US EPA: AG 101
http://www.epa.gov/agriculture/ag101/cropdrainage.html
Drainage
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Operations and Timing
Equipment Used
Potential Environmental Problems
Best Management Practices
Impacts of poor drainage -- stunted, yellow plants
Source: Purdue University
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.
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.
Poorly drained area in crop field will damage yields
Source: Purdue University
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because they were originally made from short lengths of clay pipes known as tiles. Water would seep
into the small spaces between the tiles and drains away.
Drain tile outlet to a drainage ditch
Source: Purdue University
The most common type of "tile" is 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.
Operations and Timing
On average, about two-thirds of annual precipitation is used by crops in the eastern Corn Belt. The
rest falls at a time when it does not meet crop needs. Monthly precipitation remains fairly constant
throughout the year, while evapotranspiration (a combination of evaporation from soil and
transpiration from the crop), is much higher from June to September. From January to May, and from
October to December, precipitation is greater than evapotranspiration, creating a water surplus. The
surplus results in excess water in the crop root zone and the need for drainage. Drainage is primarily a
concern in the periods prior to the growing season (January to April) so that crops can be planted at
the optimum time.
(Note: Even in humid areas, crop water needs often exceed precipitation in July, August, and
September. Although some of the moisture deficiency is met by stored soil moisture, irrigation can
often increase yields.)
Equipment Used
Trenching machine used by drainage contractors to install subsurface drainage tile (shown in white)
Source: Purdue University
Drainage plow being pulled by farm tractor installing a tileline
Source: Purdue University
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. Agronomists and
engineers have developed recommendations for drainage depth and spacing in specific soil types
based on years of experience and knowledge of soil properties. Drainage contractors use these
recommendations to design drainage systems that economically and effectively drain a particular field.
Drainage plows that can be pulled by farm tractors are becoming more popular. But most farmers hire
contractors to design and install their tile drainage systems because of the knowledge, skills, and
experience needed to install a successful system.
Potential Environmental Concerns
The major concerns related to drainage are:
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Loss of wetlands, and
Increased loss of nitrate through tile drains.
Wetlands
Much of the Midwest landscape consisted of wetlands before large-scale drainage began in the 19th
century. Although enormous public health and economic benefits have resulted from the draining of
these wetlands over the last 150 years, there have also been negative impacts on the environment.
Wetlands have an important hydrologic function in regulating water flow and maintaining water
quality, as well as providing habitat for water-based wildlife. Recognition of their value has changed
the way our society thinks about and protects wetlands.
Drainage improvements today are rarely for the purpose of converting existing wetlands to
agricultural production. Improved drainage is 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.
Water quality
Poor Drainage
Source: Purdue University
Drainage has both positive and negative effects on water quality.
In general, less surface runoff, erosion, and phosphorus is lost from land that has good subsurface
drainage than from land without drainage improvements or with only surface drainage.
Nitrate loss can be quite high from drained land. Because nitrate is very soluble, it flows easily
through the soil and into tile lines. 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 levels, in the Gulf
of Mexico has increased concern about this nitrate source. Concentrations of nitrate in tile drains are
usually quite high (10-40 mg/l).
Pesticides can also flow into subsurface drains, but usually only in very low concentrations. Pesticides
move more easily in flow over the soil surface than through the soil, so the highest concentrations of
pesticides in tiles are often in fields that have surface inlets into the drains. In fact, subsurface
drainage may actually reduce pesticide loss to rivers and streams because it reduces surface runoff.
Best Management Practices
Source: Purdue University
Traditionally, the goals of drainage were to:
1. Maximize crop yield and
2. Minimize costs of drainage installation.
Reducing water quality effects of drainage is becoming a third objective in drainage design.
Nitrate loss is the biggest water quality concern related to tile drainage. Several new technologies can
reduce nitrate loss. Controlled drainage keeps the water table high during the off-season when
crops are not growing. The high water table increases the rate of denitrification (a process that
converts nitrate to harmless nitrogen gas (N2) as soon as the saturated soil warms up in the spring)
and reduces nitrate loss to the environment.
Controlled drainage can be combined with subirrigation to improve yields while protecting water
quality. Subirrigation is irrigation back through the subsurface drain tiles. Subirrigation may be
economical when fields are relatively level and need to be drained anyway, since additional
infrastructure consists mainly of increased numbers of tiles 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, although it is costly.