Aquatic Sciences

, Volume 59, Issue 3, pp 225–242 | Cite as

Hypolimnetic density currents traced by sulphur hexafluoride (SF6)

  • Jürg W. Schlatter
  • Alfred Wüest
  • Dieter M. Imboden
Article
  • 53 Downloads

Abstract

The artificial tracer sulphur hexafluoride (SF6) has been used to study the density-driven deep water exchange between two sill-separated basins of Lake Lucerne, Gersauersee and Urnersee. The sources of the density gradients between the two basins are (1) salinity differences between the major inlets due to the different geology of their drainage areas, and (2) temperature differences due to spatial variation of wind forcing. Wind speeds are generally larger in Urnersee, especially in spring during the so-called Föhn events, when winds blow from the south. In contrast, Gersauersee is protected form these winds. In spring 1989, a total of 630 g of SF6 was released at 80 to 120 m depth in the small Treib Basin located between Urnersee and Gersauersee. During about 100 days the distribution of SF6 in the lake was determined by gaschromatography. Two models are used to quantify the exchange flow, (1) a one-box mass balance model for SF6 in the deep part of Treib Basin, and (2) a one-dimensional diffusion/advection model describing the temporal and vertical temperature variation in Urnersee. According to the first model, the flow into the deep hypolimnion of Urnersee, decreases from 21·106 m3·d−1 at the end of March to about 8·106 m3·d−1 in late April. The second model yields similar flow rates. The decrease of the flow rate during spring, confirmed by both approaches, is consistent (1) with the decreasing strength of the density gradient above the sill during spring and early summer, and (2) with hydrographic information collected in Lake Lucerne during other years.

Key words

Lake Lucerne sulphur hexafluoride lakes density-driven flow interbasin exchange differential mixing tracer 

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Copyright information

© Birkhäuser Verlag 1997

Authors and Affiliations

  • Jürg W. Schlatter
    • 1
  • Alfred Wüest
    • 1
  • Dieter M. Imboden
    • 1
  1. 1.Environmental PhysicsSwiss Federal Institute of Environmental Science and Technology (EAWAG) and Swiss Federal Institute of Technology (ETH)DübendorfSwitzerland

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