Mass balance of Devon Ice Cap, Canadian Arctic Andrew SHEPHERD, 1* Zhijun DU, 1 Toby J. BENHAM, 1 Julian A. DOWDESWELL, 1 Elizabeth M. MORRIS 2 1 Scott Polar Research Institute, University of Cambridge, Lensfield Road, Cambridge CB2 1ER, UK E-mail: andrew.shepherd@ed.ac.uk 2 British Antarctic Survey, Natural Environment Research Council, Madingley Road, Cambridge CB3 0ET, UK ABSTRACT. Interferometric synthetic aperture radar data show that Devon Ice Cap (DIC), northern Canada, is drained through a network of 11 glacier systems. More than half of all ice discharge is through broad flows that converge to the southeast of the ice cap, and these are grounded well below sea level at their termini. A calculation of the ice-cap mass budget reveals that the northwestern sector of DIC is gaining mass and that all other sectors are losing mass. We estimate that a 12 489 km 2 section of the main ice cap receives 3.46  0.65Gt of snowfall each year, and loses 3.11  0.21 Gt of water through runoff, and 1.43  0.03Gt of ice through glacier discharge. Altogether, the net mass balance of DIC is –1.08  0.67 Gt a –1 . This loss corresponds to a 0.003 mm a –1 contribution to global sea levels, and is about half the magnitude of earlier estimates. 1. INTRODUCTION The 3980 km 3 Devon Ice Cap (DIC) in the Canadian Arctic is among the largest of Earth’s ice caps, and forms a significant fraction of all land ice volume that lies beyond the Antarctic and Greenland ice sheets. While DIC’s geographical lo- cation has exposed it to a range of climatic conditions (Paterson and others, 1977; Dowdeswell and others, 1997), general circulation models (Church and Gregory, 2001) consistently predict continued warming throughout the coming century. Moreover, recent studies (Braun and others, 2004) have shown that fluctuations in the mass of the world’s smaller ice bodies will constitute the greatest cryospheric component of future sea-level rise, so a know- ledge of the present state of balance of DIC is a subject of considerable interest (Burgess and Sharp, 2004). Persistent monitoring of the DIC surface mass balance began in the early 1960s with sparse (10 –2 sites km –2 ) field surveys of accumulation and other parameters (Koerner, 1966). More recently, a change in the ice-cap volume has been reported (Abdalati and others, 2004) based on repeat sorties of an airborne laser altimeter, and an estimate of the DIC surface mass balance has been produced (Mair and others, 2005) using accumulation data from eight boreholes and a model of ablation. From these surveys, it was concluded that the ice cap lost 0.8 km 3 of ice each year between 1995 and 2000, and 1.6  0.7 km 3 a –1 over the longer period 1963–2000. Another study (Burgess and others, 2005) used satellite interferometric synthetic aper- ture radar (InSAR) and other data to estimate the rate of mass loss due to iceberg calving between 1960 and 1999, and concluded the ice cap was losing 0.57  0.12 km 3 of ice each year through glacier outflow. Although separate studies, these works put the long-term (30 year) mass balance of DIC at about –2.2  0.7 Gt a –1 , a loss sufficient to raise global sea levels by 0.006 mm a –1 . Here, we use InSAR measurements of ice discharge, in situ records of snow accumulation, and a positive degree-day model of summer ablation, to reach a new, absolute estimate of the DIC mass balance. 2. DATA We use European Remote-sensing Satellite (ERS) synthetic aperture radar (SAR) data recorded during the tandem-repeat phase to produce repeat-pass interferograms from two separate satellite ground tracks. No ERS data were recorded during descending orbits at DIC, so we used data from ascending orbits only (their trajectory is shown in Fig. 1a). The distribution of our SAR dataset allowed us to form interferograms with temporal baselines of 1 or 35 days in spring 1996 (Table 1); data were not available for other time periods. The width of the SAR images was 100 km, so it was necessary to mosaic data from two adjacent ERS tracks in order to survey the entire DIC. 2.1. InSAR DEM Although a recent airborne survey has provided models of the DIC surface and bedrock elevations (Dowdeswell and others, 2004), their 1 km horizontal resolution is lower than that required for InSAR processing. Instead, we derived a new, fine spatial resolution digital elevation model (DEM) to facilitate our InSAR estimates of the ice-cap surface deformation, ice-flow direction and mass flux. We formed the DEM using differential interferograms from the two ERS tandem (1 day) pairs (Table 1). Their baselines were constrained (Zebker and others, 1994) with ground-control points (GCPs) of known elevation. The spatial resolution of the new InSAR DEM is 40 m, 250 times finer than that of the airborne survey. To assess its accuracy, we re-sampled the InSAR DEM to the same ground resolution as the airborne DEM, and the root-mean-square (rms) deviation between the two models was 18 m. This was comparable to the precision of the airborne dataset, so we conclude that the two elevation models were indistinguishable. 2.2. Ice surface velocity We used InSAR to measure the DIC surface displacement in the look direction of the SAR sensor (e.g. Joughin and others, 1996; Shepherd and others, 2001) from data recorded during ascending orbits only (Table 1). In the absence of data from alternate viewing geometries, it was necessary to make assumptions as to the ice-flow direction in order to derive Annals of Glaciology 46 2007 * Present address: School of GeoSciences, University of Edinburgh, Drum- mond Street, Edinburgh EH8 9XP, UK. 249