Jökull - 01.12.1990, Blaðsíða 138

Figure 17. The Geitland-
Hallmundarhraun ground-
water basin. Explanations:
See Fig. 16— Grunnvatns-
kerfi á Geitlandi og í Hall-
mundarhrauni. Skýringar:
Sjá mynd 16.
THE LANGJÖKULL GROUNDWATER BASIN
As mentioned above, near to 70 % of the visible
outflow from the Langjökull basin system is ground-
water, issued in springs (Fig. 16). The mean discharge
ratio is near to 1001/s/km2, corresponding to approx-
imately 3,000 mm/year as mean infiltration. These
very high values have led to the assumption of a very
high precipitation over the whole area, with a peak on
the glacier Langjökull itself (>4,000 mm/year) and
possibly also on the high mountains, marginal to the
Southem lowlands (Einarsson, 1988).
In reality there are certainly much sharper fea-
tures in the distribution of the precipitation. The
present author’s long and various experience of that
area, connected with diverse other indications, points
to a relatively low precipitation in the flat and low
lying parts of the Skjaldbreiður lavafield, perhaps as
low or even lower than at the meteorological station
at Þingvellir. This is indeed what could be expected
in such a mountain sheltered basin (Einarsson, 1988).
On the other hand, the excessive downpours on the
steepsided, high mountains of the region are a well
known feature. Even though such observations are
of a subjective nature, they are strongly supported by
the theory, that the precipitation is much higher over
or behind steep mountains than over extensive flat or
lowlying parts of a region (Sigfúsdóttir 1990/1987).
This must be taken into account, when trying to es-
timate the infiltration into the distinct groundwater
basins of the region.
The demarcation of the basins is in most cases far
from clear, at least on hydrogeological grounds alone.
The comparison of the discharge and the assumed in-
filtration may throw some light on the extension of
the basins, while indications of the probable hydroge-
ological conditions can be found in the chemistry.
THE GEITLAND - HALLMUNDARHRAUN BASIN
West of Langjökull up to 30 m3/s are issued from
springs to the river Hvítá between Kalmanstunga
and Stóri As (Measurements and estimates by Ami
Hjartarson and Freysteinn Sigurðsson in June 1985,
Fig. 17). It has for a long time been a common be-
lief (see Örn and Örlygur, 1989), that this water was
drained from the river Norðlingafljót, which indeed
does not by far carry enough water for that purpose (in
September 1988 only 3.5 m3/s), as has already been
recognized on observation a long time ago (Þorsteins-
son, 1987). Árnason(1976)presumesadrainagebasin
for these springs on the northem Langjökull glacier
134 JÖKULL, No. 40, 1990