Jökull


Jökull - 01.01.2004, Page 33

Jökull - 01.01.2004, Page 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
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