# Irrigation Formulas
### Water Measurement 1 gallon = 8.3 pounds 1 cubic foot = 7.48 gallons = 62.4 pounds 450 gallons per minute (gpm) = 1 cubic foot per second (cfs) = 1 acre-inch per hour 1 acre-inch = 27,000 gallons = 1 acre flooded 1 inch deep 1 acre foot = 12 acre-inches = 43,560 cubic feet = 325,900 gallons 1 gallon = 3.785 liters = 0.003785 cubic meters 1 cubic meter = 1000 liters = 264.2 gallons ### Pressure 1 column of water 2.31 feet high exerts a pressure of 1 psi 1 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 ### Horsepower Water horsepower (WHP) - power required to lift a given quantity of water against a given total dynamic head
WHP = | Q X H 3960 | Q = flow rate (gpm) H = total dynamic head (feet)
Brake horsepower (BHP) = required power input at the pump
BHP = | Q X H 3960 X E | Q = flow rate (gpm) H = total dynamic head (feet) E = pump efficiency as a decimal (assume .75 unless known)
### Water Application Application amount (H) inches:
|  | gpm 450 | X | time
H = | QT = A |
|  | acres
Q = flow rate in acre-in./hr = gpm/450 T = length of application (hours A = area irrigated in time T (acres) Maximum application rate for outer sprinklers of a center pivot:
AR = | Q X 122.5 R x r | Q = gpm of sprinkler system R = length of center pivot (ft.) r = radius of sprinkler heads at distance R AR = application rate (inches/hr)
Furrow irrigation set size (acres):
No. of rows X row width (feet) X row length (feet) 43,560
Approximate acreage covered by a pivot:
Radius of pivot wetted area*,feet)2X 3.14 43,560
*Wetted radius includes radius of end gun Flood irrigation set time:
T= | A X D Q | T = pumping time to flood a field to depth (D), in hours D = depth of flood (inches)** A = area of field to be flooded (acres) Q = flow rate acre-inches/hr = GPM
**Allow for 1" additional water for seepage and other losses on newly flooded fields.
Maximum economical pipe flow capacities: RULE OF THUMB - Based on maximum velocity of 5 ft/sec.
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