Rit (Vísindafélag Íslendinga) - 01.06.1976, Blaðsíða 54

extensive isotopic exchange between the snow crystals and drain
water was observed at the same time as the fine-grained snow
changed to coarser form. This laboratory model then led to the
development of a theoretical model which was designed to describe
the exchange processes in melting snowpack quantitatively (Búa-
son 1969).
When water (rain and meltwater) percolates through a snow-
pack, an isotopic equilibrium is only partially reached. In the
theoretical model of Búason a pair of linear first-order partial dif-
ferential equations describe the exchange process at each stage in
the melting snow column. The parameter, t, is a time constant,
which is expressed as a fraction of the total melting time of the
snow column. By choosing the proper time constant, the agreement
between the laboratory experiment and the theoretical model was
excellent.
The theoretical model was then tested under natural conditions
by comparison with isotopic data obtained from samples of snow-
cover, precipitation and snow-lysimeter drainage at the experi-
mental site Weissfluhjoch, Switzerland (Árnason et al. 1973).
Using the model of Búason, the isotopic composition of the drain
water was computed. The computed data were then compared with
results obtained by direct measurements.
Two experiments were carried out, in 1970 and 1971. In the
1970 experiment, only deuterium was measured. In the 1971 ex-
periment, both deuterium and tritium were measured. Fig. 10
shows directly measured data and data obtained by least squares
adjustment, together with data computed by using different time
constants and either 10% or 20% freewater content of the snow-
pack. The solid lines on the three diagrams to the right in the
figure give the hypothetical case that the meltwater percolates the
snowpack without any isotopic exchange (t = oo). The agreement
between the theoretical model and experimental results was good.
In regions with great seasonal changes in isotopic composition,
the summer thaw and rain is initially much richer in the heavier
isotopes than the winter snow into which it percolates. In such
regions, the exchange process is often quite complicated and has
therefore been overlooked. This is, for instance, the case for numer-
ous glaciers in North America and in the Alps. In the glaciers of
50