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# Lake retention time

Lake retention time (also called the residence time of lake water, or the water age or flushing time) is a calculated quantity expressing the mean time that water (or some dissolved substance) spends in a particular lake. At its simplest this figure is the result of dividing the lake volume by the flow in or out of the lake. It roughly expresses the amount of time taken for a substance introduced into a lake to flow out of it again. The retention time is especially important where pollutants are concerned.

## Global retention time

The global retention time for a lake (the overall mean time that water spends in the lake) is calculated by dividing the lake volume by either the mean rate of inflow of all tributaries, or by the mean rate of outflow (ideally including evaporation and seepage). This metric assumes that water in the lake is well-mixed (rather than stratified), so that any portion of the lake water is much like any other. In reality, larger and deeper lakes are generally not well-mixed. Many large lakes can be divided into distinct portions with only limited flow between them. Deep lakes are generally stratified, with deeper water mixing infrequently with surface water. These are often better modeled as several distinct sub-volumes of water.

## 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.

## List of residence times of lake water

The residence time listed is taken from the infobox in the associated article.

Lake Location Basin countries Residence time
Klingnauer Stausee Aargau Switzerland 0.13 days
Lake Hévíz Hévíz Hungary 1 day (?)
Lake Wohlen Canton of Berne Switzerland 2.1 days
Lake St. Clair Canada, United States 7 days (2-30 days)
Lake Arapuni North Island New Zealand 1 week
Råcksta Träsk Sweden 2-3 weeks
Lake Cristallina Ticino Switzerland 2-4 weeks
Prospect reservoir Prospect, New South Wales Australia 30-40 days
Sicklasjön Sweden 0.1 year
Devils Lake Lincoln County, Oregon United States .15 years
Lough Derg Ireland 0.15 years
Lake Biel Switzerland 58 days
Hacksjön Sweden 0.2 years
Kamloops Lake British Columbia Canada ca. 0.2 years (20-340 days)
Trehörningen Sweden 0.27 years
Lake Lesina Province of Foggia, Puglia Italy 100 days (summer 306 days, winter 31 days)
Hüttnersee Hütten, Canton of Zurich Switzerland 120 days
Lake Lauerz Canton of Schwyz Switzerland 0.3378 years
Magelungen Sweden 4-5 months
Lappkärret Sweden 5 months
Mettmenhaslisee Canton of Zurich Switzerland 180 days
Egelsee Bubikon, Canton of Zurich Switzerland 180 days
Lac de Vouglans France 180 days
Clinton Lake DeWitt County, Illinois United States 6 months
Lake Coeur d'Alene Northern Idaho United States 0.5 years
Kyrksjön Sweden 7 months
Lake Silvaplana Switzerland c. 250 days
Långsjön Sweden 9-10 months
Lake Sarnen Switzerland 0.8 years
Lake Lovozero Kola Peninsula, Murmansk Oblast Russia 10 months
Drevviken Sweden 10-11 months
Judarn Sweden 11 months
Khanka Lake Heilongjiang Province,China and Primorsky Krai, Russia China, Russia 1 year
Greifensee Switzerland 408 days
Lake Zurich Switzerland 440 days
Tegernsee Bavaria, Germany Germany 1.28 years
Deer Creek Reservoir Utah United States 1.3 year
Lake Walen Switzerland 1.4258 years
Lake Murten Switzerland 1.6 years
Oeschinen Lake Bernese Oberland Switzerland 1.61 years
Ältasjön Sweden 1.8 years
Faaker See Carinthia Austria 1.8 vears
Lake Ossiach Carinthia Austria 1.8 vears
Long Lost Lake Clearwater County, Minnesota United States 1.8 years
Lake Thun Canton of Berne Switzerland 684 days
Gömmaren Sweden 1.9 years
Loch Lomond Scotland 1.9 years
Türlersee Switzerland 730 days
Lake Tutira Hawke's Bay New Zealand 2 years
Pfäffikersee Switzerland 2.085 years
Wilersee Canton of Zug Switzerland 2.2 years
Lake Sils Switzerland 2.2 years
Päijänne Finland 2.5 years
Lake Erie Canada, United States 2.6 years
Lake Brienz Canton of Berne Switzerland 2.69 years
Ammersee Upper Bavaria Germany 2.7 years
Trekanten Sweden 3.1 years
Lake Champlain Canada, United States 3.3 years
Lake Lucerne Switzerland 3.4 years
Flathead Lake Montana United States 3.4 years
Lake Hallwil Aargau Switzerland 3.9 years
Flaten Sweden 4 years
Lake Annecy France 4 years
Lake Maggiore Italy, Switzerland Italy, Switzerland 4 years
Lake Baldegg Switzerland 4.2 years
Lake Constance Germany, Switzerland, Austria Germany, Switzerland, Austria, Liechtenstein 4.3 years
Corey Lake Michigan United States 4.4 years
Lake Como Northern Italy Italy, Switzerland 4.5 years
Sea of Galilee Israel 5 years
Sebago Lake Cumberland County, Maine United States 5.1 to 5.4 years
Bay Lake Crow Wing County, Minnesota United States 4-7 years
Lago di Mergozzo Piedmont Italy 6 years
Lake Ontario Canada, United States 6 years
Lago di Candia Province of Turin Italy 6 or 7 years
Lac La Nonne Alberta Canada 6.5 years
Ägerisee Canton of Zug Switzerland 6.8 years
Moose Lake Alberta Canada 7.5 years
Millstätter See Carinthia Austria 7.5 years
Lake Lugano Ticino, Lombardy Switzerland, Italy 8.2 years
Lake Neuchâtel Switzerland 8.2 years
Lake Orta Piedmont Italy 8.9 years
Weissensee Carinthia Austria 9.2 years
Quesnel Lake British Columbia Canada 10.1 years
Lake Taupo North Island, New Zealand New Zealand 10.5 years
Wörthersee Carinthia Austria 10.5 years
Lake Chelan Washington, USA United States 10.6 years
Lake Geneva Switzerland, France 11.4 years
Lake Wakatipu South Island New Zealand c. 12 years
Higgins Lake Michigan United States 12.5 years
Lake Zug Switzerland 14.7 years
Lake Sempach Switzerland 16.9 years
Skaneateles Lake United States 18 years
Cayuga Lake United States 18.2 years
Bear Lake Idaho/Utah United States 19.6 years
Lake Huron Canada, United States 22 years
Lake Garda Northern Italy Italy 26.8 years
Crystal Lake Benzie County, Michigan United States 30 to 60 years
Okanagan Lake British Columbia Canada 52.8 years
Lake Ohrid the Republic of Macedonia,
Albania
70 years
Lake Michigan United States 99 years
Clinton Lake Douglas County, Kansas United States 100 years
Arendsee Saxony-Anhalt Germany 100 years
Gull Lake Alberta Canada >100 years
Crater Lake Oregon United States 157 years
Lake Superior Canada, United States 191 years
Caspian Sea Azerbaijan, Iran, Kazakhstan, Russia, Turkmenistan 250 years
Lake Baikal Russia 350 years
Lake Tahoe California, Nevada United States 650 years
Lakes Qaban Russia several thousand years
Lake Poyang People's Republic of China 5,000 years
Lake Vostok (Antarctica) 1,000,000 years