Jökull - 01.01.2016, Page 12

Eyjólfur Magnússon et al.
Figure 8. The glacier thickness of Drangajökull rel-
ative to the glacier surface on 20 July 2011 (Lidar).
Ice divides of the main ice catchments are shown
with red lines (key dimensions given in Table 1). –
Þykkt Drangajökuls m.v. jökulyfirborð þann 20. júlí,
2011. Lykilstærðir Drangajökuls og helstu ísasviða
hans (rauðar línur á mynd) eru í 1. töflu.
The main source of errors in both glacier thick-
ness and bedrock elevation are related to the interpo-
lation between measured bedrock elevations, incom-
pleteness of 2D migration and uncertainties in the
propagation velocity of the radar signal through the
ice, which may vary over the ice cap (e.g. Bradford
et al., 2009). Here the electromagnetic wave propa-
gation velocity of Cgl=1.70×108 m s−1, which seems
the appropriate average for ∼100 m thick glacier in
the ablation area of Drangajökull is likely to decrease
with increasing glacier thickness (decreasing ratio be-
tween winter snow and ice thickness). This could
therefore result in slightly too thin glacier for thick-
ness <100 m and slightly too thick glacier for thick-
ness >100 m in the glacier ablation zone. In the accu-
mulation area of the ice cap, the firn pack underneath
the winter snow should however raise Cgl, which may
result in some underestimation of the glacier thick-
ness. We assume this variability in Cgl to be less than
5% over the ice cap (Lapazaran et al., 2016), which
corresponds to wider range of Cgl than indicated by
e.g. Bradford et al. (2009) when Cgl is vertically aver-
aged through the glacier at each location. This would
correspond to possible error of 14 m where the ice
cap is thickest. Based on this, the comparison shown
in Figure 4 and the error assessment of the interpola-
tion when splitting the RES-data into two subsets (see
Data and Methods), we expect the uncertainty in the
bed elevation as well as in glacier thickness to range
Table 1. Key dimensions of Drangajökull ice cap and its main ice catchments (Figure 8) relative to the glacier
surface on 20 July 2011 (Lidar). The estimated uncertainty of the volume and average thickness is 2.5% of given
values. – Flatarmál, rúmmál, meðal- og hámarksþykkt Drangajökuls og helstu ísasviða m.v. jökulyfirborð þann
20. júlí, 2011. Óvissa í uppgefnum rúmmálum og meðalþykktum er um 2.5%.
Name Area km3 Volume km3 Average thickness (m) Max. thickness (m)
Drangajökull ice cap 144 15.4 107 284
Leirufjarðarjökull 28 3.3 120 248
Kaldalónsjökull 35 4.6 131 284
SW-Drangajökull 27 3.0 112 221
SE-Drangajökull 22 1.4 64 180
Reykjarfjarðarjökull,
Þaralátursjökull and
Ljótarjökull 27 2.7 100 252
Northern tip 4 0.3 67 160
12 JÖKULL No. 66, 2016