Jökull - 01.01.2012, Blaðsíða 144

E. Magnússon et al.
To draw the water divides between subglacial
drainage basins (Figure 9), we assume static water
potential ϕ with water pressure equal to the ice over-
burden pressure (see e.g. Paterson, 1994):
ϕ = ρwgzb + ρigH
where ρw =1000 kg m−3 and ρi = 900 kg m−3 is
the density of water and ice, g = 9.82 m s−2 the ac-
celeration due to gravity, zb the bedrock elevation
and H= zs-zb is the ice thickness. To calculate H the
surface elevation (zs) was derived using the LiDAR
DEM with 100 m x 100 m grid size, filtered with a
same kind of circular filter as described above with
width equal to the ice thickness at each location. By
applying a filter of this size we are assuming that, due
to the strength of the ice, the weight of an ice column
affects the ice overburden pressure over a distance
equal to the ice thickness. The ice and water divides
often coincide since the locations of water divides
are also commonly forced by the rugged topography.
In areas of more gentle topography the water divides
are derived using a trace algorithm with the water
potential as an input.
Discussion
The distribution of ice with elevation is considerably
different from one outlet of Öræfajökull to another
(Figure 8b). Due to this difference we would expect
these outlets to respond differently to current climatic
conditions. If we assume that in the past decade the
equilibrium line altitude (ELA) of Öræfajökull outlets
has been similar to the observed ELA at the neigh-
bouring Breiðamerkurjökull it would be typically be-
tween 1100 and 1200 m a.s.l. (Björnsson and Pálsson,
2008). This along with the area distribution indicates
accumulation area ratio (AAR, the ratio of the accu-
mulation area to the total glacier area) close to 0.6
for the glacier on the west side in addition to Kvíár-
jökull, while the derived AAR for the south and east
outlets excluding Kvíárjökull is 0.05–0.2 lower. Typ-
ical AAR values for Vatnajökull outlets at zero mass
balance are 0.55–0.65 (Björnsson and Pálsson, 2008).
In order to investigate how the variable area distribu-
tions, which presumably may lead to east-west trend
in AAR, affect the balance of these glaciers we did a
rough comparison between the LiDAR DEM and el-
evation observation from 2002–2006. The older data
covers only the ablation area of the glaciers and in-
cludes DEMs from Loftmyndir ehf. obtained in 2002–
2003 and GPS profiles in 2005–2006 obtained during
the RES point observations. This comparison reveals
typically 3–10 m yr−1 lowering for elevations below
500 m a.s.l. but shows no clear trend from west to
east. This indicates that current meteorological condi-
tions can sustain a glacier with lower elevation distri-
bution on the east side of Öræfajökull than on the west
side, due to lower ELA and greater mass turnover on
the east side caused by higher precipitation rate (Cro-
chet, 2007). More quantitative comparison of multi-
temporal elevation observations and analysis of mete-
orological data are however needed to conclude fur-
ther on this subject, which is beyond the scope of this
paper.
Figure 8. a) The ice thickness of Öræfaökull (50 m contours) and ice divides of the main ice catchments (red
lines). b) Cumulative area distribution of ice surface (blue) and bedrock (brown) with elevation. The letters
refer to the ice catchments marked in a. The base of the brown fill indicates the minimum bedrock elevation,
which reaches below sea level (broken blue line) at four ice catchments. The red block indicates the range of
typical ELA on Breiðamerkurjökull outlet glacier (east of Öræfajökull) in recent years (Björnsson and Pálsson,
2008). c) The ice volume for every 50 m surface elevation interval for the same ice catchments. The legend
gives total ice volume of each catchment. Note different scales on the x-axis in both b and c. – a) Þykkt jökuls
og ísasskil. b) Uppsöfnuð flatardreifing með hæð jökulyfirborðs (ljósblátt) og botns (brúnt) fyrir tilsvarandi
íssvæði merkt á mynd a. Botn brúnu þekjanna sýnir lægstu botnhæðir hvers ísasvæðis. Rauði borðinn sýnir
dæmigerða hæð jafnvægislínu á Breiðamerkurjökli undanfarin ár. c) Ísrúmmál á hverju 50 m hæðarbili fyrir
sömu ísasvæði. Heildarrúmmál hvers ísasvæðis er merkt inn á línuritin.
142 JÖKULL No. 62, 2012