Jökull - 01.12.1990, Side 126

Figure 3. Hydrological regime of glacial ground-
water. Explanations: 1) Directly measured values.
2) Variously estimated values. 3) Values calculated
from other factors. D: Surfacial runoff. E:
Evapotranspiration. F: River flow. G: Groundwater.
I: Infiltration. P: Precipitation. a: ”Abyssal“. f: Fluvial.
g: Glacial. t: (Sub-)terranean. A: Change in balance.
— Vatnabúskapur jarðvatns frá jökulsvœðum.
Skýringar. 1) Mœld gildi. 2) Gildi metin með
ýmsu móti. 3) Gildi reiknuð út frá öðrum
þáttum. D: Afrennsli á yfirborði. E: Raungufun.
F: Árrennsli. I: írennsli. P: Úrkoma. a: I djúpum
jarðar. f: Fallvatnarennsli. g: Af jökulrœnum toga.
t: Neðanjarðar. A : Forðabreytingar.
linear, horizontal orientation. The lava fields are es-
sentially horizontal and very extensive at the surface,
but shallow in depth. These differences might be seen
in the quantity of the water as well as in its properties,
thus giving some clues to its origin.
HYDROLOGICAL REGIME OF THE
GLACIAL GROUNDWATER
The infiltration of groundwater from the glaciers
can be assessed with the help of measured or estimated
values for the miscellaneous factors of the groundwa-
ter regime (Fig. 3).
Of these only the flow of rivers and the discharge
of springs are measured directly. The precipitation on
the glaciers and in the glacial groundwaterbasins must
be estimated from measurements at meteorological
stations in the lowlands and some few totalisers in the
highlands (Veðráttan, 1989) (Fig. 4).
The distances from the meteorological stations and
the size of the probable error in the measurement of
the precipitation (Sigurðsson, 1990/1987) renders the
estimate of the precipitation on the glaciers rather in-
accurate (Einarsson, 1988). This can to some degree
be helped off by the measurements of the snowcover
on the glaciers, which the Glaciological Society has
for some decades carried out regularly on Vatnajökull,
especially at Grímsvötn, (Bjömsson, 1985) and the
systematical measurements shortly started by NEA on
Hofsjökull. (Sigurðsson, 1988). Sofar these investi-
gations have yielded valuable potential correctives for
the calculated or estimated distribution of precipita-
tion. Yet at present not enough data have been col-
lected to allow any exact estimates of the groundwater
infiltration as a rest member after the subtraction of
the surficial runoff.
Still less is known about the short term changes
in the hydrological balance of the glaciers. The reg-
istration of the position of many glacier snouts by the
Glaciologial Society during the latest decades gives
very valuable hints but hardly a quantitative estimate
(Sigurðsson, 1989). The above mentioned observa-
tions and the investigations carried out with the ISSJA
(”íssjá“ is the Icelandic name for a radio echo sounding
equipment, developed by H. Björnsson and his collab-
orators and used with great success on the Icelandic
glaciers, literally meaning ”glacioscope“) in the later
years on the Hofsjökull and the western Vatnajökull
glaciers (Björnsson 1988) have demonstrated that the
balance could, at least technically, be calculated from
direct measurements, so that it is only a question of
appropriate means, how densely this can be done in
time and space, but on this density rests the accuracy
of the calculated changes.
A factoral analysis of the flow of the glacial and
springfed rivers originating in the glacial groundwater
basins, which takes into account hydrogeological, me-
teorological and other natural conditions, would cer-
tainly ascertain the weight of the groundwater com-
ponent, and even, when if indirectly, the role of the
glacial groundwater in that factor. This requires a
thorough knowledge of the areas in question and is a
122 JÖKULL, No. 40, 1990