Water Source - Understanding Lake Data

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• PHYSICAL CHARACTERISTICS - Page 3

• Water source

The source of a lake's water supply is very important in determining its water quality and in choosing management practices to protect that quality. If precipitation is the major water source, the lake will be acidic, low in nutrients, and susceptible to acid rain. (This includes many seepage lakes.)

If groundwater is the major water source, the lake is usually well buffered against acid rain and contains low to moderate amounts of nutrients. (This includes all groundwater drainage lakes and some seepage lakes.) Local septic systems or other groundwater contamination could cause problems. Water exchange is fairly slow.

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If streams are the major source of lake water, nutrient levels are often high and water exchange takes place more rapidly. These lakes have the most variable water quality depending on the amount of runoff and human activity in the watershed (land that drains toward the lake).

Managing the watershed to control nutrients and soil that enter the lake is essential to protecting water quality. Controlling water that runs from the land's surface into the lake is important for drainage lakes and impoundments, and some seepage and groundwater lakes. Protecting groundwater quality is particularly important for seepage and groundwater drainage lakes.

Watershed management becomes especially critical in impoundment lakes. If a stream is dammed the natural movement of water will be restricted, causing soil and nutrients to collect in the impoundment.

Lake managers measure inflow and outflow to determine a lake's water budget. As shown in the formula below, a water budget consists of several elements. Precipitation in Wisconsin averages 30 inches per year. Evaporation depends on the type of summer weather, but is usually about 21 inches. Groundwater flow is more difficult to measure, but can be estimated.

The water budget can be expressed in percent or in volume. A typical water budget for a drainage lake follows:

    30%    +      10%     +   60%  =     5%     +    11%    +   84%
Groundwater  Precipitation  Surface  Groundwater Evaporation  Stream
inflow	                    runoff   outflow                  outlet

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Produced by: Wisconsin Department of Natural Resources, Bureau of Watershed Management
More information on this topic: James Vennie Lake Data Manager, Watershed Management Bureau 608-266-2212

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