Cindy Hagley, Great Lakes Environmental Quality Educator, Minnesota Sea Grant, (218) 726-8106

In the language of lakes, residence time (also called retention time) is the period required to completely replace a lake’s water with an equal volume of “new” water. If you compare a lake to a bathtub, residence time might be easier to understand. Filling a bathtub takes just a few minutes if the drain is closed. The 5 or so minutes it takes would be the bathtub’s residence time. However, if the drain is open or the faucet is only dripping, filling the tub will take longer; its residence time could be hours or possibly days. Of course, residence time also depends on how big the tub is. So, a lake’s residence time depends on 3 major factors, the rate of water inflow, the capacity of the lake to hold water (its volume), and the rate of water outflow.

Residence time can vary greatly in lakes, from a few days in many reservoirs to hundreds of years. Lakes with small volumes and high flow rates have short residence times, and lakes with large volumes and low flow rates have long residence times. For example, Lake Superior has a residence time of roughly 191 years compared to Lake George, in Uganda, with a residence time of four months.

Why do we care about a lake’s residence time? The longer a lake’s residence time, the longer it takes to refresh its waters. Pollutants tend to hang around a lot longer in lakes with longer residence times. If your lake association has worked to reduce inputs of nutrients or other contaminants into your lake, knowing your lake’s residence time will help you gauge how long it will take to see improvements in water quality. There are many other factors that impact water quality, so residence time by itself will not answer your questions, but it is an important characteristic to understand about your lake. It is necessary to know your lake’s residence time to determine annual lake budgets for water, nutrients, heat, oxygen, contaminants, and herbicides.

A lake’s residence time is calculated by dividing the lake’s volume by its average annual water outflow. Lake managers calculate outflow on an annual basis so that seasonal variation doesn’t unduly influence results. Volume (V) is usually expressed in acre-feet, and mean outflow is expressed as acre-feet/year. So the formula looks like this:

Residence time (years) = lake volume (acre-ft) / mean outflow (acre-ft/yr)

In lakes with very short residence times (i.e., a lake with a small volume and high inflow and outflow rates), algae may get flushed out of the lake so fast that they don’t accumulate. Intermediate residence times allow algae and aquatic plants plenty of time to take advantage of the nutrients that are present. In lakes with longer residence times, phosphorus coming into the lake tends to have more time and opportunity to bind to particles, either through biological activity or through chemical and physical processes. These particles settle out of the water and are deposited in the sediments, making the attached phosphorus at least temporarily unavailable to algae and other plants. So lakes with long residence times can have lower phosphorus concentrations in the water. Residence time is one of the major variables that scientists incorporate into models predicting phosphorus retention and impacts in lakes. Determining residence time for your lake is probably not a job you can take on without some expert help, but it is an important characteristic of your lake.

Sources:

• Water on the Web, Module 8, Lecture 1. www.waterontheweb.org/curricula/ws/unit_03/U3mod8_9.html
• Holdren, C., W. Jones, and J. Taggart. 2001. Managing Lakes and Reservoirs. N. Am. Lake Manage. Soc. and Terrene Inst., in coop. with Off.Water Assess.Watershed Prot. Div. U.S. Environ. Prot. Agency, Madison, WI.
• Wedepohl, R.E., D.R. Knauer, G.B. Wolbert, H. Olem, P.J. Garrison, and K. Kepford. 1990. Monitoring Lake and Reservoir Restoration. EPA 440/4-90-007. Prep. By N. Am. Lake Manage. Soc. for U.S. Environ. Prot. Agency,Washington, DC.

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