Prepared by:
Robert Evans, Extension Agricultural Engineering Specialist
J.W. Gilliam, Professor of Soil Science
Wayne Skaggs, William Neal Reynolds Professor
Department of Biological and Agricultural Engineering

Published by: North Carolina Cooperative Extension Service

 Publication Number: AG 443

Last Electronic Revision: June 1996 (KNS)


North Carolina farmers grow crops on over 2 million acres of poorly drained soils. Thesefields represent nearly 40 percent of the state's cropland. Cropland drainage has long been one of the most important components of land management in the coastal plain and tidewater regions of the state beginning soon after colonization more than 300 years ago (28). *

 Many of these artificially drained soils are adjacent to estuaries and freshwater lakes and streams, areas that are environmentally sensitive and ecologically important. Agricultural runoff has contributed to the decline of water quality in many of the state's surface water systems (36, 45, 46, 50). As shown in Figure 1, agricultural production, tourism, recreation, and fishing contribute more than $10 billion to the state's economy and $4 billion to the economy of coastal counties. Since these important industries must coexist, future agricultural practices must be designed and managed in a way that will protect water quality.

 Restrictions on land development and drainage imposed by the 1985 Food Security Act have limited the possibility of expanding farming operations by clearing and draining land. Instead, farmers are using more intensive management practices to increase yields and improve production efficiency on land already in cultivation.

 *Numbers in parantheses refer to the list of references at the end of this publication.

 Figure 1. Contribution of agriculture, recreation, tourism, and marine fishing to the economy of North Carolina. (All values are gross direct receipts to source that is, paid to farmers, fisherman, etc.- and do not include indirect revenues such as processing, packaging, transportation, distribution, and retailing.)

Source: MC. Agricultural Statistics, USDA, 1988; North Carolina Department of Natural Resources, Division of Parks and Recreation; NC. Department of Commerce, Division of Travel and Tourism; and NC. Division of Marine Fisheries.

Water Quality Concerns

The hydrologic, biological, and chemical cycles in coastal rivers and estuaries are extremely complex. Although fresh water and nutrients are essential components of these systems (30, 40, 50), they may become unbalanced as a result of human activities. These activities include 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).

 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.