Tile drainage (in agriculture) is an agriculture practice that removes excess water from soil subsurface. Whereas irrigation is the practice of adding additional water when the soil is naturally too dry, drainage brings soil moisture levels down for optimal crop growth. While surface water can be drained via pumping and/or open ditches, tile drainage is often the best recourse for subsurface water. Too much subsurface water can be counterproductive to agriculture by preventing root development, and inhibiting the growth of crops. Too much water also can limit access to the land, particularly by farm machinery. In terms of access, most modern agriculture depends on the usage of large machinery—tractors and implements—to prepare the seedbed, plant the crop, carry out any cultivation and applications during the growing season, and ultimately, to harvest the crop. Operating most machinery in excessively wet conditions may result in soil degradation because of excessive soil compaction, and get stuck.
Increased crop yields
Most crops require specific soil moisture conditions, and do not grow well in wet, "mucky" soil. Even in soil that isn't "mucky" the roots of most plants will not grow much deeper than the water table. Early in the growing season when water is in ample supply, plants are small and don't require much water. During this time, the plants do not need to develop their roots to "reach" for the water. As the plants grow and use more water during the growing season water becomes more scarce. During this time, the water table starts to fall. Plants suddenly need to start developing roots to reach to the water. During dry times, the water table can fall faster than the plants can develop roots to "reach" for the water. This can seriously stress the plants.
By adding drain tile, the water table is effectively lowered, and plants can properly develop their roots. The lack of water saturation allows oxygen to exist in the soil around the roots. Drain tile prevents the roots from being under the water table during wet periods that could cause excessive plant stress. By removing excessive water, crops use water they have more effectively.
An increase in crop yield can be simply summarized by the following: Simply by forcing the plants to have more developed roots, the plants can more effectively absorb more nutrients and water.
The same principle is employed by containers that hold house plants: they have drain holes in the bottom to allow oxygen to the roots, and prevent soil saturation. By placing drain tile under a field in a grid style layout, the same effect can be employed on a several hundred acre field.
Plumbing of drain tile
In a tile drainage system, a sort of "plumbing" is installed below the surface of agricultural fields, effectively consisting of a network of below-ground pipes that allow subsurface water to move out from between soil particles and into the tile line. Water flowing through tile lines is often ultimately deposited into surface water points—lakes, streams, and rivers—located at a lower elevation than the source. Water enters the tile line either via the gaps between tile sections, in the case of older tile designs, or through small perforations in modern plastic tile.
Soil type greatly affects the efficacy of tile systems, and dictates the extent to which the area must be tiled to ensure sufficient drainage. Sandier soils will need little, if any, additional drainage, whereas soils with high clay contents will hold their water tighter, requiring tile lines to be placed closer together.
History of tile drainage
Both Cato and Pliny have described tile drainage systems, in 200 BC and the first century AD, respectively. According to the Johnston Farm website, tile drainage was first introduced to the United States in 1838, when John Johnston brought the practice from his native Scotland to his farm in Seneca County, New York. Johnston labored to lay 72 miles (116 km) worth of clay tile on 320 acres (1.29 km2). The effort paid off by increasing his wheat yield from 12 bushels per acre to 60 bu/acre. Johnston, "Father of American Tile Drainage", continued to advocate tile drainage throughout his life, attributing his success as a farmer to the formula "D,C, and D" (dung, credit, and drainage).
The expansion of drainage networks was an important technical aspect of Westward Expansion in the 19th century. Although land in the United States was parceled out in accordance with the Public Land Survey System as established by the Land Ordinance of 1785, development, especially of agricultural land, was often limited by the rate at which it was made capable for cultivation. For example, although Iowa was made a state in 1846, maps depicting land ownership show below-average population densities in the northwestern region as late as the 1870s, a corner of the state that today is still noted for its high water table and numerous lakes, marshes, and wetlands.
States throughout the region faced similar limits to agricultural intensification. Many states offered government incentives to improve land for farming. For example, legislation in Indiana prompted an Act of Congress in 1850 that provided for swamplands to be sold at a discount to farmers on the condition that they drain the land and bring it into agricultural productivity. To facilitate this process, most states set up government agencies to oversee and regulate the installation of tile drainage systems. Even today, ballots for elections in rural America often include candidates for local drainage supervisory boards.
The decades following the American Civil War saw rapid expansion of drainage systems. For example, historical literature from Ohio notes that in the year of 1882, the number of acres drained was about equal to the area of land drained in all previous years. In the 1930s, the Civilian Conservation Corps contributed to the tile network throughout the Midwest, much of which is still in use.
Advances in drainage technology
Throughout the twentieth century, the technology of tile installation remained similar to the methods first used in 1838. Although cement sections later replaced the original clay tiles, and machines were used to dig the trenches for the tile lines, the process remained quite labor-intensive and limited to specialized contractors.
The introduction of plastic tile served to reduce both the cost of tile installation, as well as the amount of labor involved. Rather than set individual sections of cement tile end-to-end in the trench, tile installers had only to unroll a continuous section of lightweight, flexible tile line. Towards the end of the twentieth century, when large, four-wheel-drive tractors became more common on American farms, do-it-yourself tile implements appeared on the market. By making tile installation cheaper and allowing it to be done on the landowner's schedule, farmers are capable of draining localized wet spots that may not create enough of a problem to merit more costly operations. In this way, farmers may enjoy increases in crop yield while saving on the capital costs of tile installation. Perhaps the most useful implement in drainage history was James B. Hill's Buckeye Traction Ditcher, which laid drainage tiles at a record pace. Hill's ditching machine drained the Great Black Swamp in Ohio, vast stretches of Louisiana, and Florida's swampland.
Social and ecological effects of tile drainage
The ability for farmers to install their own tile can be problematic. First of all, private installations may reduce the ability of local drainage supervisory boards to regulate tile installation, which in some areas of the country requires proper documentation before a contractor can continue. This leads to the second potential conflict, the unintentional interruption of existing tile networks. Most "do-it-yourself" tile plows do not dig trenches but rather split the soil enough to squeeze the tile line in; thus, a farmer would not be aware if he breaks a line of tile that might serve his neighbors, as well (Mutual tile lines are often dictated by topography rather than land ownership, and the location of many old, but still effective, tile lines are unknown.). The potential for across-the-fence disputes is obvious.
Ecologically, the expansion of drainage systems has had tremendous negative effects. Hundreds of thousands of wetland species experienced significant population declines as their habitat was increasingly fragmented and destroyed. Although market hunting within the Central Flyway was a contributing factor in the decline of many waterfowl species' numbers in the early decades of the twentieth century, loss of breeding habitat to agricultural expansion is certainly the most significant. Early maps of midwestern states depict many lakes and marshes that are either nonexistent or significantly reduced in area today. Channelization, a related process of concentrating and facilitating the flow of water from agricultural areas, also contributed to this degradation.
Tile drainage and the corresponding changes to the landscape - draining wetlands, wet soils, and channelizing streams – have contributed to more erosive rivers. This response of rivers due to drainage is the result of shortening the residence time of water on the landscape. For example, precipitation used to be held in wetlands and in/on the surface of soils, continuously evaporating or being used via transpiration of plants. Water would slowly drain through the landscape and eventually drain to rivers. The process of tile drainage, used to dry soils quickly and efficiently, results in an efficient transmission of water to the river – so efficient, in fact, that higher volumes of water are delivered to rivers. The effect of higher volumes of water is more energy in water - the dynamic equilibrium state that rivers existed in for centuries (slowly changing shape and continuously transporting limited sediment) was, and currently is, out of balance. The result of this loss of equilibrium is extreme amounts of bank erosion which results in over-burdensome sediment loads and critical impacts to natural environments and riverine habitats.
Drainage tile sometimes decreases soil erosion and runoff of some nutrients, including phosphorus. Phosphorus is an important nutrient to control because it is the limiting nutrient in most aquatic ecosystems. Thus phosphorus is the main culprit in eutrophication of surface water; however, the other limiting nutrient, nitrogen, causes substantial damage to waters. For example, nitrogen has been implicated in the gulf hypoxia. Drainage tile sometimes increases water quality because the water flows into the ground then the tile, instead of running off the field into a ditch, carrying soil and nutrients with it. The soil has a chance to filter the water before it enters the streams and rivers. However, by bypassing surface improvements like conservation tillage or riparian buffers, tile drainage can also create problems with water quality  and outflow from tile drainage tends to be extremely high in nitrogen. Furthermore, some tile drainage sometime contains very high levels of other chemicals. Since surface forms of conservation agriculture are less effective in tile-drained systems, other practices such as controlled drainage or constructed wetlands may be more effective. In very flat areas, where the natural topography does not provide the gradient necessary for water flow, "agricultural wells" can be dug to provide tile lines sufficient outlet. In these cases, it is the groundwater that stands to be polluted by unfiltered tile output.
Intensive livestock operations (ILO) have led to challenges of livestock effluent disposal. Livestock effluent contains valuable nutrients, but the misapplication of these materials can lead to serious ecological problems, such as nutrient loading. Injecting effluent directly into the ground is one method employed by manure applicators to improve nutrient uptake. Drainage tiles may increase injected manure seepage into surface waterways from manure injection because liquid manure seeps through soils and then drains out of the field and into waterways via drainage tiles.
Today, a number of state and federal initiatives serve to reverse habitat loss. Many programs encourage and even reimburse farmers for interrupting the drainage of localized wetholes on their property, often by breaking tile intakes or removing the tile completely. Landowners are often partially or fully compensated for forfeiting the ability to grow crops on this land. Such programs and the cooperation of landowners across the country have had significant positive effects on the populations of a wide variety of waterfowl.
- Drainage equation
- Drainage system (agriculture)
- Watertable control
- Salinity control by subsurface drainage
- Drain spacing equation using the energy balance of groundwater flow
- Drainage research
- This site  gives free downloads of articles and software on land drainage
- University of Minnesota drainage fact sheet
- Schottler et al. Hydrological Processes: Twentieth century agricultural drainage creates more erosive rivers
- Agricultural Drainage Publication Series: Issues and Answers
- Lemke et al. Evaluating Agricultural Best Management Practices in Tile-Drained Subwatersheds of the Mackinaw River, Illinois
- Lemke et al. Evaluating Agricultural Best Management Practices in Tile-Drained Subwatersheds of the Mackinaw River, Illinois