Jökull - 01.12.1992, Side 29

data, exceeded 1200 mm, the highest annual value
ever observed during the 62 years of precipitation ob-
servations at Húsavík.
SNOW MELTING
The mountains Grísatungufjöll lie partly within the
catchment area of lake Höskuldsvatn. These moun-
tains are known for heavy snowfall. In early summer,
their snow cover exceeds that of mountains of similar
elevation farther inland, as Lambafjöll and Gæsafjöll.
Several small snow patches do not melt during the
summer, but no glaciers have developed. There is
probably minor accumulation of snow and ice during
periods of unusually cool summers, as occurred fre-
quently between 1960 and 1990, but this accumulation
ls assumed to be insignificant relative to the annual pre-
cipitation. The summer of 1991 was slightly warmer
than the average, causing some of the accumulated
snow patches of previous years to melt. However, this
ts certainly of no significance with respect to the total
amount of water flowing towards lake Höskuldsvatn.
The meltwater during spring and early summer is
generally transported by surface flow, while later in the
summer, rain water enters the ground without forming
surface streams. This is a characteristic of the part of
Iceland covered by Pleistocene to Recent volcanics,
where the ground is very permeable, but a thin layer
°f ice or frozen soil prevents the water from seeping
down during spring and early summer. However, this
has no effect on the total amount of water in the area,
although the direction of surface flow may be different
from the groundwater flow. It is quite possible, that
a fraction of the rain water that seeps into the ground
within the 30 km2 catchment area of lake Höskuldsvatn
as defined by the topography will not flow towards
the lake, but take a subsurface path leading elsewhere.
Similarly, rain water falling outside the catchment area
°f a lake may take a subsurface path towards the lake.
SEDIMENT TRANSPORT
It has been suggested that the bottom of lake
Höskuldsvatn has become less permeable in recent
years, causing less water to seep out of the lake. This
effect is caused by sediments brought into the lake by
meltwater streams during spring and early summer.
It is certainly not known whether more sediment has
been brought into the lake in recent years than earlier.
Most of the catchment area of lake Höskuldsvatn has
been subject to wind erosion in the past, removing
most of the topsoil. Only limited wind erosion has
been occurring in recent years.
Erosion by meltwater certainly exists, bringing
some sediments into the lake. There is, to the best
of my knowledge, no evidence for more rapid stream
erosion in recent years than earlier. Increase in the
number of grazing sheep in the area during the last
four decades may have damaged the limiled vegeta-
tion, resulting in gradually increasing erosion. How-
ever, this is a pure speculation, based on very limited
information.
TECTONIC PROCESSES
The meteorological conditions in Iceland are char-
acterized by lowtemperatures and high humidity. This
means that evaporation is not very significant in the
hydrological cycle and any lake with no surface out-
let must have a subsurface drainage. In the case of
Höskuldsvatn, this subsurface drainage must carry sig-
nificant amount of water, at least 1 m3/s on the average.
This estimate is based on the catchment area which is
about 30 km2 and precipitation in excess of evapora-
tion which is estimated to be more than 1000 mm per
year.
If the subsurface drainage is confined to shallow
conduits, then the surface topography will indicate the
hydraulic gradient and hence the path of this drainage.
Maps show that the saddle point to the east or southeast
of the lake exceeds 280 m elevation, but not 300 m.
Another saddle point to the southwest exceeds 300 m,
but not 320 m. A third saddle point to the northwest,
on the Húsavík fault, exceeds 360 m elevation.
Drainage to the east would carry the water through
Holocene lavas into the Þeistareykir fissure swarm,
and towards north, east of the Tjörnes mountains.
Great fresh water springs are found at sea level in
this area.
Drainage to the southwest would carry the water
through Pleistocene lava and glacial moraines towards
Reykjahverfi, where it would supposedly create rather
JÖKULL,No. 42, 1992 27