Jökull - 01.12.1969, Side 125

Lake-Ice of Mývatn
SIGURJÓN RIST,
HYDROLOGICAL SURVEY, NATIONAL ENERGY AUTHORITY, REYKJAVÍK, ICELAND
There are three basic reasons for my present
discussion of ice in lake Mývatn.
Firstly the farmer of Grímsstadir at Mývatn
Jóhannes Sigfinnsson has for 36 years recorded
the freeze-up and break-up of the ice cover in
the lake. His uninterrupted 36 years of ob-
servation allow discussion of the duration of
the lay-time and its mean value. Comparing
its border dates with meteorological reports
might also be highly instructive.
Secondly ice occurring in Mývatn lias greater
financial impact for the nation than most
people suspect. This impact was felt in the
past and is still one of the two main ice pro-
blems harassing the power plants in Laxá.
The third reason for my discussion of ice
in Mývatn is the dedication of this symposium
to the memory of Jon Eythorsson who is known
to have appreciated few things tnore highlv
tlian the observing and recording of natural
phenomena by a self-educated amateur with
such diligence as to render them scientific
value. Exactly this is what Sigfinnsson has
accomplished in my opinion.
Mývatn covers an area of 36 km2. A greater
Fig. 1. Lay-time of ice cover stationary on the
middle of the lake Mývatn for 36 years. The
earliest freeze-up tirne in autumn is Sept.ember
29 (’54 ancl ’63), but the ice cover broke-up
again. The earliest date of freeze-up of an ice
cover, which remains the whole winter is Octo-
ber 6 (’68), the median value is November 5.
Dates of break-up are listed in a correspond-
ing sequence, earliest April 6 (’64), latest June
21 (’49), median value May 14.
Duration of the ice cover: Shortest lay-time is
143 days (the winter ’33—’34) and longest 244
days (T5—'16 and ’48—’49) the median value
of the lay-time duration for 36 years is 190
days.
part of the lake is 2.5 to 3 meters deep; its
deepest part reaching only 4.5 m. The heat
storage of the lake therefore is small so freez-
ing starts early in the autumn and summer
temperature of the water is high. The lake is
wholly surrounded by postglacial lava flows,
which also constitute its bottom. Mývatn must
be considered a through-flow lake. The inflow
originates from a spring horizon extending for
many km along the east bank of the lake,
ordinary groundwateií in Kálfastrandarvogar
Is á Myvatni
Iftr
1915/16
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32/33
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34/35
3 5/36
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61 /62
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JÖKULL 19. ÁR 121