Jökull - 01.12.1971, Síða 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