Although drainage benefits agricultural production, there are concerns about its potential environmental impact. Fortunately, there are ways to reduce the negative impacts, while retaining its agronomic benefits.
Drainage issues and ways to address them
Subsurface drainage systems have a positive impact because they generally decrease the amount of surface runoff, reducing the loss of substances generally transported by overland flow.
However, there are concerns about the potential negative impacts of drainage on the:
Hydrology of watersheds.
Water quality of receiving water bodies.
Amount and quality of nearby wetlands.
Drainage systems are designed to alter field hydrology (water balance) by removing excess water from waterlogged soils. There are concerns about the downstream hydrological effects caused by draining this excess water.
Anecdotal evidence indicates streams and ditches have become “flashier” over time, spilling over their banks and causing localized crop damage.
Some research articles suggest a landscape’s most dramatic hydrological changes occur when it’s converted from native vegetation to agricultural production, and that subsurface drainage may reduce peak flows in some situations. In fact, a regional publication concluded subsurface drainage:
Reduces surface runoff by 29 to 45 percent.
Reduces peak flows from watersheds by 15 to 30 percent.
Has little impact on the total annual flow from watersheds.
A publication summarizing drainage studies from several countries concluded that subsurface drainage generally decreases peak flows in fine-textured soils, but often increases those flows in coarser, more permeable soils. The publication also found that subsurface drainage often increases base flow to streams.
However, we need locally based research to better understand the impact drainage can have at watershed scales. In addition, surface inlets’ impact on watershed hydrology is an important issue currently being examined.
Surface drainage (enhancing overland runoff) tends to increase the loss of nutrients and sediment that occurs with surface runoff. Subsurface drainage, however, can decrease surface runoff, reducing sediment losses by 16 to 65 percent and phosphorus losses by up to 45 percent.
The main water quality concern about subsurface drainage is the increased loss of nitrates and other soluble constituents that can move through soil to drainage systems and end up in nearby surface water. In addition, surface intakes—common across southern Minnesota and northern Iowa—provide a fairly direct pathway for sediment and other contaminants in surface runoff to reach nearby surface waters.
Despite the fact that various regulations protect wetlands, it’s estimated that more than 60,000 U.S. wetland acres are lost each year.
The loss of wetland ecosystems—valued for their wildlife habitat, water storage and, increasingly, their potential role to improve water quality—isn’t easy to quantify. However, it’s likely that agricultural and urban drainage activities both cause wetland loss.
Many current drainage research and Extension programs throughout the country are trying to find ways to reduce the potential environmental impacts of agricultural drainage, while retaining its agronomic benefits.
Some management practices have been effective, and others are presently being examined. We describe both below.
Properly managing crop nutrients (nutrient source, application rate and timing) is an important way to help control the loss of nutrients through surface runoff and subsurface drainage water.
It’s been shown that applying nitrogen fertilizer at rates higher than those recommended by the University of Minnesota increases the amount of nitrate removed through subsurface drainage systems. University recommendations are based on an optimum economic return, so over-applying nitrogen fertilizer will also be less profitable.
However, note that drained agricultural soils have significant nitrate losses from the natural process of organic matter mineralization. Improved nutrient management can potentially reduce nitrate losses on drained lands by up to 30 percent.
Row crops such as corn and soybeans experience considerably more nitrate loss through subsurface drainage flow than perennials such as alfalfa and brome grass. Incorporating alfalfa or other perennials into crop rotations could significantly decrease nitrate losses to nearby surface water.
While alfalfa may be a financially sound crop for some operations, it isn’t an economically viable solution for many Minnesota farmers.
Subsurface drainage systems are designed to remove excess water from soil quickly enough to minimize crop stress in most years. Agricultural engineers have developed depth and spacing guidelines for installing drainage pipes.
For example, recommendations for the clay-loam soils prevalent in much of southern Minnesota call for placing drainage pipes approximately 3 feet deep and 60 feet apart or 4 feet deep and 80 feet apart. Either design should remove water at the same rate and give similar crop yields.
It’s been proposed that placing drainage pipes at shallower depths might result in less nitrate loss. This would happen because nitrate would be more likely to reach a biologically active but saturated zone and be converted to nitrogen gas by denitrifying bacteria.
The conversion of nitrate-nitrogen to nitrogen gas would prevent the nitrate from reaching the drainage pipes and nearby surface waters. If proven effective, this practice has the advantage of being applicable anywhere drainage systems are installed. It also requires no new management or capital investment.
Controlled drainage has become recognized as an effective practice—and in other states, a best management practice—for mitigating nitrate losses from drainage systems (Figure 1).
This practice involves placing simple water control structures at various locations in the system to raise the water elevation. The elevated water causes the water table in the soil to rise, which decreases the field’s drained depth.
Researchers from North Carolina, Ohio, Michigan and Canada have demonstrated that controlled drainage:
Decreases the volume of water drained by 15 to 35 percent.
Slightly increases surface runoff, because soils have less space to store water.
Significantly decreases (by up to 50 percent) nitrate losses seen in conventionally drained fields. This has primarily been attributed to reductions in the volume of water drained and, to a somewhat lesser extent, increased denitrification in the soil.
If managed properly, controlled drainage has the potential to improve crop yields by making more water available to plants.
However, topography limits the application of controlled drainage techniques. The process is economically unfeasible on land slopes greater than about one percent because more water control structures are needed as slopes increase. In addition, controlled drainage adds new management requirements to systems (also increasing with slope) that some view as a disadvantage.
Minnesotans are more frequently using alternatives to the traditional open inlet. One design involves digging a trench, placing drainage pipe at its bottom and filling the trench with small rock. These rock, or blind, inlets slow the flow of water (compared to open inlets) and may reduce the amount of sediment reaching the drainage system.
Another design involves installing subsurface drainage pipes in a very tight pattern in a small area in the middle of a wet spot. Another more traditional technique involves replacing open inlets with perforated risers.
All designs have the potential to do a better job protecting water quality than open inlets, while still providing adequate drainage so crops don’t drown.
Wetlands have been proposed as a means of treating water from drainage systems before it’s released into nearby rivers or lakes.
Biological activity in wetlands can be effective at removing nitrate by converting it to nitrogen gas through a denitrification process similar to what occurs in soils. Researchers in Iowa suggest wetlands can remove from 20 to 80 percent of the annual nitrate in subsurface drainage water, depending on the ratio between the areas of drained land and wetland.
This approach to “treating” drainage water presents some challenges. Site topography may pose difficulties in getting subsurface drainage waters to the surface and into wetlands. Land requirements and construction costs are also important economic factors.
Finally, the bulk of nitrate losses from drained lands in Minnesota occur in early spring when wetlands aren’t functioning at their peak nitrate-removing capacity, because of low temperatures and high water flow rates. The potential effectiveness of wetlands in treating drainage water in colder climates requires more research.
Research and other efforts
An array of new and ongoing research and Extension projects target important drainage issues in Minnesota.
These projects involve research facilities at the University of Minnesota’s St. Paul Campus, University Research and Outreach Centers. Projects have included the following.
Crop nutrient management and cropping systems
Subsurface drainage plots established more than 20 years ago in Waseca continue to examine the impact of the following on the amount of nutrient loss from drainage flow:
Fertilizer and manure application rates and timing.
Nitrification inhibitor use.
Drainage system design
The University of Minnesota Southern Research and Outreach Center (SROC) in Waseca is measuring the effects of drainage depth and spacing on crop yield and surface and subsurface water quality, using field-sized plots of 2 to 6 acres.
Researchers are combining computer simulations with field research to estimate these practices’ long-term effects. It’s anticipated that this research will lead to better drainage design recommendations for southern Minnesota.
The SROC is investigating the effects of controlled drainage on crop response and water quality on six half-acre plots. Researchers hope to see if this practice is feasible, so they can develop water management strategies for southern Minnesota. Computer simulations will also be used to estimate the long-term performance of this technique.
Alternative design for surface inlets
Farmers, contractors, local governmental units and university researchers have been examining alternative surface inlet designs to see if they’ll provide adequate drainage and control the delivery of contaminants to receiving bodies of water. These projects include laboratory work, small research plots and on-farm field-scale research.
They’re evaluating rock inlets for their effectiveness in removing water and trapping sediment. Plus, a design and monitoring process is in place to assess these techniques’ longevity and efficacy.
Computer simulation of drainage systems
A variety of projects have used computer modeling to investigate the performance of drainage systems and landscapes over several (often many) years. This work complements field research that’s typically conducted over a shorter time frame. Projects include:
Simulating the effects of various management practices on drained watersheds.
Assessing the hydrology of drainage systems.
Evaluating best management practices for drained fields.
Wetland area for mitigation and water treatment studies
Natural and constructed wetlands at SROC and the University of Minnesota Southwest Research and Outreach Center (SWROC) in Lamberton measure wetlands’ potential to mitigate nutrient loss in southern Minnesota.
The Agriculture Ecology Research Farm at SROC has wetlands that receive drainage water from approximately 100 acres of land. These wetlands function in a total water management system designed to improve water quality and reduce peak flows.
Drainage ditch design
Significant amounts of denitrification can occur in ditches, which can reduce the amount of nitrate that reaches area rivers.
SWROC has a project that’s comparing the ability of two similar ditches to remove nitrogen under varying physical and flow characteristics. Plus, there’s research being conducted on a drainage ditch at SROC.
Yield and hydrologic impact of subsurface drainage
Research on farms and plots at the University of Minnesota Northwest Research and Outreach Center in Crookston is investigating the impact of subsurface drainage on crop yields, water quality and hydrology in the Red River Valley.
Binstock, L. Minnesota Land Improvement Contractors. Personal communication.
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Reviewed in 2018