Jökull - 01.10.1998, Page 41

Fjöllum run through lava fields and it is generally be-
lieved that considerable glacial runoff enters the
groundwater storage. Further, groundwater is dis-
charged from springs downstream of the streamflow
gauges considered in this study. Hence, these stream-
flow gauges may not measure the entire glacial
runoff. Only in the rivers draining Brúarjökull do the
river gauges measure all the glacial melt.
The purpose of the present study is to estimate the
mass balance of drainage basins that drain ice and
water, respectively, toward the various river systems.
The boundaries of the ice catchment areas were
drawn upstream from the water divide between the
rivers at the glacier edge and upstream to the highest
point (Björnsson, 1988; Björnsson and Pálsson, 1991;
Bjömsson and others, 1992). The boundaries of the
main ice catchment basins were delineated using sur-
face elevation maps. In these maps the ice flow lines
have been drawn perpendicular to smoothed elevation
contours. The boundaries of the water drainage basins
were delineated by using maps of water pressure po-
tential that was defined as the sum of the graviational
potential and a water pressure at the glacier bed which
was assumed to be approximately equal to the ice
overburden pressure (Bjömsson, 1988). This approxi-
mation is justified at water divides where the dis-
charge of water is small.
For most drainage basins the two types of drainage
systems are similar except on Tungnaárjökull where
their location is significantly different (Fig. 2). The
water drainage basin of Tungnaá is mainly located in
the ablation area and generally its net mass balance is
negative in all years (Bjömsson, 1988).
METHODS
Measurements of the surface mass balance on a
large ice mass like Vatnajökull are impractical in
terms of cost with conventional techniques and sam-
pling density that are typically used on small glaciers.
The spatial variability of the mass balance may, how-
ever, be predictable on the flat large outlets of such an
ice cap given data on several profiles extending over
the elevation range of the glacier. The precipitation
generally increases with elevation and decreases with
the distance from the coast, but both the distribution
of snowfall and redistribution of snow by drift de-
pends on the prevailing wind direction during the
winter. The summer melting depends mainly on ele-
vation and on the albedo of the glacier surface. There-
fore, we have used observations along a limited num-
ber of flowlines which span the elevation range of the
outlets to assess areal estimates of surface mass bal-
ance. Each profile describes the variation with eleva-
tion, but together they also describe the lateral varia-
tion of the mass balance. Recently, modem over snow
vehicles and helicopters have allowed fast traverses to
ensure successful field work in spite of frequently
poor weather conditions. Error limits for the area inte-
grals of the mass balance components must neverthe-
less be assigned no lower than 15%.
The winter mass balance (bw) is defined as the
mass of snow accumulated during the winter months,
the summer balance (bs) is the mass balance during
the summer, and the net balance (b„) is defined as their
sum. The specific mass balance is expressed in terms
of the equivalent thickness of water. All mass balance
components apply to a time interval between given
measurement dates which are not fixed from one year
to another. The dates in the autumn are separated by
approximately one calender year which roughly coin-
cides with the hydrolocical year defined as October 1
to September 30. The measurements in the spring
were done in late April to mid-May. Snow cores were
drilled through the winter layer and profiles of the
density were measured. The summer balance was de-
rived in the autumn from measurements of the
changes in the snow core density during the summer
in the accumulation area and from readings at stakes
and wires drilled into the ice in the ablation areas.
On the western outlets of Vatnajökull, the values
for the specific balances (bw, bs and bn) at a central
flowline were taken as representative mean values at
each elevation. This proved to be justified for Tungna-
árjökull in 1985-86 when the mass balance compo-
nents were also measured on a traverse line (Bjöms-
son, 1988). On the northem outlets Dyngjujökull and
Brúarjökull, however, maps were drawn of the specific
mass balance which described their lateral variations
(except 1991-1992). Volumes of mass balance were
obtained by integrating the specific mass balance over
the glacier area. Mean values of the mass balance were
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