1400 m cannot be excluded.
2. Heinabergsjöklar 1903—1945. Heinabergs-
jöklar are two outlet glaciers of Vatnajökull
in Austur-Skaftafellssýsla. Tlieir lower part, be-
low about 720 m on the southern glacier and
below about 460 m on the northern glacier,
was measured and the reduction in volume
provecl to be 2192 X 10° m3 during these 42
JÖKULL 17. ÁR 275