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DRIP IRRIGATION SYSTEMS

An Israeli engineer Symcha Blass is credited with inventing
"drip" or trickle irrigation in the 1940s. Since that time, it
has come to be regarded as the least resource intensive and most
beneficial form of irrigation. Drip irrigation differs from
other forms in that it applies water at a very slow rate,
directly to the root system of the crops, thus reducing the
amount of runoff and evaporation. This makes more water
available to the growing plant. Because of the slow rate of
application (.05-.10 inches/hour), drip irrigation is done much
more frequently, in some areas every day. This keeps the
moisture content near the plant at more constant levels
decreasing the likelihood of moisture shock. In this section, we
address the specifics of planning and implementing a drip
irrigation system, as well as discussing some ethical questions
about irrigation.

1. Drip Irrigation Advantages & Disadvantages

Advantages

* Requires 30 - 50% less water than other methods.

* Requires less energy for pumping, Both volume and
pressure is less than other systems.

* Because flow rate and pressure are lower, smaller pipe and
lower pressure rated pipe can be used.

* Enables better control of soil moisture and higher
precision in moisture placement

* Plant foliage and fruit doesn't get wet which helps
control pests and prevents disease.

* Area between the rows is not irrigated, reducing weed
problems.

* Less labor intensive than other irrigation methods.

* Field operations, such as harvesting, can be continued
while irrigating.

* Fertilizer can be supplied through the system.

* Irrigation can be done on sloping terrain without runoff
and accompanying erosion.

* Wind does not affect the wetting pattern.

[Diagram 5 Typical Drip Irrigation Setup]

Disadvantages

* Requires very clean water. The small openings or paths in
an emitter can be clogging by soil particles, organic matter,
minerals and sediment in the water and algae growth.

* Soil moisture distribution is limited. The volume of the
soil wetted is affected by soil type, emitter or orifice
discharge and distance between emitters.

* System is subject to damage by animals, rodents, insects,
and machines.

* Generally, not suited or recommended for close growing
crops like grasses.

* Requires a higher level of design and management than
other systems.

* May require higher initial and annual investment than
other types of irrigation.

2. Soil Water Loss

Soils loose water by
1. surface evaporation and
2. plant absorption
a. some to growth and development
b. some lost through transpiration

Evapotranspiration Rate (ET) is a measure of how quickly
soil is losing water. It is calculated using average
temperature, wind speed, humidity, precipitation, and percent of
ground covered by foliage and is expressed in units of inches (or
millimeters) per unit time. Plants transpire considerably to
remain cool in dry sunny weather. Evapotranspiration rates can
vary from 0.10 - 0.25 inches per day from May to September. This
water is removed daily from soil moisture storage unless replaced
by rain or irrigation. Root depth is also an important factor
determining the depth (and volume) of soil from which moisture is
withdrawn. Shallow plants have fewer moisture reserves than
deeply rooted varieties and need more frequent irrigation.

In temperate areas, such as Appalachia, continuous
irrigation will probably not be necessary. Moisture should only
be supplemented as necessary during the middle of the growing
season, when crop moisture requirements are greatest and the
weather is driest. Many people begin irrigation in late May or
early June. Checking the soil for moisture content on a regular
basis will help ensure that irrigation is begun early enough to
avoid moisture shock to the plants.

Field Capacity is the amount of water held by a soil after being
wetted by rain or irrigation, once excess water has drained away.

SOIL MOISTURE HOLDING CAPACITIES
(in inches of water held per foot of soil depth)

very coarse sands 0.40 - 0.75
sandy loams 1.00 - 1.50
very fine sandy loams and
silt loams 1.50 - 2:30
sandy clays and clays 1.60 - 2.50
peats and mucks 2.00 - 3.00

Coarse soils have little reserve moisture.

NOTE: Using mulch in conjunction with drip irrigation can greatly
reduce moisture lost to evaporation.

3. Planning a Drip Irrigation System

There are three important elements in creating a successful
system: proper planning and design, proper installation, and
proper operation & maintenance. After deciding to install a drip
irrigation system, the first step is making a field plan.
Factors to consider include the size and shape of the area,
elevation contours, and location of water source relative to the
field. Other considerations are soil type(s), climate, power
supply, crop, row and cross row spacing, single or double row and
field layout. Of particular importance are:

1. The moisture storage capacity of soil
and the soil and plant moisture losses.
2. Moisture considerations for each crop
3. System components must be identified
and designed. The system must be laid
out for the most efficient water delivery.

Of course any decisions will be shaped by budget. In
selecting a system, it is important to choose one which is
flexible enough to meet changing conditions. However, it is not
advised to purchase more equipment than is needed. Companies
that sell drip equipment commonly either design systems for
customers or can assist in design. Before developing a plan make
a scale drawing of the garden or field to be irrigated to
facilitate determining how many feet of tubing will be required
and the number and type of emitters to be used.

4. Equipment

Water Supply

Potential water sources for drip irrigation systems included
ponds, streams, creeks, ditches, groundwater and municipal
treated water. The cleaner the water, the less filtration will
be required and the lower the potential for clogging.
Groundwater is generally high quality, but may contain sand,
sediment or chemical contamination. Running surface water
(streams, creeks, etc.) has a high concentration of suspended
particles. Large ponds will generally have lower levels of
suspended solids. Many resources recommend using municipal water
for drip irrigation because it has low particulate levels and
because chlorine in the water reduces the potential for algae
growth in the irrigation line (See Ethical Considerations). Any
potential water source should be tested for chemical
contamination before making an irrigation plan.

Pump and Power Source (or Gravity)

Most irrigation systems will require a pump and a power
unit. Extremely hilly or mountainous terrain may allow for the
use of gravity. However, in less ideal situations two types of
pump are recommended. For pumping water from a source that is on
the surface or groundwater depths of less than 15 feet, the
straight centrifugal or self-priming centrifugal pump should be
used. If the source is more than 20 feet deep, a submersible or
deep-well turbine pump is recommended.

There are several factors to be considered when choosing a
pump including total pressure of the system, volume of water
required, and the type of power unit. The total dynamic
(pressure) head is calculated using the elevation head (total
difference in elevation between the water source and the higher
emitter location), friction head (pressure drop resulting from
water flowing through the pipe, fittings, and valve) and pressure
head (pressure required at the most distant emitter).

Filters

All types of drip irrigation equipment and almost all water
sources will require filtration. Filters remove suspended
particles in water, but have no effect on dissolved minerals and
bacteria. There are two types of filters. Primary filters are
located immediately downstream near the pump. Secondary filters
are located nearer the field and several secondary filters may be
used.

Filters can be installed in ground water sources on the
intake side of submersible and turbine pumps. These will remove
sand and other suspended particles which could damage the
equipment. Pump discharge filters include sand filters, screen
filters, "y" type line strainers, line cartridges and sand
separators. The type of filter depends on the type and size of
the emitter-orifice and the water source. The manufacturer's
recommendations will be invaluable in making this decision. A
good rule is to use a filter that removes all particles greater
than 1/10 the diameter of the smallest passageway in the system.

Strainers (or screen filters) -- Most pumps have a strainer
on the suction pipe for removal of large particles.
Effectiveness of the strainer depends on the slot or screen
opening and the length or diameter of the strainer. Floating the
strainer 18 - 24 inches below the water surface will reduce the
volume of suspended particles (in surface sources). These
filters will only remove small amounts of sand and suspended
particles and should not be used if large quantities of algae are
present. Fine screens will clog more quickly than coarse ones.
To reduce the frequency of cleaning the filter install two or
more in parallel or use a screen with a large surface area.

Pressure regulators and gauges

The pressure regulator reduces system pressure to proper
operating pressure. Regulators need to be positioned on either
side of the filter and can indicate how well the filter is
functioning. A decrease in outlet pressure signals a clogged
filter. Pressure relief valves are not always necessary,
especially on flat terrains or at low pressure. They will
usually be located at higher points in the lines and at the ends
of the lines.

Vacuum relief gauges (vacuum breakers) prevent potential
contamination of a water source, which can occur if negative
pressure occurs in the line. Vacuum relief gauges are a must if
municipal or well water is used. A 1-inch vacuum relief valve
for each 25 gallons per minute of flow should be installed
downstream from the valve that controls flow into the irrigation
area.

Control valves

All systems need an on-off valve. The simplest type of
valve is a gate valve. In very small systems where pressure
fairly constant, a gate valve can be used to regulate pressure.
More sophisticated solenoid or hydraulically operated valves are
necessary to automate the operation or to zone fields. Zoning
allows separation of crops according to water needs, providing
water more frequently to zones with higher water requirements.
Solenoid valves can be activated manually but usually use an
automatic controller.

Distribution piping

The main water line carries water to the field lateral
system. It can be a length of ordinary garden hose or flexible
1/2-inch PVC tubing. The main line can be run underground, out
of sight and protected from the sun. With large drip systems it
may be necessary to run more than one main line to maintain
consistent pressure. Laterals of 1/2-inch to 3/4-inch
polyethylene pipe may be used for distribution from the main line
to the rows (13-16mm - 19-24mm).

Emitters or local delivery

In drip (trickle) irrigation water is applied in low volume
at low pressure. Because of the low volume and pressure, the
emission device used must be able to counteract pressure
differences caused by topography and friction loss and yet not
become clogged.

There are two types of emitters:

* Line Source -- Drip tubing

A line source emitter consists of holes (orifices) or tubes
of uniform size and spacing down the length of a single or double
chamber tube or small openings in a porous tube. This type of
emitter is used with closely spaced row crops and is commonly
used with annual row crops. Discharge rates for line source
emitters are normally given in gpm (gallons per minute) per 100
ft of pipe with a variance of ñ 0.1 - 1.2 gpm. The spacing of
the plants and orifices determines the volume of water delivered.
Most of these systems use PE (polyethylene) plastic pipe. Water
pressure ranges from 2-30 psi, (usually 15 psi). Row length is
limited to 1000 ft or less with installation on flat terrain, on
the contour or gentle slopes. If installed on sloping ground,
the laterals should run downhill from the supply (header) pipe.
The pipes are installed on or below the ground surface. The in-
field line source laterals are designed to be used only once.
The difficulty involved in retrieval and storage, the likelihood
of clogging, and the low cost of replacing tubing each year make
efforts to reuse tubing inadvisable.

* Point Source

Point source emitters are usually used on tree and vine
crops or ornamentals which are not spaced closely. The
individual emitter may be attached to a pipe or installed in-
line. Line pressure is dissipated through the emitter in several
ways to achieve the desired low flow rate: long narrow paths,
vortex chambers, small orifices and a variety of other means.
Some emitters will allow medium sized particles to be passed and
flushed from the system. Others are self flushing at low
pressures but others may have to be manually flushed
periodically. Operating pressure for point source emitters is
between 5 - 60 psi with the normal range being 10-25 psi. Flow
typically ranges from 0.5 - 2.0 gph (gallons per hour). Some
systems have discharge rates up to 10 gph. Most point source
systems require 100-200 mesh filtration but there are which
require as little as 30 mesh filtration. Conventional point
source emitters have uniform discharge over a narrow range of
pressure. When point source emitters are installed on rolling
topography, more expensive pressure compensating emitters are
recommended.

Controllers and moisture measuring devices

In regions where frequent irrigation is necessary many
growers automate their drip irrigation systems. Controllers,
which may consist of a simple time clock, a volume meter or
expensive computer equipment are used to turn the system on and
off automatically. Highly automated systems may even use soil
moisture measuring equipment to control the operation.

Hard Water Tip

In systems with point source emitters, hard water can
precipitate calcium carbonate which eventually causes clogging.
Rather than discarding clogged emitters, they should be soaked in
vinegar. This will dissolve deposits and extend the life of the
emitters.

5. Ethical Considerations

In certain climates, or at certain times of the year, the
decision to irrigate can mean the difference between food on the
table or losing the crop. However, with rising concern over the
availability and safety of surface and groundwater stores, how
and when the grower chooses to irrigate can have far-reaching
impacts. In the United States a major portion of available water
is used for agricultural irrigation. Because of extensive
chemical contamination -- part of the high price we pay for our
industrialized society -- safe water for human consumption is at
a premium. For this reason water uses must be prioritized, with
drinking water heading the list. Drinking water should not be
used for irrigation. Some say that drip irrigation is best
performed with clean municipal water. Of drip irrigation water
sources municipal water is obviously listed most desirable and
running surface water as the least. In rural areas where other
sources of water are available, (ponds, lakes, streams, etc.)
using sprinkler systems rather than drip irrigation is a more
common practice.

Concerns have been raised about amount of waste generated in
the use of drip irrigation systems. Although T-tapes work well,
they can generally only be used one season, after which they are
pulled up and discarded. It has been suggested that tapes could
be used a second season if great care were taken in pulling them
out of the field; this seems questionable. Tapes are usually
damaged beyond reuse simply from wear and tear in the field. If
left on the surface, heavy rains and leaf fall almost inevitably
result in clogging and the exposure to weather and sun damage the
plastic. Buried, the tubes are still exposed to clogging from
rain carried particles, as well as damage from rodents and
insects. If solid piping is used, as in point source irrigation
for tree crops, damage from external sources is less likely.
However, drip irrigation equipment placed in the ground to be
used with tree crops, berries, grapes, etc. is expected to stay
in the ground for the life of the crop -- usually 10 - 20 years.
This is not an option for annual crops.

--------------------------------

REFERENCE AND RESOURCES

Bauder, James and Darnell R. Lundstrom. Tensiometers: Their Use
Installation and Maintenance. Fargo: North Dakota State
University Extension Service, 1977.

Nick, Jean M.A. and Fern Marshall Bradley, eds. Growing Fruits
and Vegetables Organically. Emmaus, PA: Rodale Press, 1994.

Ross, David S. Trickle Irrigation for Fruit Crops in Maryland
(Agricultural Engineering FACTS 105). College Park:
University of Maryland Cooperative Extension Service,
1979.

Sanders, Douglas C. An Introduction to Drip Irrigation for
Vegetables (Horticultural Information, Leaflet No. 33-C).
Raleigh: North Carolina Extension Service, Department of
Horticultural Science, North Carolina State University, 1988.

Sneed, Ronald E. "Trickle Irrigation" Water Management,
Biological and Agricultural Engineering. Raleigh: North
Carolina Agriculture Extension Service, North Carolina
State University.