Robert Evans, Extension Specialist
andWayne Skaggs,William Neal Reynolds Professor
Department of Biological and Agricultural Engineering
Publication Number: AG 355
Last Electronic Revision: June 1996 (KNS)
The best solution to this increasing demand involves developing management strategies
to conserve and use the existing water resources more efficiently. These management strategies
can also have a pronounced effect on water quality. Thus, all management decisions should consider
strategies that maintain or improve water quality. However, the best management strategies are often geo-
graphically sensitive: what might work in one region might not work in another. Each management
strategy, therefore, should be selected based on minimizing the specific local problems.
These soil-water conditions cause problems for crops grown on about 2.3 million acres of poorly
drained soils in North Carolina, soils that make up about 40 percent of the state's total cropland. Yield
reductions of more than 50 percent may develop from stress caused by excessive soil-water conditions on
these poorly drained soils. This may result either (1) from the inability to plant and tend the crop at the
right time due to poor trafficability or (2) from direct damage to the crop when water stands too long in the
field. Excess water may also cause nutrient deficienies due to eventual leaching or denitrification.
On the average, the Coastal Plain of North Carolina receives more than 48 inches of rainfall each
year. Since only 36 inches of water are needed to satisfy potential evapotranspiration, this means that
there are more than 12 inches of excess rainfall annually. Yet, droughts ranging from a few days up to
several weeks occur in most years during June, July and August. Because this is when many crops are
most susceptible to deficit soil water, yield reductions take place often. In fact, droughts have produced
almost total crop failures in some years even on poorly drained soils. Drought reduces average corn
yields by 20 to 30 percent on these traditionally "wet" soils.
Surface drainage systems require the least initial investment and are often more effective in "tight"
clay-textured subsoils; however, maintenance is high and ditch space is nonproductive. Also, high
runoff rates characteristic of surface drainage systems contribute greatly to soil and nutrient
losses from the field.
Subsurface drainage systems using corrugated plastic tubing may also be used to remove
excess water (Figure 2). In general, 4- to 6-inch diameter tubing is buried 3 to 5 feet deep
at intervals of 50 to 200 feet. These systems are designed to remove approximately one-half inch of
water per day (Figure 3). Compared to an open ditch surface drainage
system, the subsurface drainage
system is initially more expensive and results in greater nitrate transport. But this system's benefits
include lower maintenance, less interference with tillage operations and lower peak outflow rates which
may be more desirable environmentally.
Figure 2. Schematic of a typical subsurface drainage system for eastern North Carolina
Figures 3(A) and (B). (A) Outlet pipe for a typical subsurface drainage system. (B) These systems were
designed to remove approximately 1/2 inch of water per
Many types of above-ground irrigation systems are available to help minimize drought-related stress.
These systems range from labor-intensive hand move aluminum pipe systems to self-propelled mechanical
move systems capable of irrigating 200 acres in a single pass.
Figure 4. Water rises into the root zone using subirrigation in ways similar to water rising in a
flower pot when water is added to the bottom of the pot.
In the field a structure, such as a flashboard riser is placed in the drainage ditch or tile outlet to
control the rate of subsurface drainage (Figure 5). The control structure functions in the same manner
as the shallow pan discussed earlier. With this type of system, drainage continues as ]ong as the water
table remains in the root zone. On the other hand, once the water table in the field drops below the root zone,
which is normally 12 to 18 inches below the soil surface, drainage stops (Figure 6). The water table will
continue to recede as the crop removes water by evapotranspiration. Without rainfall or irrigation to
replenish this water, the water table will eventually drop too low to supply sufficient water to the crop.
Figure 5 (A) and (B). (A) Flashboard riser control structure before installation and (B) after
installation controlling the drainage outflow.
Figure 6. Schematic of a controlled drainage system. Drainage stops when the water table drops to the same
level as the top of the control structure (weir). The water table continues to recede,
however, due to
Controlled drainage may save enough water in the soil profile to delay drought stress for a few days.
But, in most years, this method will not be sufficient to eliminate all drought stress for the entire growing
Figure 7. Water being pumped into a drainage outlet to provide
In order to provide an irrigation function, it will be necessary to add additional water to the system
(Figure 7). Rather than adding this water through an overhead irrigation system, the needed water could
be pumped into the ditch containing the control structure. This water would move into the field
through the underground tubing and maintain a water level as determined by the position of the con-
trol structure (Figure 8). (The actual height of the water table needed to eliminate drought stress is a
function of the soil and crop but normally will be between 18 to 36 inches from the surface.) The water
will then move upward from the water table to the root zone due to capillary rise. This process is known
Several studies have shown that drainage water characteristics can be influenced by the type of
drainage system used. For example, a subsurface drainage system will reduce peak flowrates by more
than 50 percent compared to a surface system. Subsurface drainage also reduces movement of sediment,
phosphorus and pesticides. However, subsurface drainage increases nitrate levels in the drainage
water flowing from the site. In these cases it is vital to manage the specific system. For example,
controlling the drainage rate on a subsurface system has been shown to reduce the nitrates leaving the
site by nearly 50 percent compared to conventional subsurface drainage.
Figure 8. Schematic of a subirrigation system. Water is pumped into the outlet ditch
then moves through the underground tubing due to gravity flow. Water moves
from the water table into the root zone by capillary
Field sites should be evaluated before setting up any water management system. One of the best
methods available to evaluate the potential benefits of subirrigation or other water management
alternatives is the water management model, DRAIN-MOD, developed at North Carolina State University.
This model allows the user to simulate the crop's response (yield) to a water management practice,
such as surface or subsurface drainage, controlled drainage or subirrigation. The actual design
and sizing of a specific system can also be evaluated. The model evaluation considers such factors as crop, site
and soil characteristics, and weather conditions, all of which are important when selecting and
sizing a specific system. The model user can then recommend the best water management alternative
based on optimizing long-term simulated yields.
Irrigation may increase long-term yields an additional 15 to 30 percent on the same soils
(Figure 9). Considering the same Rains sandy loam, when the drains were moved closer together
(50 feet apart) to provide both drainage and subirrigation, the predicted yield was over 165 bu/a.
Good water management not only increases average yields, it also reduces the year-to-year varia-
tion in yield and the risk associated with growing the crop. Finally, undesirable off-site effects can
also be reduced by selecting the appropriate alternative.
Figure 9. Long-term relative yield in response to different water management options on a
poorly drained soil in eastern North Carolina.
Seven subirrigation demonstration sites were established recently in Camden, Hyde and Pamlico
counties with federal cost/share assistance provided through the Resource Conservation Act. Tours of
these sites will be conducted routinely to demonstrate the performance and general operation and
management of these systems.
But before getting started, first seek trained help. Your county Agricultural Extension Service and Soil
Conservation Service are available to help evaluate the potential benefits of any water management
alternative for your farm. Their staff has been trained to measure the field properties-such as
hydraulic conductivity-necessary to evaluate your site. Drainage/subirrigation equipment suppliers
and contractors can also help.