Jökull - 01.12.1953, Page 16

Fig. 6. Grímsvötn on 1 July 1953. Based on
S. Sigurðsson’s map from 1942 and author’s
measurements.
Kort af Grímsvatnasvœðinu. Byggt á korti Stein-
þórs Sigurðssonar frá 1942 og mælingum höf.
ponding roughly to an annual average preci-
pitation of 2500 mm water in the Grímsvötn
area.
In the two profiles measured in 1951 by S.
Rist (one within, the other a short distance W
of the Grímsötn intake area) the accumulation
autumn 1950—24 April 1951, or during ab. 8
months, was respectively 1860 mm and 1200
mm water. This corresponds to an annual pre-
cipitation of 2800 and 1800 mm respectively,
mean value thus 2300 mm.
The average of the 6 values mentioned is
ab. 2400 mm, which thus may be regarded as a
very rough mean value for the precipitation
within the Grímsvötn intake area.
According to the topographical maps of the
Danish Geodætic Institute (sheets No. 76, 86,
scale 1:100.000), which were slightly corrected
by our expedition, the Grímsvötn intake area
is ab. 300 km2. According to Eythórsson’s map
of the subglacial topography of Vatnajökull
based on the seismic soundings carried out by
the French-Icelandic Expedition (Eythórsson
1952), the subglacial Grímsvötn intake area is
ab. 305 km2 (Thorarinsson, 1952). The total
annual precipitation within the Grímsvötn in-
take area may thus with a considerable degree
of probability be estimated at 0.7 á 0.75 km3, or
7 á 7.5 km3 in ten years, which is roughly the
mean interval between normal glacier bursts
(jökulhlaups) from Grímsvötn.
Changes in the Grimsvötn depression
since 1950/51.
One of the main objects of our expedition
was to study how the Grímsvötn depression had
changed since I flew over it on 28 Aug. 1950
and since it was visited by the French-Icelandic
Expedition in April 1951, and especially to
study the raising of the water level in the de-
pression since then.
In Aug. 1942, Steinþór Sigurðsson made a
map of Grímsvötn which was first published in
my paper on Grímsvötn in the Journal of
Glaciology (Thorarinsson, 1953). His height
figures were determined by trigonometrical
measurements. The height of Svíahnúkur eystri
he found to be 1702 m, or 23 m lower than its
height on the Geodætic Institute map. The map
published here as Fig. 6 is the same as Sigurðs-
son’s map as regards those areas which have
not changed since then. The height figures in
the Grímsvötn depression and a height figure
NNW of Svíahnúkur eystri are my corrected
altimeter readings, all based on the height dif-
ference between the cairn built by Sigurðsson
on Svíahnúkur eystri and the respective points.
I used an aeroplane altimeter (Kollsman Sensi-
tive Altimeter). All the height measurements on
the map were made within 20 hours and the
air conditions were unusually stable. The error
of the height measurements within the Gríms-
vötn depression is hardly more than + 2 m,
assuming that the height of Svíahnúkur eystri
is 1702 m. It must be emphasized that the con-
tour lines limiting the depression towards N
and NW are far from accurate on my map. Be-
fore dealing further with our observations of
Grímsvötn a few remarks should be made on the
names on my map. For the mountain as a
whole which delimits the Grímsvötn depression
towards SSE, I suggest the old name Grimsfjall
mentioned as the site of an eruption in 1684
in P. H. Resen’s Atlas Danicus, written in
1684—87 (cf. Thorarinsson 1948, pp. 98—100),
and I further propose that the name Svíahnúk-
ar (Swede’s Peaks), used for the mountain as a
whole on the Geodætic Institute map should
be limited to the two highest peaks of the
mountain. Sviahnúkur eystri (E Svíahnúkur) is
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