Overhead sprinkler systems
spray water into the air above and around the foliage of the crop in a broadcast
pattern. The water droplets should be large enough to fall through the canopy
to growing substrate or to run along the foliage and stems to the base of the
crop.
Sizes of droplets are largely determined by the sprinkler system to achieve
distribution. Overhead sprinklers are 70 to 75 percent efficient in water delivery;
evaporation occurs in the air, from the plant foliage, and from the ground surface.
The circular pattern of
application makes it difficult to achieve a high coefficient of uniformity in
delivering water to all plants. Sprinkler nozzles must be placed with sufficient
overlap (between 40 to 60 percent) to achieve nearly uniform coverage of the
surface area. More overlap is needed for windy situations.
Sprinklers usually throw water beyond the structure. Over-irrigation often occurs (i.e. the duration of the irrigation event is too long) since the containers on the fringes have to be adequately wet.
Many overwintering
structures have a single row of sprinklers down the center. These sprinklers
are much closer together in order to wet the interior area of the structure
more uniformly. Containers are spread out under the part of the broadcast covered
area where the water coverage is most uniform.
Sprinkler nozzles are designed
to function at some pressure and discharge rate which determines the diameter
of water throw. Operating pressure may range from 20 psi to 80 psi for large
sprinklers. Discharge rates may range from 0.5 gallons per minute (gpm) to 1,000
gpm for a large gun. Wetting diameters range from 35 feet to 300 feet.
The application
rate in inches per hour (iph) onto the ground is important. A rate of 0.2 iph
to 0.5 iph is common. The crop should be watered fairly quickly to reduce the
evaporative losses to the air and to reduce the time the foliage is wetted.
At the same time, the irrigation should not cause runoff or puddles in fields.
In container production an impervious surface may mean there will be runoff
but it should be controlled.
It is a challenge to try
to match sprinklers to any small or narrow area. In an overwintering (nursery) house,
the sprinkler heads are placed close together in a line to get multiple overlaps
of heads over some width of the structure. The goal is to achieve uniform coverage
over the width of the structure. The many overlaps increase the application
rate of the water and substrate wetting may suffer. Water applied too quickly
may drain through quickly. Water will be thrown beyond the structure but the
application rate will be low at the outer edge of the sprinkler pattern.
Overhead sprinklers are
efficient for bedding plants, field crops, and small container plants placed
close together. However, foliage is wet during each irrigation, so disease
pressure may be a problem. Irrigations should be shceduled in time for the foliage to dry before relative humididty rises at night (in humid environments).
In larger fields, the sprinklers can be placed in many rows so that overlapping
of nozzles is possible and application uniformity can be good.
Overhead sprinklers include
fixed installations, center pivots, traveling guns, and portable sprinklers.
Each system has its own characteristics to consider.