I posed the question to my Dad, and thought I'd post his e-mail response:
An acre is 43,560 square feet. An acre-foot is one foot deep so that makes 43,560 cubic feet of water. One cubic foot contains 7.8 gallons so 7.8 times 43,560 equals 339,768. Say 340,000 per acre foot. A lake with 25 acre-feet (5 acres x 5' average depth) would contain 8-1/2 million gallons.
To drain in 14 days would require 607,000 gal/day or 25,000 gal per hour or 421 gal per minute which is a lot. I am in the process of trying to determine how much water would siphon out of the 4" pipe you described. A lot would depend on the distance the discharge is below the water level. I am estimating at least 10 feet, maybe up to 20 feet. Each foot of water adds 0.5 psi pressure so 10 feet below would be the equivalent of 5 psi pressure going thru the 4" pipe.
He also located a great website for calculating the amount of discharge by inputing various factors, like size of pipe, number of 90 and 45 degree turns, etc. Very interesting.
http://www.sahra.arizona.edu/programs/water_cons/resources/pipeToFlow.htm
