Water
Measurement1
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 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 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 HorsepowerWater
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
ApplicationApplication
amount (H) inches:
  gpm
450  X  time  H
=  QT
=
A 
   acres 
Q
= flow rate in acrein./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 acreinches/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.  
