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"KNOWLEDGE WORKING FOR MISSOURIANS" Irrigation of Vegetable
and Small Fruit Crops
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by
Bob Schultheis, Extension Natural Resource Engineering Specialist


Introduction

Irrigation is important to profitable production of vegetable and small fruit crops in Missouri. Despite
an average annual rainfall of 36"-44" statewide, the distribution and timeliness of the rainfall is erratic,
and soil conditions in many areas are only marginally acceptable. Rocky soils, sandy soils, and those
soils which have shallow, impermeable (fragipan) layers often present special problems for keeping a
vegetable or small fruit crop in steady production.

Every year the Missouri Ozarks has a 3 to 4 week (or longer) period in which the crop water use far
exceeds the rainfall. Soils in the Ozarks will typically hold 1½ to 2 inches of water per foot of soil
depth, with water intake rates of 0.3-0.7 inches per hour. Available moisture (that the plants can use)
is about 75 percent of the soil's water holding capacity.

While a deep, loamy soil can sometimes provide enough moisture storage to carry a crop through a 2-
3 week dry spell, the typical two-foot-deep gravelly, clay soils of the Ozarks only provide a 9-15 day
supply of water. This is based on an evapo-transpiration rate of 0.25 inches per day. The summer
droughts of 1980, 1983 and 1988 showed crop water use rates at least one-third higher than this.
Shallow-rooted crops, such as blueberries, are especially at risk from dry weather. The native saying
that "the Ozarks is always just two weeks away from drought" is very appropriate.

Growers have many options available to them when designing their irrigation systems. Sprinklers are
best suited to applications where the grower needs to stretch capital equipment costs over as many
acres as possible. A tradeoff between equipment and labor inevitably occurs. Table 1 shows the
labor requirements to be expected for various types of irrigation systems.


Table 1. Irrigation System Labor Needs

Equipment
Hours/Acre per
Application
Traveling Gun Sprinkler
With Buried Mainlines
Without Buried Mainlines

0.15
0.25
Towline Sprinkler
0.54
Side-Roll Sprinkler
0.55
Hand-Carry Portable Sprinkler
0.92
Solid-Set Sprinklers or Trickle System
(1 hour/day)

Sprinkler Irrigation

Hand-carry and solid-set sprinklers do offer the possibility for frost control. Application of water to the
crop during hours of sub-freezing temperatures can give enough protection to keep the crop alive.
Efforts in frost control have been most successful for low-growing crops such as strawberries, but
some larger crops have also been protected. Overloading with ice is a major concern if prolonged
water pumping is necessary.

When considering frost control, the water supply must have continuous pumping capacity of 45 to 60
gallons per minute (GPM) for each acre, at 40-50 pounds per square inch (psi) pressure to protect
down to 24 to 20 degrees F. This compares to 4 to 7 GPM of water pumping capacity for each acre of
normal hand-carry or solid-set sprinkler irrigation.

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Often the grower has a small pond he would like to use for the sprinkler irrigation water source. A
"small pond" in the Ozarks is usually about 100 feet in diameter and 8 feet deep. Total capacity is
about 280,000 gallons when the pond is full, but only about half is available for pumping. The rest is
lost to evaporation or seepage or is not pumpable. The 140,000 gallons that is pumpable will only last
about 6 days if irrigating at 20 GPM (gallons per minute) for 20 hours per day. Consider, too, that the
water is usually least available when it is most needed during dry weather. Therefore, these ponds
are not reliable water sources.

Sprinklers will be most effective in situations where water is available from large reservoirs or creeks
with year-round flow. These situations present special filtering problems for trickle irrigation but allow
pumping at higher sprinkling rates. The type of sprinkler system and pumping schedule will affect the
minimum pumping capacity of the system. For example, if irrigating 5 acres over a 20-hour period, a
4 GPM per acre rate is possible. Reducing pumping time from 20 hours to 8 hours increases
pumping capacity needed to 10 GPM per acre. Thus, pumping requirements increase when pumping
downtime increases.

Drip Irrigation

Drip irrigation (also known as trickle irrigation, micro-irrigation, or low-volume irrigation) offers an
excellent alternative to sprinkler irrigation for vegetable and small fruit growers. Trickle irrigation
systems typically use 30-50 percent less water than sprinkler systems, and the water is rationed to the
plants as they need it. This reduces evaporation, particularly on hot, windy days, and enables the
grower to only water the desired plants and not the row alleys or roadways. Weed control is therefore
simplified, and workers are able to do fieldwork while the irrigation system is running.

The system's almost continuous operation at low flowrates and operating pressures allows the grower
to irrigate with lower-cost, smaller pumps through smaller, lightweight pipes which may deliver as little
as 15 or 20 GPM. The irrigation pumping requirement drops from the 4 to 7 GPM per acre at 40-50
psi typical for sprinklers to 2

to 5 GPM per acre at 6-20 psi for trickle irrigation systems. So a 15 GPM
capacity water well solely dedicated to supplying 3-4 sprinklers may be used to trickle irrigate 2-4
acres of vegetables or small fruits, with enough extra capacity to meet normal household needs.

Drip Irrigation Components

To better understand how a drip irrigation system works, it is useful to know what each of the basic
components of the system are. Figure 1 shows a sketch of a sample layout for a drip irrigation
system. Many types of equipment are available but the buyer should consider the following points
when purchasing a system.

a)
Water supply -- A well is best because algae and sediment problems will be minimized. The
grower may have to treat for excess water hardness or pH (a measure of alkalinity/acidity) in some
areas. Clear spring water is a good second choice. Any visible particles in the water will eventually
plug the system. Locating the water supply near the top or center of the field will reduce pump and
piping costs. Figure 1. Example Layout of Drip Irrigation System


b) Pump

A pump of sufficient capacity is needed to meet crop needs of one irrigation block.
The pump is usually powered by electricity to more easily automate the system. The pump must be
able to lift water to the highest point in the field and then be able to provide the required pressure for
the emitter which has been selected.

c)
Filter System -- A filter capable of removing sediment particles which are too large to pass
through the emitters must be installed as the first component in the system. Manufacturers will
specify how fine the mesh needs to be. Most require a filtering material equivalent to a screen in the
range of 100 to 150 mesh. The grower should provide a means for flushing the filter screen free of
debris particles. This can be done manually for smaller acreages (1/2 to 1 acre), but automatic
backflushing is preferred for larger acreages.

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d)
Pressure Regulation - The number of regulators will depend on field slope and pipe layout.
In-line pressure regulators are available. Many growers may prefer to use conventional pressure
tanks similar to those in their houses. Pumps can often be matched to a pressure tank system more
easily and local plumbers are accustomed to supplying this type of service. A pressure tank will
control the number of times that the pump will cycle per hour to prevent motor burnout. Each pump
manufacturer specifies the maximum permissible cycles per hour. Some pumps should not cycle for
more than 4 to 6 times per hour so the plumber should be careful in sizing components. Remember
that the higher the flowrate, the larger the pressure tank needs to be.

e)
Solenoid Valves -- These small, low-voltage, electrically-operated water valves determine
when water will be sent to each irrigation block. Ordinarily, a commercial system will have three to
five blocks so that the system can be operated nearly 24 hours a day. Each block will typically run for
three to five hours per day to meet mature crop needs. New plantings may need only 15 minutes per
day. It's best to install these valves above ground level wherever possible to assure easy access for
maintenance.

f)
Controller -- The controller consists of an electric time clock and switching devices to control
the solenoid valves. This device is often installed near the pump. It is sometimes mounted on a
board with the solenoid valves.

g)
Mainline -- The mainline consists of pipe (usually PVC plastic) which carries water to each
irrigation block. Typically 2" to 3" in diameter, these pipes should be selected with