Mass balance, ice velocity and ice temperature at the Inland Ice margin north:'east of Jakobshavn, central West Greenland

Glaciological field investigations were made on the Inland Ice north-east ofJakobshavn. The work is part of the hydropower investigations at Påkitsoq in a drainage basin proposed for a local hydropower project. Brief reports of the work have been given by Thomsen (1983, 1984, 1985, 1986).

Mass balance, ice velocity and ice temperature at the Inland Ice margin north:'east of Jakobshavn, central West Greenland

Henrik Højmark Thomsen
Glaciological field investigations were made on the Inland Ice north-east of Jakobshavn. The work is part of the hydropower investigations at Påkitsoq in a drainage basin proposed for a local hydropower project. Brief reports of the work have been given by Thomsen (1983Thomsen ( , 1984Thomsen ( , 1985Thomsen ( , 1986.

Mass balance measurements 1986/1987
The glaciological programme was started in August 1982.when stakes were drilled into the ice for measuring themass balance ( fig. 1). The stakes were visited by 'helicopter on13 May and 13 August. Six of the stilkes are located near the)ce margin ending in lake 187 (fig. 2) and are represented by stake 2 in fig. 1. The stakes have been plotted on the most recent map of the area, based on aerial photographs from 1985 (see Thomsen, 1988).
The winter snow cover on the ice was very patchy and confined mainly to drifts in gullies and crevasses up to an elevation of about 500 m a.s.l., but snow cover was continuous at higher elevations. The transient balance for the winter period was measured in snow pits and by depth soundings at the stakes. As there were no signs of heavy melting during the winter, the observed distribution of snow cover is probably due to wind drifting. At elevations of about 500 m a.s.1. the 1987 summer ablation is high compared to earlier years.
At lower elevations the stakes were melted out confirming strong melting during the summer. The transient and annual balances are shown in Table 1. In the caseof stakes which melted out, a minimum b'alance . figure can begiven based on the length of the stake still in the ice at the time ofthe spring stake readings. These figures are given in brackets in Table 1.

lce temperature measurements
To measure englacial temperatures two sets of thermistor strings were drilled down to depths of 202 m and 300 m in the ablation area ( fig. 2).· The holes were drilled with a newly constructed hot water drill (see Olesen & Clausen, 1988). The drilling operation was . carried out from a base camp established on the ice. The 202 m hole is situated 4.4 krri upstream from the ice margin with an ice-surface elevation of 490 m a.s.1. The l:ZJ lee margin § ' Lake and fjord g Contour lines   ice thickness at this location is 300 m according to radio echo soundings (Thorning & Hansen, 1987). The 300 mh oie which extends to the bottom of the ice in accordance with radar measurements is situaied. 3.2 km upstream from the ice margin with an ice surface elevation. of 455 m a.sJ.
Thermistors were mounted at every 25 m on the strings except for the lower end where the distance between the three lowest thermistors were 10 and 15 m, respectively. The accunicy of the temperature measurements in the ice is ± 0.2°C. Temperature readings were made several times during the two weeks duration of the drilling operation and were read five weeks later by a visiting field team from Greenland Technical Organisation (GTO). Temperature readings of thermistors drilled into the ice with a hot water drill on White Glacier, Axel Heiberg Island, showed temperatures to be sufficiently dose to the equilibrium state to allow measurements after 2 to 3 weeks (Blatter, 1985). The latest readings at Påkitsoq are assumed to be dose to the equilibrium state, but exact verification is needed next year. The temperature readings reveal negative temperatures in the whole ice body and a-small range through the profiles with a minimum temperature of -2.1°C and a maximum temperature of -0.6°C. Temperature measured ,at the bottom of the ice is -0.9°C.

lae velocity measurements
Ice velocity was measured on the glacier tongue ending in lake 187. lce velocity is measured by theodolite su~ey at stakes drilled into the ice from fixed points established on the ground. Ice velocities are given in Ta;ble 2 and stake locations are given in fig. 2. lce. movement is highest at stake 3 located at the foot of a small icefall. For the stakes near to the ice margin there is generally a higher velocity at the southem lateral part of the tongue compared to the central part. For all stakes there is a marked seasonal variation in ice movement, with mean summer velocities up to twice the mean wintervelocity. Variations in sliding velocity could be an explanation (Paterson, 1981;Andreasen, 1985) which in tum implies that the basal ice is at the mdting point and that surface meltwater can penetrate to the bed and build up high water pressure. It is not possibie at present to determine if this movement pattein applies to the whole marginal area as no velocity measurements have been made further upstream. From depth soundings in lake 187 and ice thickness measurements with radar, it is reasonable to believe that the glacier tongue is floating. Water level recordings in lake 187 show that the mean water level is 2 m higher in summer (GTO, 1983). The possibility that the move c Thomsen, H. H. 1988: Mapping  Mapping of surface hydrology and modeIling of glacier hydraulics at the margin of the Inland Ice north-east of Jakobshavn have been used for investigating glacier drainage. The work is part of the hydropower investigations at Piikitsoq in a drainage basin proposed for a local hydropower project. Excludiitg its IiIland Ice sector the basin covers an area of 33.6 km 2 and is situated at about 20Q-600 m a.sJ. ( fig. 1; Thomsen, 1988, fig. 1). The main part of the runoff from the basin is meltwater from the ice sheet draining through three lakes, 326, 233 and 187. Lake 187 and 233 are proposed as two separate reservoirs, with tunnels leading to the fjord north-west of the basin.

Glacier hydrological conditions
Meltwater drainage on the ice itself is complicated.-Over large areas meltwater drains through innumerabJe rivers whose drainage courses are infiuenced by the surface undulation and different structural features on the ice surface. In most cases the rivers escape down into moulins or crevasses, after which the meltwater drainage iscontrolled byenglacial and subglacial drainage conditions. Delineation of drainage basins requires information about supraglacial andsubglacial conditions.
[Z] Ice margin § Lake and fjord