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Module No. 9

Drip Irrigation System

Drip or trickle irrigation (also called micro irrigation) refers to the frequent low volume, low
pressure application of irrigation water on or beneath the soil surface where only the root zone is
irrigated through drippers or emitters (Figure 1). It is adaptable for most soils and in any sloping
agricultural land. It is a viable alternative in windy areas where sprinkler irrigation is not feasible.
Drip irrigation is ideal for small areas to irrigate small trees, shrubs, vines, ornamentals, vegetables
and other field crops planted in rows. Generally, however, only high-value vegetables and field
crops are considered because of the high investment cost involved in installing a drip irrigation
system.

Collaged from: Washington State Fruit Commission (2018); United States Department of
Agriculture-Natural Resources Conservation Service (n.d); Mother Earth News (n.d);
Gerak Maju Pertanian (2014) [clockwise]; Dubois Agrinovation (2018) [center]

Figure 1. Drip irrigation system

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With all factors equal, drip irrigation system is more efficient than other types of irrigation
method. Unlike flooding, furrow, border and sprinkler where the soil profile is indiscriminately
wetted, drip irrigation system is capable of precisely delivering small amounts of water close to
plants where only the portion of the soil profile in which the roots grow is irrigated. Thus, irrigation
efficiency of a well-designed and maintained drip irrigation system can be from 80 percent
(United States Department of Agriculture-Natural Resources Conservation Service, 1997) to as
high as 90 percent (Kizer, n.d). With very good management practices, application efficiency
may approach the ideal value of 100 percent (Sharma, 2017).

Aside from high application efficiency, Sawant reported that drip irrigation system also
saves water by 30 to 60 percent as compared to flooding method, increases crop yield by up to
230 percent, increases fertilizer use efficiency by 30 percent, and enables the use of saline water
for irrigation (2018). It can stretch a limited water supply to cover up to 25 percent more area
than a typical sprinkler system (Kizer, n.d). Other advantages include:

 Minimizes nutrient loss due to localized application and reduced leaching.

 Field levelling is not necessary.
 Fields with irregular shapes are easily accommodated.
 Recycled, non-potable water can be safely used.
 Maintains moisture within the root zone at field capacity anytime during the growing
period.
 Soil type plays a less important role in the frequency of irrigation.
 Reduces soil erosion.
 Controls weed growth.
 Water distribution is highly uniform, controlled by the output of each emitter.
 Labor cost is less than other irrigation methods.
 Amount of water application can be regulated by regulating the valves and emitters.
 Fertigation can easily be included with minimal waste of fertilizers.
 Reduces the risk of diseases because the foliage remains dry.
 Reduces energy cost because of low operating pressure (Wikipedia, 2018).

Drip irrigation is not without drawbacks. Initial cost can be higher than other overhead
systems. If the water is not properly filtered and the equipment not properly maintained, it can
result in clogging. Under-application or over-application is a common problem particularly for
inexperienced irrigator. In lighter soils, sub-surface drip may be unable to wet the soil surface for
germination. Without sufficient leaching, salts applied with the irrigation water may build up in
the root zone, usually at the edge of the wetting pattern. On the other hand, drip irrigation
avoids the high capillary potential of traditional surface-applied irrigation, which can draw salt
deposits up from deposits below (Wikipedia, 2018).

The principle in drip irrigation system design is to achieve minimum discharge at

maximum water flow possible for a given field condition. This large water passage in the pipe
system is essential to minimize clogging of emitters (Agiinfo.in, 2015). The main task is to properly
match the different components. Unlike in sprinkler, drip irrigation system design is much more
forgiving of design errors (Stryker, 1997). Under-sizing or over-sizing the different drip system
components has little effect on its overall performance because irrigation application is frequent
at low volume, which is usually within the field capacity range of the soil. The ultimate objective
is to come up with the best system that gives the highest return on investment. This is our concern
in this module.
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After thorough and diligent study of this module, you should be able to:

a. Differentiate drip irrigation from sprinkler irrigation and surface irrigation methods such as
basin, furrow and border strip;

b. Enumerate the general design criteria for irrigation systems;

c. Synthesize the advantages and disadvantages of drip irrigation system;

d. Draw a schematic field layout of a drip irrigation system and identify the different
components;

e. Synthesize the conditions to be met for proper operations of the emitters, laterals, sub-
mains, and mainline;

f. Design a drip irrigation system in terms of:

 Type of system application,

 Emitter type and operating pressure,
 Emitter flow characteristics,
 Number of emitters per plant and per lateral,
 Connection of emitter to lateral,
 Placement and spacing of emitters,
 Allowable elevation difference between the lowest and highest emitters,
 Lateral tube size and type,
 Length of laterals,
 Number of laterals operating simultaneously,
 Connection of laterals to sub-mains,
 Sub-mains and mainline pipe size, type and grade,
 Number of sub-mains,
 Length of sub-mains and mainline, and
 Filtration system; and

g. Prepare the bill of materials and determine the total cost of the designed drip irrigation
system.
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I. Components of Drip Irrigation System

A drip irrigation system distributes water through a network of pipes, tubings, control
valves, and emitters. Starting from the source to the emitters, the essential components include
the pump unit, filtration system, back flow preventer, fertigator, mainline, solenoid valve, pressure
regulator, sub-mains, laterals and emitters (Figure 1). Other necessary control devices and
management tools and equipment are specifically mentioned, where appropriate, in the
discussions on these components.

Source: United States Department of Agriculture-Natural Resources Conservation Service (1997)

Figure 2. Typical field layout of drip irrigation system showing the different components

A. Pump unit

Like sprinkler irrigation system, drip irrigation system distributes water under pressure. This
pressure forces water through the pipe system towards the emitters. A piston, diaphragm, or
centrifugal type pump is commonly used to provide this pressure, which is driven by either an
electric motor, or a gasoline or diesel internal combustion engine. Recently, solar pump is being
promoted to popularize drip irrigation. Since the pressure requirement of emitters is a lot lower
than sprinkler head requirements, an elevated water tank or a pressurized tank is installed
between the pump and the filtering system.

Pressure gauge and flow meter are installed at the discharge side of the pump. For
accuracy of reading, the meter should have a straight, unobstructed section of pipe after
upstream equivalent in length to at least 5 to 10 times the pipe diameter, and 2 to 4 times the
pipe diameter downstream of the flow meter.
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B. Filter

Filtration is a must in drip irrigation systems to remove debris, sediments and organic
materials that may clog the emitters. The suitable type of filter depends on the emitter type and
the impurities found in the water. There should be pressure gauges installed upstream and
downstream of the filtration system to determine when cleaning is necessary, which is
manifested by a drop in pressure between the inlet and outlet of the filtering unit.

1. Gravel and sand media filter

These filters are effective against inorganic suspended solids, biological substances and
other organic materials. This type of filter is essential when the source of water is from an open
reservoir where there is usually algae growth. The dirt is removed and accumulates inside the
filtration system, which consists of small basalt gravel and sand placed in a metal cylindrical
tank. As shown in Figure 3a, the water enters from the top and flows through the filter media. The
filtered clean water is discharged at the bottom.

The filter is cleaned by reversing the direction of flow or back flushing (Figure 3b). To
determine as to when, pressure gauges are fitted at the tank inlet and outlet. When filtered
impurities accumulate inside the tank, the pressure difference between the inlet and outlet will
increase. When the pressure difference is about 0.5 to 1.0 kg/cm2 (7 – 14 psi), back flushing
should be done (Agriinfo.in, 2015).

(a) Filtering (b) Back flushing

Source: Alibaba.com (2018)a

Figure 3. Typical gravel and sand filtration system

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2. Screen filter

These are installed with or without gravel and sand filter, depending upon the quality of
water. The screens are usually cylindrical in shape and are made of non-corrosive metal or
plastic materials (Figure 4). Screen filters are specified as below (Agriinfo.in, 2015):

a. By the diameter of inlet and outlet (¾” to 4”).

b. By the recommended flow rate (3, 5, 7, 10, 15, 20, 30, 40 m3/hr).

c. By the size of holes in the screen (mm, micron or mesh number/in2). The most common
mesh selected for drip irrigation system is 100 to 200 mesh screens (0.15 to 0.07 mm, or 140
to 74 microns). Note that the higher the mesh number, the smaller the diameter of the
hole is.

d. By the total surface area of the filter (m2) or by the active or net filter area, which is
usually about 1/3 of the total filter surface area.

e. By the cleaning method (manual or automatic). The head loss across the filter should not
be more than 3 m (10 ft) H2O (4.33 psi), otherwise cleaning is needed. The screen filter is
cleaned by back flushing (see Figure 3). After cleaning, the screen is checked for tears
and the gasket should be checked and replaced when necessary.

Source: Drip Depot, Inc. (2018) and UVAR Holland b.v. (n.d.)

Figure 4. Typical screen filtration system

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3. Disc filter

A disc filter is similar to a screen filter, except that the filter cartridge is made of a number
of plastic discs of varying mesh numbers, stacked on top of each other. Figure 5 shows some of
the typical disc filters (orange color) used in drip irrigation systems. Each lateral must be installed
with a disc filter placed downstream of the control valve (see Figure 10).

Each disc is covered with small grooves or bumps which allow raw water to pass through
it and the impurities trapped behind. The discs have a hole in the middle, forming a hollow
cylinder in the middle of the stack where the filtered water passes through towards the outlet.
They come in different sizes from ¾” to 6” and capacities ranging from 4 m3/hr (17.6 gpm) to 160
m3/hr (704 gph). Pressure drop is about 5 psi. Depending on the amount of organic impurities in
the water source, the bigger disc filters are used as primary filtering unit in lieu or in addition to
either the media or the screen filter.

Disc filters have better cleaning capability than the media and screen filters because of
its multi-stage filtration process. The larger outer meshes operate as screen filters and collects the
larger particles, and the smaller meshes trap the fine particles, mainly organic matter. The filter
elements can be easily cleaned manually, or by back flushing.

Source: Mani (n.d)

Figure 5. Secondary disc filters for drip irrigation system

C. Back flow preventer

To prevent water from flowing back especially when fertilizers, pesticides, and pipeline
cleaning chemicals are injected into the system, a check valve (Figure 6) is an essential
component to avoid contaminating the water source. It should be installed upstream of the
fertigator.
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Source: IndiaMart, Ltd. (n.d)

Figure 6. Back flow preventer or check valve

D. Fertigator

Application of fertilizer through drip irrigation system increases fertilizer use efficiency with
significant savings on fertilization labor cost to boot. This is achieved through a fertilizer applicator
or fertigator, which according to the Food and Agricultural Research and Extension Institute may
increase crop yield by 40 to 50 percent and the quality of produce is improved (n.d). Aside from
soluble fertilizers, pesticides can be injected too into the system.

When water with significant amounts of dissolved minerals (hard water) is used, it may
leave mineral deposits, which over time restrict or impede flow and clog the emitters. In such
cases, mild solution of either chlorine or calcium hypochlorite is injected into the system including
other chemicals to treat biological growth and water quality problems.

The construction of a fertigator is very simple and can be fabricated locally. It consists of
a fertilizer tank and a venturi injector (Figure 7) that can be installed in the mainline. The
differential pressure created by the flowing water in the mainline sucks the fertilizer or chemical
solution in the tank to enter the mainline through the venturi tube and mix with the irrigation
water.

The tapping of the fertigator into the system can be as shown in Figure 8. It must be
located between the back flow preventer and the mainline. A screen filter should be placed
before the mixture enters the mainline. Fertilizer application rate can be regulated by the third
control valve in the figure, which should be located upstream of a flow meter (not shown). The
first and second control valves are used to divert flow.
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Source: Wassertec Ozone Systems (2018)

Figure 7. Cross-section of venturi injector

Source: Food and Agricultural Research and Extension Institute (n.d)

Figure 8. Installation of fertigator into the drip irrigation system

E. Mainline

The mainline supplies water from the filtration system and fertigator to the sub-mains. In
order to minimize corrosion and clogging, a polyvinyl chloride (PVC) pipe is used (Figure 8).
Usually, the mainline is buried at least 60 to 90 cm (2-3 ft) so that it will not interfere with farm
operations. The pipe diameter should be based on the pump capacity and irrigation water
requirement. The maximum permissible flow velocity should not be greater than 2.0 mps (Gajjar,
2013) and the friction head loss should be less than 5 m (16.4 ft) H2O column per 1000 m length
of pipeline (Agriinfo.in, 2015). The maximum discharge for the mainline (also sub-mains) must not
exceed 45 gpm (Terry, 2010). As a rule, the total pipe length must not exceed 120 meters
(Stryker, 1997).
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F. Sub-mains

The sub-main distributes water from the mainline to the laterals. However, when the field
is too big and impractical to irrigate it at one time, the field is divided into several manageable
sub-plots called irrigation zones. In such case, sub-mains are necessary to supply water from the
mainline to the irrigation zones wherein each zone is served by several laterals. For small trees,
shrubs and vines, a single plant or hill is a drip zone. It may have one or more emitters depending
on the discharge and soil type. In this case, an irrigation zone is consist of several drip zones.

Like the mainline, the sub-mains are also made of PVC pipe preferably of the same
diameter and placed 60 to 90 cm (2 - 3 ft) below the ground surface. Total head loss in the sub-
mains should not be more than 1.5 m (4.92 ft) H2O column and maximum permissible velocity is
2.0 mps (Gajjar, 2013). A control valve is installed between the mainline and sub-main for better
water management. Each sub-main (also laterals) can be fully automated, wherein a compact
programmable control panel or a timer can be installed to operate the solenoid valve to start
and stop irrigation. A manual priority switch should be necessarily installed also to over-ride the
control panel or the clock switches when irrigation run needs to be postponed or when there is a
need to add some more water.

In many cases, laterals may be level or nearly so, but the sub-main that feeds them is not.
Where slope is 5 percent or steeper, the sub-mains must be modified by any one of the following
techniques to prevent the pressure variation from being too high (United States Department of
Agriculture-Natural Resources Conservation Service, 1997):

a. Divide the sub-main into shorter lengths so it does not have more than 10 ft elevation
drop between the inlet and the lowest outlet. Then size the sub-main so total friction loss
about equals the elevation pressure gain.

b. Install a pressure regulator along the sub-main to reduce pressure variation due to
elevation.

c. Install a control valve between the sub-main and each lateral. Adjust to equalize flow
into each lateral.

d. Connect smaller diameter laterals to the sub-main. By selecting the proper length and
diameter, the flow to each lateral can be regulated.

e. Use pressure compensating emitters.

G. Solenoid valve

A solenoid valve (Figure 9) is installed between the mainline (or sub-mains as explained
above) and the laterals. It is an electro-mechanically actuated or controlled valve. It has a
solenoid, which is an electric coil with a movable ferromagnetic core (called plunger) in its
center. As soon as the coil is electrically energized, a magnetic field is created which pulls the
plunger up towards the center of the coil. This opens the orifice so that the fluid can flow
through. In rest position, the plunger closes off the orifice. A solenoid valve is turned on and off
by a timer. The electric current sent to the valve solenoid is just used to jump-start the plunger
movement of closing and opening the orifice.
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If for some reasons a solenoid valve is not used, a simple gate valve can be installed
instead. What is important, is that, the flow in each lateral can be regulated. As shown in Figure
10, this control valve is the first of a series of components in the laterals.

(a) Closed (b) Open

Source: Tameson.com (n.d)

Figure 9. Direct operated solenoid valve

Modified from: Vila (2013)

Figure 10. Different lateral components

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H. Pressure regulator

A pressure regulator is a very important component of a drip irrigation system for two
reasons. First, whenever the solenoid valve is turned off by the timer, it shuts off the flow of water
abruptly, which triggers a hydraulic phenomenon called “water hammer.” Water hammer is
what happens when all that fast flowing water is forced to stop moving almost instantly. Water
hammer creates tremendous pressure surge in the system that may damage the pipe system
components.

Second, since drip irrigation systems operate at relatively low pressures, even a small
variation in pressure can have a significant effect on the uniformity of water application. This
pressure variation is affected by the relative elevations of the emitters. For every 2.31 feet of
difference in elevation, the water pressure in a pipe system will vary by one psi. Therefore, if a
field has a variation of 10 feet in elevation from the highest to the lowest point, then the emitter
at the lowest point will be operating 4.33 psi (10 ft/2.31 psi/ft) greater than the highest emitter. In
a drip irrigation system operating at low pressure, then this pressure variation between the lowest
and the highest emitters is an extremely very large variation.

To prevent pressure surge and extreme pressure variation from happening, a pressure
regulator should be installed in the system. It is used to lower the pressure and then keep it at
that pressure, even if the incoming water pressure varies up and down. They come in varying
discharge capacity from 0.1 to 32 gpm and pressures from 15 to 40 psi There are two types – the
non-adjustable ones with a factory pre-set outlet pressure (Figure 11a), and the ones with user
adjustable pressure settings (Figure 11b). Either type may be used for drip irrigation systems. As a
general rule, the non-adjustable type are used for drip systems with not more than three laterals
operating simultaneously. The adjustable-type pressure regulators allow more flexibility and are
usually more accurate. The outlet pressure is adjusted through the knob on top of the pressure
regulator.

(a) Non-adjustable type (b) Adjustable type

Source: The Toro Company (2018) Source: Amazon.com, Inc. (2018)

Figure 11. Pressure regulator for drip irrigation system

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The non-adjustable type must be installed after the control valve of each lateral. It may
be damaged if placed in any other constantly pressurized location. The adjustable type can be
placed anywhere in the system. In fairly level fields, pressure regulator is not needed when
pressure compensating emitters are used.

I. Laterals

From the source, the mainline carries water to a system of sub-mains which then goes to
manifolds (also called headers), and then to laterals and emitters. The manifold is used to
distribute water to two or more laterals (Figure 12).

Drip irrigation laterals are small diameter flexible low-density polyethylene (PE) tubes that
are connected to the mainline, or to the sub-mains in large drip irrigation systems. Tubing used
for drip irrigation is normally 5/8” (16 mm) PE where the emitters are installed (see Figure 15). If
laid above ground as is the usual practice, the PE tubing should be colored black to avoid
algae growth. If buried for aesthetics and other reasons, PVC pipe is used instead.

Source: Lowe’s (2018)

Figure 12. Lateral manifold

There are two categories of lateral based on field application (Figure 13). These are the
line-source (in-line) and the point-source (on-line) laterals. According to Kizer, line-source lateral,
also called drip tube or drip tape, are used when plants are closely spaced within a row, with
the rows separated several feet apart, as with most vegetables and other field crops (n.d). It is
also used for watering crops in pots and containers in nurseries and greenhouses. Line-source
tubings can be run only 60 to 90 m (200 - 300 ft) on level terrain.

A permanent drip irrigation system with above ground point-source emitters and buried
main and sub-mains is common for small fruit trees, shrubs and vines. Point-source laterals can be
150 to 300 m (500 – 1000 ft) long on relatively flat terrain (United States Department of
Agriculture-Natural Resources Conservation Service, 1997).
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(a) On-line or point-source (b) In-line or line-source

Source: Rountree (2016) Source: irrigationglobal.com (n.d)

Figure 13. Categories of drip irrigation lateral

Flow velocity in laterals should not exceed 1.5 mps (Hunter Industries, Inc., 2012). The
operating pressure variation between two extreme points (fist and last emitters) of the lateral
should not be more than 20 percent, and the discharge variation should not be more than 10
percent (Agriinfo.in, 2015). There should be no more than 20 percent discharge variation within
the irrigation zone (United States Department of Agriculture-Natural Resources Conservation
Service, 1997).

On sloping ground, the laterals should be placed along the contour with 10 percent
extra length allowance for sagging when measuring length (United States Department of
Agriculture-Natural Resources Conservation Service, 1997). At the distal end of each lateral,
there should be an end cap cum flush or drain valve installed.

Vacuum relief valve is also necessary to prevent pipe collapse at shut down due to
negative pressure in the pipeline. Negative pressure can also result when soil particles are
sucked into the emitters. Each lateral should have a vacuum-air combination relief valve
installed downstream of the solenoid or gate valve (United States Department of Agriculture-
Natural Resources Conservation Service, 1997). If there is any portion in the lateral higher than
this tapping point (see Figure 10), the relief valve should be installed at this highest point.

J. Emitters

Emitters are the main component of drip irrigation system for discharging water from the
lateral to the soil (Figure 14). They are mechanical devices located either within the drip tube, or
externally. These locations of the emitter relative to the lateral are referred to as line-source or in-
line and point-source or on-line emitters, respectively, as discussed in the previous section and
shown in Figure 13 above. The rate of water application depends upon the operating pressure,
discharge and spacing of the emitters. Generally, the application rate should be between the
infiltration rate of the soil and the minimum discharge of the emitter.
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Source: Alibaba.com (2018)b

Figure 14. Different types of emitter

Emitters are classified into groups based on their operating principle whether the
discharge is regulated or non-regulated, and whether the emitters are pressure compensating or
non-compensating (standard). Since pressure increases as elevation decreases and vice versa,
a pressure compensating emitter is used in sloping lands with elevation difference of more than
1.5 m because it applies water at a fairly constant rate over a wide range of operating pressures
(United States Department of Agriculture-Natural Resources Conservation Service, 1997).

Emitters are designed to discharge low volume of water at low pressures ranging from 13
to 23 psi for non-pressure compensating, and from 10 to 41 psi for pressure compensating types.
Allowable pressure loss is 4 to 6 psi for non-compensating, and 10 to 22 psi for pressure
compensating (Table 1). Discharge rates can be 0.5 gph for vegetables and other field crops, 1
to 2 gph for grapes, and 2 gph for small trees and shrubs. For line-source emitters or drip tape,
the unit of discharge is gph/100 ft lateral tubing.
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Table 1. Recommended maximum pressure ranges for typical emitters1/

Type of Emitter Design Pressure Pressure Range2/ Pressure Variation
Non-pressure compensating 15 psi 13 – 17 psi 4 psi
20 psi 17 – 23 psi 6 psi
Pressure compensating3/ 15 psi 10 – 20 psi 10 psi
20 psi 13 – 28 psi 15 psi
30 psi 19 – 41 psi 22 psi
1/ Based on 20 percent flow rate variation. It is used as a guide to allowable line losses.
2/ The allowable pressure range is an estimate for typical point-source emitters, and is
included to illustrate the advantages of pressure-compensating emitters only. If available,
manufacturer's discharge data should be used instead.
3/ Pressure compensating emitters are available with allowable maximum pressures up to 50
psi or more.
Source: United States Department of Agriculture-Natural Resources Conservation Service (1997)

Table 2 serves as a guide in the selection of type and number of emitters needed, and in
setting the distance between emitters. The number of emitters required is based on the
percentage of potential rooting volume to be watered. For small fruit trees, shrubs, or vines, on-
line emitters are often used. The United States Department of Agriculture-Natural Resources
Conservation Service recommends at least 25 to 60 percent of the root zone area be wetted.
The root zone area can be estimated by the projection of the canopy onto the ground. Avoid
wetting the tree trunk continually as it may cause crown rot (1997).

Table 2. Suggested type and spacing of emitter

Type of Crop Emitter Recommendations Spacing
Dwarf trees 1 gph on-line emitter per The distance between
plant. Placed at least 18” (46 emitters is determined by the
cm) away from the tree trunk. size of the drip zone and the
type of soil. Obviously, the
Vine and berries 0.5 to 1.0 gph on-line emitter size of the drip zone will be
per plant. Placed at least 18” smaller when the plant is
(46 cm) away from the tree young and will increase in size
trunk. as the plant grows. Bigger
drip zone needs more
Semi dwarf and 1 gph on-line emitter per tree. emitters. In sandy soils,
standard trees May need 3 or more emitters emitters need to be spaced
in sandy soils depending on closer together because the
canopy area. Placed at least water does not move as far
18” (46 cm) away from the horizontally. In clay soils,
tree trunk. where the water moves
farther sideways, the emitters
may be farther apart.*
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Table 2. (continued)
Type of Crop Emitter Recommendations Spacing
Field crops and vegetables Use either in-line drip tape, or Usethe recommended
on-line emitter. spacing based on emitter
discharge and soil type are
given in Table 3.

* Table 3 also applies on these dwarf trees, vine and berries, and semi dwarf and standard trees.
Source: United States Department of Agriculture-Natural Resources Conservation Service (1997)

Table 3. Typical emitter spacing based on discharge and soil type

Soil Type Emitter Spacing Based on Discharge
0.5 gph 1 gph
Coarse soil (sand and loamy 30 cm (12”) 60 cm (24”)
sand)

Medium soil (sandy loam, 60 cm (24”) 100 cm (36”)

loam, silt loam and silt)

Fine soil (silty clay loam, silty 100 cm (36”) 130 cm (48”)
clay and clay)

Source: Stryker (1997)

For high-value vegetables and other field crops which are usually planted in rows, the
emitters are placed in rows as well. Most often, drip tape is used but on-line emitters also can be
used. It is recommended that the entire root zone area be wetted. Drip tapes comes in 5/8” Ø
with pre-determined emitter spacings of 4”, 6”, 8”, 12”, 18”, 24”, 36”, 48” and 60”.

On-line emitters for small fruit trees, shrubs and vines can be connected to the lateral in
different ways either (Figure 15): (a) emitter directly attached to the lateral; (b) emitter directly
attached to the lateral but with extended spaghetti tubing to reach the plant; or (c) emitter
attached at the end of the spaghetti tubing which is connected to the lateral. The emitters
should be at least 1” above the ground to prevent soil from being sucked into the system, which
results to negative pressure during shut down.

Figure 15. Connection options for on-line emitters

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The shape of the water inlet hole of the emitter is indicative of its dependability and
quality. According to Stryker, a round hole is easily clogged by a grain of sand or other trash in
the water. An oblong (–) or cross (+) shaped hole is much more resistant to clogging. Some
emitters even have multiple inlet holes of different and odd shapes. Multiple holes and odd
shaped holes make it much less likely the inlet will become clogged. The outlet hole is not as
critical as the inlet (1997).

K. Soil moisture sensing devices (for automatic system)

A fully automatic drip irrigation system is equipped with a soil moisture sensor to provide
the triggering mechanism to start-stop an irrigation run. This is achieved by simply connecting a
pump relay in the water pump circuit.

There are several soil moisture sensing apparatuses but two are commonly preferred by
irrigators, namely: tensiometer and electrical resistance meter.

1. Tensiometer

A tensiometer (Figure 16) is a sealed, water-filled tube with a porous ceramic tip on the
lower end and a vacuum gauge on the upper end. The tube is installed in the soil with the
ceramic tip placed at the desired root zone depth and with the gauge above ground. In dry
soil, water is drawn out of the instrument, reducing the water volume in the tube and creating a
partial vacuum which is registered on the gauge. The drier the soil, the higher the reading. When
the soil receives water through rainfall or irrigation, the action is reversed. The vacuum inside the
tube draws water from the soil back into the instrument which, in turn, results in lower gauge
readings. The amount of vacuum reflected by the gauge is a direct measure of soil water
tension or soil suction (Harrison, 2012).

2. Electrical resistance meter

An electrical resistance meter (Figure 17a) determines the soil water by measuring the
electrical resistance between two wire mesh rings embedded in porous gypsum block (Figure
17b) or similar material that is semi-permanently put in place in the soil (Figure 17c).

The electrical resistance of the block varies with its water content, which in turn is
dependent upon the water content of the soil in contact with it. As the soil dries, the block loses
water and the electrical resistance increases. Therefore, resistance changes within the block as
measured by the meter can be interpreted in terms of soil water content. The blocks, which
have stainless steel electrodes embedded in them, are installed semi-permanently in the soil at
one-third and two-thirds of the rooting depths of the crop being irrigated (Werner, 2002).
Insulated wires from each block are brought above the soil surface where they can be plugged
into a portable meter for reading. The resistance blocks are generally calibrated in terms of soil
water tension so as to make readings applicable across soil textures. Blocks should be calibrated
for each soil type. The way blocks manufactured by different companies respond to changes in
soil water tension varies considerably. For this reason each manufacturer furnishes calibration
curves for their own instruments and blocks (Harrison, 2012).
 19

Source: Kerr (2017) Source: Shreeja (n.d)

Figure 16. Components of tensiometer

 20

(a) Electrical resistance (b) Gypsum block sensors (c) Placement of sensors in
meter the root zone

Collaged & modified from: Brouwer, Prins & Heibloem (1989) and Washington State University (n.d.)

Figure 17. Electrical resistance meter

The standard unit of soil water tension in the metric system of measurement is bar where
one bar is equal to 0.987 atmosphere (atm) and also equal to 14.504 pounds per square inch
(psi). Most soil moisture meters are calibrated in hundredths of a bar (called centibars) and
graduated from 0 to 100. Tensiometer and electrical resistance meter operate in a range of 0 to
80 centibars. In order to absorb and use the soil water in the root zone, plant roots must
overcome the soil water tension. Table 4 gives the guidelines in interpreting soil moisture meter
gauge readings.

Table 4. Interpretation of soil moisture meter readings

Gauge Reading (centibar) Interpretation
0 to 5 This range indicates a nearly saturated soil and often occurs for
one to three days following a heavy and prolonged rainfall
event or excessive irrigation application. Waterlogging sensitive
crops suffer from lack of oxygen in the root zone if readings in
this range persist.

5 to 20 This range indicates field capacity. Discontinue irrigation in this

range to prevent waste of water by percolation and also to
prevent leaching of nutrients below the root zone.
 21

Table 4. (continued)
Gauge Reading (centibar) Interpretation
20 to 60 This is the usual range for starting irrigation. Most field crops
having root system 18 inches deep or more will not suffer until
readings reach the 40 to 50 range. Starting irrigations in this
range ensure maintaining readily available moisture at all
times. It also provides a safety factor to compensate for
practical problems such as delayed irrigation, or inability to
obtain uniform distribution of water to all portions of the field.

70 and higher This is the stress range for most soils and crops. Deeper rooted
crops in medium textured soils may not show signs of stress
before readings reach 70. A reading of 70 does not necessarily
indicate that all available water is used up, but that readily
available moisture is below that required for maximum growth.

Source: Harrison & Tyson (2004)

II. Data and Information for Drip Irrigation System Design

As in sprinkler, designing a drip irrigation system is an engineering task which requires
competency in mathematics and knowledge of Bernoulli’s principle, hydraulic head, and
pumps and pumping. Understanding agronomy and soil science particularly soil-water-plant
relationship, irrigation water management and irrigation scheduling are bonus competencies.

All data and information needed in the design of sprinkler irrigation system are also
needed in drip irrigation system design, unless otherwise specifically discussed below:

A. Field elevation and area

A drawn-to-scale location and topographic maps should be prepared. The location

map should include dimensions of the field and the topographic map should preferably have
0.5 m contour intervals showing elevations with respect to the water source. These maps are
needed in determining the point-of-connection, drip zones, and orientation of the system.

B. Soil type and water movement

Soil texture affects water movement, available water content (AWC), and maximum
allowable depletion (MAD) for crops to be irrigated. Water Infiltration rate is also a function of soil
texture. These are important considerations in selecting the type and spacing of emitters.

Figure 18 shows the wetting patterns for sand and clay soils with high and low emitter
discharge rates. Evidently, the width of the wetted zone in sandy soils is relatively narrower than
in clayey soils when the emitter discharge is high; and when the emitter discharge is low, the
depth of application in sandy soils is relatively deeper than in clayey soils. These wetting patterns
 22

should be considered in selecting the appropriate emitter size and in determining the emitter
spacing based on soil texture and rooting depth of the crop.

(a) Sand (b) Clay

Source: Brouwer, Prins, Kay & Heibloem (n.d)

Figure 18. Wetting pattern for sand and clay soils with high and low discharge rates

During irrigation, the water applied seeps slowly in the soil profile until equilibrium is
reached. The soil wetted at equilibrium by a single emitter is given in Table 4. This wetted
diameter and the root zone area to be wetted are used to determine the number of emitters.

Table 4. Diameter of soil wetted by a single emitter

Soil Texture Wetted Diameter (ft)
Coarse sand, sand, fine sand 2
Loamy sand, loamy coarse sand, loamy fine sand 3
Loamy very fine sand, sandy loam 4
Fine sandy loam, very fine loam 4.5
Loam, silt loam, sandy clay loam 5
Clay loam, silty clay loam 6
Sandy clay, silty clay, clay 7
Source: United States Department of Agriculture-Natural Resources Conservation Service (1997)
 23

C. Crop

The type of crop, size at planting and maturity, root depth, recommended planting
distance between hills and between rows, and crop evapotranspiration (ETc) should also be
known prior to designing a drip irrigation system.

Water needed per day per plant is calculated based on the ETc, maximum allowable
depletion (MAD) and available water content (AWC) of the soil. ETc is computed using the
procedure in Laboratory Exercise No. 3 of AE 163 (Hydrology). For drip irrigation, MAD is set at 25
percent instead of the usual 50 percent for irrigation scheduling purposes using other methods.
MAD and AWC are needed in irrigation scheduling, which is discussed in Module No. 6 of this
course.

D. Water source

The location, quality and safe yield of the water source should be determined. If there is
no existing water system and a new one is to be developed, this should be located as much as
possible in the highest portion of the area so that the mainline can be laid downhill, not uphill
which will require higher pressure.

Clogging of emitters is the most serious problem in drip irrigation. Testing water quality is a
must to properly plan the filtration system needed, which depends on the impurities present in
the water. These impurities may clog the emitters and significantly affect the operation and
longevity of the system. The physical, chemical and biological parameters are given in
Appendix Table 1. Values less than or equal to the “low” column are ideal for drip irrigation.

III. Design Process

As in sprinkler, the main objective in the design of drip irrigation system is to provide
sufficient capacity to adequately supply the crop water requirement and apply this as uniform
as possible. Non-uniform application is caused by the pressure differential in the pipe system due
to friction losses, elevation change, clogging of emitters, and variability in manufacturers’
emitter design and construction.

Proper matching of the different drip irrigation system components is essential. All the
designer needs to do is to make a rational choice on the specific type of emitter, and the
discharge per emitter that will provide the most effective system. The choice depends mainly on
the amount of water application as a function of number of emitters per plant and irrigation run
time. The overall irrigation schedule should be based on the water holding capacity of the soil
and crop water requirement.

The application time must be sufficient to apply the water that has been consumed
since the previous irrigation run. The irrigation run time can be determined after the following are
known: (a) crop water requirement; (b) irrigation interval; and (c) application rate. After which,
the size of pipe to carry the design flow per lateral, sub-mains and mainline are determined.
Then, the total system capacity is determined to meet the design crop water requirement, which
is the basis in selecting the appropriate pump for the system.
 24

A. Crop water requirement

1. Small trees, shrubs and vines

For cacao, coffee, avocado, citrus and grapes, ETc can be determined using the
procedure outlined in Laboratory Exercise No. 3 of AE 163 (Hydrology) using Appendix Table 2 of
Module No. 8 of this course. This ETc has the same value as the crop water requirement (Cwr)
although they refer to different physical quantities as discussed in the previous module. For other
small trees and shrubs, Table 5 can be used.

Since the unit of ETc using the FAO Penman-Monteith equation is in mm per day, convert
this to gallons per day by multiplying it by the root zone area, which can be estimated by
projecting the canopy onto the ground.

2. Vegetables and other field crops

For vegetables and other field crops, the daily water requirement per 100 ft of row crop is
determined by (United States Department of Agriculture-Natural Resources Conservation
Service, 1997):

Cwr = 50 x ETc x S (Eq. 1)

where: Cwr Crop water requirement, gallons/day per 100 ft of row

ETc Crop evapotranspiration, ipd
Average Kc for vegetables is 0.8, which is already factored in in the
conversion factor 50.
S Row spacing, ft
 25

Table 5. Daily water requirements of trees, shrubs and vines

Plant Type Agro- Water Requirement (gallons per day) Based on Plant Canopy Diameter (ft)
ecological 1.0 1.5 2.0 2.5 3.0 3.5 4 5 6 8 10 12 15 20
Zone
Young trees, shrubs, Coastal 0.1 0.2 0.4 0.7 1.0 1.3 1.7 2.7 4 7 11 16 24 43
vines, perennials, flower
beds, thirsty ground Inland 0.17 0.4 0.7 1.1 1.4 2.1 2.8 4.3 6 11 17 25 39 69
cover

Fruit trees, mature Coastal 0.08 0.2 0.3 0.5 0.7 0.9 1.2 1.9 2.7 5 8 11 17 30
ornamental trees
Inland 0.12 0.3 0.6 0.9 1.2 1.5 2.0 3.2 4.8 8 13 17 29 51

Citrus, avocadoes, Coastal 0.07 0.1 0.3 0.5 0.7 0.9 1.2 1.9 2.7 5 8 10 17 30
large shrubs, drought
tolerant trees, shrubs Inland 0.12 0.2 0.5 0.8 1.1 1.5 1.9 3.0 4.4 8 12 17 27 48
and vines, drought
tolerant ground cover

Mature drought tolerant Coastal 0.04 0.1 0.2 0.3 0.4 0.5 0.7 1.1 1.6 2.8 4.3 6 10 17
trees and shrubs,
established native trees Inland 0.07 0.2 0.3 0.4 0.6 0.8 1.1 1.7 2.5 4.4 7 10 16 28

Source: Harmony Farm Supply and Nursery (2018)

 26

B. Irrigation interval

In drip irrigation, water application is normally done on a daily basis. This practice,
however, may cause drought stress to plants particularly when the system breaks down
unexpectedly, or when it needs servicing. Thus, according to the United States Department of
Agriculture-Natural Resources Conservation Service, a 3-day moisture requirement must be
supplied in the root zone as a minimum safety factor (1997).

When the drip irrigation system is desired to operate more frequently or less frequently,
the time per irrigation run can be shortened or stretched longer, respectively, provided the run
time for the latter case should not be more than 18 to 22 hours per day. Increasing the amount
of water applied per irrigation run can be accomplished also by adding more number of
emitters per plant. Note that in drip irrigation it is better to use more low-discharge emitters per
plant than to use a single emitter with high discharge.

C. Application rate

Emitters are designed to discharge low volume of water at low pressure where only a
portion of the root zone is irrigated. Usually, they come in 0.5, 1 and 2 gpm capacity suitable for
vegetables and other field crops, grapes, and small trees and shrubs, respectively. For versatility
of applications, Stryker espoused that a “one-size-fits-all” drip system with 1 gpm emitters can be
designed (1997), which can be used for any crop in any field condition. With this versatile system,
only the frequency of application and irrigation run time are adjusted accordingly to suit various
field applications and requirements.

D. Irrigation run time

Maintaining a fairly constant moisture level in the root zone within the field capacity of
the soil ensures consistent, healthy and vigorous plant growth and development. To attain this,
irrigation water must be applied at the right amount at the right time. The right amount should
be based on the crop water requirement at a given time span. The right time implies as to when
to start and stop water application based on the capacity of the soil to hold the water applied,
Application time can be determined by:

(Eq. 1)

where: T Irrigation run time, hr/day (maximum of 22 hours to provide time for
soil aeration)
Cwr Crop water requirement, gallons per day per plant
Q Discharge rate, gph per emitter
N Number of emitters per plant
 27

E. Net depth of application

The net depth of application is determined by (United States Department of Agriculture-

Natural Resources Conservation Service, 1997):

(Eq. 2)

where: Fn Net depth of irrigation water application, in

C Units conversion factor = 1.604
Q Discharge rate, gph for on-line or point-source emitter, or per foot
run of lateral for in-line /line-source drip tape
N Number of emitters, or total length of lateral, ft
T Irrigation run time or hours of operation per day (maximum of 22
hours and referable less than 12 hours to provide time for soil
aeration)
E Overall field application efficiency, decimal (maximum 90 percent)
A Areas served by the number of emitters, ft2
f Percent of total area to be wetted, decimal
Use the canopy coefficient corresponding the percent canopy
shading. Refer to Table 6.

Table 6. Percentages of ground shade and corresponding canopy coefficients

Ground shaded (%) 10 25 50 60 and above
Canopy coefficient 0.3 0.6 0.9 1.0
Source: United States Department of Agriculture-Natural Resources Conservation Service (1997)

Example 1 – Determining net depth of water application

Problem: Suppose a farmer desires a “one-size-fits-all” drip irrigation system and

commissioned you to design it for use in a loamy sand vineyard located
near the seashore in Agoo, La Union. Determine the net depth of water
application when it is used to irrigate:

a. a newly established drought-sensitive grapes with canopy diameter of

5 ft; and
b. when the canopy diameter will become 10 ft five years later; and

What would be the appropriate emitter type to use when the expected
lowest and highest emitter setting is 10 ft? How will these emitters be
connected to the lateral?

Assume other data, if necessary.

 28

Solution:

A. Net depth of water application when the canopy diameter is 5 feet

Make rational Since on-line emitters can be used not only for
decisions on the small trees, shrubs and vines but for vegetables
specifications of and other field crops as well, the type of the
the different drip irrigation system designed for versatility is = On-line or point-
components source drip
based on industry Note that this on-line drip irrigation system can irrigation
standards: be used not only in vineyards but also for any system
crops in any field applications and conditions,

Based on the recommendation of Stryker

(1997), the “one-size-fits-all” on-line drip
irrigation system can be equipped with emitters
that have a rated discharge (Q) = 1 gph

From Table 5, For drought-sensitive grapes with canopy

determine crop diameter of 5 ft, Cwr from Table 5 = 2.7 gallon per
water requirement day
(Cwr): Note that in this particular item, we used
tabular values of Cwr. In design practice,
however, Cwr is determined using the FAO
Penman-Monteith equation where numerous
climatic, crop, soil and other field factors are
considered.

From Table 4, For loamy sand, the diameter of soil wetted by

determine a single emitter = 3 ft
diameter of soil
wetted per
emitter:

Determine percent Per USDA-NRCS (1997) recommendation, the

of root zone area percent of root zone area to be wetted (p.15) = 25 to 60 %
to be wetted:
Maximum value is assumed = 60 %

Solve for root zone Substituting values in the equation:

area to be wetted
using the area of a
circle (A) formula,
thus: A(circle) = π(5 ft)2/4 = 19.63 ft2

Root zone area to be wetted = 0.60 x 19.63 ft2 = 11.78 ft2

 29

Solve for diameter Substituting values in the equation:

(d) of the root
zone to be wetted d = √4A/ π = √[(4)(11.78 ft2)]/π = 3.87 ft
using the area of a
circle (A) formula,
thus:

Solve for number Substituting values in the equation:

of emitters (N)
needed per plant,
thus:

N = 3.87 ft/3 ft per emitter = 1.26 ≈ 2 emitters/

plant
Note that always round-off to the next higher
number to avoid under-coverage.

Note also that the placement can be opposite

each other, 1.5 ft (18”) away from the trunk,
and 1” above the ground surface.*

Solve for irrigation Substituting values in the equation:

run time (T) using
Equation 1, thus:

T = 2.7 gal/day/
[(1 gph/emitter)(2 emitters)] = 1.35 hrs/day

or 1 hr
21 min/day
Per United States Department of Agriculture-
Natural Resources Conservation Service (1997)
recommendation, a 3-day moisture
requirement must be supplied. Therefore, the

Required irrigation run time (TReqd) = 1.35 hrs/day

x3= 4.05 hrs/day

or 4 hrs
3 min/day

From Table 6, For 60 percent and above canopy shading, f = 1.0

determine canopy
coefficient (f)
corresponding the
percent canopy
shading:
 30

Solve for net depth Substituting values in the equation:

of water
application (Fn)
using Equation 2,
thus:
Fn = [1.604 (1 gph)(2 emitters/plant)
(4.05 hr/day)(0.90)]/[(19.63 ft2)(1.0)] = 0.60 in

or 15 mm

Therefore, the net depth of water application using this on-line or point-source drip irrigation
system with 1 gph emitters is 0.60 inch. Using the same system through the years as the grapes
grow and develop, the net depth of water application remains almost the same. Of course,
as the vines grow and develop they need more water. To meet this increased water
requirement, all the user needs to do is to install additional emitter(s) and adjust the time of
application accordingly. This is illustrated in the succeeding item.

B. Net depth of water application when the canopy diameter is 10 ft

The following conditions in Item (A) above also apply:

 Type = On-line or point-source “one-size-fits-all” drip irrigation system

 Emitter rated discharge (Q) = 1 gph
 Grapes variety = Drought-sensitive
 Soil type = Loamy sand where the diameter of soil wetted by a single emitter is 3 ft
 Maximum root zone area to be wetted = 60 %
 Canopy coefficient = 1.0

From Table 5, For drought-sensitive grapes variety with

determine crop canopy diameter of 10 ft, Cwr from Table 5 = 11 gallons per
water requirement day
(Cwr):

Solve for root zone Substituting values in the equation:

area to be wetted
using the area of a
circle (A) formula,
thus: A(circle) = π (10 ft)2/4 = 78.54 ft2

Root zone area to be wetted = 0.60 x 78.54 ft2 = 47.12 ft2

Solve for diameter Substituting values in the equation:

(d) of the root
zone to be wetted d = √4A/π = √[(4)(47.12 ft2)]/π = 7.75 ft
using the area of a
circle (A) formula,
thus:
 31

Solve for number Substituting values in the equation:

of emitters (N)
needed per plant,
thus:

N = 7.75 ft/3 ft per emitter = 2.58 ≈ 3 emitters/

plant
Note that the placement can be a triangular
pattern around the trunk, 1.5 ft (18”) away from
the trunk, and 1” above the ground surface.*

Solve for irrigation Substituting values in the equation:

run time (T) using
Equation 1, thus:

T = 11 gal/day/
[(1 gph/emitter)(3 emitters)] = 3.67 hrs/day

or 3 hr
40 min/day
Per United States Department of Agriculture-
Natural Resources Conservation Service (1997)
recommendation, a 3-day moisture
requirement must be supplied in the root zone
as a minimum safety factor. Therefore, the

Required irrigation run time (TReqd) = 3.67 hrs/day

x3= 11 hrs/day

Solve for net depth Substituting values in the equation:

of water
application (Fn)
using Equation 2,
thus:
Fn = [1.604 (1 gph)(3 emitters/plant)
(11 hr/day)(0.90)]/[(78.54 ft2)(1.0)] = 0.60 in

or 15 mm

Indeed, the net depth of water application using the same drip irrigation system remains
almost the same at 0.60 inch per irrigation run. Only the number of emitters and irrigation run
time are adjusted proportionally. This is only true when the grapes are already fully developed.
Of course, during the early stages of vineyard establishment, water application is relatively
less. As a drip irrigation system designer, you must prepare an operations manual regarding
this so that the user can make the necessary adjustments when necessary.

* Note that loamy sand is a coarse soil and the emitter spacings indicated above are within
the recommended limits in Table 2.
 32

C. Type of emitter

From Table 1, The following maximum allowable difference in

determine type of elevation between the lowest and highest
emitter to use: emitters shall be used to select the appropriate
type of emitter:

Pressure Variation Equivalent elevation

(psi)* (ft)
4 9.24
6 13.86
10 21.10
15 34.65
22 50.82
* From Table 1
1 psi = 2.31 ft H2O column

Since the expected elevation difference

between the lowest and highest emitters is 10 ft,
the most economical emitter type to use = Non-pressure
compensating rated
at 20 psi operating
pressure
Or, if the vineyard owner is willing to
compromise higher initial investment cost for
higher safety factor, the emitter type can be = Pressure
compensating rated
Note that lower operating pressure will require at 15 psi operating
lower power rating of water pump, which in pressure
turn redound to lower energy cost. Thus, the
supposed higher initial investment cost for
pressure compensating emitters is recouped in
the long run through lower energy
consumption.

Note also that the type of emitter whether

regulate flow or non-regulated flow is not yet
resolved. This will be determined later in
Example 2.
 33

D. Connection of emitters to the lateral

Determine possible Since the number of emitters vary according to

connection growth stage, directly connecting the emitters
options of emitters to the lateral will not be effective. Thus, the
to the lateral: emitters can be either = Directly attached to
the lateral but with
extended spaghetti
tubing
or
Attached at the end
of the spaghetti
tubing

F. Discharge capacity of laterals, sub-mains and mainline

To determine the discharge capacity of laterals, multiply the number of emitters (N) on a
lateral by the rated discharge rate (Q) of the emitter at average operating pressure. Adding the
capacities of all laterals attached to a sub-main and operating simultaneously will give the peak
flow requirement for this sub-main. Subsequently, the peak flow rate for the mainline is
determined by adding the capacities of the sub-mains operating simultaneously.

Designing the pipe system involves the selection of pipe size that is capable to carry the
required flow rate at acceptable pressure head losses due to friction and elevation. The
allowable friction head loss in plastic irrigation pipes made of PVC and PE are given in Appendix
Tables 2 and 3, respectively. In drip irrigation system, the emitters are normally set above and
close to the ground, and the elevation of which does not usually differ much with the elevation
of the pump system. However, in cases where the elevation difference is significant, say 5
percent slope or steeper, the elevation head loss is readily and easily controlled within allowable
limits by any of the techniques in Item I.F on page 10 hereof. For this reason, there is no equation
presented in the succeeding discussion to calculate the elevation head loss, unlike in sprinkler
irrigation system design where the sprinklers are set at a significant height through the risers.

G. Friction head loss in laterals and sub-mains

The following formulas are used to determine head loss due to friction in drip irrigation
piping system (United States Department of Agriculture-Natural Resources Conservation Service,
1997):

1. In laterals and manifolds up to 1½”

P = 0.0006 Q1.75 D-4.75 (L + N Le) Fe (Eq. 3)

 34

where: P Pressure drop in the pipe, psi

Q Total flow rate, gpm or the number of emitters multiplied by the
average flow rate per emitter
D Pipe inside diameter, in (see Table 7)
Note that flexible polyethylene (PE) tube is normally used for laterals
for ease of proper placement of emitters.
L Total pipe length, ft
Le Emitter equivalent length factor to correct for added resistance
from the emitters (see Table 8)
N Number of emitters
Fe Outlet correction factor to account for the discharge through
emitters along the pipe (see Table 9)

Table 7. Plastic pipe diameters

Polyethylene (PE) Polyvinyl Chloride (PVC)
(Any grade) (Standard Dimension Ratio, SDR 26)*
Nominal Diameter Inside Diameter (in) Nominal Diameter Inside Diameter (in)
3/8” 0.375 2” 2.193
15 mm 0.580 2½” 2.655
½” 0.622 3” 3.230
16 mm 0.630 4” 4.154
20 mm 0.800 6” 6.115
¾” 0.824
1” 1.049
1¼” 1.380
1½” 1.610
2” 2.067
* PVC SDR 26 is usually used for drip irrigation; while SDR 21 for sprinkler irrigation.
Source: United States Department of Agriculture-Natural Resources Conservation Service (1997)

Table 8. Equivalent length factor (Le) for typical emitters

Nominal Pipe Diameter Le (ft)*
3/8” 0.9
12 mm 0.6
15 mm 0.4
½” 0.3
 35

Table 8. (continued)
Nominal Pipe Diameter Le (ft)*
¾“ 0.2
> ¾“ 0
* Assume Le = 0 for emitter spacing 20 times Le or more apart. For example, assume Le = 0 for ½”
pipe when emitters are spaced 6 ft (= 20 x 0.3) or farther.
Source: United States Department of Agriculture-Natural Resources Conservation Service (1997)

Table 9. Outlet correction factor (Fe)

Number of Outlets Fe
1 1.00
2 0.65
3 0.55
4 0.50
5 0.47
6 0.45
7 0.44
8 – 11 0.42
12 – 19 0.40
20 – 30 0.38
31 – 70 0.37
> 70 0.36
Source: United States Department of Agriculture-Natural Resources Conservation Service (1997)

2. In 1½” or larger pipes, or small tubings (Hazen-Williams Equation)

P = 4.53 (Q/C)1.85 D-4.8655 L (Eq. 4)

where: C Roughness coefficient

= 140 for polyethylene (PE) pipe
= 150 for polyvinyl chloride (PVC) pipe

H. Pipe size of laterals, sub-mains and mainline

After the flow rate and friction head loss are known, the pipe size of laterals, sub-mains
and mainline can be determined using either Appendix Table 2 (PVC), or Appendix Table 3 (PE).
In selecting the appropriate pipe size, the tabular value must be equal to or less than the design
 36

pressure drop (P) in the pipeline at the corresponding flow rate. If design P is higher than the
tabular value, it can be reduced by selecting a bigger pipe diameter.

The design flow rate must be the basis in the selection of appropriate pump as discussed
in Module No. 7 (Pumps and Pumping) of this course.

I. Filtration system

Perhaps the most limiting factor in the use of drip irrigation system is water quality as it
defines the performance of the emitters. Thus, the filtration system component to prevent
clogging of the emitters must be given special consideration. Table 10 gives the general
recommendation as to the number of media filter tanks and diameter to use based on the
system flow rate. The tanks are connected in series with provision for individual cleaning. Per
Agriinfo.in report, drip irrigation system flow rate usually has a design capacity of 10 to 50 m 3/hr
(44 – 220 gpm) at 10 to 50 psi operating pressure (2015).

Table 10. Number and diameter of media filter tank required based on system flow rate
System Flow Rate (gpm) Number of Media Filter Tank and Diameter
50 2 – 18”
100 3 – 18”
150 3 – 24”
200 – 250 3 – 30”
300 – 400 4 – 30”
450 – 550 4 – 36”
600 – 750 3 – 48”
800 – 1000 4 – 48”
Source: United States Department of Agriculture-Natural Resources Conservation Service (1997)

Example 2 – Determining pipe size of laterals, sub-mains and mainline

Problem: Determine the proper pipe size of laterals, sub-mains and mainline for the
on-line drip irrigation system in Problem No. 1. The system has two-30 m
sub-mains operating one after the other. Each sub-main supplies 16-25 m
laterals with 60 emitters each but only eight laterals can be operated
simultaneously through two 4-feed manifolds. What would be the total
system flow capacity and specifications of the media filter tank?
 37

Solution:

A. Pipe size of laterals

Solve for lateral Substituting values in the equation:

discharge (QLateral),
thus: QLateral = Qemitter x N = (1 gph)(60 emitters) = 60 gph
or
QLateral = 60 gph x 1 hr/60 min = 1.0 gpm

Consider Equation P = 0.0006 Q1.75 D-4.75 (L + N Le) Fe

3, thus:

From Appendix For 1.0 gpm lateral discharge, the minimum or

Table 3, determine most economical pipe size (Dmin) to use = ½ in Ø PE tube
pipe size (D) that
can carry the Question: Will this ½” PE tube work under the
design lateral circumstances given in the problem? This will be
discharge: answered later.

From Table 7, For ½” Ø PE, Dnominal = 0.622 in

determine nominal
diameter (Dnominal):

From Table 8, For ½“ Ø PE, Le = 0.3

determine
equivalent length Note that the spacings between emitters in
factor (Le): Problem No.1, Items A and B can never be
more than 6 ft because the emitters are to be
set 1.5 ft away from the grapes trunk as
recommended. Therefore Le ≠ 0.

From Table 9, For 60 emitters, Fe = 0.37

determine outlet
correction factor
(Fe):

Solve for pressure Substituting values in the equation:

drop in the lateral
tube (P) using P = 0.0006 Q1.75 D-4.75 (L + N Le) Fe
Equation 3, thus:
P = 0.0006 (1.0 gpm)1.75 (0.622 in)-4.75 [(25 m x
3.28 ft/1 m) + (60 emitters)(0.3)](0.37) = 0.212 psi

Since the tabular value for ½”Ø PE tube in

Appendix Table 3 is 0.448 psi which is higher
than the design allowable pressure drop of
0.212 psi, this ½” PE tube will not work properly
under the given circumstances in the problem.
 38

Therefore, a bigger pipe size should be

selected.

From Appendix There are five possible pipe sizes (¾” up to 2” Ø)

Table 3, select a with discharge rate of 1 gpm and friction head
bigger pipe size loss less than 0.212 psi but the smallest one is the
with allowable most economical. Thus, use = ¾ in Ø PE
friction head loss
less than 0.212 psi Note that the allowable friction pressure loss of
and can carry the this ¾” Ø PE tube is 0.124 psi at 1 gpm
required flow rate discharge. Since the required emitter flow rate
of 1.0 gpm: is only 0.5 gpm, this can be reduced by using a
regulated flow type emitters which in effect
further lower the friction loss.

Solve for flow This is in necessary to meet the condition that

velocity (V) in the the ”flow velocity in laterals should not exceed
lateral: 1.5 mps” (Hunter Industries, Inc., 2012).

Convert V = 1.5 m/sec x 100 cm/1 m = 150 cm/sec

Q = 1.0 gal/min x 3.78 li/1 gal x 1000 cm3/1 li x

1 min/60 sec = 63.0 cm3/sec

A = π d2/4 = π (0.824 in x 2.54 cm/1 in)2/4 = 3.44 cm2

Substitute values in the continuity equation:

Q = AV
V = Q/A = (63.0 cm3/sec)/(3.44 cm2) = 18.31 cm/sec

Since the flow velocity in the ¾“ Ø PE tube

carrying 1.0 gpm is less than 1.5 mps or 150
cm/sec, the condition is met.

Note that the other conditions regarding

variation in operating pressure, and variations in
discharge between emitters and within
irrigation zones are already factored in in
Equation 3.

Therefore, the size of the laterals should be ¾” Ø PE tube with 60 regulated flow type emitters
per lateral, either a non-pressure compensating rated at 20 psi operating pressure, or a
pressure compensating rated at 15 psi operating pressure, and either directly attached to the
lateral but with extended spaghetti tubing, or attached at the end of the spaghetti tubing
which is connected to the lateral.
 39

B. Pipe size of sub-mains

Solve for sub-main Substituting values in the equation:

discharge
(Qsub-main), thus: Qsub-main = Qlateral x No. of laterals = 1 gpm x 8 = 8 gpm

From Table 7, The minimum pipe diameter for sub-mains = 2 in Ø PVC

determine
minimum pipe size Question: Will this 2ӯ PVC pipe work under the
used for sub- circumstances given in the problem? This will be
mains: answered later.

From Appendix Allowable friction head loss of 2” Ø PVC pipe at

Table 2, determine 8 gpm discharge = 0.11 ft H2O
allowable friction
head loss of 2” Ø Note that the tabular values are for standard
PVC pipe that dimension ratio (SDR) = 21. To find friction head
carries 8 gpm: loss in PVC pipe having an SRD = 26, multiply
the values in the table by 0.91, thus:

0.11 x 0.91 = 0.10 ft H2O

Convert to psi = 0.10 ft H2O x 1 psi/2.31 ft H2O = 0.043 psi

Solve for pressure Substituting values in the equation:

drop in PVC sub-
mains using P = 4.53 (Q/C)1.85 D-4.8655 L
Equation 4, thus:
P = 4.53 (8 gpm/150)1.85 (2.193)-4.8655 (30 m x
3.28 ft/m) = 0.043 psi

Since the corrected tabular value for 2ӯ PVC

pipe in Appendix Table 2 is 0.10 ft H2O or 0.043
psi which is equal to the design allowable
pressure drop of 0.043 psi, this 2” PVC pipe will
work properly under the given circumstances in
the problem.

Therefore, the pipe size of the sub-mains should

be = 2 in Ø PVC

Therefore, the size of the two-30 m sub-mains should be 2” Ø PVC pipe grade SDR 26 that
supplies 8 gpm to eight-25 m laterals with 60 emitters each.
 40

C. Pipe size of mainline

Since the on-line drip irrigation system has only two-30 m sub-mains that operate one after the
other, the mainline capacity is the same as the 8 gpm capacity of the sub-mains. Therefore,
the mainline pipe size should also be 2” Ø PVC Grade SDR 26.

D. Filtration system

Solve for total Since there is only one sub-main operation at

system capacity, one time,
thus:
Qsystem = Qmain = Qsub-main = Qlateral = 8 gpm

From Table 10, For system flow rate < 50 gpm, filtration system
determine number required = One-18” Ø media filter
and diameter of tank
media filter tank Note that this system filtration requirement is
required: over and above the disc filters installed in each
lateral.

The final design specifications of the drip irrigation system under consideration are, as follows:

Type of drip irrigation system: On-line or point-source

Application: Vineyard
Field condition: Sloping land near the seashore

Emitter:
Type and operating pressure: Either
 20 psi non-pressure compensating, or
 15 psi pressure compensating
Flow characteristics: Regulated flow
Capacity: 1 gpm
No. of emitters per lateral: 60 emitter
No. of emitters per plant: 2 to 3
Connection to lateral: Either
 directly attached to the lateral but with
extended spaghetti tubing, or
 attached at the end of the spaghetti
tubing
Placement: One inch above the ground and at least
1.5 ft away from trunk
Allowable elevation difference of emitters: Not to exceed 10 ft

Lateral:
Size and type: ¾” Ø PE tube
No. of laterals 16
Length: 25 m per lateral
No. of laterals operating simultaneously: 8
Connection to sub-mains: Two 4-feed manifolds
 41

Sub-mains:
Size and type: 2” Ø PVC pipe
Grade: SDR 26
No. of sub-mains: 2
Length: 30 m per sub-main
No. of sub-mains operating simultaneously: 1

Mainline:
Size and type: 2” Ø PVC pipe
Grade: SDR 26
Length: Not to exceed 30 m

Filtration system:  One-18” Ø media filter tank, and

 Disc filter for each lateral
 42

Passing Score: 75 points

Due Date: ASAP but not later than ____________________________________.
Penalty for Late Submission: 5 points deduction per day of delay

If space is not enough, continue at the left-side directly opposite the item being answered.

1. Differentiate drip irrigation from sprinkler irrigation and surface irrigation methods such as
basin, furrow and border strip (15 pts).
 43

2. What are the general design criteria for irrigation systems? (5 pts)

3. What are the advantages and disadvantages of drip irrigation system? (5 pts)
 44

4. Draw a field layout of a drip irrigation system and identify the different components.
(10 pts)
 45

5. Prepare a list of conditions for proper operations of the emitters, laterals, sub-mains, ad
mainline (10 pts).

6. Design a drip irrigation system for off-season tomato grown in a greenhouse in terms of
(40 pts):

 Type of system application,

 Emitter type and operating pressure,
 Emitter flow characteristics,
 Number of emitters per plant and per lateral,
 Connection of emitter to lateral,
 Placement and spacing of emitters,
 Allowable elevation difference between the lowest and highest emitters,
 Tube size and type of laterals,
 Length of laterals,
 Number of laterals operating simultaneously,
 Connection of laterals to the sub-mains,
 Pipe size, type and grade of sub-mains and mainline,
 Number of sub-mains,
 Length of sub-mains and mainline, and
 Filtration system.

Assume other data, if necessary.

46
47
 48

7. Prepare the bill of materials of Item No. 6 and determine the total cost of the designed drip
irrigation system (15 pts).

BILL OF MATERIALS

_______________________Sprinkler Irrigation System

(specify type)

Qty Unit Specifications Unit Cost Total Cost

(PhP)* (PhP)*

* Prices as of November 2018

 49

Appendix Table 1. Quality of irrigation water used in drip irrigation

Parameter Clogging Potential
Low Moderate High
A. Physical (suspended solids, ppm) < 30 30 – 100 > 100
B. Chemical
pH < 7.0 7.0 – 7.5 > 7.5
Total dissolved solids, ppm < 500 500 – 2,000 > 2,000
Iron, ppm < 0.1 0.1 – 1.5 > 1.5
Manganese, ppm < 0.1 0.1 – 1.5 > 1.5
Calcium, ppm < 40 40 – 80 > 80
Carbonate density, ppm < 150 150 – 300 > 300
Hydrogen sulfide, ppm < 0.2 0.2 – 2.0 > 2.0
C. Biological (bacteria population, #/ml) < 10,000 10,000 – 50,000 > 50,000
ppm = parts per million = mg/li
Source: Hunter Industries, Inc. (2012)
 50

Appendix Table 2. Friction head loss in plastic irrigation pipes made of polyvinyl chloride (PVC)*
Flow Rate Pipe Size
(Q, gpm) 1” 1-1/4” 1-1/2” 2” 2-1/2” 3” 3-1/2” 4” 5” 6”
Friction Head Loss (Pf, feet water column)
2 0.15 0.04 0.02
4 0.54 0.17 0.09 0.03 0.01
6 1.15 0.37 0.19 0.06 0.02
8 1.97 0.63 0.32 0.11 0.04 0.01
10 2.98 0.95 0.49 0.16 0.06 0.02 0.01
15 6.32 2.03 1.04 0.35 0.14 0.05 0.02 0.01
20 10.79 3.46 1.78 0.60 0.23 0.09 0.04 0.02
25 16.30 5.22 2.70 0.91 0.36 0.13 0.07 0.04 0.01
30 22.86 7.32 3.78 1.27 0.50 0.19 0.10 0.05 0.02
35 9.75 5.03 1.70 0.67 0.25 0.13 0.07 0.02 0.01
40 12.46 6.46 2.18 0.86 0.32 0.17 0.09 0.03 0.01
45 15.51 8.02 2.71 1.07 0.40 0.21 0.12 0.04 0.01
50 18.87 9.75 3.30 1.30 0.49 0.25 0.14 0.05 0.02
55 22.48 11.64 3.94 1.54 0.59 0.30 0.17 0.06 0.02
60 13.64 4.62 1.81 0.69 0.36 0.20 0.07 0.03
65 15.85 5.36 2.10 0.80 0.41 0.23 0.08 0.03
70 18.19 6.14 2.42 0.92 0.47 0.27 0.09 0.04
75 20.65 6.99 2.75 1.06 0.55 0.31 0.11 0.04
80 23.28 7.86 3.10 1.19 0.62 0.35 0.12 0.05
 51

Appendix Table 2. (continued)

Flow Rate Pipe Size
(Q, gpm) 1” 1-1/4” 1-1/2” 2” 2-1/2” 3” 3-1/2” 4” 5” 6”
Friction Head Loss (Pf, feet water column)
85 8.81 3.47 1.33 0.69 0.39 0.14 0.05
90 9.79 3.85 1.48 0.77 0.43 0.15 0.06
95 10.82 4.25 1.64 0.85 0.48 0.17 0.07
100 11.89 4.69 1.80 0.93 0.52 0.19 0.07
110 14.21 5.59 2.14 1.11 0.63 0.22 0.09
120 16.69 6.56 2.52 1.31 0.74 0.26 0.10
130 19.35 7.63 2.92 1.53 0.85 0.30 0.12
140 22.21 8.73 3.36 1.75 0.98 0.35 0.14
150 9.94 3.82 1.99 1.11 0.40 0.16
* Based on standard dimension ratio (SDR) = 21. To find friction head loss in PVC pipe having a standard dimension ratio other than 21,
the values in the table should be multiplied by the appropriate conversion factor shown below, which can be interpolated:

SDR Conversion Factor

13.5 1.35
17 1.13
21 1.00
26 0.91
32.5 0.84
41 0.785
51 0.75

Abridged from United States Department of Agriculture-Natural Resources Conservation Service (1997)
 52

Appendix Table 3. Friction head loss in plastic irrigation pipes made of polyethylene tube (PE)*
Flow Rate Pipe Size (in)
(Q, gpm) ½ ¾ 1 1¼ 1½ 2 2½ 3 4
Friction Head Loss (psi)
1 0.448 0.124 0.038 0.010 0.005 0.001
2 1.76 0.448 0.138 0.036 0.017 0.005
3 3.72 0.948 0.293 0.077 0.036 0.011
4 6.34 1.61 0.498 0.131 0.062 0.018
5 9.58 2.44 0.753 0.198 0.094 0.028
6 13.42 3.42 1.06 0.278 0.131 0.039
7 17.85 4.54 1.40 0.370 0.175 0.052
8 22.85 5.82 1.80 0.473 0.224 0.066
9 28.42 7.23 2.23 0.588 0.278 0.082
10 34.53 8.79 2.72 0.715 0.338 0.100
11 10.48 3.24 0.853 0.403 0.119
12 12.32 3.80 1.000 0.473 0.140
13 14.28 4.41 1.160 0.549 0.163
14 16.38 5.06 1.330 0.629 0.187
15 18.61 5.75 1.510 0.715 0.212
16 20.97 6.48 1.710 0.806 0.239
17 23.46 7.25 1.910 0.901 0.267
18 8.06 2.120 1.000 0.297
19 8.90 2.340 1.110 0.328
 53

Appendix Table 3. (continued)

Flow Rate Pipe Size (in)
(Q, gpm) ½ ¾ 1 1¼ 1½ 2 2½ 3 4
Friction Head Loss (psi)
20 9.79 2.58 1.22 0.361 0.152
22 11.68 3.07 1.45 0.431 0.181
24 13.72 3.61 1.71 0.506 0.213
26 15.91 4.19 1.98 0.587 0.247
28 18.24 4.80 2.27 0.673 0.283
30 5.46 2.58 0.764 0.322
32 6.15 2.90 0.861 0.363
34 6.88 3.25 0.964 0.406
36 7.65 3.61 1.070 0.451
38 8.45 3.99 1.180 0.499
40 9.29 4.39 1.300 0.548 0.191
42 10.17 4.80 1.420 0.600 0.209
44 11.08 5.24 1.550 0.654 0.227
46 12.04 5.68 1.690 0.710 0.247
48 6.15 1.820 0.768 0.267
50 6.63 1.970 0.828 0.288
52 7.13 2.110 0.891 0.310
54 7.65 2.270 0.955 0.322
56 8.18 2.430 1.020 0.355
 54

Appendix Table 3. (continued)

Flow Rate Pipe Size (in)
(Q, gpm) ½ ¾ 1 1¼ 1½ 2 2½ 3 4
Friction Head Loss (psi)
58 8.73 2.59 1.09 0.379
60 2.76 1.16 0.403 0.108
62 2.93 1.23 0.429 0.114
64 3.11 1.31 0.455 0.121
66 3.29 1.38 0.481 0.128
68 3.47 1.46 0.508 0.136
70 3.66 1.54 0.536 0.143
75 4.16 1.75 0.610 0.162
80 4.69 1.98 0.687 0.183
85 5.25 2.21 0.768 0.205
90 5.83 2.46 0.854 0.228
95 6.45 2.72 0.944 0.252
100 7.09 2.99 1.040 0.277
* For polyethylene tube meeting ASTM-D-2239. Values are based on Hazen-Williams formula, C=140. Pressure loss in psi per 100 feet.
Abridged from United States Department of Agriculture-Natural Resources Conservation Service (1997)
 55

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