Is there a hole in Lake Lanier?

Gainesville averages 8.61 inches of rainfall in July and August. This year, Gainesville has had 10.40 inches of rainfall from July 1 through August 20th.

During that time period, while Gainesville has received above average rainfall, Lake Lanier has dropped 15.77 inches. Why?

Source for rainfall totals and averages: http://www.nws.noaa.gov
Source for actual lake level: http://lanier.uslakes.info/Level.asp
This picture included is not an actual picture on Lake Lanier.

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