Lake retention time (also called residence lake water, or water age or rinsing time ) is a calculated quantity that stating the mean time that water (or solute) is spent on a particular lake. Simply put, this number is the result of dividing the volume of the lake with the incoming or outflow of the lake. It roughly states the amount of time it takes for a substance put into the lake to flow out of it again. Retention time is very important where pollutants are concerned.
Video Lake retention time
Global retention time
global retention time for a lake (the overall mean time spent on lake water) is calculated by dividing the lake volume by the average inflow rate of all tributaries, or with an average outflow (ideally including evaporation and seepage). This metric assumes that the water in the lake is well mixed (not terraced), so that every part of the lake water is just like any other. In fact, larger and deeper lakes are generally not well mixed. Many large lakes can be divided into sections with only a limited stream between them. The lakes in general are grouped, with less water mixing with surface water. This is often better modeled as several different water sub-volumes.
Maps Lake retention time
Residence time is more specific
It is possible to calculate residence times more specific to a particular lake, such as the individual residence times for sub-volumes (eg a particular arm), or the distribution of residence times for different layers of stratified lakes. These numbers are often better to express the hydrodynamics of the lake. However, such an approach remains a simplification and should be guided by an understanding of the processes that operate on the lake.
Two approaches can be used (often in combination) to explain how a particular lake works: field measurement and mathematical modeling. One common technique for field measurement is to introduce the tracer to the lake and monitor its movements. It can be a solid tracker, like a buoy built to float neutrally in certain water layers, or sometimes fluids. This approach is sometimes referred to as using the Lagrangian reference frame. Another field measurement approach, using the Eulerian frame of reference, is to capture the various properties of lake water (including mass movement, water temperature, electrical conductivity and dissolved levels, usually oxygen) in various fixed positions in the lake. From this it can build an understanding of the dominant processes that operate in different parts of the lake and its range and duration.
Field measurements alone are usually not a reliable basis to generate residence time, especially as they represent a small part of the location and conditions. Therefore, measurements are generally used as inputs 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 the entry of water particles to exit the lake. One can then calculate the traveling time of the particle using the Lagrangian method. However, this approach outweighs the available detail in current hydrodynamic models and current computer resource capacity. In contrast, residence models developed for the dynamics of gases and fluids, chemical engineering, and bio-hydrodynamics can be adjusted to produce residence times for lake sub-volumes.
List of lake water dwellings
The residence time is taken from infobox in related articles unless otherwise specified.
See also
- Water cycle: Residence time
References
External links
- EPA Great Lakes Factsheet # 1
- EPA Great Lakes Atlas
- Jody Richardson, Alistair McKerchar. "Fish locked on land and residence time in the lake" . Retrieved May 22 2007 . - The relationship between residence time of New Zealand lake and koaro, population smelly and general bullying.
Source of the article : Wikipedia
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