Jökull - 01.01.2004, Side 33

Glaciological application of InSAR topography data of W-Vatnajökull
Figure 11. The stream lines (arrow lines) around the depression in Figure 10. The two perpendicular lines show
cross section used for calculating the flow in and out of the depression. – Straumlínur kringum sigketil og
þversnið sem voru notuð til að reikna ísflæði inn og út úr katlinum.
area southwest of Grímsvötn. By comparing esti-
mated summer balance on the glacier with measured
discharge in these rivers we conclude that most of the
water from this area drains to Hverfisfljót.
From a comparison between the estimated sum-
mer balance and the estimated ratio between the dis-
charge in the eastern and western outlet of Skaftá we
conclude that the derived water potential incorrectly
predicts water divides between these outlets. The de-
rived water potential fails either due to the simplicity
of the static model or/and because of unknown sub-
glacial landscape features.
The water divides of Tungnaá derived from our
DEM were also compared with water divides obtained
from the surface DEM of 1981 which was based on
barometric precision altimeter profile measurements.
The estimated area of the sub-glacial river basin of
Tungnaá is 95 km2 found from our 1998 DEM while
the 1981 DEM gave 149 km2. This change is caused
by the surge in Tungnaárjökull in 1994–1995. The
difference in summer flow from June to September for
the two estimated river basins is between 10 and 15
m3/s, draining now into Skaftá instead of Tungnaá.
The predicted water potential was also used to
draw main sub-glacial watercourses for Tungnaá,
Skaftá, Hverfisfljót, Brunná and Djúpá. The predicted
location of the river outlets for Skaftá and Hverfisfljót
agreed fairly well with the actual outlets observed in
JÖKULL No. 54 33