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Planning an agricultural subsurface drainage system

Jerry Wright and Gary Sands

The Agricultural Drainage series covers such topics as basic concepts; planning and design; surface intakes; economics; environmental impacts; wetlands; and legal issues.

General considerations

Many soils in Minnesota and throughout the world would remain wet for several days after a rain without adequate drainage, preventing timely fieldwork, and causing stress on growing crops. Saturated soils do not provide sufficient aeration for crop root development, and can be an important source of plant stress. That's why artificial drainage of poorly draining soils has become integral to maintaining a profitable crop production system. Some of the world's most productive soils are drained, including 25 percent of the farmland in the United States and Canada.

Planning an effective drainage system takes time and requires consideration of a number of factors, including:

The U.S. Department of Agriculture (USDA) Food Security Act and the farm bills of 1985, 1990, and 1996 created many special wetlands restrictions and mandates that all drainage projects, including upgrades, must follow. It's also very important that the landowner, system designer, and contractor understand other applicable federal laws, as well as the local watershed and state laws dealing with drainage. People considering installation of a drainage system should also know their rights and responsibilities concerning the removal of water from land and its transfer to other land. So the first steps of any installation project should always include visits to the offices of the Soil and Water Conservation District (SWCD), the Natural Resources Conservation Service (NRCS), and the local watershed administrative unit.

While developing a drainage plan and specifications, it's useful to consult a number of information sources. These include county soil and site topography surveys, the Minnesota Drainage Guide1, local drainage experts, Farm Service Agency aerial photos, and ditch and downstream water management authorities. It's also a good idea to do some surface and subsurface evaluation of a field.

Economics

To decide whether a new drainage system (or improving an existing system) makes economic sense, it's necessary to determine or estimate the following: (1) what the crop response might be for the area to be drained, (2) the impact of a system on the timeliness and convenience of field operations, and (3) changes in inputs and other costs associated with a drainage system. Needless to say, it's not easy to estimate some of these factors. Data gathered from a combine yield monitor may offer good information on the yield range and variability of a field, as well as crop response to previous drainage activities. Crop response information from Iowa, Ohio, and Ontario specialists (Table 1) could also be helpful.

Table 1. Crop yield response to subsurface drainage for various regions (bu/acre increase).

Crop

Iowa2
1984-1986
Ohio3,4
1962-1980
Ontario5
1979-1986
Corn 10 - 45 20 - 30 26
Soybeans 4 - 15 7 - 14 7
Spring Grain     22
Winter Wheat     17

Other potential sources for yield response information related to improved drainage include neighbors, county Extension educators, and the SWCD office. Many county soil surveys have also identified the potential yield for each soil type for common crops using sound management practices. A detailed financial analysis using the Ohio crop response information can be found in "Minnesota Farmland Drainage: Profitability and Concerns."6 A simplified on-line profitability analysis, developed by the University of Minnesota Extension Service, can be performed at the following website: www.prinsco.com/article.cfm?ID=96. Advanced Drainage Systems (ADS) also offers a CD version of a simplified profitability analysis for drainage investments. Contact your local dealer for more information. These simplified analyses can give you a first guess at overall profitability, but lack the sophistication required to fine-tune investment decisions.

System capacity and drainage coefficient

To protect crops, a subsurface drainage system must be able to remove excess water from the upper portion of the active root zone 24 to 48 hours after a heavy rain. (See Agricultural Drainage Publication Series: Soil Water Concepts for more information on excess, or drainable, soil water.) The drainage system capacity selected for most northern Midwest farmlands should provide the desired amount of water removal per day, commonly referred to as the "drainage coefficient." This figure is often between 3/8 and 1/2 inch of water removal per day. Table 2 shows drainage coefficients guidelines for crop production for land that has adequate surface drainage. (The figures are from Chapter 14 of the NRCS Engineering Field Handbook).

Any refinement of these drainage coefficient guidelines should be done after consulting with drainage experts and local drainage contractors. NRCS literature suggests the drainage coefficient may need to be increased where one or more of these situations occur:

Table 2. General drainage coefficients (inches/24 hours).

Without surface inlets
  Soil type Field crops Truck crops  
Mineral 3/8 to 1/2 1/2 to 3/4
Organic 1/2 to 3/4 3/4 to 1 1/2
With surface inlets

Soil type

Field Crops
Truck crops
Blind inlets Open inlets Blind inlets Open inlets
Mineral 3/8 to 3/4 1/2 to 1 1/2 to 1 1 to 1 1/2
Organic 1/2 to 1 3/4 to 1 1/2 3/4 to 2 2 to 4

Topography and system layout

Where it is necessary to convey surface water to the subsurface drainage system through surface inlets. NRCS literature suggests use of the drainage coefficients in the bottom half of Table 2, depending on inlet and soil type. The selected coefficient should be applied to the entire watershed contributing runoff to the surface inlet unless a portion of the runoff is drained by other means.

Figure 1. Various drainage system layout alternatives.

The goal of drainage system layout and design is to provide adequate and uniform drainage of a field or area. Field topography and outlet location/elevation are typically the major factors considered in planning drainage system layout, with topography greatly influencing what layout alternatives are possible. It's best to create a topography map of the field showing the elevations of the potential or existing outlet(s). A number of methods may be used to create the map, including standard topography surveys, a GPS or a laser system. The topography map helps the designer assess overall grade and identify the high or low spots in a field that might pose challenges.

The system outlet, whether an open channel or a closed pipe, must be large enough to carry the desired drainage discharge from a field quickly enough to prevent significant crop damage. Drainage outlets are typically located three to five feet below the soil surface. Sometimes pumping is required to create an adequate outlet. The bottom of an outlet pipe should be located above the normal water level in a receiving ditch or waterway. It is expected that floods or high water levels may submerge the outlet briefly. Drainage outlets must be kept clean of weeds, trash, and rodents. Outlets must also be protected from erosion, damage from machinery and cattle, and ice in flowing water.

Figure 2. Alignment of field laterals with contours.

Although there may be many possible layout alternatives for a given field (see Figure 1), specific drainage goals should be evaluated to find the best layout. These goals include removing water from an isolated problem area, improving drainage in an entire field, intercepting a hillside seep, and so on. Farmers and designers should approach system layout and drainage needs in a broad, comprehensive manner, anticipating future needs where possible. Even if a drainage system is installed on an incremental basis – some this year, more next year, and so on – system planning should not be piecemeal. Additions to a system will be much easier to make if the established mains are already large enough and located appropriately.

When selecting a layout pattern for a particular field or topography, lateral drains, or field laterals, should be oriented with the field's contours as much as possible. This way, laterals can "intercept" water as it flows down-slope. Mains and submains (also called "collectors"), on the other hand, can be positioned on steeper grades, or in swales, to facilitate the placement of laterals (Figure 2).

Drain depth and spacing

A close relationship exists between soil permeability and the recommended spacing and depth of drains. When a system of parallel laterals is used, the drain spacing and depth should be considered simultaneously, based on soil type, soil permeability and stratification, the crops to be grown, the desired drainage coefficient, and the degree of surface drainage. If there is an abrupt transition from lighter to heavier soil, it's better to keep the drains above the heavy layer, when possible. Spacing drains closer together results in a higher drainage coefficient and faster drainage. The answer to the question "How close is close enough?" involves balancing costs and benefits. Simply stated, the increased cost associated with narrower drain spacings can only be justified to a point. After that, the only result is decreasing profits.

An ideal drainage system would have a uniform drain depth. In the real world, topography and system layout determine the actual depths of drains. A system layout that matches poorly with field topography will result in a wide variation of drainage depths and uneven field drainage. Avoid a system layout with many points of minimum cover (2 - 2 1/2 ft) and excessively deep cuts.

Figure 3. Minnesota Drainage Guide drainage spacing recommendations for a blue Earth Series soil, for 36- and 48-inch depths and four drainage coefficients.

Make decisions on drain spacing and depth after consulting NRCS literature and talking to people in the area with drainage experience. Table 3 shows the most general spacing and depth options that might be considered during the early planning phase of a new or improved system. The Minnesota Drainage Guide1 contains a table of drain spacing recommendations for many soils in Minnesota. Figure 3 shows an example for a Blue Earth soil.

Table 3. General parallel drain lateral spacing and depths for different soils.

Soil Type
Subsoil permeability
Drain spacing (ft.)
Drain depth (ft.)
Fair drainage 1/4 in.
Good drainage 3/8 in.
Excellent drainage 1/2 in.
Clay loam Very low 70 50 35 3.0-3.5
Silty clay loam Low 95 65 45 3.3-3.5
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

Drain sizing

The maximum amount of water a drainage pipe can carry (its capacity) depends on the pipe's inside diameter, the grade or slope at which it's installed, and what the pipe is made of (e.g., smoother pipe has a greater flow capacity, all else being equal). Typically, full-flow pipe capacities for specific grades, pipe sizes, and pipe materials can be obtained from a number of sources:

To estimate the required flow capacity (Q) in cubic feet per second (cfs), multiply the area to be drained by the desired drainage coefficient (dc) and divide by the conversion factor (23.8).

Q(cfs) = [area (acres) x dc (inches per day)] / 23.8

(To use the equation in this form, area and dc must be in units of acres and inches/day, respectively.) Once Q is determined, pipe grade, material, and (ultimately) diameter can be selected to provide the required flow capacity. Topographical constraints typically determine pipe grade, so the pipe size is determined after the material is selected (e.g., corrugated polyethylene pipe, smooth interior pipe, etc.).

Besides flow capacity, drainage systems should also be designed to provide a certain minimum velocity of flow so that "self-cleaning" or "self-scouring" takes place. Where fine sands and silt are present, the minimum recommended velocity is 1.4 feet per second to keep sediments from accumulating in the system. Drainage systems in more stable soils can tolerate slower flow velocities, as low as 0.5 feet per second. Table 4 shows the minimum grades recommended for various pipe sizes when using these flow velocities. These grades are supported by the American Society of Agricultural Engineers – ASAE EP260 standards. Flatter grades result in slower flow and run the risk of failure, and reverse grades, of course, must always be avoided.

Table 4. Minimum recommended grades (percent) for drainage pipes.

Drain inside diameter (in.)
Drains not subjected to fine sand or silt (min. velocity 0.5 ft/s)
Drains where fine sand or silt may enter (min velocity 1.4 ft/s)
Tile
Tubing
Tile
Tubing
3 0.08 0.10 0.60 0.81
4 0.05 0.07 0.41 0.55
5 0.04 0.05 0.30 0.41
6 0.03 0.04 0.24 0.32
8-12*   0.07    
12 and larger*   0.05    
*Recommendation for drain sizes is from NRCS - Minnesota Drainage Guide. For smooth interior CPT, use the "Tile" column.
Example: Find the flow capacity needed to drain 80 acres with a 1/2 inch/day drainage coefficient:
Q(cfs) = 80 ac x 0.5 in/day divided by 23.8 = 1.7 cfs

Because excess water velocities could cause some pressure problems at drain joints or tube openings that might result in unwanted erosion of the soil around the drain, there are also suggested maximum grades for drain sizes and soil types. These suggestions are outlined in Chapter 4 of the Minnesota Drainage Guide1.

Tables 5-7 show the potential land area that can be drained with various grades, drain sizes, and pipe materials using 1/4-, 3/8-, and 1/2-inch drainage coefficients. For other grades, sizes, materials, and drainage coefficients, consult one of the sources mentioned above. When computing drain size with any tool or chart, always round an intermediate size to the nearest larger commercially available size. For example, if a calculation calls for a 6.8-inch diameter pipe, select an 8-inch pipe, assuming a 7-inch pipe is not available.

Table 5. Potential acres drained by drain size, type, and grade for a drainage coefficient of 1/4-inch per day.

 

% Grade
(ft/100 ft)

 

Drain type

Drain size (in.)
4
5
6
6
10
12
15
18

 

0.1

CPE 5.0 9.0 14.6 32 50 82 126 206
Smooth 7.5 13.5 22 47 .86 140 253 411

 

0.2

CPE 7.0 12.7 21 45 71 116 179 291
Smooth 10.5 19.1 31 67 121 197 358 582

 

0.3

CPE 8.6 16 25 55 87 142 219 356
Smooth 12.9 23 38 82 149 242 438 712

 

0.4

CPE 10 18 29 63 101 164 253 411
Smooth 14.9 27 44 95 172 279 506 823

 

0.6

CPE 12 22 36 77 124 201 310 504
Smooth 18 33 54 116 210 342 620 1008

 

0.8

CPE 14 25 41 89 143 232 358 582
Smooth 21 38 62 134 243 395 715 1163

 

1

CPE 16 28 46 100 160 260 400 650
Smooth 24 43 69 150 271 441 800 1301

 

1.5

CPE 19 35 57 122 195 318 490 797
Smooth 29 52 85 183 332 540 980

1593

 

2

CPE 22 40 66 141 226 367 566 920
Smooth 33 60 98 212 384 624 1131 1840
CPE denotes corrugated polyethylene pipe (3"-8", n=0.015; 10"-12", n=0.017; >12", n=0.02) smooth denotes smooth-wall CPE, concrete or clay tile (n=0.01).

Table 6. Potential acres drained by drain size, type, and grade for a drainage coefficient of 3/8-inch per day.

% Grade
(ft/100 ft)
Drain type
Drain size (in.)
4
5
6
6
10
12
15
18

0.1

CPE 3.3 6 9.8 21 34 55 84 137
Smooth 5 9 15 32 57 93 169 274
0.2 CPE 4.7 8.5 14 30 48 77 119 194
Smooth 7 12.7 21 45 81 132 238 388
0.3 CPE 5.7 10 17 36 58 95 146 237
Smooth 8.6 16 25 55 99 161 292 475
0.4 CPE 7 12 20 42 67 109 169 274
Smooth 9.9 19 29 63 114 186 337 548
0.6 CPE 8 15 24 52 82 134 207 336
Smooth 12 22 36 77 140 228 413 672
0.8 CPE 9 17 28 59 95 155 238 388
Smooth 14 25 41 89 162 263 477 776
1 CPE 10 19 31 67 106 173 267 434
Smooth 16 28 46 100 181 294 533 867
1.5 CPE 13 23 38 81 130 212 327 531
19 35 57 122 222 360 653 1062

1593

2 CPE 15 27 44 94 150 245 377 613
Smooth 22 40 66 141 256 416 754 1226
CPE denotes polyethylene pipe (3"-8", n=0.015; 10"-12", n=0.017; >12", n=0.02) smooth denotes smooth-wall CPE, concrete or clay tile (n= 0.01).

Table 7. Potential acres drained by drain size, type and grade for a drainage coefficient of 1/2-inch per day.

% Grade
(ft/100 ft)
Drain type
Drain size (in.)
4
5
6
6
10
12
15
18

0.1

CPE 2.5 4.5 7.3 16 25 41 63 103
Smooth 3.7 6.8 11 24 43 70 126 206
0.2 CPE 3.5 6.4 10 22 36 58 89 145
Smooth 5.3 9.6 16 33 61 99 179 291
0.3 CPE 4.3 8 13 27 44 71 110 178
Smooth 6.5 12 19 41 74 121 219 356
0.4 CPE 5 9 15 32 50 82 126 206
Smooth 7.5 14 22 47 86 140 253 411
0.6 CPE 6 11 18 39 62 101 155 252
Smooth 9 17 27 58 105 171 310 504
0.8 CPE 7 13 21 45 71 116 179 291
Smooth 11 19 31 67 121 197 358 582
1 CPE 8 14 23 50 80 130 200 325
Smooth 12 21 35 75 136 221 400 650
1.5 CPE 10 17 28 61 98 159 245 398
Smooth 14 26 43 92 166 270 490

797

2 CPE 11 20 33 71 113 184 283 460
Smooth 17 30 49 106 192 312 566 920
CPE denotes polyethylene pipe (3"-8", n=0.015; 10"-12", n=0.017; >12", n=0.02) smooth denotes smooth-wall CPE, concrete or clay tile (n= 0.01).

Use of drain envelopes (socks)

A drain envelope, or "sock," is a material placed around a drain pipe to provide either hydraulic function, which facilitates flow into the drain, or barrier function, which prevents certain sized soil particles from entering the drain. Drain envelopes are not filters. Filters become clogged over time; drain envelopes do not. Many types of envelope material exist, from thick gravel and organic fiber to thin geotextiles. The useful life of a synthetic drain envelope is quite long, provided it is not left in the sun for a long time and exposed to too much ultraviolet radiation.

Fine-textured soils with a clay content of 25 to 30 percent are generally considered stable, so they don't need drain envelopes. A geotextile sock is recommended for coarse-textured soils free of silt and clay. These soils are considered unstable even if undisturbed, so that particles may wash into pipes. The need for an envelope in intermediate soils (clay contents less than 25 to 30 percent) is best left to a professional contractor or soil and water engineer because soil movement is more difficult to predict.

Environmental impacts

Subsurface tile drainage systems can convey soluble nitrate-nitrogen (N) from the crop root zone. Implementation of nitrogen fertilizer Best Management Practices (BMPs) can reduce the potential loss of nitrate-N. Adding perennial crops to the rotation may also reduce N losses to surface waters in addition to decreasing water drainage. Farmers installing new or improved field drainage systems should consider using crop management practices and landscape structures that reduce nitrogen, sedimentation, and water discharge rates.

Surface inlets (intakes)

Surface inlets remove ponded water that forms in closed basins or potholes in a field. These inlets, however, can provide a direct pathway for surface waters that may carry sediment and other pollutants to drainage ditches and other downstream surface water. The general public, resource managers, and others are concerned about the potential impacts of surface inlets to both the quality and quantity of downstream waters.

From a water quality perspective, almost any inlet configuration is preferable to using an open pipe that's flush with the ground surface. Of the traditional intakes available, the slotted or perforated riser is a good option because it promotes some settling of sediments in the basin during flow events.

Farmers in some areas have begun replacing traditional inlets with "blind" or "rock" inlets. These have the advantage of being farmable, and anecdotal evidence suggests they can remove water effectively. There are still questions, however, about the effective life of rock inlets. University of Minnesota researchers are currently investigating the performance characteristics of these and other alternative surface inlet designs. This work will ultimately lead to a better understanding of their effectiveness and longevity.

Installation quality

A great deal of careful consideration goes into installing a drainage system. Drain depth, grade, pipe size, and field layout are all extremely important design factors that will determine how well a system performs. But the installation method is also key to a successful system. It's why special care should be taken to ensure that every installation is on grade and of high quality.

Because quality installation is important, an experienced installer is usually an asset. It's also important to know the limitations of equipment. Although pull-type and tractor-mounted drainage plows or trenchers can often perform adequately, they face limitations in the field that, when improperly accounted for, can result in installation and performance problems. Field irregularities such as dead furrows, lines, swales and rocks can pose installation problems for these machines. In addition, operators have found it difficult to make cuts deeper than five feet.

Summary

Improved surface and subsurface drainage is necessary for some Minnesota soils to optimize the crop environment and reduce production risks. To assure an effective and profitable system, it's important to couple a good design process with the thorough evaluation of suconion on-site factors as soil type, topography, outlet placement and existing wetlands. This, and a quality installation will ensure a drainage system that will perform effectively for many years to come.

References

Other resources

Send your additional questions to: Gary R. Sands, Extension Engineer
213 Biosystems & Ag Engineering
1390 Eckles Ave. University of Minnesota
St. Paul, MN 55108
Phone:612-625-4756
Fax:612-624-3005
E-mail:grsands@umn.edu
Web: http://www.extension.umn.edu/
AgDrainage or Department of Bioproducts and Biosystems Engineering

The authors

Jerry Wright is an Extension Engineer, University of Minnesota Extension Service, West Central Research & Outreach Center, Morris 320-589-1711 or jwright@umn.edu

Gary Sands is an Extension Engineer, University of Minnesota Extension Service, Department of Biosystems and Agricultural Engineering, St. Paul 612-625-4756, grsands@umn.edu

Produced by Phyllis Wolk Unger, Sr. Production Editor, Communication and Educational Technology Services, University of Minnesota Extension.

BU-07685 Reviewed 2009
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