Emitters: Wetted Diameter,
Spacing, & Number/Tree
Natural Resources Conservation Service
Why should I be concerned
with wetted diameter and
spacing?
 From a practical
perspective, one needs to get
water to a significant portion of the potential
root zone of the tree/shrub/plant so it
doesn’t die from an inability to intake water,
or fall over during a windstorm from lack of
an anchoring root system
 From a standards and specifications
perspective:
PS 441 (Irrigation System, Trickle, 4/82) requires
that “Emitters shall be located to provide an
overlap of the wetting pattern within the root
zone.”
 Draft PS 441 (Irrigation System, Microirrigation,
2/00) requires that “The number and spacing of
emitters along the lateral line shall be adequate to
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provide
water distribution to the plant root zone

What exactly is Pw?

Pw is the “average horizontal area wetted in the top 6 to 12
inches of the root zone as a percentage of the total crop area.”
(NEH 623, Ch. 7)
For a lone tree
Pw=Aw/Arz
emitter
Tree
trunk
Surface area
wetted
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Aw
Arz
Percent Wetted Area & Number of
Emitters
Sw
e= # of
emitters/tree
Sp
Se
Sr
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Wetted diameter overlap
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Percent Wetted Area & Number of
Emitters
eSeSw
Pw 
(100)
SpSr
PwSpSr
e
(100) SeSw
Eq. 7-1, NEH 623
33Pw50 NEH 623
40Pw50 NEH 652
Se.8Sw
NEH 623
BUT, what is Sw (wetted diameter) ?……….
Natural Resources Conservation Service
Wetted diameter is a function
of:
 emitter
type, if not a point source
 emitter discharge
 duration of operation
 soil layering (horizontal)
 soil type
 any others?…..
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Wetted diameter is a function of soil
Also see Table 6-16 of NEH 652
and BE CAREFUL
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Wetted diameter is a function of soil
layering
Area in ft2 wetted from point source emitter
discharge of 1 gph
Type of Layering
None
1.8
Low
High
Density Density
5.0
9.8
Taken from Table 7-2, NEH 623
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Wetted diameter is a function of
discharge & duration - to a limit
1 gph for 2 hrs
1 gph for 8 hrs
1 gph for 2 hrs
4 gph for 2 hrs
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Wetted diameter is a function of emitter
type
Micro sprinkler
Point source emitter
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Considerations for Emitter type and
#
#
increases wetted area
 zigzag, pigtail etc…..increases #
 pulsing
 micro sprays
 cost (more emitters w/ less q)
 increase duration of flow
 FIELD TEST
Natural Resources Conservation Service
Emitter Spacing & Number
Recommendations for Windbreaks
Low Shrubs
2-3 ft tall
Shrubs or trees
-5 ft tall
Shrubs or trees
5-10 ft tall
Trees
>10 ft tall
one or two 1gph
emitters
Two or three 1gph
emitters
Three to four 1
gph emitters or
one or two 2 gph
emitters
Four to six 1 gph
emitters, or two or
three 2 gph
emitters
Placed 6-12 inches
from base of plant
Place 1 ft from
base of plant
Equally spaced 23 ft from base of
plant
Equally spaced
aobut 4-8 ft from
trunk
Taken from Table 7-2, NEH 623
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