20 gallons per minute is equal to 28800 gallons per day (that number comes from 60 minutes in one hour, 24 hours a day times 20 gallons per minute).
28800 gallons is equal to 3850 cubic feet.
385 cubic feet is equal to 0.0884 acre-feet.
I'm not a mathematician so if you'd like to check my math, you can at this website.