Lake Retention Time - More Specific Residence Times

More Specific Residence Times

It is possible to calculate more specific residence time figures for a particular lake, such as individual residence times for sub-volumes (e.g. particular arms), or a residence time distribution for the various layers of a stratified lake. These figures can often better express the hydrodynamics of the lake. However, any such approach remains a simplification and must be guided by an understanding of the processes operating in the lake.

Two approaches can be used (often in combination) to elucidate how a particular lake works: field measurements and mathematical modeling. One common technique for field measurement is to introduce a tracer into the lake and monitor its movement. This can be a solid tracer, such as a float constructed to be neutrally buoyant within a particular water layer, or sometimes a liquid. This approach is sometimes referred to as using a Lagrangian reference frame. 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. From these can be constructed an understanding of the dominant processes operating in the various parts of the lake and their range and duration.

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. Therefore the measurements are generally used as the input for numerical models. 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. One could then calculate the traveling times of the particles using a Lagrangian method. However, this approach exceeds the detail available in current hydrodynamic models and the capacity of current computer resources. Instead, residence time models developed for gas and fluid dynamics, chemical engineering, and bio-hydrodynamics can be adapted to generate residence times for sub-volumes of lakes.

Read more about this topic:  Lake Retention Time

Famous quotes containing the words specific, residence and/or times:

    I recognize in [my readers] a specific form and individual property, which our predecessors called Pantagruelism, by means of which they never take anything the wrong way that they know to stem from good, honest and loyal hearts.
    François Rabelais (1494–1553)

    My residence was more favorable, not only to thought, but to serious reading, than a university; and though I was beyond the range of the ordinary circulating library, I had more than ever come within the influence of those books which circulate round the world, whose sentences were first written on bark, and are now merely copied from time to time on to linen paper.
    Henry David Thoreau (1817–1862)

    All that we did, all that we said or sang
    Must come from contact with the soil, from that
    Contact everything Antaeus-like grew strong.
    We three alone in modern times had brought
    Everything down to that sole test again,
    Dream of the noble and the beggarman.
    William Butler Yeats (1865–1939)