Drainage Water Management Online Advisory

Water Table Management

Drainage has long been an important component of agricultural 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 enhance 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) (see below image) or underground systems comprised of drain tile or tubing designed to lower the water table by subsurface flow. A surface drainage system typically consists 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.

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 groundwater 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 though 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.