The environmental impacts of these activities vary with the status and
circulation of the receiving water. For example, juvenile marine organisms
depend on the headwaters of the estuarine system to provide shelter and
food. These areas also provide natural outlets for rural and urban stormwater
drains and agricultural drainage systems. Dilution of seawater by fresh
water creates the medium salinity waters that produce most of the economically
important species of fish and shellfish (50). Some studies have shown,
however, that high outflow rates associated with intensive artificial drainage
further reduce the salinity of head-waters (30, 50), sometimes resulting
in stress, disease, or depressed production of certain pelagic species
(surface feeders such as Atlantic menhaden) (40, 44). Other more intensive
studies directed specifically at evaluating hydrodynamic effects showed
that freshwater out-flow from land- based activities such as artificial
drainage have little influence on estuarine salinity (51, 52, 53). Rather,
salinity fluctuations were dominated by natural circulation patterns caused
by tides, wind, and rainfall. Recently, freshwater outflow was found to
stimulate growth of some demersal species (bottom feeders such as spot
and croaker) (41, 42).
damming of rivers and streams;
discharge of municipal and industrial waste directly into rivers, streams,
urban and rural development, resulting in more intense stormwater runoff
that may carry nutrients and suspended solids from lawns, roads, and other
artificial drainage to promote agriculture and forestry;
introduction of nutrients from agricultural fertilization and from live-
stock and domestic waste:
alteration or conversion of wetlands to other uses such as agriculture,
rural and urban development, recreation, and tourism.
Nitrogen and phosphorus levels in many tidewater rivers, streams,
and estuaries have become high enough that a very delicate balance exists
between undesirable species such as blue-green algae (BOA) and other desirable
flora (5, 27, 30, 31, 47, 48, 49). Typically, water bodies receiving excessive
nutrient loads are most susceptible to blooms of blue-green algae. These
algae are very prolific when excessive levels of nitrogen, phosphorus,
or both are present (5, 27, 30, 31, 47, 48, 49). Blue-green algal dominance
may alter the aquatic food chains. The algae blooms are un-sightly (Figure
2) and may pose problems (such as toxicity and bad taste or odor) to recreational
users of the water. They can also consume much of the dissolved oxygen,
leaving the water anoxic (deprived of oxygen). This problem is more acute
when the waters are stagnant or have slow circulation (48, 49). Anoxic
conditions are stressful and sometimes fatal to fish, which depend on oxygen
to survive (Figure 3). When fish come to the surface and gasp for air,
it often indicates anoxic conditions.
Figure 2. Blue-green algae mat on surface of Perquimans River
near Hertford, North Carolina, July, 1985.
Figure 3. A fish kill resulting trom anoxic conditions(low
dissolved oxygen level) following decomposition of blue-green algae.
Agricultural cropland is a major nonpoint source of nitrogen and phosphorus
contributing to the nutrient enrichment of tidewater rivers and estuaries
in North Carolina (5, 45, 46). Best Management Practices must be adopted
to reduce the amount of nitrogen and phosphorus discharged to sensitive
surface waters. Reductions of at least 30 percent in nitrogen and 50 percent
in phosphorus have been recommended to minimize blue-green algae blooms
in the Lower Neuse River during the summer months (49).
Agricultural Activities Influencing Water Quality
Many agricultural practices contribute to environmental problems. These
include tillage practices, fertilizer and pesticide application methods
and rates, and drainage and irrigation practices. For example, typical
North Carolina nitrogen fertilization rates for corn have been between
150 and 200 pounds of nitrogen per acre annually (35). Although these rates
were based on a potential grain yield of 175 to 200 bushels per acre per
year, annual corn yields more typically average less than 100 bushels per
acre because of soil and climatic conditions. Therefore, nitrogen fertilization
rates exceed yield expectations in many years, resulting in the application
of excess nitrogen that is not removed by the crop. This nitrogen is carried
from the field and may cause environmental problems (20, 21, 25). Clearly,
one very important Best Management Practice is to apply fertilizers at
rates consistent with sustainable yields rather than potential yields.
Although several Best Management Practices can be employed to
minimize the environmental impacts of crop production, this publication
focuses on strategies related to water table management.
Water Table Management
Although excessive soil water is often a problem on poorly drained soils,
weather conditions are extremely variable, and crops sometimes suffer from
drought, which can also reduce yield. Intensive drainage systems are necessary
to ensure access to many fields during wet periods. But past drainage practices
have not always encouraged water conservation. As a result, these systems
have tended to overdrain many areas and increase drought damage during
dry growing seasons (9).
These problems are resulting in a transition from conventional
drainage methods to water table management systems. The latter provide
drainage during wet periods but also include control structures to manage
the water level at the drainage outlet, making it possible to reduce overdrainage.
In some cases, the system can be used to provide subirrigation during dry
periods. Collectively, these management practices are referred to as water
table management (14) and include any combination of management practices
such as surface drainage, subsurface drainage, controlled drainage, or
subirrigation- that influence the level of the shallow water table.
Research on the use of subirrigation and controlled drainage to
provide water for crops and to meet drainage needs has been conducted in
North Carolina since the early 1970s (54, 55). Using results of this research,
methods and guidelines for designing drainage systems for different soils,
crops, and weather conditions have been developed (10, 11, 12, 16, 19,56,57,58,61,64,65,66).
Controlled Drainage: A Best Management Practice
Studies have shown that water table management systems can improve drainage
water quality when properly designed and carefully managed (7, 8, 17, 22,
23, 24, 62, 63). On the basis of these studies, water table management-in
particular, controlled drainage-has been designated a Best Management Practice
(BMP) for soils with improved drainage. It therefore qualifies for cost-share
assistance under the North Carolina Agricultural Cost Share Program(NCACSP).
As of July 1, 1989, more than 2,500 control structures have been
installed to provide controlled drainage on approximately 150,000 acres
in eastern North Carolina. The North Carolina Agricultural Cost Share Program
has helped bear the cost of approximately 1,800 of these water control
structures. An additiona1 1 million acres of cropland in North Carolina
are suited for controlled drainage. This acreage includes most of the cropland
in the lower coastal plain and tidewater regions.
Unlike many BMPs, controlled drainage benefits both production
and water quality. The production benefits make it a popular practice with
farmers, while the water quality benefits help meet environmental goals.
The North Carolina Agricultural Cost Share Program is not designed
to benefit agricultural production but rather to hasten the adoption of
BMPs to promote soil and water conservation, provide habitat for wildlife,
and protect or restore the environment through improved water quality.
The NCACSP seeks these environmental benefits by providing financial incentives
to those who implement controlled drainage systems.
The implementation of controlled drainage, or any other management
practice, does not by itself satisfy the objectives of the North Carolina
Agricultural Cost Share Program. The program's purposes are met only after
the environmental concerns or problems for a given site have been taken
into account and incorporated in a management strategy for that site.
For water table management or controlled drainage practices, this
means taking into account any water quality problems of the receiving fresh
waters or estuaries and developing management strategies that will minimize
the adverse effects of drainage water flowing from agricultural lands.
To do this successfully, it is necessary to know (1) about any problems
in the receiving waters, (2) the characteristics of the drainage water
and how specific management strategies might influence these characteristics,
and (3) the subsequent impact on receiving waters.
Figure 4. Typical hydrologic cycle for eastern North Carolina.
East of I-95 (which parallels the fall line between the coastal plain and
the piedmont) average annual rainfall ranges from 46 to 56 inches, depending
on location. Actual evaporation ranges from 34 to 36 inches annually. Therefore,
the "excess" rainfall, most of which returns to surface waters, ranges
from 12 to 20 inches per year.
Influence of Water Table Management on Drainage Water Quality
Precipitation and Drainage Outflow
Rainfall in eastern North Carolina averages from 46 to 56 inches annually,
depending on location. Potential evapotranspiration ranges from 38 to 41
inches, although actual evapotranspiration is typically 34 to 36 inches
annually because short-term droughts often occur. Therefore, excess rainfall
(the difference between rainfall and actual evapotranspiration) ranges
from 12 to 20 inches annually. A small amount of this excess (usually less
than 1 inch per year) percolates through the soil to recharge the deep
groundwater aquifer systems (29). Most of the excess rainfall, however,
eventually returns to the ocean through the surface water system of streams,
rivers, estuaries, and sounds.
The rate at which rainfall leaves a site depends on rainfall intensity,
topography, infiltration, soil permeability, vegetative cover, the distance
from a drainage outlet, and the location of restrictive horizons. Intense
rainstorms often result in surface runoff, whereas rainfall from milder
storms usually infiltrates the soil. Much of eastern North Carolina is
underlain by marine clay sediments at depths less than 30 feet from the
soil surface (6, 29). These sediments restrict the rate of deep groundwater
recharge. As a result, excess rainfall that infiltrates the soil moves
laterally above them as shallow groundwater flow until it eventually discharges
to the surface water system.
This process of rain, surface runoff, infiltration, evaporation,
shallow groundwater flow, and deep groundwater recharge is referred to
as the hydrologic cycle. Figure 4 illustrates the hydrologic cycle for
eastern North Carolina. The total annual volume of outflow from a field
is about the same for sites that drain well naturally, typical of the upper
coastal plain and piedmont, and for those that do not, typical of the lower
coastal plain and tidewater regions. The main difference is in the rate
of outflow and the pathway by which excess water leaves the site. On well-drained
sites, outflow occurs soon after rainfall and the flow duration is relatively
short, usually a few days. On poorly drained sites, the outflow is more
gradual but may last several weeks. Artificial drainage tends to convert
a poorly drained site into a well- drained site.
Characteristics of Drainage Water
Excess rainfall leaves a field either as surface drainage (surface runoff)
or as subsurface drainage (shallow groundwater flow). This difference is
important from a water quality standpoint because the characteristics of
the drainage waters differ. In practice, it is usually difficult to differentiate
between surface and subsurface drainage because the outflow in drainage
ditches or canals is a combination of the two. For the remaining discussion,
drainage systems in which the majority of outflow has drained through the
soil profile are referred to as subsurface drainage systems (14).
Systems where surface runoff is the primary drainage mechanism are
called surface drainage systems.
The following paragraphs summarize the characteristics of drainage water
based on approximately 125 site-years of data collected at 14 locations
in eastern North Carolina.
Agricultural development using artificial drainage increases total annual
outflow at the field edge by 5 to 10 percent (26, 59), depending on rainfall
(Figure 5). On the average, subsurface drainage increases annual outflow
slightly compared to surface drainage, but the increase is usually less
than 5 percent. Peak outflow rates at the field edge are lower by a factor
of 1 to 4 (26, 37, 59, 60, 62), depending on the storm intensity, with
subsurface drainage systems than with surface drainage systems.
Drainage waters from surface drainage systems contain higher concentrations
of organic nitrogen, phosphorus, and sediment than those from subsurface
drainage systems (8, 13, 20, 21, 24, 60). Outflow from subsurface drainage
systems contains higher concentrations of nitrates than that from surface
drainage systems (8, 13, 20, 21, 23, 24, 60). Both systems result in increased
nutrient concentrations in drainage outflows compared to undeveloped sites
(24, 26, 38), a result of fertilizer use on the developed sites.
Controlled drainage, when managed all year, reduces total outflow by approximately
30 percent (17, 23) compared to uncontrolled systems, although outflows
vary widely depending on soil type, rainfall, type of drainage system and
management intensity (8, 17, 23, 60). For example, control only during
the growing season typically reduces outflow by less than 15 percent (23).
The effect of controlled drainage on peak outflow rates varies seasonally.
Drainage control reduces peak outflow rates during dry periods (summer
and fall) but may increase peak outflow rates during wet periods (winter
and spring), depending on the control strategy.
Drainage control has little net effect on total nitrogen and phosphorus
concentrations in drainage outflow (8, 17, 22, 23). Controlled drainage
may reduce nitrate-nitrogen(NO3-N) concentrations in drainage outflow by
up to 20 percent, but total Kjeldahl nitrogen (TKN) concentrations are
somewhat increased. Controlled drainage tends to decrease phosphorus concentrations
on predominately surface systems but has the opposite effect on predominately
subsurface systems (8, 17, 22, 60). Seasonal variations may also occur,
depending on rainfall, soil type, and the relative contribution of surface
or subsurface drainage to total outflow.
Controlled drainage reduces nitrogen and phosphorus transport at the field
edge (8, 17, 22, 23, 60), primarily because of the reduction in outflow
volume. In 14 field studies, drainage control reduced the annual transport
of total nitrogen (NO3-N and TKN) at the field edge by 9 pounds per acre,
or 45 percent (Figure 6), and total phosphorus by 0.11 pound per acre,
or 35 percent (Figure 7). Again, the reductions at individual sites were
influenced by rainfall, soil type, type of drainage system, and management
Figure 5. Average annual outflows measured from 14 sites in
eastetn North Carolina. The values shown represent approximately 125 site-years
Figure 6. Average annual nitrogen tranaport (TICN and NO3 N)
in drainage outflow as measureci at the field edge for 14 soils and sites.
Figure 7. Average annual total phosphorus transport in drainage
outflow as measured at the field edge for 12 soils and sites. Values shown
are for mineral soils only. Two sites with organic soils were not included.
Figure 8. Algae bloom in drainage ditch with controlled drainage.
Controlled drainage helps keep agricultural nutrients within the field
boundaries and out of sensitive receiving waters.
Effects of Artificial Drainage on Receiving Waters
Agricultural development slightly increases the total volume of flow reaching
the receiving surface water system in coastal areas. Drainage reduces the
time that excess rainfall remains on site before it reaches the receiving
system, resulting in higher outflow rates (more water over a shorter time)
than from undeveloped sites. However, in modelling studies (37) peak outflow
rates reaching receiving streams were lower than peak outflow rates at
the field edge. The impact on the receiving waters of these hydrologic
changes is still unclear, but it almost certainly depends on the drainage
network and circulation of the receiving water.
From a water quality standpoint, the most dramatic effect of agricultural
development is a significantincrease in nutrient concentrations in drainage
water and nutrient transport from fields. This increase is not a direct
consequence of drainage activities; rather, it results from fertilization
and other production activities made possible by improved drainage. The
net effect of fertilization combined with artificial drainage is an increase
in nitrogen and phosphorus reaching coastal waters. However, the amounts
reaching receiving waters are less than the amounts leaving the field edge
(2, 3, 4, 20, 32, 33, 34) because some of the nitrogen and phosphorus is
assimilated and removed en route by natural mechanisms.
Any management strategy that can help keep nutrients on site and
prevent them from reaching sensitive receiving surface waters is a positive
step. However, to be effective these strategies must reduce nitrogen by
30 to 40 percent and phosphorus at least this much (47, 48, 49). Smaller
reductions may not adequately improve water quality.
The transport of nitrogen and phosphorus from artificially drained
fields can be minimized by applying fertilizers at a rate consistent with
sustainable yields, selecting the appropriate water management system (such
as controlled drainage), and properly managing that system.
Management Considerations and Guidelines
The successful management of controlled drainage systems rests on two important
objectives. The first is achieving optimum production efficiency (15);
the second is attaining maximum water quality benefits (18). In selecting
the best management strategy from the standpoint of water quality, the
characteristics of the receiving surface waters must be considered. For
coastal rivers and streams, the primary concern is eutrophication. The
management strategy should therefore effectively reduce the transport of
nitrogen and phosphorus in drainage water. The condition of the receiving
waters will determine whether it is more important to reduce nitrogen or
phosphorus, or whether it is necessary to reduce both. In estuaries, nutrients
and freshwater inflow (resulting from peak drainage outflow rates) are
the primary concerns.
The question most frequently asked by farmers, yet the one most difficult
to answer is: At what depth should the water table be controlled? In humid
regions, there is no "optimum depth because the water table may fluctuate
several inches from day to day in response to such variables as rainfall,
evapotranspiration, and drainage. Williamson and Kriz (1970) report that
a wide range of water table depths (1 to 4 feet) will result in optimum
yields for many crops, depending on soil type, profile layers and their
hydraulic properties, weather conditions, type of crop, crop development,
and rooting depth. The starting point recommended for the sandy loam soils
of North Carolina is 2 feet (12, 1S, 64). Most crops can readily tolerate
a 6-inch increase or decrease in the water table. Although not well documented,
experience has shown that short- term fluctuations in the water table will
not reduce the yield on most soils provided that the water table depth
does not undergo long periods (typically 24 hours or more) at a depth less
than 1 foot or greater than 3 feet.
Information in earlier sections suggests that total drainage outflow
could be decreased (to levels near those of undeveloped sites) if the control
elevation is raised to the soil surface. Since nutrient transport has been
shown to be nearly proportional to drainage outflow, minimizing outflow
would minimize the potential transport of fertilizer nutrients. In addition,
very high water table elevations during the growing season may increase
the potential loss of nitrogen by denitrification (21), thus reducing the
nitrate levels in drainage outflow. While this situation might be desirable
from a water quality standpoint, it would not be desirable for crop production.
Thus, some compromise in potential water quality benefits is necessary
to maintain productivity, although the compromise for water quality is
not nearly as severe as might be expected.
Nitrate concentrations in drainage water could be reduced by keeping
the water elevation as high as possible in the outlet ditch and subsequently
in the field. However, doing so would increase the transport of phosphorus
by increasing the proportion of surface runoff in total flow. Although
total outflow would be reduced (usually less than S percent), there would
be an increase in peak flows of shorter duration (a property of surface
runoff), which would be likely to cause salinity fluctuations near primary
nursery areas. Furthermore, a shallow field water table would restrict
root growth and development (39), thereby reducing evapotranspiration (26)
and nutrient uptake, leading in turn to increased losses in drainage water.
Thus, most water table management strategies aimed at improving production
on cropland also have a positive effect on drainage water quality. The
primary difference between the two management objectives is that management
during the nongrowing season is also beneficial for water quality (22,
Another important consideration involving water table management,
production, and water quality is trafficability, which is essential for
efficient production. Farmers may severely impair the production potential
of their fields by trying to till the soil when it is too wet. With improved
drainage, tillage can begin sooner after rainfall. For example, the wet
spring of 1989 presented serious trafficability problems for many North
Carolina farmers. Because of poor traffficability, less than 50 percent
of the acreage planned for corn was actually planted. However, most fields
with good artificial drainage were planted on time.
In some cases, farmers have attempted to resume tillage operations
too soon after rains stop. In doing so, they may damage the soil structure
and promote the development of tillage pans (43), thereby increasing surface
runoff and reducing infiltration, root development, and evapotranspiration.
These effects are undesirable for crop production and also adversely affect
drainage water quality. This problem occurs most frequently on soils with
a clayey subsoil and a shallow sandy loam or loamy sand surface horizon
less than 2 feet thick (43). The problem may be compounded by controlled
drainage if farmers are reluctant to lower the control elevation. Many
times this problem is not apparent during tillage because there is no immediate
trafficability problem, particularly with high-flotation equipment.
The interaction between water table management, trafficability,
and the development of tillage pans has not been studied extensively. Farmers
in North Carolina have not experienced any apparent trafficability problems
when the water table was at least 3 feet deep (12). Based on this information,
farmers have been encouraged to lower the water control elevation to at
least 3 feet deep two days or more before beginning tillage operations.
In some cases, the soil can bear traffic when the water table is about
2 feet deep. However, the effect on the development of tillage pans or
impairment of soil structure is not known.
Controlled drainage in North Carolina has predominantly been practiced
with a crop rotation of corn, wheat, and soybeans, although increasing
acreages of potatoes, peanuts, and vegetable crops are being included.
Table 1 summarizes a recommended management strategy for a two-year rotation
of corn, soybeans, and wheat; this strategy is designed to improve both
production and drainage water quality. The values shown are weir elevations
of the control structure relative to average soil surface elevations. Water
table levels in the field may be considerably different from the weir elevation,
depending on whether the system is in a drainage or recharge cycle; however,
the values shown are also the average target elevations of the water table
in the field.
Table 1. General Water Table Management
Guidelines to Promote Water Quality and Optimum Crop Yields for a Two-Year
Rotation of Corn-Wheat-Soybeans
Most of the control elevation adjustments shown in Table 1 are
related to trafficability and seasonal fluctuations in rainfall. For example,
during a wet spring (April and May), the weir elevation should be approximately
1 foot lower than the values shown to improve trafficability; increase
potential storage for infiltration; and reduce the potential for surface
runoff, sediment and phosphorus transport, and higher peak outflow rates.
During summer months, evapotranspiration will usually cause the
water table in the field to decline to an elevation considerably below
that of the weir unless water is added to the system by subirrigation.
Intense summer thunderstorms sometimes exceed the infiltration rate of
the soil, resulting in loss of much- needed water by surface runoff. Under
these circumstances, the weir elevation can be raised temporarily to retain
this water in the field ditches, from which it will then move back into
the field by subsurface flow. However, if the water level in the field
ditches has not receded to at least 1 foot within 24 hours, lower the weir
elevation to the suggested levels shown in Table 1.
To prevent serious crop damage from heavy rainfall, do not raise
weirs in the outlet ditch above 1 foot and leave them unattended for more
than 24 hours during the growing season. When adjusting the weir level
to remove excess water, never lower it more than 6 inches within a 12-hour
period because the ditch banks will be saturated and may be unstable. Lowering
the water level too quickly may result in sloughing and erosion of the
banks. Also, lowering the weir in small increments minimizes peak outflow
The primary benefit of controlled drainage in lands that discharge
to freshwater rivers and streams is a reduction of total outflow and nutrient
loading. These goals can be accomplished by setting the weir at a specified
level and making minor adjustments to accommodate production requirements,
as shown in Table 1. Without attentive management, the potential production
and water quality benefits of drainage control will not be realized. Controlled
drainage and subirrigation systems are normally designed with closer drain
spacings than conventional drainage systems to provide the drainage capacity
needed for traffficability and crop protection when the water table is
elevated. As a result, drainage outflow rates and nutrient transport can
be significantly greater than those from conventional systems if the system
is not properly monitored.
As discussed earlier, operating a controlled drainage system with
a constant weir elevation will reduce outflow rates during recharge periods
but will sometimes increase outflow rates during discharge periods. Thus,
if drainage waters are discharged near estuarine areas, the weir elevation
must be more intensively managed if peak outflow rates are to be reduced
during the critical spring period.
All water table management systems function primarily in the drainage
mode during the spring when water table elevations are high and rainfall
exceeds evaporation. Peak outflow rates are usually higher during this
period because of the higher water table elevations resulting from controlled
drainage. To reduce peak outflow rates and minimize surface runoff by draining
the soil between rains, set the weir elevation at or near the soil surface
when rainfall is anticipated or forecast. After the rainfall and as soon
as all surface water has infltrated, lower the weir elevation gradually
(about 6 inches per day) to its lowest possible elevation or until the
next rainfall. This strategy will allow the soil profile to drain gradually
but uniformly and will also provide soil storage capacity in preparation
for the next rainfall. Management at this level of intensity is necessary
from late February to May. During the remainder of the year, the procedures
outlined in Table 1 can be followed.
The potential for reducing peak outflow rates by means of this
intensive management strategy is not known, but it is believed that outflow
rates at the field edge can be reduced to approximately one-half of those
for conventional control strategies. The major disadvantage is the greater
attention that must be given to managing the system, especially during
the nongrowing season.
Monitoring the System
The water table elevation in the field may be considerably different from
the water level in the outlet ditches or the weir elevation. From the standpoint
of crop production, the water table level in the field is the important
consideration. Intensive management of these systems is required for both
production and water quality because many of the management indicators
are hidden from view and the response to weir adjustments is not usually
The response time for water table fluctuations in the field may
be several days longer than for similar fluctuations in the outlet ditch.
For example, during the summer the water table in the field sometimes drops
below 5 feet. At this depth, most soils in the tidewater region can store
2 to 4 inches of rainfall. Yet, as discussed previously, a 2-inch intense
thunderstorm may result in surface runoff and outflow. Based on outflow
at the control structure, one would assume that the soil was saturated.
However, after soil moisture redistribution, the water table may rise only
to 3 feet, which in many soils is not high enough to supply the crop's
water needs. Thus observation wells that allow the level of the field water
table to be observed are essential for proper system management. Recommendations
for the construction, location, and installation of observation wells have
been reported, as have suggestions for monitoring frequency (12, 15).
Be a Steward of Your Soil and Water Resources
Although agricultural activities have undoubtedly contributed to coastal
water problems, they are not the only source of these problems. Society
has been slow to recognize that freshwater and estuarine ecosystems have
a limited capacity to absorb waste products produced by human activities.
Likewise, more efficient harvesting methods, especially the non-selective
methods practiced over the past two decades, have recognized the production
and self-stocking capacity of the estuarine system. The problems that now
exist in coastal waters did not develop overnight; they are the cumulative
effects of man's activities over many years. For example, although nutrient
levels in the Chowan River reached levels conducive to blue-green algae
blooms by the mid-1950s (1), the first significant nuisance bloom did not
occur until 1972, nearly 20 years later.
Similarly, coastal water problems cannot be corrected overnight.
Nutrient- enriched sediment can now be found at the bottom of many streams,
rivers, and estuaries (47), resulting from many decades of agricultural,
municipal, and industrial discharges. As a consequence, it will take coastal
waters several years to cleanse themselves through natural biological,
chemical, and flushing processes.
Implementing Best Management Practices on farms may not immediately
produce visible improvement in the coastal environment, but it will begin
to reverse the damage. The first step is to reduce inputs so that the receiving
systems can beg in to cleanse themselves. By integrating good water management
with other Best Management Practices, such as reducing fertilization rates
to the minimum levels needed for reliable yields, we may be able to achieve
drainage water quality similar to that of undeveloped sites. Equally important,
attaining the target yields for which fertilizer rates are chosen also
requires proper water management. Agriculture's harmful impacts on the
coastal water environment can be significantly reduced, but only when all
landowners and producers exercise stewardship of land and water resources.
Thus far, Best Management Practices for soil and water conservation
and environmental protection have been implemented on a voluntary basis,
supported by incentive programs such as the North Carolina Agricultural
Cost Share Program. However, with the passage of the swampbuster and sodbuster
provisions of the 1985 Farm Bill, Congress demonstrated that it is not
deaf to continuing pleas to protect the environment through regulations.
Farmers still have the opportunity to implement practices such as controlled
drainage voluntarily. But without conscientious attention to such practices,
future implementation may become mandatory and financial incentives may
If water quality is to be improved, water control structures
must be managed throughout the year, not just during the growing season.
This means raising the weir level to within 12 to 18 inches of the soil
surface after the crop has been harvested instead of leaving the flashboards
lying on the ditch bank over the winter.
Strategies for water table management are complex. To ensure maximum
production and water quality benefits from controlled drainage, seek professional
advice. Staff members of the North Carolina Agricultural Extension Service
and the Soil Conservation Service in each county have been trained in water
management and can recommend strategies for effficient operation.
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Doty, C. W., R. O. Evans, R. D. Hinson, H. J. Gibson, and W. B. Williams.
1986. Agricultural water table management: A guide for eastern North Carolina.
U.S. Department of Agriculture, Soil Conservation Service and Agricultural
Research Service, North Carolina Agricultural Research Service, and North
Carolina Agricultural Extension Service, Raleigh. 205 p.
Evans, R. O., P. W. Westerman, and M. R. Overcash. 1984. Drainage water
quality from land application of swine lagoon effluent. Transactions of
the American Society of Agricultural Engineers 27(2):473-480.
Evans, R. O. and R. W. Skaggs. 1985a. Agricultural water table management
for coastal plain soils. Publication AG-355. North Carolina Agricultural
Extension Service, Raleigh. 12 p.
Evans, R. O. and R. W. Skaggs. 1985b. Operating controlled drainage and
subirrigation systems. Publication AG-356. North Carolina Agricultural
Extension Service. Raleigh. 12 p.
Evans, R. O., R. W. Skaggs, and R. E. Sneed. 1988. Economics of controlled
drainage and subirrigation systems. Publication AG-397. North Carolina
Agricultural Extension Service, Raleigh. 20p.
Evans, R. O., J. W. Gilliam, and R. W. Skaggs. 1989a. Effects of agricultural
water table management on drainage water quality. Report no. 237. North
Carolina Water Resources Research Institute. Raleigh. 87 p.
Evans, R. O., J. W. Gilliam, and R. W. Skaggs. 1989b. Managing water table
management systems for water quality. ASAE Paper 89-2129. 24 p.
Evans, R. O., and R. W. Skaggs. 1989. Design guidelines for water table
management systems on coastal plain soils. Applied Engineering in Agriculture
Gambrell, R. P., J. W. Gilliam, and S. B. Weed. 1974. The fate of fertilizer
nutrients as related to water quality in the North Carolina coastal plain.
Report no. 93. North Carolina Water Resources Research Institute, Raleigh.
Gambrell, R. P., J. W. Gilliam, and S. B. Weed. 1975. Nitrogen losses from
soils of the North Carolina coastal plain. Journal of Environmental Ouality
Gilliam, J. W., R. W. Skaggs, and S. B. Weed. 1978. An evaluation of the
potential for using drainage control to reduce nitrate loss from agricultural
fields to surface waters. Tech. Report no. 128. North Carolina Water Resources
Research Institute, Raleigh. 108 p.
Gilliam, J. W., R. W. Skaggs, and S. B. Weed. 1979. Drainage control to
reduce nitrate loss from agricultural fields. Journal of Environmental
Ouality 8(1): 137-142.
Gilliam, J. W., and R. W. Skaggs. 1985. Use of drainage control to minimize
potential detrimental effects of improved drainage systems. In:Proceedings
of the Specialty Conference "Development and Management Aspects of Irrigation
and Drainage Systems." Ir Div., ASCE. pp. 352-362.
Gilliam, J. W. 1988. Mtrate in North Carolina ground water. Proceedings
of the Soil Science Society of North Carolina. Raleigh. Vol. 31, pp 70-78.
Gregory, J. D., R. W. Skaggs, R. G. Broadhead, R. H. Culbreath, J. R. Bailey,
and T. L. Foutz. 1984. Hydrologic and water quality impacts of peat mining
in North Carolina. Report no. 214. North Carolina Water Resources Research
Institute, Raleigh. 215 p.
Harrison, W. G., and J. E. Hobbie. 1974. Nitrogen budget of a North Carolina
estuary. Report no. 86. North Carolina Water Resources Research Institute,
Raleigh. 172 p.
Heath, R. C. 1975. Hydrology of the Albemarle-Pamlico region of North Carolina.
USGS Water Resources Investigation 9-75. 98p.
Heath, R. C. 1980. Basic elements of groundwater hydrology with reference
to conditions in North Carolina. U.S.Geological Survey, Water Resources
Investigations, Open-File Report no. 80-44. 87 p.
Hobbie, J. E., B. J. Copeland, and W. G. Harrison. 1972. Nutrients in the
Pamlico River estuary, N.C., 1969-1971. Report no. 76. North Carolina Water
Resources Research Institute, Raleigh. 242 p.
Hobbie, J. E. 1974. Nutrients and eutrophication in the Pamlico River estuary,
North Carolina, 1971-1973. Report no. 100. North Carolina Water Resource
Research Institute, Raleigh.239 p.
Jacobs, T. C., and J. W. Gilliam. 1983. Nitrate loss from agricultural
drainage waters: implications for nonpoint source control. Report no. 209.
North Carolina Water Resource Research Institute, Raleigh. 99 p.
Jacobs, T. C., and J. W. Gilliam. 1985a. Headwater stream losses of nitrogen
from two coastal plain watersheds. Journal of Environmental Quality 14(4):
Jacobs, T. C., and J. W. Gilliam. 1985b. Riparian losses of nitrate from
agricultural drainage waters. Journal of Environmental Quality 14(4):472-478.
Kamprath, E. J. 1986. Nitrogen studies with corn on coastal plain soils.
Technical bulletin no. 282. North Carolina Agricultural Research Service.
Raleigh. 15 p.
Kirby-Smith, W. W., and R. T. Barber. 1979. The water quality ramifications
in estuaries of converting forest to intensive agriculture. Report no.
148. North Carolina Water Resources Research Institute, Raleigh. 70 p.
Konyha, K. D., R. W. Skaggs, and J. W. Gilliam. 1988. Hydrologic impacts
of agricultural water management. In: The Ecology and Management of Wetlands.
Vol. 2. pp 148-159.
Kuenzler, E. J., P. J. Mulholland, and L. A. Ruley. 1977. Water quality
in North Carolina coastal plain streams and effects of channelization.
Report no. 127. North Carolina Water Resources Research Institute, Raleigh.160
McDaniels, V., and R. W. Skaggs. 1988. Corn root response to high water
tables. ASAE Paper No. 88-2609. 15 p.
Miller, J. M. 1985. Effects of freshwater discharges into primary nursery
areas for juvenile fish and shellfish: criteria for their protection. In:
W. Gilliam, J. Miller, L. Pietrafesa, and W. Skaggs, Water Management and
Estuarine Nurseries. UNC Sea Grant Publication no. UNC-SG- WP-85-2. pp.
Miller, J. M., B. M. Currin, and M. L. Moser. 1988. Broad Creek report
- Faunal studies. In: M. F. Overton, J. S. Fisher, J. M. Miller, and L.
J. Pietrafesa, Freshwater Inflow and Broad Creek Estuary, North Carolina.
UNC Sea Grant Special Report. pp. 213-247.
Moser, M. L. 1987. Effects of salinity fluctuation on juvenile estuarine
fish. Ph.D. dissertation. North Carolina State University, Raleigh. 150
Naderman, G. C. 1985. Subsurface compaction and subsoiling in North Carolina.
Publication AG-353. North Carolina Agricultural Extension Service, Raleigh.20p.
Noga, E. J., M. J. Dykstra, and J. F. Levine. 1989. Fish diseases of the
Albemarle-Pamlico Estuary. Report no. 238. North Carolina Water Resources
Research Institute, Raleigh. 81 p.
NRCD. 1982. Chowan River water quality management plan. Division of Environmental
Management, North Carolina Department of Natural Resources and Community
NRCD. 1987. Surface water quality concerns in the Tar- Pamlico River basin.
Report no. 87-04. Division of Environmental Management, North Carolina
Department of Natural Resources and Community Development. Raleigh.
Paerl, H. W. 1982. Environmental factors promoting and regulating N2 fixing
blue-green algal blooms in the Chowan River, N.C. Report no. 176. North
Carolina Water Resources Research Institute, Raleigh. 76 p.
Paerl, H. W. 1983. Factors regulating nuisance blue-green algal bloom potentials
in the Lower Neuse River, N.C. Report no. 188. North Carolina Water Resources
Research Institute, Raleigh. 48 p.
Paerl, H. W. 1987. Dynamics of blue-green algal (Microcystis aeruginosa)
blooms in the Lower Neuse River, North Carolina: Causative factors and
potential controls. Report no. 229. North Carolina Water Resource ResearchInstitute,
Pate, P. P., and R. Jones. 1981. Effects of upland drainage of estuarine
nursery areas of Pamlico Sound, North Carolina. Working paper no. 81-10.
UNC Sea Grant, Raleigh. 25 p.
Pietrafesa, L. J. 1985. Response of Rose Bay to freshwater inputs. In:
W. Gilliam, J. Miller, L. Pietrafesa, and W. Skaggs, Water Management and
Estuarine Nurseries. UNC Sea Grant publication no. UNC-SG-WP-85-2. pp.
Pietrafesa, L. J., G. S. Janowitz, T. Y. Chao, R. H. Weisberg, F. Askari,
and E. Noble. 1986. The physical oceanography of Pamlico Sound. UNC Sea
Grant publication. UNC-WP-86-5. 125 p.
Pietrafesa, L. J., F. Askari, and C. Gabriel. 1988. On salinity fluctuations
in Broad Creek. In: M. F. Overton, J. S. Fisher, J. M. Miller, and L. J.
Pietrafesa, Freshwater Inflow and Broad Creek Estuary, North Carolina.
UNC Sea Grant Special Report. pp. 62-212.
Skaggs, R. W., and G. J. Kriz. 1972. Water table control and subsurface
irrigation in mineral and high organic coastal plain soils. Report no.
67. North Carolina Water Resources Research Institute, Raleigh. 63 p.
Skaggs, R. W., G. J. Kriz, and R. Bernal. 1972. Irrigation through subsurface
drains. Journal of the Irrigation and Drainage Division, ASCE 90:363-373.
Skaggs. R. W. 1973. Water table movement during subirrigation. Transactions
of the American Society of Agricultural Engineers 16(5):988-993.
Skaggs, R. W. 1978. A water management model for shallow water table soils.
Report no. 134. North Carolina Water Resources Research Institute, Raleigh.
Skaggs, R. W. 1980. A water management model for artificially drained soils.
Technical bulletin no. 267, North Carolina Agricultural Research Service,
Raleigh. 54 p.
Skaggs, R. W., J. W. Gilliam, T. J. Sheets, and J. S. Barnes.1980. Effect
of agricultural land development on drainage waters in the North Carolina
Tidewater Region. Report no. 159. North Carolina Water Resources Research
Institute, Raleigh, 164 p.
Skaggs, R. W., and J. W. Gilliam. 1981. Effect of drainage system design
and operation on nitrate transport. Transactions of the American Society
of Agricultural Engineers 24(4)929-934, 940.
Skaggs, R. W. 1982. Field evaluation of a water management simulation model.
Transactions of the American Society of Agricultural Engineers 25(3): 666-674.
Skaggs, R. W., and A. Nassehzadeh-Tabrizi. 1982. Effect of drainage system
design on surface and subsurface runoff from artificially drained lands.
Proceedings of the Inst. Symposium on Rainfall-Runoff Modeling. Mississippi
State University, Mississippi State, Miss. pp. 337-354.
Skaggs, R. W., A. Tabrizi, and G. R. Foster. 1982. Subsurface drainage
effects on erosion. Journal of Soil and Water Conservation 37(3): 167-172.
Skaggs, R. W., and A. Tabrizi. 1983. Optimum drainage for corn production.
Technical bulletin no. 274. North Carolina Agricultural Research Service,
Skaggs, R. W., and A. Tabrizi. 1986. Design drainage rates for estimating
drain spacings in North Carolina. Transactions of the American Society
of Agricultural Engineers 29(6):1631-1640.
Skaggs, R. W., R. O. Evans, and A. Tabrizi. 1989. Short cut methods for
the design of subirrigation systems. Transactions of the American Society
of Agricultural Engineers (in review).
Williamson, R. E., and G. J. Kriz. 1970. Response of agricultural crops
to flooding, depth of water table and soil gaseous composition. Transactions
of the American Society of Agricultural Engineers 13(2):216-220.