Irrigation Formulas 



Water Measurement1 gallon = 8.3 pounds1 cubic foot = 7.48 gallons = 62.4 pounds 450 gallons per minute (gpm) = 1 cubic foot per second (cfs) = 1 acreinch per hour 1 acreinch = 27,000 gallons = 1 acre flooded 1 inch deep 1 acre foot = 12 acreinches = 43,560 cubic feet = 325,900 gallons 1 gallon = 3.785 liters = 0.003785 cubic meters 1 cubic meter = 1000 liters = 264.2 gallons Pressure1 column of water 2.31 feet high exerts a pressure of 1 psi1 pound per square inch (psi) = 2.31 feet of water 1 atmosphere = 14.7 psi = 33.95 feet of water 1 inch of mercury (hg) = 1.13 feet of water psi X 2.31 = feet of head Total dynamic head (TDH) in feet includes: pumping lift in feet elevation change in feet friction loss in feet system operating pressure in feet TDH = lift + elevation change + friction + pressure HorsepowerWater horsepower (WHP)  power required to lift a given quantity of water against a given total dynamic head
Brake horsepower (BHP) = required power input at
the pump
Water ApplicationApplication amount (H) inches:
Q = flow rate in acrein./hr = gpm/450
Maximum application rate for outer sprinklers of
a center pivot:
Furrow irrigation set size (acres):
Approximate acreage covered by a pivot:
*Wetted radius includes radius of end gun Flood irrigation set time:
**Allow for 1" additional water for seepage and other losses on newly flooded fields.



