Boat with measuring equipment (Variosens)
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The water circulation is one of the governing factors
for the eutrophication state of a lake and therefore of major importance.
Numerical water circulation models are tools to better understand
the water circulation and particle transport in lakes. However, they
can only provide reliable results when calibrated on experimental
data. In the framework of a co-operation between the University of
Ljubljana in Slovenia and the Institute of Hydrology, a tracer experiment
was conducted at Lake Bohinj, to investigate the water circulation
and provide calibration data for such a model. The lake is located
in the Triglav National Park in the central part of the Eastern Julian
Alps in Slovenia at an altitude of 525 m a.s.l.. It is dimictic, has
a surface area of 4.3 km2, a mean depth of 30 m and is surrounded
by steep, high mountains. In September 1997 the main inflow to the
lake was traced with 30 kg of the fluorescence tracer uranine. During
the following nine days the tracer distribution in the lake was recorded
by a team of scientists. Two boats, equipped with in situ tracer measuring
devices, shipped the lake during day times. Vertical profiles of the
tracer distribution were recorded in different locations, while the
boats' position was determined by GPS. The in situ measurements were
supplemented by samples from the boats and at the lake outlet for
laboratory analysis. Another method of observation was photography.
Due to the clearness of the lake the horizontal tracer distribution
during the first days with high tracer concentrations was visible.
The field experiment resulted in differentiated insights into the
water circulation pattern of the lake. The lake circulation was mainly
induced by the main inflow, which forms different distinct vertical
layers in different depth. These layers have very different dynamics,
decreasing from the top to the bottom. The experimental results were
an important basis to develop and calibrate a numerical lake circulation
model. |