Rit (Vísindafélag Íslendinga) - 01.06.1976, Side 84

surface streams, flowing from the inner and upper parts of the
Vestfirðir plateau and collected on the lowland, cannot be used
here to predict the deuterium content of the precipitation on the
plateau because melted snow often contributes significantly to the
streams and therefore results from the rivers give no reliable in-
formation about the mean ðD-value of the groundwater in this area.
Although some districts are somewhat inadequately covered by
sampling points for a thorough survey, the results obtained show
that the deuterium content of the precipitation has a most sys-
tematic geographic distribution and that it is possible to connect
places with the same ðD-value by isometric lines. In areas such as
in the Ódáðahraun and on the Vestfirðir high plateau, the isolines
are, however, drawn as broken lines to indicate the uncertainty
of the lines in these areas.
The map shows well how the öD-values generally decrease from
the coastal areas inland. The highest 8D-values are found in the
coastal areas, 8D = — 50%o on the south coast and 8D = — 64%0
on the north coast, and lowest in northern central Iceland, where
8d = — 106%0 in the northern part of the Vatnajökull glacier. The
decrease in 8D towards the centre of the country can be accounted
for by the combined effect of altitude and distance.
The difference between the 8D-value on the north and south
coasts, A8d = 14%0, can be accounted for as a result of a latitude
effect. This effect appears to be approximately 7.5%0 per degree
of latitude which is similar to the value 8%0 per degree of latitude,
found by Dansgaard et al. (1973) along the coast of Greenland.
The influence of the landscape is also clearly reflected on the
map. When air masses travel over wide lowlands, as in Southwest
Iceland, 8D decreases much more slowly inland than it does when
the air masses have to pass over high mountains such as the 2000 m
high glacier Öræfajökull in Southeast Iceland. The effect of the
landscape is also reflected in local minima found in high moun-
tains and in areas which are shielded from the coastal areas by
high mountains. An example of this are the local minima in the
area north of the glacier Mýrdalsjökull. Most of the precipitation
in this area is brought by southerly winds and the air masses there-
fore first have to pass high mountains, where the moisture becomes
heavily depleted in deuterium.
80