Journal of Irrigation and Drainage Engineering

Fertilized cropland is a potential nonpoint source of nitrogen and phosphorus, which can contribute to the nutrient enrichment of surface water ecosystems. Research has shown that agricultural drainage water may contain fertilizer nutrients and pesticides. Nitrogen and phosphorus are present in drainage outflow primarily because of the addition of fertilizer, which results from the change in land use following drainage improvements instead of from the mere installation of drainage. Yet, public concern for the environment has led to a general criticism of agricultural drainage for degrading water quality. Controlled drainage has been recognized in some states as a best management practice to reduce the transport and delivery of nitrogen and phosphorus to sensitive surface waters. This article summarizes and compares water quality impact associated with conventional and controlled drainage practices. Nitrogen and phosphorus reductions of 30% to 50%, resulting from controlled drainage are reported in several studies. Several researchers believe further reductions are possible by carefully managing fertilization rates and timing to match crop uptake. Further minimization of the off-site impact of agricultural production will require a better understanding of the mechanisms of fate and transport of nutrients and pesticides and their interaction with drainage and other water-management alternatives such as controlled drainage.

A review of literature and summary of research results are presented on the effects of drainage on water quality in North Carolina, South Carolina, Georgia, and Florida. Principal findings from the predominate coarse-textured soils of the Atlantic Coastal Plain include increased nitrate-nitrogen losses associated with improved subsurface drainage. Benefits of drainage include potential reductions in organic nitrogen (N) and phosphorus (P) losses from these mineral soils. Results of drainage investigations on organic soils in the Everglades Agricultural Area of Florida indicate that phosphorus is the primary limiting factor for eutrophication problems in Lake Okeechobee. P losses can be potentially reduced by using slow versus fast drainage, retaining drainage water from vegetable and sugarcane fields (on sugarcane or fallow areas), and minimizing water-table fluctuations. Some of these potential practices and their resultant effects need additional verification for application to other areas and conditions.

The soils and the climate of the Great Lakes and Cornbelt states dictate that drainage is required to carry out economically viable farming activities. When drained, the soils are very productive and this eight-state region accounts for nearly 80% of the agricultural production of the United States. Drainage played an important role in the development of the region and a historical perspective is included to indicate the impetus for drainage and the amount of drainage application. Research results of agricultural drainage effects on water quality indicate that agricultural subsurface drainage has both positive and negative impacts; i.e., reduction in sediment and phosphorous, and increase in nitrate-nitrogen delivery to receiving waters. Research is needed to evaluate the full potential of controlled drainage and water-table management systems for managing agricultural effects on water quality. This information is needed by state and federal agencies to help landowners meet existing and impending water-quality requirements. Drainage is an important management practice for improving water quality while sustaining agricultural viability.

Agricultural drainage first began in Washington State around 1830 and in Oregon around 1840. The first efforts to drain soils for agricultural use began by constructing shallow ditches spaced 18–20 m apart to remove excess surface waters from croplands. Farming of poorly drained soils was done by these systems of beds until the 1930s. These beds interfered with the use of modern machinery and in the 1930s the establishment of beds was diminishing in favor of subsurface tile systems. Between 1930 and 1980 there was a substantial increase in subsurface-tile-drainage activity, which peaked during the 1960s. Since 1985, installations of subsurface and surface drainage have gradually diminished, and constuction of drainage systems has been limited to replacement of failed or deteriorated systems. By reducing saturated soil conditions through subsurface drainage, surface-water quality has benefited by reductions of nutrients and pesticides associated with surface runoff. With the increase of agricultural (dairy) waste applications to cropland, additional research is needed to quantify the impacts of subsurface drainage on ground- and surface-water quality.

Drainage is an essential land-management practice for growing crops in the Lower Mississippi Valley and Gulf coastal areas. Current crop production and water-management methods pose significant potential for surface and ground–water contamination. Management practices need to be identified, evaluated, and refined so that threats to water quality can be eliminated without significant loss in productivity. Sediment and nutrient (N, P, K) losses were measured from research plots with surface drainage only and from plots with both surface and subsurface drainage for all or part of the decade 1982–1991. During the 1987 growing season, losses of pesticides (atrazine and metolachlor) were also measured. Subsurface drainage was effective in reducing surface runoff by an average of 35%, and thus reduced soil loss by 31%, nitrogen loss by 17%, phosphorus loss by 31%, and potassium loss by 27%. For the 1987 season, losses of preemergent applied herbicides from the plots with subsurface drainage were less than one-half that from plots with surface drainage only: 1.4% versus 3.2% of applied for atrazine, and 1.1% versus 2.4% of applied for metolachlor.

Drainage has been used in the northeastern United States and Canada since colonial times. It has only been since the 1970s that substantial quantities of subsurface drains have been installed annually in Quebec. Drainage in Ontario was first installed in the early 1900s. Today there is an increasing concern that drainage is harmful to the environment. It has only been in recent years that projects have been initiated to evaluate the environmental impacts of agricultural drainage. Both nitrates and pesticides have been detected in agricultural drainage waters, sometimes at concentrations exceeding drinking-water standards. Research has identified the need for integrated management of water quality and quantity. There is a need to quantify the role of preferential flow as a mechanism of pollutant transport in surface and subsurface drainage water and to develop a modeling approach that is applicable at the field and watershed scale.

Agricultural drainage development in the form of drainage alone and controlled drainage– subirrigation (CD-SI) has created environmental concerns. The primary concern is increased loading of nutrients, pesticides, and sediments to environmentally sensitive waters. Productive steps to address these concerns are hampered by the lack of research data and the lack of public knowledge of the history, development, design, and water-quality impacts of these systems. The research activities of the 1970s and 1980s in some states have provided information on the adverse and beneficial impacts of agricultural drainage and overall water-table-management systems. This paper reviews the history, magnitude, institutional and social constraints, and the water-quality impacts of agricultural drainage in the Mid-Atlantic states (New Jersey, Delaware, Maryland, and Virginia). It also provides examples of the results from selected research activities in these states. Finally, this paper provides some thoughts on research needs. Overall, this study concludes that the research data on the impact of agricultural drainage on water quality and design guidelines for the specific geologic conditions of the Mid-Atlantic states is limited and in many instances nonexistent. Drainage legislation in these states has a history of success in forming drainage districts and associations, although accurate research data are needed to address the environmental concerns (negative and positive) of these systems.