Subsurface drainage in the Red River Valley
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Since the late 1990s, subsurface (tile) drainage system installations have increased in the Red River Valley of North Dakota and Minnesota due to a wet climate cycle, increased crop prices and increasing land values.
As a fairly new practice in the region, many questions are being asked about tile drainage. Here, we attempt to briefly answer some of the more commonly asked questions.
Determining if drainage is right for you
Over the last 20 years, tile drainage installation has accelerated in the Red River Valley drainage basin and other parts of North Dakota (Figure 1). A number of factors have sparked the region’s recent interest in this practice, including increased rainfall, seasonally high water tables, higher land values and higher crop prices.
In the spring, many farmers have difficulty planting crops in a timely manner due to the wet conditions. Soil salinity, another problem in the Red River Valley, is related to water table behavior and soil moisture. Soil salinity in the Red River Valley alone encompasses more than 1.5 million acres and accounts for about $50 to $90 million of lost revenue.
Tile drainage is a management practice that offers the potential to control and reduce salinity in poorly drained soils.
Tile drainage has been successfully practiced on a wide range of soil textures, from sandy to clayey.
You can drain coarser soils (silts and sands) with wider drain spacings, whereas finer soils (loams and clays) require narrower drain spacing. Soils with significant coarse silt or fine sand content may need a sock envelope around the pipe to prevent soil particles from entering the tile.
For a 4-foot drain depth and a drainage coefficient of 0.25 inches per day, a Fargo clay might require a drain spacing of around 32 feet, while drain spacing for a Ulen fine sandy loam would be around 120 feet.
Soils where shrinking-swelling clays or peat predominate, or sodic soils, may need special consideration with regard to tile drainage. Soils are classified sodic when the pH is over 8.5 and the amount of sodium in the soil complex is much greater than the combined amount of calcium and magnesium.
You can drain level fields as long as you maintain minimum grades of 0.08 to 0.1 percent for tile laterals and mains. A tile at 0.1 percent grade has 1 foot of fall per thousand feet. On level ground, this means the tile depth would vary by 1 foot over 1,000 feet.
Many parts of the Red River Valley have a natural field slope of around 0.1 percent. A typical drainage system provides an outlet where tile can freely drain (by gravity) into a surface ditch.
Design and installation
Where outlet ditch topography or depth doesn’t allow for a gravity outlet, use pumped outlets, provided a surface waterway exists to discharge the drainage water (Figure 2). A pumped outlet, or lift station, provides the lift required to get the drainage water from the tile’s elevation to the ground surface or higher and into the receiving waterway.
Pumped outlets increase the initial investment, operation and maintenance costs of the tile drainage system, but may be economically feasible in many situations. A pumped outlet station includes a sump, pump, discharge pipe and usually an electric control panel. Important design features include the sump storage volume and pump capacity (flow rate).
Do-it-yourself (DIY) tiling is certainly an option that’s being considered by many farmers and landowners (Figure 3). With good equipment, good design and the necessary commitment of time and resources, DIY tiling may be a sound option and may save on installation costs.
However, like any other field operation, you need to invest in specialized equipment and knowledge. Tiling typically requires at least:
A four-person crew.
A tile plow.
Electronic controls (GPS and plow control).
Several large- and medium-sized tractors.
Professional tiling contractors bring experience and familiarity with design procedures and standards of tile drainage systems. Pipe depth and grade, pipe size and field layout are all extremely important in design and will determine the system’s performance quality.
Above all, it’s important to properly design and install the tile system so it’ll perform well for many years.
Your need for an envelope(sock), or narrower slots, on the drainage pipe depends on the soil texture at the tiling depth.
In general, poorly graded fine sands and coarse silts require sock envelopes. Clay, silty clay, sandy clay, silty clay loam and loams generally don’t require envelopes because of their natural cohesiveness.
Use the Natural Resource Conservation Service Web Soil Survey website to determine the soil texture at the tiling depth.
If you have doubts or questions, do a soil sieve or particle size analysis. This is a relatively easy mechanical procedure that can be performed by a commercial soil-testing lab or by the soil-testing lab at North Dakota State University (NDSU).
The analysis will determine the soil’s sand, silt and clay fractions, and the range of soil particle sizes. You don’t need a sock if the clay fraction is greater than 30 percent. You may need a sock if the medium to very coarse sand fraction (0.5 to 2.0 millimeter particle size) comprises more than 20 percent of the total.
Controlled, or managed, drainage systems incorporate structures that allow you to raise the outlet elevation at strategic locations in the drainage system. This way, you can control the release of drainage water and potentially maintain a shallower water table when desired (Figure 4).
Controlled drainage systems potentially conserve soil water in the root-zone and reduce drainage flows and the loss of dissolved nutrients (nitrogen and phosphorus) from the field. If the rainfall timing is favorable, it creates the potential to store water for drier periods during the growing season.
How it works
Use one or more control structures, or the pumped outlet itself, to control the drainage system. Control structures use stop-logs or baffles to set the desired water table elevation at the structure’s location. Turning off a pumped outlet creates the same effect.
It’s important to consider the option of drainage water management in the drainage system’s initial design. This way, the system layout accommodates the goal of drainage management to the fullest extent and maximizes the practice’s effectiveness.
Typically, fields with an average field grade from 0 to 0.5 percent are best-suited for the practice, but other factors such as field slope uniformity and access to control structure locations are important too. Fields that are nearly flat may only require one control structure (or the pumped outlet) to implement the practice, while fields with more grade may require several control structures.
The benefit of drainage water management is it gives producers one more tool to manage production risks. The drainage water management philosophy is to only drain the necessary amount to create adequate field conditions and retain water that may contribute to crop production.
Under certain conditions, water retained with the control structures may offer the potential to increase crop yield.
Subirrigation is the practice of providing water to the root zone through a drainage water management system. If a source of irrigation water is available and the drainage system is appropriately designed, water can be introduced into the water control structures (or the sump of the pumped outlet) to raise the water table and make water available to the crop.
To make this practice work, you need a sufficient water source to supply the crop’s water needs, usually during July and August. As with drainage water management, you must design the subirrigation system before installing the tile for this practice to be effective.
A system designed for subirrigation will generally require closer drain spacing than a system only designed for conventional drainage.
Benefits and effects
Planting and field operations
On poorly drained soils, tile drainage will promote faster soil warm-up and drying in the spring, and intermittent wet spots in fields will more uniformly dry out.
A significant negative effect of inadequate drainage relates to the timeliness of spring and fall field operations. Inadequate drainage can delay spring field operations from days to weeks and interrupt field traffic patterns due to nonuniform drying.
Machinery traffic on soils that are too wet will increase soil compaction. Planting delays mean a shorter growing season and fewer heat units for the crop. Once the crop has been planted, inadequate drainage can cause stunted and shallow root growth and, sometimes, complete crop failure due to excess water stress (lack of oxygen in the root zone).
The combined stresses of delayed planting, soil compaction, and excessive water can significantly impact crop yield.
The magnitude of the yield impact for a growing season depends on crop and variety, soils and the season’s rainfall pattern.
Soluble salts may accumulate in the root zone when high water tables occur over several years.
Salinity can be measured by its ability to conduct electricity and may be expressed in units of millimhos per centimeter (mmhos/cm). Before measuring, dry the soil sample and mix equal parts water and soil. Conductivity readings will be higher with higher salt concentration.
What to expect
Expect yield reductions for most crops with salinity levels above 1 mmho/cm. Studies have shown that leaching water through the profile and removing the salt via tile drainage may reduce salt concentration in the root zone over time.
Depending on seasonal rainfall or the ability to irrigate, it may take a few years before the salt in high-concentration areas reduces enough for optimum agricultural production. This effect may occur more quickly in years with higher rainfall, and may not occur at all in dry years.
It’s important to reclaim the land with a sequence of more tolerant crops, such as barley, before planting a salt-sensitive crop.
Saline seeps may occur where soil water from high land slowly and laterally seeps to lower areas and carries dissolved minerals (salts) with it.
If the water comes near, or seeps out of the surface in the low area, it may evaporate and leave the salts behind. Over time, salts can increase to a point at which the soil can no longer support crop growth.
Tiling these low areas along with the side slopes (to intercept saline water before it reaches lower areas) will lower the water table. Plus, it may eventually leach the salts, depending on the precipitation amount. A targeted drainage system of relatively few tile lines may be all that’s needed to address a saline seep situation.
The economics of tile drainage systems depend on:
Crop yield response.
Initial capital investment for the materials and system installation.
Any annual operation and maintenance costs, such as electricity for pumped outlets.
Although you can directly assess crop yield response to drainage, the impacts of inadequate drainage on soil quality (structure, microbial activity, etc.) are more difficult to measure and assign economic value.
Many field crops positively respond to drainage (on previously poorly drained soils). Often the best response is from combining surface and tile drainage. The yield increase level for a given year greatly depends on how poorly drained the soil was prior to drainage, and the timing of seasonal rainfall.
Research has shown that over many growing seasons, average yields may increase around 10 to 15 percent, depending on the aforementioned factors. Typical yield increases might be 10 to 30 bushels per acre for corn and 4 to 8 bushels per acre for soybeans.
Research on a clay loam soil has shown that wheat yield will reduce by 42 percent and sugarbeets by 29 percent when the water table stays 15 to 20 inches below the surface for extended periods during the growing season.
In addition to yield increases associated with adequate drainage, you may also reduce your farm’s operating expenses due to reduced cropping inputs, less power consumption and timely field operations. Several drainage pipe manufacturers have tools for evaluating drainage investments.
Tile drainage doesn’t remove plant-available water from the soil, it merely removes gravitational water that would naturally drain, if unimpeded by confining layers in the soil.
The greatest benefits of tile drainage are typically realized in wet years. However, because drainage promotes deep root development, crops will often have better access to soil moisture in dry years. During extremely dry growing seasons, it’s certainly possible that a tile-drained field might have less available water at some point during the growing season than a non-drained field.
Whether or not such an effect would offset the early-season positive effects of drainage is unknown, and highly site- and year-specific. In general, where poorly drained soils exist, crop yields will be more uniform from year to year with tile drainage.
You can install drainage control structures (also known as controlled drainage or drainage water management) so you have the potential to limit the release of drainage water to conserve more soil water in the root zone. Similarly, you can turn off the pump in a lift station if you’re concerned about drier growing conditions.
The water quality impacts of tile drainage can be both positive and negative.
In general, compared to surface drainage, tile-drained fields may reduce phosphorus and sediment losses via surface runoff, while losses of nitrate-nitrogen and other dissolved constituents from the root zone tend to be greater.
The extent to which these constituents increase or decrease also depends on farm management practices. The magnitudes of the losses highly vary from year to year, primarily due to the variability in annual precipitation.
Conservation drainage practices
A number of conservation drainage practices have been (and are being) developed to address water quality concerns. Some of these practices include:
Modified agronomic and crop production practices (fertilization timing/rate and crop selection).
Drainage water management or controlled drainage.
Optimized drainage design.
Alternative surface inlets.
Two-stage drainage ditches (Figure 5).
For more than a century, the impact of tile drainage on downstream flow and flooding has been the subject of much debate. The way tile drainage influences streamflow involves complex processes that depend on many factors.
Generalizations such as saying tile drainage “causes” flooding or tile drainage “prevents” flooding oversimplify the issue. Important factors that’ll determine the impact of tile drainage on downstream flow and flooding include:
Rainfall (or snowmelt) amount and intensity.
Point of interest (near the field outlet or over a larger watershed).
Time frame of interest.
Existing soil moisture conditions.
The extent of surface drainage, including surface intakes.
Despite this complexity, there are areas of general agreement from the research on tile drainage and streamflow.
For poorly drained, low permeability soils—where tile drainage is typically used in the Upper Midwest—tile drainage will lower the water table. This increases soil water storage capacity and infiltration, and reduces the amount of surface runoff and the peak flows coming from the field.
Rain or snowmelt events
For small or moderate rain or snowmelt events, this may help reduce downstream peak flows that are often a concern for flooding. However, discharge from tile drainage occurs over a longer time period than surface runoff, so baseflows (streamflows between storm or snowmelt events) tend to increase from tile drainage.
Some computer modeling-based studies suggest the total water leaving the field (surface runoff plus tile and shallow groundwater flow) under a tile-drained scenario may increase approximately 10 percent. These studies haven’t been verified with field data.
For large rain or snowmelt events or extended rain events on wet soils that exceed the soil’s infiltration ability—which are typically related to catastrophic flooding—surface runoff drives streamflows, and tile drainage has minimal impact on downstream flows and flooding (Figure 6).
Moving beyond the field scale to larger watershed scales, the complexity greatly increases with more variation in all factors contributing to streamflow. It thus becomes much more difficult to isolate the impacts of tile drainage at these scales. This is why the influence of tile drainage on streamflow and flooding at these larger scales isn’t yet well understood.
Skaggs, R.W., Brevé, M.A., & Gilliam, J.W. Hydrologic and water quality impacts of agricultural drainage. (1994). Critical Reviews in Environmental Science and Technology, 24(1), 1-32.
Reviewed in 2018