17
®
16
PGP
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Rotor
PRECIPITATION RATE
Two formulas are shown below, the first is most useful when comparing precipitation rates between different types of sprinklers
or calculating precipitation rates on areas with a single type of sprinkler and uniform head and row spacing. The second method
is better suited to areas where sprinkler head flows or spacing varies. Metric versions are shown in parenthesis.
Precipitation Rate - Method #1 - Individual Head Method
Where:
Pr = precipitation rate in inches per hour
GPM = flow for a given sprinkler of any arc, in gallons per minute
Degrees Arc = the arc of the given sprinkler in degrees
Head Spacing = the space between the heads in a row, in feet
Row Spacing = the space between rows of heads, in feet
34650 = constant for conversion of area and flow into common units
Example:
What is the precipitation rate for a 270 degree sprinkler with 6.8 gpm spaced at 28' by 30'?
Pr =
34,650 x 6.8
Pr =
1.04 in./hr.
270 x 28 x 30
Precipitation Rate - Method #2 - Total Area Method
Where:
Pr = precipitation rate in inches per hour
Total GPM = total flow from all sprinklers in the given area in gallons per minute
Total Area = the given irrigated area in square feet
96.25 = constant for conversion of area and flow into common units
Example:
What is the average precipitation rate for a section of turf 325' by 80' if the total flow from all sprinklers in the
area is 112 gpm.
Pr = 96.25 x 112
Pr =
0.41 in./hr.
(325 x 80)
SPRINKLER RUN TIME
The sprinkler run time formula calculates the number of minutes required to apply enough water to replace the water lost by
evapotranspiration for a specific crop irrigated with a system at a particular precipitation rate and efficiency.
Where:
T
=
sprinkler run time in minutes
Et
o
=
reference evapotranspiration rate, in inches
K
c
=
crop coefficient, percent
Pr
=
precipitation rate of the area, in inches per hour
E
a
=
application efficiency of the system, percent
60
=
Constant for conversion of area, flow, inches per hour and inches per day into common units
Example:
Determine the sprinkler run time for an athletic field with an accumulated Et
o
of 0.45 inches (0.15"/day for 3
days) and a crop coefficient for the warm season turf of 0.70. The sprinkler precipitation rate is 0.50 in./hr. with an applica-
tion efficiency of 75%.
T = 60 x 0.45 x 0.70 T = 50.4 min.
0.50 x 0.75
