Jökull - 01.12.1990, Page 140

Figure 18. The Seyðisá groundwater basin. Explanations: 1) Postglacial volcanic fissure. 2) Springs with
temperature 5-10 °C. 3) Springs with temperature >10 °C. 4) Inferred groundwater currents. 5) Spring area on
Seyðisá, temperature < 5°C. 6) Mountain areas. — Grunnvatnskerfi Seyðisár. Skýringar: 1) Gossprungur frá
nútíma. 2) Lindir með vatnshita 5-10 °C. 3) Lindir með vatnshita >10 °C. 4) Aœtlaðir grunnvatnsstraumar.
5) Lindasvœði við Seyðisá, vatnshiti < 5 °C. 6) Fjalllendi.
on the glacier Langjökull. This can not be ruled out,
although the glacial contribution is probably limited (a
considerable surficial runoff from the glaciers occurs
in this area). On the other hand a neglect of the alti-
tudinal effects of the mountains on Arnason’s (1976)
deuterium map could have led to an overestimate of
the glacial component.
A quantitative assessment of the hydrological
regime of the Langjökull — Þingvellir basin is at
present still a play with numbers. The icefree part
of the basin is probably 700-800 km2, excluding the
shortly mentioned, marginal mountain zone (Fig. 16).
The precipitation could be expected to be in the range
1,000-3,000 mm/year. The greater part of this area
lies in the precipitation shadows from the surround-
ing mountains. A mean infiltration of 2,000-2,500
mm/year (possibly too high!) would supply 45-65
m3/s of the minimal 80 m3/s, which are issued from
springs into lake Þingvallavatn. The rest would be
supplied by the glacial part of the basin. Its exten-
sion is not known, but an area of 150-300 km2 seems
plausible, depending on varying assumptions for the
hydrogeological structures below the glacier. This is
in a good agreement with Árnason’s (1976) results,
although his interpretations seem to include an idea of
confined water conduits from the glacier Langjökull
to the lake Þingvallavatn, which is more than ques-
tionable in view of the hydrogeological conditions.
The surface runoff from this part of the glacier is
infiltrated in front of the glacier, only a small portion
reaching the great rivers flowing out ofthe basins. The
runoff is instead almost totally added to the ground-
water. Assuming an infiltration of 4,000 mm/year, the
glacial component could be 20^10 m3/s, correspond-
ing to approximately 25-50 % of the total outflow
from the basin. A part of this high, assumed infil-
tration might be due to a negative glacial balance, as
the major glaciers have been retreating for decades
136 JÖKULL, No. 40, 1990