Horticulture Principles and Practices

growers should take to determine and mitigate irrigation water quality. The first is to
select a good source of water. The second is to have the water tested for contaminants
and relative concentrations of nutrients. And the third step is to use filtering technology
in all the necessary irrigation supply lines (i.e. lines that will be used for drip/trickle
irrigation or overhead sprinkler/emitter delivery).
Compared to other forms of irrigation such as furrow or overhead irrigation, drip
irrigation is 90% efficient in delivering the amount of water applied to the soil compared
to approximately 50% with the other forms of irrigation.
Drip tape is manufactured with specific characteristics based on thickness of the
plastic tape, distance between orifices (holes where water exits through the drip tape),
amount of water volume the drip tape will deliver (standard delivery rate is 0.45
gallons/minute/100 linear feet of tape), pressure compensation and self cleaning orifices.
Drip tape operates very efficiently at low pressures of 6 to 10 pounds per square inch
(psi) and water volume of 30 to 40 gallons/minute/acre. Pressure compensation refers to
a drip line's ability to maintain a specified emission rate over a range of pressures.
A pressure compensating line will emit water at the same flow rate over a range of
pressures. A non-pressure compensating line emits water at a rate that increases linearly
with pressure. Commonly used drip lines fall somewhere in the middle and are called
“partially pressure compensating”. For example, many drip lines will experience a 10%
increase in emission rate when pressure is increased 20%. Drip tubes with internally
attached emitters are fully pressure compensating. These emitters are more complicated
to manufacture and are more expensive (Figure 3).
Filters are essential to the operation of a drip system. Many devices and management
techniques are available for cleaning irrigation water. Depending on the water
source, settling ponds, self-cleaning suction devices, sand separators, media filters, screen
filters, and disk filters are used to filter any organic debris in the water before delivered to
the drip irrigation system. Keeping a drip system free of debris is critical because most
types of clogging of the orifice will irreparably disable a system.
In the last 10 years in North America, there has been mild to severe droughts in
many locations of North America. Concurrent with drought has been an increased
demand for residential water as a result of a significant increase in the population of the
United States. Demand for water has not only caused water prices to dramatically
increase, especially in the western U.S., but also has impacted the type of crops many
growers are producing. For those vegetable and small fruit growers that have been using
raised beds with plastic mulch, addition of drip irrigation was a necessity. However,
there has been an ever increasing use of drip irrigation with crops grown on bare ground
such as grapes, sweet corn and small fruit. The use of drip irrigation even on bare soil
compared to overhead irrigation can reduce water consumption for crop production by
50 per cent. In addition, use of drip irrigation with crop production on bare soil can also
reduce incidence of foliar diseases and the total amount of fertilizer applied to produce
crops (Figure 4).
(This item omitted from WebBook edition)
FIGURE 3 Use of drip irrigation
tape to water broccoli and
romaine lettuce crop in high
tunnel. Note wetting pattern on
soil from water being emitted
from drip tape. (Photo courtesy of
Catie Rasmussen, Penn State Center for
Plasticulture)
20.3 Regionalized Production of Vegetables 617