Robert Evans, Extension Specialist
andWayne Skaggs,William Neal Reynolds Professor
Department of Biological and Agricultural Engineering
Publication Number: AG 356
Last Electronic Revision: June 1996 (KNS)
Dual-purpose drainage/subirrigation systems are now being used on poorly
drained soils to reduce water-related stress on crops. The mechanics of this
dual-purpose system is discussed in Agricultural Water Management for Coastal
Plain Soils, AG-355. During wet periods this system operates as a drainage
system (Figure 1). Here, excess water is removed from the field through a
system of underground drain tubes which outlet to a main drain tube or open
ditch. When a structure (such as a flashboard riser) is used in the outlet
ditch to regulate the drainage rate, the system may function in either the
controlled drainage or subirrigation mode. Usually a weir is placed in the
control structure so that the water level in the drainage outlet has to rise
higher than the weir crest before the water will flow out of the field.
The process called controlled drainage occurs when the structure is used
to conserve water by reducing drainage outflows and when no additional
water is pumped in. During dry periods, water may be pumped into the control
outlet where it moves back through the drainage network, thus raising
the water level in the field. In this mode the system is being used for
The dual-purpose system will normally fluctuate between the drainage, controlled
drainage and subirrigation modes several times during one cropping season.
Because the role of the system often changes, intensive monitoring and management
of the system is necessary for effective operation.
Figure 1. Three modes of operation for a water management system:
drainage, controlled drainage and subirrigation.
The best crop response for a drainage/subirrigation system will be realized
with a system that is specifically designed to serve both purposes. Systems
whose designs focus on drainage are, in general, not adequate to function in
both modes. The physical limitations of the system can, however, be overcome
somewhat. One way would be to increase system management. Another way to
overcome some limitations would be to increase the drainage intensity of the
system by installing additional tile lines between existing lines. This
additional drainage capacity is needed when the water table rises, a
condition which decreases the drainage capacity of any subsurface system. Land
shaping to improve surface drainage may also be needed, especially on
an existing system with limited drainage capacity.
If a new system is to be installed for both drainage and subirrigation, the
system's size and layout must satisfy the water management needs of the
specific site. Many factors influence this size and layout. For example, crop
rooting depth and tolerance to water stress are important considerations. Several
soil properties such as water-holding capacity, hydraulic conductivity, and
location of layers within the profile should also be measured for each site.
Furthermore, rainfall distribution within the geographic region under
consideration will influence the design and operation of the system. Finally,
farm operators should carefully evaluate the time available to "look after"
the system. To reduce the initial cost, design the system for an intensive
level of management. However, intensive management may not be realistic since
the subirrigation system will often compete for time with other vital farm
The final design of the drainage/subirrigation system should be evaluated using
the water management simulation model, DRAINMOD. For initial planning purposes,
subirrigation/drainage requires about 65 percent of the spacing normally
needed for drainage alone. Drain tubing depth should be about 3 to 4 feet which
is somewhat shallower than the depth for conventional drainage. Subirrigation
offers little advantage to placing the tubing deeper unless the lower layer is
more permeable than the overlying layers. The control structure should be
sized and managed so that the ditch or waterway can carry its full capacity
during high flow periods.
In eastern North Carolina, removing excess water from the field is the system's most important
role. Thus, in areas where droughts are temporary, the system functions mainly
as a drainage system. During the traditionally wet winter and early spring
period, the water-level control structure should be set below the level of the
tile outlet to provide maximum drainage capacity. This water level should be
maintained until after the crop has been planted and most of the trafficable
operations have been completed. To maintain water quality in receiving streams,
it may be desirable to operate in the controlled drainage mode during most of
the winter in order to reduce nitrate outflows on some sites.
Once the system is operational, the most important management decisions include:
These decisions not only may differ for every site, they may also vary with the
crop. Depending on soil type, root depth, drain depth and control structure
setting, from 1 to 4 inches of plant available water can be held in the profile
that would otherwise drain (Figure 2). (Plant available water is any water
retained in the soil that plants can use.) This amount represents from 3 days to
2 weeks of water storage that would not need to be supplied by pumping. How
important this water is to the crop depends on the capabilities and limitations
of the system. The following situations based on system capabilities should be
considered as a guide for the initial decisions.
Figure 2. Water retained in the soil profile as a result of managing the
Controlled drainage-Conserving water by controlled drainage is most
critical on systems where supplemental water is not available. If dry condi-
tions are anticipated, it would be desirable to raise the weir soon after
planting to conserve as much water as possible. On the other hand, the system
should be operated so that prolonged saturation of the root zone does not occur,
especially when the crop is young. In North Carolina, late April and early May
are sometimes "rainy periods." Therefore, on systems which do not have adequate
drainage capacity, water table management too early in the season may result in
excess water stress when the crop is young.
Raising the water table too soon or controlling the water table at a shallow
depth below the soil surface will also discourage deep root growth, an effect
which could make the crop more susceptible to drought later (Figure 3). Holding
the water table too high will also encourage denitrification which could result
in a nitrogen deficiency later in the season. Thus, the long-term average
production benefit of controlled drainage greatly depends on the system's
drainage capacity in addition to the severity of the drought stress.
Consider, for example, a conventional, predominately surface drainage system
with inadequate drainage capacity during wet periods. In a very dry season,
little benefit would be realized from controlled drainage (as compared to
drainage alone) on a crop such as corn since very few soils can store enough
water to grow a crop without rainfall for 6 to 8 weeks. However, a controlled
drainage system can store enough water to reduce drought stress for short
periods. For this reason, the greatest benefit will be realized in years which
have frequent small rains (often less than 1 inch) with only short droughts.
But in a very wet year, yields will probably be reduced if the water table is
held too high because of inadequate drainage. The long-term average benefit of
this type of system (as compared to conventional drainage alone) would be
Operating the system in the controlled drainage mode can provide considerable
yield responses if the system is designed for subirrigation or controlled
drainage. This situation occurs when the system has adequate drainage for water
table management, but the supplemental water supply has not been developed. As
before, a very dry year would offer little or no benefits. But in years with
frequent rains and short droughts, the potential yields could be 10 to 20
percent higher than for drainage alone. This situation will occur about 1
year in 4. In a very wet year, few or no benefits would be realized from saving
water, but since drainage is adequate, there would be no harm in controlling the
water table elevation as in the earlier example. Over the long run, this type of
system would produce average yield increases of 2 to 5 percent above yields with
a conventional drainage system for a crop such as corn.
Figure 3. Root development in response to good and poor water table
Subirrigation-Saving water early in the season is not as important for
high yields if the system is designed for drainage and subirrigation and if
adequate supplemental water is available. Any water saved and used later in a
dry period will delay the need for irrigation and reduce associated pumping
costs. On the other hand, saving water might increase the risk of some early
wet stress and discourage root development.
The risk associated with raising the structure either too soon or too high on
any system can be reduced by increasing the intensity of system management. For
example, in case of heavy rains and systems which do not have adequate drainage,
operators must be prepared to manually lower the structure to increase the
drainage rate. This drainage control is critical when the crop is young and
should be done as soon as it is clear that too much rain will fall. Controlling
such drainage could mean lowering the structure in the middle of the night.
The structure should not be raised again until the water level midway between
drains or ditches has dropped at least 12 inches below the soil surface.
This level, however, depends on soil type and crop age as discussed earlier.
On systems which are designed for controlled drainage or subirrigation, the
water control structure does not need to be lowered after heavy rains provided
the system is being operated and managed as it was designed. Lowering the level
will increase the drainage rate and may reduce slightly the risk of excess water
stress if the crop is young; however, this effect is likely only in unusually
wetseasons. As soon as the rain stops and the water table in the field drops
below the soil surface, the structure should be reset to the design height.
Drain spacings can be increased slightly (up to 10 to 15 percent) on a system
if the operator wishes to devote more time to manage the system.
Local rainfall patterns will influence the timing and positioning of the control
structure level. From a conservation and utilization standpoint, the structure
should not be raised until the water table in the field has dropped to the
design level. Such timing would reduce the potential damage due to early
excess water and still conserve water for a subsequent dry period. Of course,
this situation is never known in advance, and the longer you delay raising
the structure, the greater the risk that rainfall wil not adequately fill the
storage capacity of the profile. Therefore, the best approach is to compile
past rainfall records for your specific location and use this information
together with weather forecast information to decide when to begin raising the
The optimum water table control level will depend on the crop, stage of growth
and soil type. This level will need to be higher for sandy soils or shallow-rooted
crops. For corn, the water level at the control structure needs to be between 18
and 30 inches for most soils. At planting, begin with the structure adjusted to
the bottom of the drainage outlet. This level will usually be between 3 to 5
feet. Until experience is gained and the crop response to the system can be
observed, raise the level of the structure incrementally about 6 inches every
7 to 14 days in order to reach the final desired elevation about mid-May. In
most years this practice will allow the profile to store its maximum potential
at the onset of the drought period and reduce the risk of early crop damage due
to inadequate drainage.
Several years of system operation may be required before learning the "final"
and best setting. Keep accurate records of any apparent wet stress, dry stress
and yields at several locations in the field to determine the best setting.
Compare yields at points directly above the drains to yields midway between
drains. The greatest water stress should be observed midway between drains.
The main concern here is not to let the soil get "too dry" before starting to
irrigate. The crop responds more slowly to a subirrigation system than it does
to a conventional overhead system. This is because as the soil dries out, the
hydraulic conductivity of the soil decreases drastically and the volume of water
needed per unit rise of water table height will also increase. In comparison,
2 to 3 days are normally required for water in a saturated (wet) soil to move
from the-drain to midway between drains. On the other hand, the water may
require 2 to 3 weeks to travel this same distance once the soil dries out.
Most subirrigation water travels laterally in a zone 3 to 6 feet below the
surface. In general, the water table should not be allowed to drop below this
zone during any part of the growing season so that when irrigation becomes
necessary, only a few days will be needed to "start up." In most poorly drained
soils in eastern North Carolina, the water table will not drop below 6 feet
except during dry periods that begin in late May and continue into the
Two approaches can be used to manage the subirrigation system. First, the water
level in the outlet is maintained near a constant level, for example, 20 inches
deep. This level is allowed to fluctuate only 1 to 2 inches and is easy to
manage by using float switches to start and stop the irrigation pump. However,
in this case, the crop relies on irrigation for much of its water, so this
approach could present a problem. If the water table is held too high when this
method is used, it would reduce the profile storage space above the water table,
storage space that is at a minimum. Consequently, most rainfall that occurs
during the irrigation season drains from the profile.
With the second approach, the water table is raised to the desired level, then
the pump is shutoff (Figure 4). The water table will then be allowed to recede
due to evapotranspiration until it drops to some lower allowable limit. At this
point, pumping starts and the water table is again raised to the upper level
where the process repeats itself. With this method, the water table is allowed
to fluctuate 12 to 18 inches depending on soil type. This approach has greater
potential to store and use the rainfall that occurs when the water table is in
the deeper range.
Figure 4. Cyclic method of water table control during subirrigation.
Pumping raises the water table into the drainage outlet, then the water table
recedes due to evapotranspiration.
In using either method, the water table should be maintained at the greatest
depth that will still supply adequate water to the crop. This level will provide
the maximum storage potential for-and more efficient use of-rainfall. To take
advantage of this rainfall, delay pumping as long as possible, especially when
the growing season begins. It should not be necessary to add water to a crop
such as corn before mid- to late May in most years, but don't let the soil get
too dry for reasons discussed earlier.
Observation wells should be established at several locations in the field to
monitor the position of the water table. These wells should be located midway
between the drain lines and at least at two different locations in the field.
Additional wells should be set up for each soil type being irrigated. Small
diameter pipe 4- to 5-feet long can be used for this purpose (Figure 5). Small
holes should be bored in the sides of the pipe so water can move easily into
and out of the pipe. This pipe should extend above the ground surface, and the
soil should be crowned around the sides and at the top to prevent surface water
from running in along the sides of the pipe. If 4-inch diameter pipe is used, a
typical toilet bowl float with a wire or small rod will just fit inside the pipe
and can be used to measure the water level.
Figure 5. Observation well used to monitor position of water table on a
controlled drainage or subirrigation system.
The water level should be measured daily throughout the first growing season
that the system operates. The wells can be monitored less frequently once (1)
experience has been gained, and (2) the response of the water table to rainfall
and changes in the control structure level have been observed. At least for the
first season of operation, maintain records of rainfall, water table position,
control structure level, pumping rate and crop performance. This information
will tell how quickly the system responds and will help managethe system in
As mentioned earlier, water table management will respond differently for
various soils and crops. For this reason, it is impossible to suggest precise
settings and/or operational guidelines that will work at every location. Until
experience is gained in operating the system, seek professional advice.
The North Carolina Agricultural Extension Service and Soil Conservation Service
in each county are available to help with your water management decisions. The
SCS can design the system to satisfy particular water management needs, and they
can provide more specific recommendations based on system capacity and