Jökull - 01.12.1972, Page 57

notice the relative high importance of sensible
heat in spite of high insolation. No condensa-
tion took place during these days. The second
week of July hacl light winds with prevailing
northerly directions.
During the second half of July in 1968 and
early August the ablation was generally higher
than before. The turbulent processes provided
niore to the melting than the net radiation.
During this period south to westerly winds pre-
vailed and temperatures were above normal.
There was a clear trend to increased wincl
speed throughout the period.
In July of 1967 the net radiation accounted
for about 60% of the ablation but 55% for the
observation period in August. July was ex-
tremely cold and dominated by northerly winds
whereas August was slightly warrner and south-
erly winds more frequent. During this cold
summer the turbulent energy exchange was
suppressed ancl in spite of the high albedo the
net radiation was the dominating source of
the ablation.
The difference in the ablation conditions
during the observation periods in 1967 and
1968 was on first hand caused by the differ-
ence in the general air circulation. This on the
other ltand influenced the albedo. Greater
amount of snowfall in summer of 1967 resulted
in a relatively high albedo throughout the
summer. On the contrary the relatively low
albedo in sunimer of 1968 was partly caused
by dust transported over the glacier. As known
this non-meteorological factor can complicate
all interpretation of the relationship between
climatic factors and glacier variations in Ice-
land.
Comparison with otlier measurements is dil'-
ficult because the observation periods are selec-
tive and methods of computation are different.
Only earlier measurements in Iceland will be
compared liere. (For comparison see Paterson
1969, Hoinlies 1964).
Ahlmann and Thorarinsson (1943) liave esti-
mated that on South-Vatnajökull the net radia-
tion provides 10 percent of the melting in the
lower ablation area but up to 40 percent in
the accumulation area. Measurements by Lister
(1959) on Breidamerkurjökull gave that 50.2
percent of the melting was accounted for by net
radiation, 27.6 percent by convection and 22.2
by condensation for shorter periods in August.
These results indicate that the net radiation
is a more important component in the energy
budget for glaciers in northern Iceland than
on South-Vatnajökull. The orientation of the
glaciers and albedo variations should therefore
be of greater importance for the summer bal-
ance of glaciers on North- than South-Iceland.
GLACIER HYDROLOGY
Full understanding of the physics of the
glacier as a hydrological system is essential for
use of water balance studies to estimate glacier
mass balance and to forecast glacier run-off.
Some observations from Bægisárjökull describe
this important problem.
A water balance stucly for a period of con-
tinous melting of an isothermal glacier at 0 °C
(except for some few hours) mid-summer in
1968 proved that the run-off (R = (3.24 ± 0.16)
10° m3) was definitely liigher than the sum of
measured rain volume (P = 0.40 10° m3) and
the change in snow and ice storage (AS =
(2.36 ± 0.24) 106 m3) (see Björnsson 1971, p.
20 and fig. 2). The error in R is $ 5% and
5? 10% in AS. Assuming that P is underesti-
mated by 20% or 0.08- 108m3, which is rather
reasonable in our windy climate, the three
water balance quantities with these specified
error limits just balance. There is however no
obivous reason why R should be overestimated
by 5% and AS underestimated by 10%. Tlrese
results and reports on similar results by Schytt
(1970), Stenborg (1970) and Tangborn et al.
(1971) indicate tliat the observed discrepancy
might be caused by delayed runoff of melt-
water stored in the glacier earlier in the
summer. In the present case rapid melting was
observed from May 22 to June 3 and July 9
to 16.
A closer examination of the water balance
components can be made from Fig. 8 which
shows the daily variation of the run-off (r),
volumes of precipitation (p) and computed
meltwater (q) released from the glacier surface.
The melting of the ice phase was calculated
frorn the energy budget estimates (see Fig. 3).
The free water content was not measured but
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