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Drip
Irrigation Design Manual |
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Calculating water requirements for dense plantings using A1 drip line |
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Example: |
- A1 drip line flow rate = .5 gph.
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- A1 drip spacing 12" apart,
and in more than one lateral drip line layout.
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- Lateral spacing 16" apart.
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Formula to determine the application
rate per hour |
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Example: Assume our ground cover area is |
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10' x 8' = 80 sq.ft. |
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Assume we have sandy soil. We can
see from table A1
that we can use A1 drip line with 1 gph drippers spaced 12"
apart. Let's stay on the safe side and use 16" spacing between
the laterals and we will start 4" from the edge of the area
being watered(see drawing) |
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8" = ie, four inches from the edge of the area being
watered on each side |
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10' x 12" =
120" - 8" = 112" |
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Using 16" lateral
spacing divided by 112" allows for 7 drip laterals |
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We plug all the numbers into the
formula and determine that our application rate is 1.20"
per hour. The example shows a higher precipitation rate than
for a sprayer or a sprinkler.
If you wish to lower the precipitation rate, increase the spacing
between drippers and laterals. |
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Average
precipitation rate for a sprayer or a sprinkler is around .50
inches per hour |
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Formula # 1 |
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After determining
the application rates, we need to determine the run time necessary
to apply 1.20" per hour |
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Formula to determine
system run time per day |
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From the first formula, our application
rate is 1.20" per hour. Assume a daily ET rate of
0.28" per day from table
A2 |
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Formula # 2 |
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The last thing to do is to compensate
for the system application efficiency by dividing the run time
of 23 minutes ( see formula 2) by application efficiency . For
this example we will use high efficiency number 90%: 23 : 90
=25.5 minutes per day
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System
run time based on formula 2 is 25.5 minutes per day of run time
or 51 minutes every other day |
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Calculating
water requirements per zone |
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Once we calculate our water
requirements, we need to calculate water requirements per zone
so we can select the size of the valve , pressure regulator,
filter and main lateral pipe. |
Calculate total feet of A1
drip line. Example drawing 1 zone: 7
drip laterals, each 8' long = 56' total of drip line divided
by 1' for drip spacing = 56 drippers with flow rates of 1 gph
= 56 gallon per hour divided by 60 minutes = .93 GALLON PER MINUTE
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System water requirements per zone .93 gallon per minute |
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Size the valve, filter and
pressure regulator according to the flow rate |
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- Recommended filter mesh for
A1: 155 mesh, available in 3/4", 1", 1 1/2" and
2" sizes
- Recommended filter flow rates:
3/4' - 10 gpm, 1" - 16 gpm, 1 1/2" - 60 gpm and 2"-
80 gpm
- Recommended pressure
for A1: 25 to 45 PSI
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Table
A1 |
A1 drip line recommended
spacing |
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Sandy soil |
Loamy soil |
Clay soil |
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Dripper flow rates |
1 gph |
.6 - 1 gph |
.6 gph |
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Spacing |
Drip line |
Laterals |
Drip line |
Laterals |
Drip line |
Laterals |
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Ground cover & shrubs |
12" |
14" - 18" |
12" - 16" |
16" - 24" |
16" - 20" |
18" - 24" |
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Flower bed |
12" |
12" - 16" |
12" |
16" - 18" |
18" |
16" - 20" |
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Turf |
12" |
12" - 16" |
12" |
16" - 20" |
18" |
18" - 24" |
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Table
A2 |
Generic estimates of ET
( inches per day) |
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AZ |
.22 to .37 |
FL |
.22 to .27 |
KS |
.25 to .27 |
NM |
.28 to .35 |
UT |
.20 to .32 |
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CA |
.15 to .27 |
GA |
.22 to .25 |
NE |
.18 to .25 |
OR |
.17 to 22. |
WA |
.14 to .21 |
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CO |
.15 to .20 |
ID |
.20 to .25 |
NV |
.20 to .25 |
TX |
.26 to .32 |
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The total amount of water that should
be supplied to a landscape must be sufficient to replace water
loss from the soil surface by evaporation, humidity, temperature
and water loss by the plant during the transpiration processes:
this is called evapotranspiration or ET. ET is usually expressed
in terms of inches of water per day or inches of water per month.
These rates vary with the season and realistically they vary
by the hour. But for lawn irrigation purposes, average ET rates
are more than enough to schedule watering. The best ET data per
a week or a month is usually obtained from local sources such
as the Cooperative Extension Service, local newspapers or National
Weather Service, or you can do it yourself by measuring the depth
of water evaporated from an open pan. |
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