Lake retention time

Two approaches can be used (often in combination) to elucidate how a particular lake works: field measurements and mathematical modeling.

This can be a solid tracer, such as a float constructed to be neutrally buoyant within a particular water layer, or sometimes a liquid.

Another field measurement approach, using an Eulerian reference frame, is to capture various properties of the lake water (including mass movement, water temperature, electrical conductivity and levels of dissolved substances, typically oxygen) at various fixed positions in the lake.

[1] Field measurements alone are usually not a reliable basis for generating residence times, mainly because they necessarily represent a small subset of locations and conditions.

In theory it would be possible to integrate a system of hydrodynamic equations with variable boundary conditions over a very long period sufficient for inflowing water particles to exit the lake.

Renewal time simply becomes a question how quickly could the inflows of the lake fill the entire volume of the basin (this does still assume the outflows are unchanged).