A SF6 TRACER STUDY OF HORIZONTAL MIXING IN LAKE CONSTANCE

Horizontal mixing processes in the hypolimnion of the western part of Lake Constance are studied by measuring the dispersion of a sulfur hexafluoride (SF6) tracer plume. Only 1 liter gaseous SF6 (STP) was released at a central hypolimnic depth of 60 m in August 1990. Over a period of 3 months the horizontal dispersion of the tracer plume was measured by 19 surveys using a new, vertically integrating sampling device. The observed horizontal dispersion is marked by strong storm-induced stirring events. Nevertheless mean turbulent diffusion coefficients for the whole period can be computed. They rise about linear from 0.7.10(5) cm(2)/s to 3.0.10(5) cm(2)/s with the distance from the western end of the lake. For the hypolimnion of Uberlingersee, a sill-separated basin in the western part of Lake Constance, a simple budget model gives an exchange time of 67 +/- 6 days with the main basin (Obersee).