Jökull - 01.01.2016, Side 1

Reviewed research article
The subglacial topography of Drangajökull ice cap,
NW-Iceland, deduced from dense RES-profiling
Eyjólfur Magnússon1, Joaquín M. C. Belart1, Finnur Pálsson1, Leif S. Anderson1,
Ágúst Þ. Gunnlaugsson1, Etienne Berthier2, Hálfdán Ágústsson3 and Áslaug Geirsdóttir1
1Institute of Earth Sciences, University of Iceland, Askja, Sturlugata 7, 101 Reykjavík, Iceland
2Laboratoire d’Etudes en Géophysique et Océanographie Spatiales,
Centre National de la Recherche Scientifique (LEGOS – CNRS), Université de Toulouse, France
3Icelandic Meteorological Office, Bústaðavegi 9, 150 Reykjavík, Iceland
Correspondence to: eyjolfm@hi.is
Abstract — Systematic mapping of the bedrock beneath the Icelandic ice caps began in 1980. We continue this
effort by presenting a detailed bedrock digital elevation model (DEM) of Drangajökull ice cap in NW-Iceland,
surveyed using radio echo sounding (RES) in March 2014. About 590 km of RES profiles, generally with 200–
300 m spacing, were surveyed. This is to date the densest set of RES profiles measured on an Icelandic ice
cap. We describe the processing of the radar profiles and construction of the bedrock DEM. We also analyse
possible errors of surveyed glacier thickness along the profiles and discuss the accuracy of the final bedrock
DEM. The bedrock DEM reveals topography with the same general character as other areas of NW-Iceland
(Vestfirðir). The new bedrock DEM and a series of surface DEMs allow estimates of total ice cap volume
and relative volume change since the mid-20th century. The bedrock data combined with the most accurate
available surface DEM of Drangajökull, from July 2011, reveals a total ice cap volume of 15.4±0.4 km3, of
which ∼71% was stored on the west side of the ice cap. At that time the average thickness of Drangajökull
was 107±3 m and the maximum observed thickness, found on Kaldalónsjökull outlet glacier, was 284±14 m.
In 1946–2014, the total ice cap volume (in autumn) was reduced from ∼18 km3 to ∼15 km3. Approximately
half of this volume was lost in 1994–2011. We describe and discuss several interesting phenomena revealed by
the RES data and the derived bedrock DEM. The presented bedrock data also helps to explain curious surface
features in the accumulation area of NE-Drangajökull.
INTRODUCTION
Well constrained glacier geometry is fundamental for
most glaciological studies. Digital elevation models
(DEMs) of surface and subglacial topography are nec-
essary prerequisites for glacier modelling (e.g. Gillet-
Chaulet et al., 2012; Aðalgeirsdóttir et al., 2011) and
subglacial hydrology studies (e.g. Björnsson, 1988,
Flowers, 2015). Glacier thickness also provides a
measure of freshwater storage in glaciers, relevant re-
garding use of glacier water for irrigation and electric
power production (e.g. Radic and Hock, 2013) as well
as to estimate the possible contribution of glacier melt
to sea level rise (e.g. Vaughan et al., 2013).
Various methods are available to accurately map
the glacier surface, but glacier thickness and sub-
glacial topography is typically constrained using
radar, often referred to as radio echo sounding (RES).
RES has been used to measure glacier thickness and
map the bedrock of cold glaciers (ice temperature
<0◦C) since the 1960s (Bailey et al., 1964; Robin
et al., 1969; Gudmandsen, 1969). In the 1970s sci-
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