Count the number of gallons as you pour water into the large container until it is full, using the gallon jug. Record the number and empty the container.
Measure the volume of water that flows from your pipe by simultaneously placing your empty large container to catch the complete flow of water and starting your stop watch. Stop the watch when the container is full and record the time.
Multiply the number of gallons the container holds times the number of seconds it took to fill your container, then divide by 60. The resulting number is the water flow in gallons per minute.
Multiply the flow in gallons per minute by 60 to give you gallons per hour. Then multiply gallons per hour by 24 to give you gallons per day, and multiply gallons per day by 365 to give you gallons per year. For example, 5 gallons per minute x 60 x 24 x 365 = 2,628,000 gallons per year.
Divide gallons per year by 327,549.52. Your answer is the number of acre-feet per year provided by the water flow you measured if it flows regularly at that rate. Using the above example, 2,628,000 gallons / 327,549.52 = 8.0 acre-feet per year, rounded to the nearest tenth.
If you want to know the acre-feet of water flow provided for a certain number of days, divide the number of days by 365 and multiply your answer by the number of acre-feet per year. For example, your water flows at five gallons per minute for 60 days and you want to know how much the total flow is in acre-feet. 60/365 = 0.16438. You found through applying the above steps that a steady flow of five gallons per minute equals 8.0 acre-feet per year. 8.0 x 0.16438 = 1.3 acre feet, the total flow of your water over the 60-day time period.