Question: What is the difference between Cubic Feet per Second (CFS), Gallons per Minute (GPM), Gallons (G), and Acre Feet (AF)?
Answer: Cubic Feet Per Second (CFS) and Gallons Per Minute (GPM) are both ways of expressing instantaneous rate of diversion. Gallons (G), or Acre Feet (AF) describe volume of water applied to land, stored in reservoirs, or consumed in a time period, such as a month.
Water use quantities need to be reported as monthly volumes, rather than as rates of diversion.
To find the volume of water diverted in a month, multiply the instantaneous rate times the amount of time that diversion occurred during the month. Examples:
· Diverting at 10 GPM, for 8 hours a day for 10 days in a month = 10 gallons per minute x 60 minutes per hour x 8 hours per day x 10 days = 48,000 gallons for the month.
· Diverting at 2.5 CFS, for 12 hours per day, 5 days per month = 2.5 cubic feet per second x 60 seconds per minute x 60 minutes per hour x 12 hours per day x 5 days = 540,000 cubic feet for the month.
You may report your monthly usage as any one of the following: Acre Feet (AF), Gallons (G), Thousand Gallons (KG), Million Gallons (MG), Cubic Feet (CF), or Million Cubic Feet (MCF).
Regardless of the unit of measurement originally reported, for comparison purposes, the Department converts all water use quantities to AF. One Acre Foot is the volume of water that will cover an acre of ground one foot deep.
Some conversion factors:
1 cubic foot (CF) = 7.48 Gallons
1 CFS = 448.8 gallons per minute (GPM)
1 Acre Foot (AF) = 325,851 Gallons (G)
1 Acre Foot (AF) = 43,560 Cubic Feet (CF)
For continuous 24-hrs per day diversions: (CFS) x (1.98) x (# of days used per month) = AF per month