Irrigation Facts

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 horespower (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 horespower (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

*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.

James G. Thomas, Extension Agricultural Engineer

Mississippi State University does not discriminate on the basis of race, color, religion, national origin, sex, age, disability, or veteran status.

Form 804
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A. Brown, Director

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