Mapping of a hydrological ice-sheet drainage basin on the West Greenland ice-sheet margin from ERS-1/-2 SAR interferometry, ice-radar measurement and modelling A.P. AhlstrÖ m , 1 C. Egede BÖ ggild , 1 J. J. Mohr, 2 N. Reeh , 2 E. Lintz Christensen , 2 O. B. Olesen , 1 K. Keller 3 1 Geological Survey of Denmark and Greenland,Thoravej 8, DK-2400 Copenhagen, Denmark 2 Òrsted-DTU, Electromagnetic Systems,Technical University of Denmark, DK-2800 Lyngby, Denmark 3 National Survey and Cadastre, Rentemestervej 8, DK-2400 Copenhagen, Denmark ABSTRACT . The hydrological ice-sheet basin draining into the Tasersiaq lake, West Greenland (6613’ N, 5030’ W), was delineated, first using standard digital elevation models (DEMs) for ice-sheet surface and bedrock, and subsequently using a new high- resolution dataset, with a surface DEM derived from repeat-track interferometric synthetic aperture radar (SAR) and a bedrock topography derived from an airborne 60 MHz ice-penetrating radar. The extent of the delineation was calculated from a water-pressure potential as a function of the ice-sheet surface and bedrock elevations and a hydraulic factor k describing the relative importance of the potential of the ice over- burden pressure compared to the bedrock topography.The meltwater run-off for the basin delineations was modelled with an energy-balance model calibrated with observed ice-sheet ablation and compared to a 25 year time series of measured basin run-off. The standard DEMs were found to be inadequate for delineation purposes, whereas delineations from high-resolution data were found to be very sensitive to changes in k in a non-linear way, causing a factor 5 change of basin area, corresponding to a doubling of the modelled run- off. The 50% standard deviation of the measured basin run-off could thus be explained by small year-to-year variations of the k-factor. INTRODUCTION Hydropower is an attractive alternative energy source in Greenland, and feasibility studies have been carried out since the 1970s. The early investigations were limited, how- ever, by the difficulty of carrying out field operations on the Greenland ice sheet and focused on the smaller basins in the immediate vicinity of towns. Discharge measurements were initiated at a few large basins adjoining the Greenland ice sheet, but only recently with global positioning system and remote-sensing technology has it become possible to study the meltwater contribution from the ice sheet. It is desirable to develop a method for determining the ice-sheet melt- water contribution and its variability, as it forms the bulk of the water flux in the largest basins in West Greenland. The aim of the present study is to investigate the impact of surface and bedrock elevation models on the assessment of meltwater output from the ice sheet. Evaluation of the error in calculating the ice-sheet run-off arising from using stan- dard digital elevation models (DEMs) of surface and bedrock instead of using high-resolution data was facilitated by a detailed survey of the ice-sheet segment draining into the Tasersiaq lake, West Greenland (6613’ N, 5030’ W). The Tasersiaq basin is the largest basin inWest Greenland in terms of hydropower potential (Weidick and Olesen, 1978), with a discharge of 19.5+9.06 10 8 m 3 a ^1 measured by the Green- land Survey since 1975. The long time series provides an opportunity to test the validity of the basin delineations by comparing modelled to measured discharge.Two sets of eleva- tion and bedrock models are used as input to a model delin- eating the hydrological basin on the ice sheet. The amount of meltwater from these ice-sheet basins is then estimated by means of an energy-balance model and compared to the meas- urements of discharge carried out by the Greenland Survey. ELEVATION MODELS Basin delineation requires knowledge of the ice-surface and bedrock elevations. Two sets of ice-sheet surface and bed- rock DEMs were tested covering the area shown in Figure1. The first set consists of the 1km resolution ice-sheet DEM of Ekholm (1996) and the 5km resolution bedrock DEM of Bamber and others (in press). The second set consists of a bedrock elevation model acquired with an airborne 60 MHz ice-radar system on a Twin Otter aircraft described in Christensen and others (2000), and a DEM derived from repeat-track synthetic aperture radar interferometry (InSAR) and resampled to a grid resolution of 330 m. The ice-radar measurements were collected from flight tracks approximately 2.5 km apart to resolve the highly undulating bedrock. The DEM was generated from two descending-orbit European Remote-sensing Satellite-1 and -2 (ERS-1/-2) tandem mode image pairs. The ERS-1 images were acquired on 20 October and 29 December 1995, respectively. Baselines of the pairs were ^68 and 122m. The raw data of Annals of Glaciology 34 2002 # International Glaciological Society 309