Jökull - 01.01.2004, Side 34

Eyjólfur Magnússon et al.
PolSAR images, which were also acquired by EMI-
SAR in 1998. It was not possible to locate the outlets
for the other rivers in the PolSAR images.
The ice flowlines in W-Vatnajökull were estimated
from EMISAR DEM and from them the ice divides of
outlets and glacier parts in W-Vatnajökull were drawn.
Compared with older results based on DEMs in the
1980s, no significant changes in the ice divides were
observed except around the sub-glacial volcano Gjálp,
which erupted in 1996 causing considerable decrease
in the ice basin of the Eastern Skaftá cauldron.
Based on our ice divides and the new DEM area
distribution with elevation was calculated for the main
outlets of W-Vatnajökull. Sylgjujökull is most sensi-
tive to changes in equilibrium line elevation due to
its narrow elevation range. Accumulation area ratios
(AAR) for Köldukvíslarjökull and Tungnajökull in
the recent years were calculated based on the new area
distribution and mass balance measurements on these
glaciers. The lowest AAR for both glaciers was de-
rived for the glacier year 1997–1998 when it dropped
below 0.2 on Tungnaárjökull and below 0.4 on Köldu-
kvíslarjökull. Due to recent surges has the accumula-
tion area percentage of the total area been close to 5%
lower than if no surges had occurred, for Sylgjujökull,
Tungnaárjökull, Skaftárjökull and Síðujökull.
A previously undetected depression, probably cre-
ated by geothermal activity, was observed close to the
margin of Köldukvíslarjökull. It falls on an approx-
imately straight line with the Skaftá cauldrons and
a large geothermal area west of Köldukvíslarjökull.
The magnitude of the geothermal power at this site
could be some tens of MW but not higher.
Acknowledgements
This work is to a great extent part of Eyjólfur Magn-
ússon’s Master thesis, which the Icelandic Centre
for Research, the Institute of Earth Sciences, Uni-
versity of Iceland and the Southern Region Institute
for Advanced Learning have supported by research
grants. Magnús Tumi Guðmundsson is gratefully
thanked for his reviews and comments. The pro-
cessing of the SAR data was financed by the Sci-
ence Institute, University of Iceland, the National
Land Survey of Iceland, the Soil Conservation Service
of Iceland, the Agricultural Research Institute, the
Icelandic Museum of Natural History, the National
Power Company, the Nordic Volcanological Institute,
the National Energy Authority and the South Iceland
Forestry. We thank Bjarni Kristinsson and Kristinn
Einarsson at the National Energy Authority for sup-
plying discharge data.
ÁGRIP
Kynntar eru jöklafræðilegar athuganir byggðar á
nýju stafrænu hæðarlíkani af vesturhluta Vatnajökluls.
Hæðarlíkanið var unnið úr háupplausnar radar bylgju-
víxlgögnum sem safnað var í ágúst 1998. Gögn-
unum var safnað með radar frá Tækniháskóla Dan-
merkur sem þota frá Konunglega danska flughern-
um flaug með. Hæðarlíkanið var notað til að endur-
skoða legu ísskila og hæðardreifingu skriðjökla í V-
Vatnajökli. Sökum breyttrar hæðardreifingar í kjöl-
far framhlaupanna í Sylgju-, Tungnaár-, Skaftár- og
Síðujökli 1993–1996, hefur hlutfall afkomusvæðis af
heildarflatarmáli þessara jökla verið um 5% lægra
seinustu ár en ella hefði verið ef engin framhlaup
hefðu orðið. Út frá nýja hæðarlíkaninu var vatnsmætti
við botn reiknað, sem síðan var notað til að draga
vatnaskil jökuláa sem renna frá V-Vatnajökli og helstu
rennslisleiðir undir jökli. Staðsetning vatnaskila milli
Skaftárkvíslanna annars vegar og milli Djúpár og
Hverfisfljóts hins vegar var yfirfarin sérstaklega með
hliðsjón af rennslismælingum. Samanburður hinna
nýju vatnaskila við eldri vatnaskil, byggð á kortum frá
því á 9. áratugnum, sýna umtalsverðar breytingar, sér-
staklega á vatnaskilum Tungnaár og Skaftár, sem rekja
má til framhlaupsins í Tungnaárjökli 1994. Í hinu nýja
hæðarlíkani sést áður óþekktur sigketill í jaðri Köldu-
kvíslarjökuls sem væntanlega er myndaður af jarðhita.
Ketillinn sem þó er ekki nýmyndaður, fellur nokkurn
veginn á línu með Skaftárkötlunum og háhitasvæðinu
í Hágöngulóni.
34 JÖKULL No. 54