Jökull - 01.12.1990, Page 125

Ti MV0DjK9«fs
1—Cj so.oe.02M öo
Figure 2. Glaicers and hy-
drogeological formations.
Black areas show major
spring regions. Shaded areas
show permeable formations
which correspond roughly to
the Late Quaternary. —
Jöklar og lekar jarðmyndan-
ir. Helstu lindasvæði eru
svört. Vel lekar jarðmyndan-
ir eru skyggðar. Þær sam-
svara í megindráttum síð-
kvarterum og nútíma gos-
myndunum.
ter basins (Fig. 2). The surface water divides are in
most cases determined by the high mountains carry-
Jng the glaciers. The groundwater divides may differ
considerably from the surficial ones. Summarily each
glacier can be regarded as lying in one or more com-
posite groundwater basins with effluent spring areas
marking the boundary of the basin.
The glaciers Langjökull and Flofsjökull are simi-
lar in size (both close to 1,000 km2), the composite
groundwater basins, in which they are located, are
also of similar size (probably 3,000-3,500 km2) and
the discharge of the great rivers flowing out from the
hasins is also similar (300-350 m3/s), but the ground-
water component in the very same rivers is probably
200-250 m3/s in those from Langjökull, and only 50
m /s at best in those from Hofsjökull. The ground-
water component is perhaps 70 % in the first case but
only 15 % in the latter one. All numbers are esti-
mates, those on the discharge are based on data from
the National Energy Authority’s (NEA) Hydrological
Survey (Orkustofnun-Vatnamælingar 1989).
The westem-northwestem part of the glacier
Vatnajökull rests on permeable rocks. The sharp
difference between rivers originating from partially
glaciated water basins with permeable rocks and such
from basins, where the rocks are semi- or impermeable
may impressively be seen in the comparison between
the rivers Jökulsá á Fjöllum and Jökulsá á Dal, both
carrying near to 150 m3/s, when leaving the glacially
influenced groundwater basins. In Jökulsá á Dal the
base flow in winter seems to be less than 10 m3/s, even
as low as 5 m3/s, but in Jökulsá á Fjöllum it is proba-
bly near to 50 m3/s, the difference being one order of
magnitude.
The distribution of spring areas and the hydro-
geological structures of the groundwater basins with
glaciers indicate, that groundwater might be con-
ducted away from the glaciers for kilometres or tens
of kilometres along flow channels of two main types:
Lavaflows and fissure swarms. In the first case the in-
filtration on the icefree lava fields will be added with
full weight to the groundwater current, e.g. between
Langjökull and lake Þingvallavatn, between Vatna-
jökull and Skjálfandafljót, between Vatnajökull and
lake Þórisvatn-river Tungnaá. In the latter case the
addition of infiltrated water is probably much more
restricted. Both types are quasi-planar aquifers. The
fissures are near to vertical planes but with a distinct
JÖKULL, No. 40, 1990 121