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Soil Management Guide  

Drainage Management

Background

Improving drainage on agricultural land not only enhances crop production but also has a role in soil conservation. Agricultural drainage improvement can help reduce year-to-year variability in crop yield, which helps reduce the risks associated with crop production. Improved field access through enhanced drainage also extends the crop production season and reduces damage to equipment and soil that can occur under wet conditions. Maintaining existing agricultural improvements and improving the drainage on wet agricultural soils presently in agricultural production helps minimize the need for producers to convert additional land to agricultural production. The main objective of agricultural drainage is to remove excess water quickly (within 24 to 48 hours) and safely to reduce the potential for crop damage. Drainage is important to avoid excess water stress to the crop. Excess water has been shown to decrease yields of wheat, oats, barley and flax by an average of 14, 18, 23 and 4 bu/ac respectively (Rigaux and Singh, 1977). Other benefits of drainage include: earlier spring seeding (see Table 6.1), warmer soils in spring, increased soil air in root zone, increased availability of nutrients, reduced risk of delayed harvesting, less damage to equipment, less overlapping of inputs during field operations and more effective weed control.

Table 6.1  Effect of delayed planting on Manitoba crop yields (MASC)  

Planting Date % Yield Reduction
Corn Canola Flax Peas
1st week May - - - -
2nd week May 5 - - 5
3rd week May 10 5 5 15
4th week May 20 10 15 20
1st week June 30 20 25 30

 

The use of surface and subsurface drainage improvements is not limited to agricultural lands. Many residential homes use subsurface drainage systems, similar to those used in agriculture, to prevent water damage to foundations and basements. Golf courses make extensive use of both surface and subsurface drains. Houses, streets and buildings in urban areas depend heavily on surface and subsurface drainage systems for protection. These generally are a combination of plastic or metal gutters, and concrete pipes or channels.

There are two principle types of field drainage – surface drainage and tile (or subsurface) drainage. In general, surface drainage is conducted on heavier-textured soils and tile drainage, along with surface drainage, is used on lighter-textured soils. A. Surface Drainage The purpose of using surface drainage is to minimize crop damage from water ponding after a precipitation event, and to control runoff without causing erosion. To accomplish this, one must follow a few drainage design standards:

  • Proper grades are 0.1 - 0.3%. Grades >0.2% should have grassed bottom and sides
  • Side slopes of ditches should be <10% 
  • For deep, permanent ditches and major landscaping, topsoil should be removed first and stored separately until earth moving is complete. Topsoil should be added back on the surface with minimal mixing of subsoil to ensure crop productivity is protected.

Shortcomings of surface drainage include: erosion and filling in of ditches (which requires ongoing maintenance), increased risk of salinization in areas affected by artesian pressure, and potential water quality impacts because water is not filtered through soil.

B. Tile Drainage The purpose of installing tile drainage is to lower the water table in order to increase the productivity of the drained land. Water tables that are close to the surface in the spring restrict seeding operations and impede crop growth and development. Rising water tables during the growing season can damage actively growing crops, resulting in yield losses. Capillary rise can carry salts into the root zone and contribute to soil salinity. In Manitoba, tile drainage has a particular fit in the wet, sandy soils used to produce high value crops. However, for tile drainage to be effective, a network of properly designed and maintained surface drains must also be in place.

  • Some common concerns and explanations regarding tile drainage:
  1. Overdraining (drying out) soils - tile drains are only able to remove excess water that flows by gravity (i.e. water above field capacity) from the portion of the soil profile that is above the depth of the tile drain. This water is unavailable for plant uptake and restricts oxygen availability. Available water (between field capacity and permanent wilting point) is held in the soil under tension and cannot enter tile drains until conditions become saturated (refer to Chapter 3 on moisture management). If soils do experience droughtiness after drainage, these are usually soil types that have both wetness (W) and moisture (M) limitations. Tiling as shallow as possible (30 to 36 inches, or 90 to 105 centimetres) should address the wetness issues on these soils; producers should implement moisture conservation practices and, if necessary, irrigation to address droughtiness issues.  
 Comparison of water table and root development in tiled and untiled conditions Figure 6.1  Comparison of water table and root development in tiled and untiled conditions (Sands, 2001)

Table 6.2  Benefits of tiling wet, sandy soils  

  Untiled Tiled
Soil moisture in root zone  Saturated throughout Field capacity above tile, saturated below tile
Potential for water uptake by crop Negligible Full
Oxygen availability Negligible Full

In summary, a soil that is tiled has less total water but more water available to the plant because the depth of the rooting zone is greater than the same soil in the untiled condition (Figure 6.1). Tiled soils also have increased capacity for storing water in the profile, since soil moisture is usually less than field capacity with a growing crop, rather than above field capacity or at saturation.

  1. Downstream flooding - conceptually, if large acres of land were tiled overnight, the drainage water could overwhelm existing municipal drains. However, with proper design of tile drainage systems and municipal drains, water leaving agricultural lands (as surface runoff or through tile drains) in the summer would be tempered because:
  1. a soil that is tile drained has more water storage capacity (i.e. soil moisture is usually less than field capacity with a growing crop, rather than above field capacity or at saturation);
  2. a healthy, actively growing crop will utilize any subsequent precipitation that brings soil moisture up to field capacity;
  3. water must flow through the soil and enter the tile before it leaves the property, rather than as overland flow directly into surface drains (exceptions would include very coarse textured soils or soils with deep, extensive cracks and root channels).

The use of small dams in specific watercourses and designated selected lands as wetlands or water storage areas would provide additional buffer to minimize downstream flooding. In sensitive areas, tile drains could be closed at crucial times of the year.

  1. Surface water quality - water that moves vertically through the soil may pick up dissolved salts, nitrates, etc. and these constituents may reach surface watercourses at the tile outlet. These soils require more intensive nutrient management practices, including soil testing, nutrient applications based on reasonable crop yield targets and nutrient budgets. Improving the water management of a field should result in more stable or improved crop yields, greater nutrient uptake and reduced risk of nutrient losses to the environment.  
  2. Cost-Benefit - installation costs for tile drainage systems can be $400 to $600 per acre or higher. For high value crops such as potatoes and other vegetable crops on coarse-textured soils susceptible to wetness limitations, the payback from increased crop yields and reduced yield variability could be realized in only a few years, especially when compared to payback from irrigation infrastructure.  
  3. Proper design - depending on field conditions, tile drains placed 30 to 36 inches (90 to 105 centimetres) deep (and properly spaced according to soil type) are effective in keeping the water table below the portion of the soil profile with the most root activity and most crucial for crop growth. Tiles placed deeper may drain more water or can be spaced further apart, but response time to heavy precipitation events may be too slow to prevent crop damage due to wetness.  
  4. Drain maintenance to prevent freezing - tiles need to be “dry” in the fall and the outlets unobstructed so that the drainage system is able to drain water early in the spring. Wet fall seasons will increase the risk of frozen tiles in the spring. If tiles freeze, they may be damaged and have their useful life reduced. In addition, frozen tiles will be unable to enhance drainage during spring thaw, but they should thaw in time to reduce the negative impacts of precipitation events later in the growing season, which may be the most harmful to crop performance.

Consult soils report when assessing lands for improved drainage

Clearly distinguish between wet land, which can be managed by drainage, cultivation and cropping systems, and wetlands, which should be conserved. “True” wetlands, like bogs, marshes and swamps, have saturated soil conditions over a sustained period of time during the year to maintain water-loving vegetation (rushes, cattails, sedges, willows) and wildlife habitat. These areas, once their benefit is assessed, should be protected from development. Wetlands are valuable for groundwater recharge, nutrient filtering and recycling and supporting wildlife habitat. Water control through backflood irrigation and proper management when haying or grazing wetlands can have multiple, long term benefits.  

Wet land A Wetland
Figure 6.2  Wet land Figure 6.3  A wetland

Table 6.3  Distinguishing wet land from wetlands using agriculture capability ratings of soils  

Limitation Wet Land Wetlands
Wetness (W)
  • Imperfectly drained soils (Class 2W-4W)
  • Poorly drained gleysols (Class 5W)
  • Soils with agriculture capability subclasses 3MW and 4MW; water tables within 3 to 6 feet (1 to 2 meters) during the growing season as stated in soil series description
  • Very poorly drained soils (Class 6W)
  • Marsh (Class 7W)
  • Open water (coded "ZZ" in soil survey reports)
Salinity (N)
  • Soils that display secondary (human-induced) salinity
  • Soils with primary (natural) salinity
  • Very strong (u) salinity (Classes 5N and 6N)
  • Salt flats (Class 7N)
Inundation (I)
  • Land inundated relatively infrequently (Class 2I, 3I)
  • Land inundated most of the season (Class 7I)

“Wet land” is agricultural land in production that has some crop limitations due to wetness limitations (see Table 6.3). Drainage of wet land by soil texture:

  1. Clays - poorly drained soils (such as Osborne soils) have their agriculture capability upgraded from 5W to 3W through properly designed surface drainage.
  2. Wet sands - imperfectly drained soils (such as Almasippi soils) benefit from properly designed surface and tile drainage when the drainage infrastructure is sufficient for effective field outlets.

Site visit

  • Acquire elevation data for the selected field to assist in determining the design capacity of the drainage system.
  • Consider soil texture, natural soil drainage, hydraulic conductivity, depth to water table, flooding frequency, depth to impermeable barrier, depth to bedrock, % slope, nature of the surface runoff, location of outlets before proceeding with drainage enhancement.
  • Confirm the occurrence of soil salinity in previously non-saline soils using dilute hydrochloric acid (HCl) and observing salt-tolerant plant species (such as foxtail barley and kochia) and established alfalfa growth patterns (refer to Chapter 5 on soil salinity).

Other factors to consider are: size of area, location in ecosystem, relative size and productivity compared to other areas considered for agricultural development and/or wildlife conservation.

Laboratory analysis

A variety of factors are required to determine the appropriate drain spacing for a given soil type. Soil texture, permeability and depth to water table, along with possible changes of these properties with depth, can influence the drain spacing and overall cost of the project. If a project becomes too expensive to have drains spaced relatively close together, the drains could be placed deeper in the subsoil or the overall capacity of the drainage system may have to be reduced with wider drain spacing.

Recommendations when considering a drainage project

A. Surface drainage:

  1. Determine purpose/goal
  2. Obtain a detailed topographic survey (elevation map) of selected field(s)
  3. Conduct a detailed cost/benefit analysis
  4. Obtain a drainage license from Manitoba Water Stewardship, which will include obtaining sign-off from those impacted (private and/or municipal).
  5. Stake out drainage path beforehand
  6. Start at outlet and work backwards, maintaining proper grade
  7. Establish buffer strips/grassed waterways of deep-rooted, perennial plants (forages, trees, shrubs) to control erosion and salinity; incorporate other appropriate erosion prevention and control measures as needed.
  8. Consider outlet control to reduce runoff velocity or to control outflow timing.

B. Tile drainage:

  1. Determine purpose/goal
  2. Obtain a detailed topographic survey (elevation map) of selected field(s)
  3. Identify site criteria to confirm tile drainage is the most appropriate solution
  4. Conduct a detailed cost/benefit analysis
  5. Obtain a drainage license from Manitoba Water Stewardship
  6. Tile drainage design:
  1. The outlet should be higher than lowest point in municipal ditch to drain water from the field without pumping into the ditch. (Manitoba Water Stewardship generally allows a maximum of one 16 inch (40 centimetre) diameter outlet per quarter section.)
  2. An appropriate alternative use to consider is runoff collection on private land and other uses such as irrigation.
  3. The tiles must be deep enough to prevent damage from tillage and keep costs down (spacing can be further apart), but shallow enough to respond quickly to precipitation events.
  4. Grade - >0.05% (depends on achieving correct flow velocity, depth, reasonable cost, etc.)
  5. Flow velocity - greater than 0.5 cu.ft./sec (14 L/sec) to prevent sedimentation, but less than 1.4 cu.ft./sec (40 L/sec) to prevent blowouts and erosion.
  6. Spacing – 40 to 50 feet (12 to 15 metres) is a general recommendation. However, the suggested spacing between tile laterals based on soil permeability conditions (modified from Beauchamp, 1955) is as follows:
  • Muck and peat: 50 to 200 feet (15 to 61 metres)
  • Sandy loam: 100 to 300 feet (30 to 91 metres)
  • Silt and silty clay loam: 60 to 100 feet (18 to 30 metres)
  • Clay and clay loam: 30 to 70 feet (9 to 21 metres)

It is recommended that producers consider the cost and benefits of installing tile drainage while designing their drainage system. Well-drained, higher areas of the field may not require tile drainage and spacing the tiles closer together than necessary is an unwarranted cost.

  1. Installation - use a laser level to remove minor humps and dips in the landscape.
  2. Design and installation of tile drainage systems should only be conducted by trained individuals.  (Workshops offered by University of Minnesota Extension Service and courses offered by the University of Manitoba are available on this subject).

Follow-up monitoring of drained fields

  • Keep records of crop yields, noting any changes in yield variability and stability prior to drainage improvements.
  • Construct nutrient budgets for N and other nutrients to compare the amount of nutrients applied with the amount of nutrients taken up by the crop and remaining in the soil.
  • Monitor water quality from drainage outlets at various times of the year. Compare with surface runoff water quality.
  • Use soil testing for salinity and nutrients.
  • Keep records of growing season precipitation events. If possible, monitor changes in water table levels over the growing season.
  • Be aware of downstream effects and options to minimize the effects, such as controlled release of runoff during critical times.
  • Be a good neighbour who is considerate of the effects of water on landowners downstream.

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