Agricultural and Environmental Impacts of Drainage
Eileen Kladivko, Department of Agronomy
Introduction
Water is a vital resource for
agriculture,
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
While enabling
Why is drainage needed?
Average annual precipitation in
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 in
How does drainage work?
Two types of drainage improvements
are commonly used in
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.
Construction of drains that are
shared by many landowners was an important process in agricultural development
in
Ultimately, the water drained from
agricultural field in
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
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
The 1985 Farm Bill, or Food
Security Act, made a dramatic change in the way the
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.
Good subsurface drainage Poor subsurface drainage
Figure 1: Flow from a watershed with poor drainage and a similar watershed with good subsurface drainage.
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
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
Conclusion
Agricultural drainage is an
essential management practice on many
For more information
More information on the topics covered in this introduction can be found in the following publications:
AY-300: Drainage Recommendations
for
AY-301: Wet Soils of
Ohio State University Bulletin 871
“Agricultural Drainage: Water Quality Impacts and Subsurface Drainage Studies
in the
Any of these publications can be
obtained from your county office of Purdue Extension, or from the
References
Estimates of
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 the
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For
more information contact Jane
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