Water Table Management

Drainage has long been an important component of land management in the Coastal Plain and Tidewater regions of North Carolina. On flat, poorly drained soils, intensive drainage is necessary to facilitate seedbed preparation and planting in order to minimize plant stress and subsequent yield reduction resulting from poor soil aeration that accompanies waterlogging. Nearly half of the cropland currently used in North Carolina requires drainage improvement for efficient production. The drainage intensity required for agricultural production is not the same in all years or all periods of the year. While wetness is the major concern, weather conditions vary such that crops periodically suffer from drought stress that may substantially reduce yields in some years. Intensive drainage systems, necessary to provide trafficability during extreme wet periods often, remove more water than necessary during drier periods, leading to temporary overdrainage (Doty et al., 1986).

Problems with drought on drained soils have resulted in a transition from conventional drainage methods to water table management systems. The latter provide drainage during wet periods, but utilize control structures to manage the water level in 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 practices are referred to as water table management and involve a combination of management practices including surface drainage, subsurface drainage, controlled drainage, and/or subirrigation.

Types of Drainage Systems: What They Do and How They Work
Drainage is accomplished by two methods — open-ditch systems designed to provide primarily surface drainage (surface runoff) (Figure 6) or underground systems comprised of drain tile or tubing designed to lower the water table by subsurface flow. A surface drainage system typically consist of 3- to 5-foot deep, open ditches installed on 300- to 600-foot intervals. Surface runoff develops when the rate of rainfall exceeds the soil’s capacity to absorb water, thereby resulting in surface ponding. Shallow surface drains (hoe drains) are often utilized to effectively convey ponded surface water to ditches. Vegetated field borders and drop inlet pipes are used to stabilize ditch banks and minimize erosion while conveying surface runoff from the surface drains into the ditch. Figure 6. Surface drainage system.

Subsurface drainage is obtained by buried tile or tubing (4- to 6-inch diameter) that is placed 3 to 5 feet deep and 50 to 200 feet apart. A subsurface system provides drainage when the water table rises above the drain depth and water flows toward and into the drain. The drainage process whereby water infiltrates into the soil and moves within the soil profile is referred to as subsurface drainage, shallow ground water flow, or sometimes interflow.

In practice, it is often difficult to differentiate between surface and subsurface drainage, particularly in eastern North Carolina, because the outflow in drainage ditches or canals is usually a combination of both surface and subsurface flow. The relative proportion of surface and subsurface flow in the total drainage volume depends on many factors. These include rainfall intensity, land surface roughness and slope, vegetation, soil permeability, and ditch or drain tubing spacing and depth. Open ditches are normally spaced farther apart than buried tubing, which typically causes subsurface flow to be slow, resulting in collection of predominately surface drainage. But in highly permeable soils, open ditches may provide significant subsurface drainage. Such is the case in the Tidewater and Lower Coastal Plain where many fields are underlain by highly permeable sands at shallow depths, typically within 3 to 6 feet of the soil surface. Under such conditions, the drainage in open ditch systems is often predominately subsurface flow even through the ditch system is referred to as a surface drainage system.

Drainage and Water Quality
Nitrogen and phosphorus are transported from land-based activities to receiving streams and estuaries by drainage of excess rainfall. The difference in drainage method (surface versus subsurface flow) is important from a water quality standpoint because the characteristics of the two drainage waters differ. Surface drainage systems result in rapid removal of excess water over a relatively short time period. This water flowing over the land surface has relatively high energy sufficient to detach and transport soil particles and constituents attached to them, such as phosphorus, organic nitrogen, and many pesticides (Gilliam et al., 1978; Skaggs and Gilliam, 1981; Deal et al., 1986). Subsurface drainage typically contains very little sediment, but contains high concentrations of soluble constituents such as nitrate-nitrogen (Gilliam et al., 1978; Skaggs and Gilliam, 1981; Skaggs et al., 1982; Evans et al., 1987; Deal et al., 1986). Field research has documented nitrogen losses in drainage water from the edge of agricultural fields to average about 20 lbs. nitrogen acre-1 year-1. Site-to-site and year-to-year variation ranges from none to over 40  lbs. nitrogen acre-1 year-1. Factors causing this variability include land use, type of drainage, drainage intensity, and variability in rainfall, soil, landscape position, fertilization rate, type of crop, and harvested crop yield.

Controlled Drainage
Water control structures, such as a flashboard riser, installed in the drainage outlet allow the water in the drainage outlet to be raised or lowered as needed. This water management practice has become known as controlled drainage. When the flashboards are lowered or removed, subsurface drainage occurs more quickly (Figure 7a and 7b). When flashboards are added to the riser, the subsurface drainage rate is decreased and the height of the water level in the ditches and surrounding fields rises. Managing the field water through the use of controlled drainage allows timely drainage but also maximum storage of water within the field for utilization by the crop. Figure 7a. Controlled drainage showing flashboard riser. Figure 7b. Water profile in drainage ditch upstream of flashboard riser (see 7a).

The transport of nitrogen from drained fields can be minimized by managing the drainage system such that only the minimum drainage water necessary is allowed to exit the field. In numerous field studies (Gilliam et al., 1978, 1979; Skaggs et al., 1982; Deal et al., 1986; Evans et al., 1989), drainage control reduced the annual transport of total nitrogen at the field edge by 9 lbs. acre-1 year-1 or 45% on average. Nitrogen reductions resulting from controlled drainage result from two processes. First, controlled drainage reduces the volume of drainage water leaving a field from 20–30% on average; however, outflow varies widely depending on soil type, rainfall, type of drainage system and management intensity. During dry years, controlled drainage may totally eliminate outflow. In wet years, control may have little or no effect on total outflow. Second, controlled drainage provides a higher field water table level which promotes denitrification within the soil profile. In some cases, nitrate-nitrogen concentrations have been 10–20% lower in outflow from controlled systems compared to uncontrolled, free-draining systems. The combined effect of reduced flow and reduced nitrate concentration results in the overall 45% reduction in nitrogen mass transport at the field edge. Controlled drainage has also been documented to reduce phosphorus transport by 0.1  lbs. acre-1 or roughly 35% (Figures 8a, 8b, and 8c). Figure 8a. Average annual outflows measured from 14 sites in eastern North Carolina. The values shown represent approximately 125 site-years of data (from Evans et al., 1991). Figure 8b. Average annual nitrogen transport (TKN and NO3-N) in drainage outflow as measured at the field edge for 14 soils and sites (from Evans et al., 1991).
  Figure 8c. 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 (from Evans et al., 1991).

Management Recommendations
The successful management of controlled drainage systems rests on two important objectives. The first is achieving optimum production efficiency and maximum nutrient utilization by the crop; the second is attaining maximum water quality benefits. Controlled drainage structures require that the topography be relatively flat. The costs to production and water quality will usually exceed benefits when the land slope exceeds 0.5%. As a consequence, controlled drainage is most practical in the Lower Coastal Plain and Tidewater regions of North Carolina. The predominant cropping sequence in these areas is a two-year rotation of corn, wheat, and soybean. General guidelines for management of controlled drainage under this cropping sequence are given in Table 5.

The general guidelines present an attempt to achieve a balance between production and water quality goals. Many of the management indicators are hidden from view and the response to adjustments is not immediate. Intensive management with long- term monitoring is necessary to develop a site-specific understanding of the system. Productivity and water goals are compatible during some years, or at least seasonally during most the year. Under some conditions, however, productivity, water quality, or both goals may need to be mutually compromised for the benefit of the other. For example, management throughout the year is necessary to achieve maximum water quality benefit and drainage control to reduce nitrogen transport is most effective during winter and early spring periods. When fields are fallow, there is no significant production cost associated with holding water levels high to achieve the maximum water quality benefit. But productivity of some crops such as winter wheat may be reduced by high winter water table levels.

Table 5. General water table management guidelines to promote water quality and optimum crop yields for a two-year rotation of corn-wheat-soybeans (from Evans et al., 1991).

Period Production Activity Control Setting*
Jan.1-Mar.15 Fallow 12-18 Minimize drainage outflow and encourage denitrification
Mar. 15 – Apr. 5  Tillage, corn seedbed preparation, planting 30-36 Just deep enough to provide trafficability and good conditions for seedbed preparation. 
Apr. 15 – May 15 Corn establishment, early growth

Nitrogen sidedressing

24 – 30


Deep enough to promote good root development.

Just low enough to allow trafficability

May 15 - Aug. 15 Corn development and maturity 18-24 Temporary adjusting during wet periods
Aug.15 - Oct. 15 Harvesting, tillage; planting of wheat 30-36 Lower enough to provide trafficabilty
Oct. 15 - Mar. 1 Wheat establishment 18-24 Lower during extremely wet periods
Mar.1 - Mar. 15 Sidedressing wheat 30-36 Low enough to provide trafficability
Mar.15 - Jun. 15 Wheat development 18-24 Temporary adjustment during wet periods
Jun. 15 - Jul. 15 Harvesting wheat; tillage, planting of soybeans 30-36 Depends on rainfall
Jul. 15 - Nov. 1 Soybean development and maturity 18-24 Temporary adjustment to allow cultivation
Nov. 1 - Dec. 15 Soybean harvesting 36-42 Low enough to provide trafficability
Dec. 15 - Mar. 15 Fallow 12-18 Minimize drainage outflow and encourage denitrification

*Values shown are the control setting (depth below average surface elevation) and should not be considered the actual water table depth in the field, which will be lower except during drainage periods.
**Most adjustments are related to trafficability and must take into account weather conditions and soil water status at the time:


Although controlled drainage may provide water quality benefits, the same stream health benefits provided by riparian buffers are not realized, particularly on large channelized streams (Correll, personal communication, 1997).

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