Managing water has been and always will be an agricultural challenge. The challenge can be too little or too much. Irrigation methods provide opportunities to address areas with too little.
In higher rainfall areas, too much water can delay or prevent fieldwork and floods can destroy planted crops. Surface and subsurface drainage can help remove excess water. Surface drainage involves the construction of waterways, ditches, and other surface structures designed to direct surface water away from tillable fields. The most common drainage system for row crops is tiling. Tiling is a constructed subsurface system of penetrable “pipes” that allows the seepage of soil water into the system to effectively dry out excessively wet soil by channeling the water to a stream, ditch, river, or lake underground. Years ago, segments of usually round clay or concrete tiles about 1.5 feet long and 6 inches in diameter were laid end-to-end, which allowed water to seep into the tile line where they met. Today, most tile lines are made of continuous plastic pipe, which contains holes for water entry (Figure 1).
The largest potential benefit from tiling is increased yield potential. Other benefits include quicker field access for timely fieldwork and planting, potential improvement in soil tilth, deeper plant rooting, reduction in compaction, reduced excess water stress on plants, improvement in emergence and final plant stands, minimized water logged conditions, and improved water infiltration during dry periods.
Research on poorly drained soils over many years has shown the potential for an average increase of 10 to 15% in crop yield when fields are tiled.1 Typical yield responses to drainage can range from 10 to 30 bu/acre for corn and 4 to 8 bu/acre for soybean.1
Environmental issues are the major concerns with subsurface drainage. Nitrate loss can increase from tiled soils because nitrate is very soluble and can easily leach into the laterals and ultimately into rivers and streams. Pesticides also have the potential to leach into tile lines; however, these are more apt to flow with surface water.
Prior to deciding on a drainage program, the local NRCS office or online at http://www.nrcs.usda.gov should be contacted to help determine the proper system for the area and if there are any cost-share programs available. Tiling itself may not be cost shared, but land work, such as waterways might be. Systems may include waterways to direct water to surface inlets for subsurface drainage.
Drainage designs should follow the topography to provide the necessary and uniform drainage. Tile laterals could be laid in fashions that are parallel, herringbone, double main, or targeted (like alternating leaves on a tree branch). Regardless of design, the main and associated laterals must be of proper size to accommodate current drainage requirements plus any future expansions. Outlets should be three to five feet deep depending on topography, but above normal water flow for the waterway, stream or river the outlet is directed into and large enough to carry the drainage capacity to drain the field quickly.
Tile depth and spacing determine the rate of water removal from the tiled soil. A close relationship exists between soil permeability, the spacing, and the depth of laterals (Table 1). Removing water quickly from the plant root zone can help with managing stress and maintaining yield potential; however, that may not be the best system for maintaining water quality. Different combinations of tile depth and spacing can achieve the same results for removing excess water but have different effects on the quality of water exiting the system. The goal of many current drainage programs is focused on optimizing yield potential while addressing water quality issues.
Reducing the amount of nitrates leaching into tile laterals is a main concern. Shallower tile lines, water control structures, bioreactors, two-stage ditches, and cover crops are methods to help manage nitrate leaching.
Laterals placed shallower and closer together have shown an ability to reduce the nitrate concentration in tile water while maintaining yield potential.2 In this system, nitrates are converted to nitrogen gas by denitrifying bacteria, preventing nitrates from reaching tile and surface water. University of Illinois research has shown that a favorable rate of return and improved water quality were achieved with a 45 foot tile spacing and at a 2 foot depth.3
A water control structure placed in the main, sub-main, or lateral drain can be used to vary the depth of the drainage outlet to manage the depth of the water table and flow of nitrates. The structure is lowered in spring and prior to harvest so the drain can flow freely to reduce soil water and raised after planting and spring field operations to store water for crop use. Research has shown that the reduction in the amount of nitrates in the tile water ranges from 15 to 75%, depending on location, climate, soil type, and cropping practices.1 Drainage water management reduces flow volumes in the system and may help reduce nitrate leaching.3 Rock inlets can be used to replace open inlets to reduce water flow to drainage inlets and limit sediment from entering the system.
Bioreactor systems use subsurface trenches filled with wood chips to filter drainage water before leaving the field and entering a body of surface water. The wood chips serve as a substrate for denitrifying bacteria to convert nitrate to nitrogen. Based on research from Iowa, Illinois, and Minnesota bioreactors typically remove about 15 to 60% of the nitrate load in a system per year.4
Two-stage ditches are drainage ditches that have been modified by adding benches that serve as floodplains within the overall channel. The twostage design mimics a more natural stream channel that leads to greater channel stability. Recent evidence has shown that the two-stage ditch has great potential to improve nutrient processing compared to conventional ditches, by creating an in-ditch bench that facilitates denitrification and nutrient uptake while enhancing the stability of the channel and reducing sediment movement.5
Cover crops can be used to pull nitrogen from the soil during late fall and possibly early spring. Stored nitrogen in the cover crop is released as crop decays and helps prevent nitrogen from seeping into tile lines.
The 12 states in the Mississippi River Basin are working together with a number of stakeholder groups and federal agencies to implement local nutrient reduction strategies to support agricultural output and improve water quality.6 An important part of the program is the 4Rphilosophy that relies on innovative and science-based approaches to protect the environment, increase production, increase farmer profitability, and improve sustainability through the use of the right fertilizer source, at the right rate, at the right time, and with the right placement to help ag retailers, crop advisors, and farmers promote improved nutrient utilization to reduce nutrient losses.
1Sands, G.R., Kandel, H., Scherer, T., and Hay, C. 2012. Frequently asked questions about subsurface (tile) drainage in the Red River Valley. University of Minnesota. http:// www.extension.umn.edu.
2 Frankenberger, J., Kladivko, E., Sands, G, Jaynes, D., Fausey, N., Helmers, M., Cooke, R., Strock, J., Nelson, K., and Brown, L. 2006. Questions and answers about drainage water and management for the Midwest. Drainage Water Management for the Midwest. WQ-44. Purdue University. https://www.extension.purdue.edu.
3 Illinois drainage guide. www.wq.uiuc.edu
4 Christianson, L. and Helmers, M. 2011. Woodchip bioreactors for nitrate in agricultural drainage. PMR 1008. Iowa State University. http://www.leopold.iastate.edu
5 Frankenberger, J. Two-stage ditches. Purdue University. https://engineering.purdue.edu
6 Mississippi River Gulf of Mexico Watershed Nutrient Task Force. http://water.epa.gov. Additional sources: Sands, G.R. Drainage fact sheet. M1292. University of Minnesota. http://www.extension.umn.edu. Wright, J. and Sands, G. Planning an agricultural subsurface drainage system. Agricultural drainage. University of Minnesota. http://www.extension.umn.edu. Drainage. Ag 101. U.S. Environmental Protection Agency. http://www.epa.gov/agriculture/ag101/cropdrainage.
All web sites verified 2/20/15.