Jökull - 01.01.2012, Side 106

S. Guðmundsson et al.
RESULTS
Surface elevation changes
Crevasse areas were used to calculate elevation
changes in the accumulation zone (Tables 1, 2 and 3).
Routines 1 and 2 give similar results, showing an aver-
age difference of 1.1 m, with routine 1 always giving
higher values (Table 4). Surface changes above 1700
m were negligible, and 4–11 m from there down to
the equilibrium line. The lowering gradually increas-
ing, 20–30 m north of Rótarfjall (Figures 4a-b and Ta-
ble 5), and reaching a maximum of 180 m in Kotárgil
(Figures 8, 9 and Table 5).
Table 2. Surface elevation changes measured by rou-
tine 1. – Tafla 2. Yfirborðslækkun á safnsvæði Kotár-
jökuls samkvæmt reikniaðferð 1.
area ∆θ(px) m/px ∆h (m)
c1 13.14 0.91 11.4
c2 5.18 1.28 6.3
c3 5.51 1.50 7.9
c4 2.72 1.73 4.5
c5 2.04 2.15 4.2
c6 5.29 1.36 6.9
c7 1.80 1.76 3.0
c8 3.06 1.72 5.0
c9 14.73 0.68 9.6
Area and volume changes
Glacial retreat and the volume loss of Kotárjökull is
based on the inner margin in Kotárgil. The terminus of
Kotárjökull has retreated 1.3 km, since the LIA maxi-
mum, from an elevation of 175 m to 350 m (Figure 9).
Rótarfjallsjökull has retreated ca. 2 km, and the ter-
minus is currently at an elevation of 660 m. The east-
ern branch, with a small part of the accumulation area
extending up to the ice cap plateau, has on average
retreated 17 m/yr, whereas the main branch shows a
mean recession of 11 m/yr. Rótarfjallsjökull receives
less ice from the caldera and has a smaller accumula-
tion area. The whole glacier area has decreased from
14.5 to 11.5 km2 (20%), and the volume loss has been
approximately 0.4 ±0.02 km3, relative to the more
limited glacier margin in Kotárgil (Figure 1). Given
the average glacier thickness of 90 m (Magnússon et
al., 2012), Kotárjökull has lost approximate 30% of its
volume. Evenly spread over the mean glacier area, the
recession corresponds to a loss of about 0.22 ±0.01 m
w.e./yr.
Table 3. Surface elevation changes measured by rou-
tine 2. Three calculations for each crevasse area, and
the lowering presented in the last two columns with
the relevant area in parenthesis. – Tafla 3. Yfirborðs-
lækkun á efra svæði Kotárjökuls samkvæmt reikni-
aðferð 2. Þrjár mælingar gerðar fyrir hvert sprungu-
svæði. Í sviga í síðasta dálki er tilgreint hvaða
sprungusvæði lækkunin á við.
crev. area α a (px) ∆h (m) ∆h (m)
c1→c9 12.3 394 10.2 (c1) 8.6 (c9)
c1→c8 13.7 429 10.5 (c1) 3.7 (c8)
c1→c6 10.6 329 10.5 (c1) 6.4 (c6)
c2→c6 14.1 441 5.8 (c2) 6.4 (c6)
c2→c8 17.2 534 5.9 (c2) 4.7 (c8)
c2 →c4 9.5 293 5.8 (c2) 4.1 (c4)
c3→c2 3.2 101 7.1 (c3) 5.7 (c2)
c3→c7 10.1 312 7.3 (c3) 2.8 (c7)
c3→c4 6.3 194 7.3 (c3) 4.1 (c4)
c4→c7 3.8 118 4.1 (c4) 2.7 (c7)
c4→c6 4.6 152 3.8 (c4) 5.9 (c6)
c7→c8 3.9 125 2.7 (c7) 4.4 (c8)
c9→c3 12.6 429 8.1 (c9) 6.7 (c3)
c9→c2 15.8 519 8.5 (c9) 5.6 (c2)
c4→c5 9.6 308 4.0 (c4) 3.7 (c5)
c3→c5 3.3 120 6.2 (c3) 3.3 (c5)
c1→c5 3.6 187 6.2 (c1) 2.3 (c5)
Table 4. Comparing the surface elevation changes of
Kotárjökull above 1100 m elevation, calculated by
the two routines. Routine 2 based on average values
from Table 3. – Tafla 4. Niðurstöður og samanburður
reikniaðferða á þykktarbreytingum Kotárjökuls ofan
1100 m hæðar. Gildi fyrir aðferð 2 eru meðaltöl úr
Töflu 3.
crevasse ∆h ∆h ∆h
area (routine 1) (routine 2) difference
c1 (1166 m) 11.4 m 9.4 m 2.0 m
c2 (1319 m) 6.3 m 5.8 m 0.5 m
c3 (1398 m) 7.9 m 6.9 m 1.0 m
c4 (1429 m) 4.5 m 4.0 m 0.5 m
c5 (1677 m) 4.2 m 3.1 m 0.9 m
c6 (1283 m) 6.9 m 6.2 m 0.7 m
c7 (1367 m) 3.0 m 3.1 m 0.1 m
c8 (1403 m) 5.0 m 4.3 m 0.7 m
c9 (1083 m) 9.6 m 8.4 m 1.2 m
104 JÖKULL No. 62, 2012