Southeastern U.S. Vegetable Crop Handbook - 2023 - 19

(75% to 90% available soil moisture), maximizes crop growth.
This may mean that more frequent irrigations of smaller amounts
are better than delaying irrigations until the soil moisture reaches
a lower level (40% to 50% available soil moisture) and then
applying a heavy irrigation. It is important to remember that different
soils also have different infiltration rates, which will affect
the frequency of irrigation events. For example, water is more
readily held in clay soils; however, clay soils have a lower water
infiltration rate as compared to sandy soils. Depending on the
soil structure, high application rates will result in irrigation water
running off the field, contributing to erosion and fertilizer runoff
particularly on heavy clay soils. Application rates should follow
values in Table 10.
Applying the proper amount of water at the correct time is
critical for achieving the optimum benefits from irrigation. The
crop water requirement, termed evapotranspiration, or ET, is
equal to the quantity of water lost from the plant (transpiration)
plus water that evaporated from the soil surface. The ET rate is
also an important and effectively strategy to schedule irrigation
events. Numerous factors must be considered when estimating
ET. The amount of solar radiation, which provides the energy to
evaporate moisture from the soil and plant surfaces, is the major
factor. Other factors include crop growth stage; day length; air
temperature; wind speed; and humidity level. Plant factors that
affect ET are crop species; canopy size and shape; leaf size, and
shape. Soil factors must also be considered. Soils having high
levels of silt, clay, and organic matter have greater water-holding
capacities than sandy soils or compacted soils (Table 9). Soils
with high water-holding capacities require less frequent irrigation
than soils with low water-holding capacities. When such
soils are irrigated less frequently, a greater amount of water must
be applied per application.
Water loss from plants is much greater on clear, hot windy
days than on cool, overcast days. During periods of hot, dry
weather, ET rates may reach 0.25 inch per day or higher. ET can
be estimated using a standard evaporation pan or using local
weather networking systems. Check with your local Extension
office for information.
In general, irrigation is beneficial in most years since rainfall
is rarely uniformly distributed even in years with above-average
precipitation. Moisture deficiencies occurring early in the
crop cycle may delay maturity and reduce yields. Shortages later
in the season often lower quality and yield. Over-irrigating, however,
especially late in the season, can reduce quality and postharvest
life of the crop. Table 8 shows the critical periods of crop
growth when an adequate supply of water is essential to produce
high-quality vegetables.
Drip Irrigation. Drip irrigation is used to maintain soil moisture
whereas other types of irrigation are used more to replace depleted
soil moisture. Drip irrigation is a method of slowly applying
water directly to the plant's root zone. Water is applied frequently,
often daily, to maintain favorable soil moisture conditions.
Even so, field operations can continue uninterrupted. Water is
applied without wetting the foliage, thereby decreasing evaporative
losses, and decreasing disease pressure due to damp foliage.
Additionally, the use of drip irrigation can limit waste and
potential contamination from overuse (or unnecessary use) of
2023 Vegetable Crop Handbook for Southeastern United States
agricultural chemicals. In most cases, drip irrigation is considerably
more uniform and efficient in its distribution of water to the
crop than other irrigation methods. In addition, fertilizers applied
through the drip irrigation system are conserved.
Drip irrigation is used on a wide range of fruit and vegetable
crops. It is especially effective when used with mulches; on
sandy soils; and on high value crops, such as cantaloupes, watermelons,
squash, peppers, eggplants, and tomatoes. Drip irrigation
systems have several other advantages over sprinkler and
surface irrigation systems. Low flow rates and operating pressures
are typical of drip systems. These characteristics lead to
lower energy costs. Once in place, drip systems require little labor
to operate, can be automatically controlled, and can be managed
to apply the precise amount of water and nutrients needed
by the crop. These factors also reduce operating costs. The areas
between rows remain dry reducing weed growth between rows
and reducing the amount of water lost to weeds.
The equipment used for drip irrigation systems must be routinely
monitored and maintained to prevent any challenges. Drip
irrigation tape and tubing can be damaged by insects, rodents,
and laborers. Pressure regulation and filtration require equipment
not commonly used with sprinkler or surface systems. The drip
system, including a pump, headers, filters, and various connectors,
must be checked and be ready to operate before planting.
Failure to have the system operational could result in costly delays,
poor plant survival, and irregular stands, reducing yield.
Calculating the length of time required to apply a specific
depth of water with a drip irrigation system is more difficult than
with sprinkler systems. Unlike sprinkler systems, drip systems
apply water to only a small portion of the total crop acreage.
Usually, a fair assumption to make is that the mulched width approximates
the extent of the plant root zone. Although the root
zone is confined, the plant canopy is vigorous and water use and
loss from evapotranspiration (ET) can far exceed the water applied
if application is based on a banded or mulch width basis.
Table 11 calculates the length of time required to apply 1-inch
of water with drip irrigation based on the drip tape flow rate and
crop row spacing. The use of this table requires that the drip system
be operated at the pressure recommended by the manufacturer.
Table
12 has been prepared to calculate the maximum recommended
irrigation period for drip irrigation systems. The
irrigation periods listed assume that 25% of the available water
in the plant root zone is depleted. Soil texture directly influences
the water-holding capacity of soils and the consequential depth
reached by irrigation water.
In drip tape systems, water is carried through plastic tubing
(which expands when water flows through it) and distributed
along the drip tape through built-in outlets or devices called
emitters. The pressure-reducing flow path also allows the emitter
to remain relatively large, allowing particles that could clog an
emitter to be discharged.
Although modern emitter design reduces the potential for
trapping small particles, emitter clogging can be a common occurrence
with drip irrigation systems. Clogging can be attributed
to physical, chemical, or biological contaminants. Proper filtration
is a must and occasional water treatment might be necessary
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Southeastern U.S. Vegetable Crop Handbook - 2023

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