Agricultural and Environmental Impacts of Drainage

Jane Frankenberger, Department of Agricultural and Biological Engineering

Eileen Kladivko, Department of Agronomy

Introduction

Water is a vital resource for agriculture,Indiana’s largest industry. The plentiful supply of water in most years helps make Indiana one of the nation’s leading agricultural states. However, at certain times of the year excess water can prevent timely farm operations, restrict plant root growth, and increase erosion.

The purpose of agricultural drainage is to remove excess water from the soil in order to enhance crop production. In some soils, the natural drainage processes are sufficient for growth and production of agricultural crops, but in many other soils, artificial drainage is needed for efficient agricultural production. About 50% of Indiana’s cropland is artificially drained. This percentage is among the highest in theU.S. The percentage of cropland that has subsurface or tile drainage in Indiana(35%) is the highest in the nation. Most of the more than 8 million acres in Indiana with drainage improvements would not be productive without the ditches or tiles that remove excess water from the field. Drainage is therefore a critical component of agricultural production in Indiana.

While enabling Indiana farmers to produce outstanding yields, drainage has led to the loss of wetlands that support wildlife and improve water quality. Tile drains also present a direct flowpath for nitrate loss, a potential water quality concern. Public concern over the loss of wetlands and water quality effects of drainage has led to serious questions about the impact of drainage improvements on Indiana’s water resources and environment. This publication discusses the need for drainage inIndiana, explains how agricultural drainage works, and addresses the environmental effects of drainage.

Why is drainage needed?

Average annual precipitation in Indiana ranges from 36 inches in the northeast to 44 inches in the southwest. Only about two-thirds of this is used by crops. Monthly precipitation remains fairly constant throughout the year, while evapotranspiration (a combination of evaporation from soil and transpiration from the crop), changes with the season. From January to May, and from October to December, precipitation is considerably greater than evapotranspiration, creating a water surplus. Crop water needs exceed precipitation only in July, August, and September. The surplus of precipitation over evapotranspiration results in excess water in the crop root zone.

If the water table (the uppermost depth at which water moves freely in the soil) is too high, crop growth is reduced. The water table can be thought of as the depth to which water will rise in a well or a hole dug in the ground. Some low-lying soils have permanent high water tables. Other soils may be poorly drained because of seepage from upslope areas, or because they are in a depressional area with no outlet. Some soils have a slowly permeable subsurface layer, which restricts vertical drainage and leads to a high water table during some portions of the year. More detailed information on soils and drainage can be found in AY-301, “Wet Soils of Indiana.”

How does drainage benefit crops?

Soils consist of solid particles (sand, silt, clay, and decomposing plant materials) and the pore spaces between the solid particles, which may be filled with water or air or both. Plant roots need oxygen to grow. When the soil is saturated with water, the plant roots will survive for a short time by using the oxygen dissolved in the water. With prolonged wetness, however, the oxygen is depleted and roots die due to a lack of oxygen. Tile drainage lowers the water table, making room for air to move back into the soil and replenish oxygen to the roots. Thus, a major function of drainage is to improve aeration for root growth.

In a poorly drained soil, root growth is restricted by the high water table early in the season, such that when the water table drops rapidly during mid-season, further root growth and hence crop growth is impaired. In the drained soil, however, the root system develops more fully in the spring, enabling the plant to have access to deeper water in the dry periods of mid-summer.

Another symptom of poor drainage that can be readily seen in some years is an overall yellow appearance in the green vegetation, especially for corn and wheat. Although the yellow color may have several different causes, one of the most common is a nitrogen deficiency in the plant. This may be due to an oxygen shortage causing poor uptake of nutrients by the roots, or it may be due to denitrification (conversion of nitrate to nitrogen gas), a loss of needed nitrogen from the soil. These yellow areas in a field can be useful indicators of places where additional drainage improvement might be needed.

Improved drainage is also vital for timely field operations in the spring. It is estimated that corn yields are reduced 1-2 bushels per acre for each day after May 10 that corn is planted inIndiana. Since driving on a wet field is detrimental to soil structure, the soil must be adequately dry before planting can take place. Improved drainage allows the farmer to get into the field several days to several weeks earlier than would be possible without drainage.

How does drainage work?

Two types of drainage improvements are commonly used inIndiana: surface and subsurface. Often a combination of these two is used, which usually maximizes drainage benefits.

Surface drainage is the removal of water that collects on the land surface. Many fields have low spots or depressions where water ponds. Surface drainage techniques such as land leveling, constructing surface inlets to subsurface drains, and the construction of shallow ditches or waterways can allow the water to leave the field rather than causing prolonged wet areas.

Subsurface drainage removes excess water from the soil profile, usually through a network of perforated tubes installed 2 to 4 feet below the soil surface. These tubes are commonly called “tiles” because formerly they were made from short lengths of clay pipes known as tiles. Water would seep into the small spaces between the tiles. Today the most common type of “tile” is actually corrugated plastic tubing with small perforations to allow water entry. When the water table in the soil is higher than the tile, water flows into the tubing, either through holes in the plastic tube or through the small cracks between adjacent clay tiles. This lowers the water table to the depth of the tile over the course of several days.

Drain tiles allow excess water to leave the field, but once the water table has been lowered to the elevation of the tiles, no more water flows through the tiles. In most years, drain tiles are not flowing between June and October.

Where does the water go?

The water is carried through the drain to an appropriate outlet, usually a stream or a ditch. The outlet is one of the most important considerations in planning and installing a drainage system.Indianahas many flat areas where it is often difficult to find an outlet sufficiently low to drain the field. In a few cases, water is pumped up to a ditch or stream, although this is much more expensive than using gravity alone.

Construction of drains that are shared by many landowners was an important process in agricultural development inIndiana. Certain drains have been designated as regulated drains by the County Drainage Board (or the Commissioners Court or Circuit Court of each county prior to 1965.) Maintenance of these regulated drains today is the responsibility of the Drainage Board, which consists of three members appointed by the County Commissioners, advised by the County Surveyor. The Drainage Board is responsible for construction and maintenance of drains, and to make judgments when there is a conflict between landowners that may use the same drain.

Ultimately, the water drained from agricultural field in Indiana flows to rivers and streams that carry it either to the Great Lakes(the northern 10% of the state) or the Mississippi River(the rest of the State).

How are drainage systems designed?

Designing and installing a drainage system is a complex process. Every field is unique and usually requires an individual design. Drainage depends on topography, crops that will be grown on the field, and soil type. Every soil type has different properties that affect its drainage. Scientists and engineers have developed recommendations for drainage depth and spacing in each soil type in Indiana based on years of experience and knowledge of soil properties. These recommendations are given in AY-300, Drainage Recommendations for Indiana Soils.

Drainage contractors use these recommendations, along with principles of sound drainage design, to design drainage systems that economically and effectively drain a particular field.

Don’t we need to protect wetlands?

There is no doubt that much of the Indiana landscape consisted of wetlands before large-scale drainage began in the 19th century. Stories about wagons sinking in the swampy ground, constant swarms of mosquitoes, and even malaria can be found in many accounts of early settlement of Indiana. About 85% of Indiana’s original wetlands have been drained. Although the public health and economic benefits resulting from the draining of these wetlands over the last 150 years are clear, there have also been negative impacts on the environment. Wetlands have an important hydrologic function in regulating water flow and improving water quality, as well as providing habitat for water-based wildlife. Recognition of the their valuable functions has changed the way society thinks about and protects wetlands.

The 1985 Farm Bill, or Food Security Act, made a dramatic change in the way the U.S. treats wetlands. The provision commonly known as “swampbuster” made anyone who converted land from wetlands to agriculture ineligible for federal farm benefits. Land converted prior to 1985 can remain in agriculture, but further conversion of wetlands is heavily restricted. More information on drainage and wetlands can be found in ID-xx, “Wetland Regulations in Indiana.”

Drainage improvements today are therefore very rarely for the purpose of converting existing wetlands to agricultural production, but are usually aimed at making existing agricultural land more productive. Some fields have drain tiles that were installed 100 or more years ago, and are broken or plugged. In many fields, only a few of the wettest spots were originally drained, while the entire field would benefit from improved drainage. More tiles are often added to improve drainage efficiency, with the goal of increasing production.

How do tile drains affect water quality?

Drainage has both positive and negative effects on water quality. In general, land that has good subsurface drainage has less surface runoff, erosion, and phosphorus transport than equivalent land without drainage improvements or with only surface drainage. Figure 1 shows a hydrograph (a graph of water flow as a function of time) from two fields that are similar in every way, except that one has good subsurface drainage while the other has poor surface drainage. The total flow from each one is about the same, but the field with poor subsurface drainage has a peak flow rate more than twice as high as the other. Higher peak flows usually result in more erosion, so sediment problems are usually reduced by good subsurface drainage. Phosphorus, which moves with eroded soil, is also reduced when more water flows with subsurface drainage rather than as surface runoff.

Nitrate movement does not depend on surface runoff, however. Because it is very soluble, it flows readily with water through the soil and into tile lines. Nitrate concentration often increases with improved subsurface drainage. For example, the nitrate concentration measured in the watersheds shown in Figure 3 was nearly three times higher in the watershed with good subsurface drainage. Nitrate flow from subsurface drains is one of the main sources of nitrate in streams and rivers in the Midwest. Concern about hypoxia, or low oxygen, in the Gulf of Mexicohas increased concern about nitrate sources. Concentrations of nitrate in tile drains are usually quite high (10-40 mg/l).

Pesticides also flow into subsurface drains, but only in very limited concentrations. Pesticides move more easily in flow over the soil than through the soil, so the highest concentrations of pesticides in tiles are often in fields that have direct surface inlets to the drains. Subsurface drainage may reduce pesticide loss to rivers and streams because it reduces surface runoff.

What can be done to minimize the impact of drainage on water quality?

Traditionally, the goal of drainage design was to maximize benefits to the crop while minimizing costs of drainage installation. Reducing water quality effects of drainage should become a consideration in future drainage improvements. Nitrate is the biggest water quality concern related to tile drainage, and several new technologies are being developed that show promise for reducing negative impacts. Controlled drainage is a system that keeps the water table in the field high during the off-season when crops are not growing. It therefore increases the rate of denitrification (a process that converts nitrate to harmless nitrogen gas when the soil is saturated) and reduces nitrate loss to the environment. Controlled drainage can be combined with subirrigation to improve yields while protecting water quality. Subirrigation is irrigation through the subsurface drain tiles, rather than more conventional methods such as using sprinklers. Subirrigation is often economical when fields would need to be drained anyway, since additional infrastructure consists mainly of increased numbers of tiles and the pumping system. One system being developed in Ohio combines a wetland for water treatment and a pond serving as a reservoir for subirrigation with a drainage system. This system has been shown to increase yields and reduce water quality impacts of drainage. It remains costly however, because of the land that is needed for the wetland and pond/reservoir.

Conclusion

Agricultural drainage is an essential management practice on many Indiana soils. Appropriate drainage improves crop growth and the efficient use of production inputs, thereby ensuring a more dependable supply of food and feed from Indiana farms. The current focus of most drainage improvements is to optimize crop production on land that is already in agricultural production. Although drainage improvements were traditionally evaluated solely for their impacts on the field in which they were installed, it is now important to consider the impacts of further drainage improvements on downstream water quantity and quality. Drainage affects the entire watershed and must be considered as one element in overall water management within the watershed.

For more information

More information on the topics covered in this introduction can be found in the following publications:

AY-300: Drainage Recommendations forIndianaSoils

AY-301: Wet Soils ofIndiana

Ohio State University Bulletin 871 “Agricultural Drainage: Water Quality Impacts and Subsurface Drainage Studies in theMidwest” provides more extensive information on water quality and practices to reduce nitrate from subsurface drain tiles.

Any of these publications can be obtained from your county office of Purdue Extension, or from thePurdueExtensionMediaDistributionCenter(1-888-EXT-INFO or Media.Order@ces.purdue.edu.)

References

Estimates ofIndiana’s drainage are from Pavelis, G., 1987. Economic survey of farm drainage. In Pavelis, G . (Editor), Farm Drainage in theUnited States: History, Status, and Prospects. Economic Research Service, USDA Misc. Pub. 1455.

Acknowledgements

Thanks to the following reviewers: Don Franzmeier, Department of Agronomy, and Bernard Engel, Department of Agricultural and Biological Engineering. Figure 1 is based on a graph from Skaggs, W., 1987. Principles of Drainage. In Pavelis, G . (Editor), Farm Drainage in theUnited States: History, Status, and Prospects. Economic Research Service, USDA Misc. Pub. 1455.

https://engineering.purdue.edu/SafeWater/Drainage/drainintro.htm

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