Jökull

Ataaseq assigiiaat ilaat

Jökull - 01.12.1971, Qupperneq 22

Jökull - 01.12.1971, Qupperneq 22
constant. At lower heights in the relatively flat middle part of the glacier the ablation increases due to higher insolation, the increas- ing effect of the turbulent energy fluxes ancl somewhat lower albedo in the autumn. The relatively low ablation in the lowermost parts of the glacier is due to a considerable moun- tain screening of the direct short wave radia- tion. On other parts of the glacier the shading effect of the surrounding mountains was fairly slight. The net balance curve changes linearly with height in the middle parts of the glacier and its form is to a high extent determined by the winter accumulation except in the lowermost parts of the glacier where its form is definitely dominated by the relatively small ablation. An important feature is that the specific net bal- ance has maximum between 1100 m and 1200 m where the glacier area is biggest. At the end of the glaciological year 1966—67 the equi- librium line of zero specific net balance lay in the height of about 1015 m. This is 100 m to 150 m below the height of the area maxim- um which is presumably the critical height for the equilibrium line. As long as the equilibri- um line lies below the height of the area maximum the existence of Bægisárjökull is secured. Comparison with the hydrological method Several attempts have been made to measure glacier mass changes by estimating the terms in the water balance equation for glacierfed areas (Rogstad 1941, 1942, 1951; Kasser 1959; Tangborn 1966, 1968). This method is often called the hydrological method. The water balance equation of a glacierized area can be written R = P — E ± AG ± AS where R is the total runoff from the basin, P is the basin-wide precipitation, E is the loss due to evaporation, AG is the change in ground- water and soil-moisture storage and AS is the change in the snow and ice storage. A test of the hydrological method was made for the observation periocl in 1968 (July 1 to August 5). About 23 percent of the well de- fined and almost impermeable drainage basin were covered with the glacier. Losses due to evaporation (E) and changes in the ground- water and soil-moisture storage (AG) were neglected. The water balance equation reduces then to R = P ± AS. The water stage in the Bægisá-river was re- corded by a limnigraph (A. Ott type) placed about 2 km from the glacier at the height of 615 m a. s. 1. The stage-discharge relationship was established by discharge measurements with a current meter and by use of the radio- isotope method (J131 isotope). The precipitation was measured every day by one standard gage and 10 Pluvius gages distributed over the basin (Fig. 2 and 10). The total precipitation over the basin was deter- mined by taking the arithmetic mean of the observations. The change in the snow and ice storage (AS) was determined on the glacier by the TABLE 7. Water balance terms (10° m3) for the period July 1 to August 5, 1968, Bægisárjökull. Area considered R Run off P Precipitation AS Change in snow and ice storage 7.5 km2, the whole basin, 23% glacier 3.24 0.40 2.36 1.7 km2, the glacier 3.04 (60 mm) 0.12 2.16 (71 mm) 20 JÖKULL 21. ÁR
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Jökull

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