# 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)2 X 3.14 43,560

 Maximum economical pipe flow capacities: RULE  - Based on maximum velocity of 5 ft/sec. 