Optimizing Subsurface Drainage Systems

  • 13;10;9;9;9;9;9;Many current university drainage research programs are evaluating ways to maintain the agronomic benefits of tile drainage and reduce the potential environmental impacts.13;10;9;9;9;9;
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  • 13;10;9;9;9;9;9;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.13;10;9;9;9;9;
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  • 13;10;9;9;9;9;9;Drainage water management is a practice that can be used to reduce nitrate loads on tile-drained soils.13;10;9;9;9;9;
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13;10;9;9;9;9;9;Subsurface drainage is extensively used in the upper Midwest in Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, Ohio, and Wisconsin.1 Subsurface drainage systems have a positive impact because they generally decrease the amount of surface runoff, reducing the loss of nutrients, sediment, and agricultural chemicals (Figure 1). However, there are concerns about the potential negative impacts on water quality from drainage reaching surface water, such as lakes and streams, and groundwater. Farmers are actively working with research groups and with state and federal agencies to find cost-effective, local subsurface drainage improvements and crop management options that maintain profitability while mitigating water quality issues.13;10;9;9;9;9;9;

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13;10;9;9;9;9;9;Increased rainfall, higher seasonal water tables, higher land values, and increased crop productivity have increased interest in tile drainage. Drainage can help fields dry out more quickly and uniformly in the spring. Some benefits of drainage include:13;10;9;9;9;9;9;

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  • Improved crop yield potential 
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  • Timely planting 
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  • Timely field operations 
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  • Better soil tilth 
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  • Deeper plant root zone 
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  • Better plant stands 
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  • Less plant stress 
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  • Minimized compaction and soil salinity 
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  • Improved harvesting conditions 
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  • Less equipment wear
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Tile drainage can promote better water infiltration. Studies indicate the potential for overall increases in water yield from 5 to 10%.2 Research on poorly drained soils over many growing seasons has shown the potential for an average increase of 10 to 15% in crop yield.3 Typical yield responses to drainage can range from 10 to 30 bu/acre for corn and 4 to 8 bu/acre for soybean.

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13;10;9;9;9;9;9;Subsurface drainage systems can have a positive impact because they generally decrease the amount of surface runoff and transport of substances with overland flow. Midwest research has shown that subsurface drainage can help reduce surface runoff by 29 to 45%, which helps reduce sediment losses by 16 to 65% and phosphorous losses up to 45%.4 There are concerns that drainage may contribute to changes in the hydrology of watersheds and the quality of water bodies receiving drainage water.13;10;9;9;9;9;9;

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13;10;9;9;9;9;9;9;Figure 1. Subsurface drainage installation13;10;9;9;9;9;9;9;
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Tile Spacing and Depth

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13;10;9;9;9;9;9;Tile depth and spacing determines the rate of water removal from soil. There is a close relationship between soil permeability and the spacing and depth of tile drains (Table 1). Many tile systems were designed to quickly remove excess water from the plant root zone to help manage stress and improve yield potential. 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.

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13;10;9;9;9;9;9;The goal of many current drainage programs is focused on optimizing yield potential while addressing water quality issues. Preliminary research results indicate that tiles at shallower depths and spaced closer together can reduce the nitrate concentration in tile water while maintaining yield potential.5 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.613;10;9;9;9;9;9;

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13;10;9;9;9;9;9;Soils with high water tables and irrigated soils can accumulate soluble salts in the plant root zone over time, causing reduced plant vigor and growth. Yield reductions for most crops can occur when salinity levels are above 1 millimhos per centimeter.3 Tile drainage can help reduce the salt concentration of the soil by improving water movement and leaching of soluble salts through the soil profile via the drainage system.3 Consideration should be given to the crop sequence during the process of reducing the soil salt concentration and growing a sequence of more salt tolerant crops, such as small grains, before growing a salt sensitive crop, such as dry beans.713;10;9;9;9;9;9;

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 Table 1. General parallel drain lateral spacing and depth suggestions
 Soil type
 Soil permeability
 Drain spacing in feet for:
 Drain depth in feet
 Fair drainage
 Good drainage
 Excellent drainage
 Clay loam
 Very low
 70
 50
 35
 3.0 - 3.5
 Silty clay loam
 Low
 95
 65
 45
 3.3 - 3.8
 Silt loam
 Moderately low
 130
 90
 60
 3.5 - 4.0
 Loam
 Moderate
 200
 140
 95
 3.8 - 4.3
 Sandy loam
 Moderately high
 300
 210
 150
 4.0 - 4.5
 Source: Wright, J. and Sands, G. 2001. Planning an agricultural subsurface drainage system. University of Minnesota.
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Management Options

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13;10;9;9;9;9;9;A tile drainage management plan should include practices to reduce nutrient movement in tile water. Agricultural drainage research and education programs are engaged in identifying viable management options to maintain crop productivity and improve drainage to help reduce water quality concerns that include:13;10;9;9;9;9;9;

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  • Nutrient best management practices 
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  • Proper tile depth and spacing for soil types 
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  • Shallow drainage to reduce nitrate loss 
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  • Controlled drainage or drainage water management 
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  • Reduced drainage intensity 
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  • Improved surface outlets 
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  • Wood chip bioreactors, rock inlets 
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  • Winter cover crops 
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  • Perennial crops in rotations 
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  • Two-stage ditches 
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  • Wetland restoration, constructed wetlands
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  • Nutrient retention basins
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13;10;9;9;9;9;9;Cover crops can be used effectively to reduce soil erosion, reduce the need for herbicides and other pesticides, protect water quality by limiting nitrogen leaching, and increase soil organic matter.13;10;9;9;9;9;9;

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13;10;9;9;9;9;9;Each of the 12 states in the Mississippi River Basin are working in concert with a broad base of stakeholder groups and federal agencies to implement local nutrient reduction strategies to support agricultural output and improve water quality.8 An important part of the program is the 4R philosophy that relies on an innovative and science-based approach to offer environmental protections, increased production, increased farmer profitability, and improved sustainability through the use of the right fertilizer source, at the right rate, at the right time, with the right placement to help promote improved nutrient utilization to reduce nutrient losses.13;10;9;9;9;9;9;

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