Full Product Manual

Table Of Contents
Determine Plant Water Requirements Page 25
Area of Plant Canopy
Use the following formula to determine the area of the plant’s canopy.
Canopy Area (sq. ft.) = .7854 x Diameter (ft.) x Diameter (ft.)
Water Requirement (GPD)
The water requirement for individual plants is measured in gallons per day. To
compute the water requirement for an individual plant, you must know the area
of the plant’s root zone, its K
c
, the worst case PET rate for the area, and the
application efficiency of the irrigation system. The formula is:
Gallons per Day per Plant = .623 x Canopy Area (sq. ft.) x K
c
x PET
Application Efficiency
EXAMPLE
You are calculating the area of the root zone for a large shrub. The mature
canopy of the shrub is 6.5 feet in diameter. To calculate the area, you multiply:
.7854 x 6.5 ft. x 6.5 ft. = 33.18315 sq. ft.
After rounding to the nearest tenth, you find that the area of the root zone is
33.2 sq. ft.
Application Efficiency
To determine the application efficiency (the efficiency with which water is
actually made available to plants) refer to the Site Data worksheet on page 11 and
identify the climate type. In the worksheet, the climate was identified as warm,
dry. From Table 3-4, a warm, dry climate results in an application efficiency of 90
percent. For use in the GPD (gallons per day) formula shown below, convert the
percentage to a decimal, i.e., 0.90.
EXAMPLE
You’re calculating the water requirement for the shrub used in the previous
examples. At this point, you know the following information:
Area of plant canopy = 33.2 sq. ft.
K
c
= 0.5
PET (worst case, mid-summer) = .20 in./day
Application Efficiency = .90
To determine the plant’s water requirement, you calculate:
.623 33.2 sq. ft. 0.5 .20 in.
.90
2.298177 GPD
×××
=
After rounding to the nearest tenth, you find that the water requirement for
this plant is 2.3 gallons per day.
Calculate the water requirement for each plant in the hydrozone, and record your
results in the spaces provided on the worksheet.
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