Jökull - 01.12.1972, Blaðsíða 42

ergmg from the above-mentioned portal in the
snout of Kverkjökull.1 *) This stream joins
several other glacial melt-water tributaries from
Dyngjujökull and Brúarjökull lobes to form
the headwaters of the Jökulsá á Fjöllum drain-
age system. Bright tones of the imagery clearly
indicate higher water temperatures near the
portal and decreasing temperatures down-
stream. On the basis of the 1966 infrared
imagery, the Kverkfjöll melt-water stream has
reached an equilibrium temperature in relation
to its environment 2 km downstream from the
portal. Although surface temperature can be
estimated only qualitatively from the imagery,
Thorarinsson’s fielcl notes for this locality in-
dicate that on 22 July 1963 he recorded a
water temperature of 13°C 50 meters inside
the mouth of the tunnel and 1° or 2° C lower
at the portal; the discharge was 1.5 m3/sec.
Advective heat transfer through
Kverkjökull outflow tunnel
From the foregoing volumetric discharge ancl
temperature data, the lieat transfer by advec-
tion from Kverkjökull outlet tongue can be
estimated if several assumptions are made about
the data and the source of the melt water.
First of all, it is assumed that Thorarinsson’s
observational data of 1963 are applicable to
the infrared survey data of 1968.
1) The simplest advective heat flow estimate
that can then be made would be based on
the assumption that the entire volume of melt-
water emerging from the portal was already in
the liquid phase as a result of nongeothermal
subglacial melting, and that is was then heated
geothermally to a temperature of 13° C from
an initial subglacial equilibrium temperature
1) Photogrammetric measurements on aerial
photographs taken in 1960 and 1968 suggest
that the snout of Ivverkjökull was in 1968
about 10 m above its position in 1960. At a
point about 350 m east of the portal of the melt-
water outflow stream, a recession of 30 m may
have occurred since 1960. The snout of Kverk-
jökull is thus about 1150 m above its position
(on the 840 m contour) of 1914 as mapped by
Trautz (1919), suggesting a recession of 50 m
per year between 1914 ancl 1937. In 1968, the
snout was entirely above the 900 m contour.
40 JÖKULL 22. ÁR
of 0° C. Taking into account the specific heat
of water (1 cal/g °C), a discharge of 1.5 m3/sec
would yield 19.5 X 10° cal/sec (or about
80 X 10° watts).
2) There is, liowever, considerable evidence
to suggest that the melt-water stream was heat-
ed subglacially to a higher temperature than
13° C measured at the glacier portal, the
interface between two different environments.
The Hveradalur steam vents and solfataras
causing perforations in the glacial cover gener-
ally upstream from the portal indicate sub-
glacial geothermal sources at temperatures pos-
sibly higher than 100° C at their exit points.
A north-northeast extension of the Hveradalur
trend intersects Kverkjökull outlet tongue
about 2(4 km upstream from the portal. If
subglacial geothermal springs at this point
approach 100° C, antl if they are indeed the
source of the melt-water, the change in tem-
perature of the stream with distance down-
stream woulcl be 0.04° C/m.
Energy exchange between the subglacial
stream and its environment (involving radia
tion, molecular heat conduction, eddy conduc-
tion to the air in the subglacial tunnel, and
latent heat of fusion of water under the
glacier) would be supplementecl by heat loss
by seepage through the stream bed. T’hese
factors could easily account for the apparent
heat loss before the stream emerged from the
portal. Thorarinsson’s observation of tempera-
tures 2° C liigher than portal temperatures
50 m inside the tunnel supports this reason-
ing ancl also suggests a subglacal heat loss of
0.02 to 0.04° C/m.i)
If we assume an initial temperature of 100°
C at the exit points of the geothermal waters,
1) In the open-air environment downstream
from the glacier portal, the infrared images
indicate a continuecl heat loss to a point ap-
proximately 2 km downstream, where a tem-
perature equilibrium hacl been established be-
tween the stream and its environment. If
grouncl and water temperatures at the equi-
librium point were 3° C, typical of ground
temperatures at 800 m elevation at miclnight
in August in the Odádahraun north of Vatna-
jökull, the rate of cooling downstream from
the portal would be 0.005° C/m.