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 |
- |
- |
- |
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 due to no “filtering” of water through soil.
B. Tile Drainage
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.
- 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.
Figure 6.1
Comparison of water table and
root development in tiled and
untiled conditions (Sands, 2001) |
![](/web/20061121033738im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbe01s07a.jpg) |
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.
- 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:
- 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);
- a healthy, actively growing crop will utilize any
subsequent precipitation that brings soil moisture up to
field capacity;
- 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.
- 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.
- 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.
- Proper design - depending on field conditions, tile
drains placed 30 to 36 inches (90 to105 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.
- 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.
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.
![](/web/20061121033738im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbe01s07b.jpg) |
![](/web/20061121033738im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbe01s07c.jpg) |
Figure 6.2 Wet land |
Figure 6.3 A wetland |
Table 6.2 Distinguishing wet land from wetlands using
agriculture capability ratings of soil
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 1-2 m (3-6 ft) 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 6.2).
Drainage of wet land by soil texture:
- Clays - poorly drained soils (such as Osborne soils)
have their agriculture capability upgraded from 5W to 3W through
properly designed surface drainage.
- 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.
- 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.
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.
A. Surface drainage:
- Determine purpose/goal
- Obtain a detailed topographic survey (elevation map) of
selected field(s)
- Conduct a detailed cost/benefit analysis
- Obtain a drainage license from Manitoba Water Stewardship,
which will include obtaining sign-off from those impacted
(private and/or municipal).
- Stake out drainage path beforehand
- Start at outlet and work backwards, maintaining proper grade
- 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.
- Consider outlet control to reduce runoff velocity or to
control outflow timing.
B. Tile drainage:
- Determine purpose/goal
- Obtain a detailed topographic survey (elevation map) of
selected field(s)
- Identify site criteria to confirm tile drainage is the most
appropriate solution
- Conduct a detailed cost/benefit analysis
- Obtain a drainage license from Manitoba Water Stewardship
- Tile drainage design:
- 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.)
- An appropriate alternative use to consider is runoff
collection on private land and other uses such as
irrigation.
- 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
- Grade - >0.05% (depends on achieving correct flow
velocity, depth, reasonable cost, etc.)
- 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
- 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.
- Installation - use a laser level to remove minor humps
and dips in the landscape
- 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 crude 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 neighbor who is considerate of the effects of
water on landowners downstream.
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