This is one possibility - a type of water use that is not returned to the lake.

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Consumptive water use

Water removed from available supplies without return to a water resources system; water used in manufacturing, agriculture, and food preparation.

http://www.lenntech.com/water-glossary.htm#ixzz3G1voB900

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The majority of water withdrawn is returned to the basin through runoff and discharge. However, some of the water withdrawn is made unavailable through evapotranspiration of incorporation into manufactured products, referred to as consumptive use. The term consumptive use refers to any quantity of water that is withdrawn from the Great Lakes system and not returned. Examples of consumptive use include water that evaporates from irrigated fields, lawns, and golf courses and water incorporated into dairy products, canned foods, drinks, and chemicals.

http://www.watershedcouncil.org/water resources/great lakes/...