Subsurface Drainage (NRCS 606)

What is it: 

Subsurface drainage is a pipe installed beneath the ground Subsurface Drainage Constructionsurface to collect and/or convey excess water. 

Where is it used: 

Subsurface drainage is best suited to fields with flat topography (<2% slope) that have poorly drained soils and/or high water tables.

Why install it: 

In Ohio, subsurface drainage is primarily used in fields with poorly drained soils and where there are seasonal high water tables or periodic saturated conditions which:

  • prevent timely crop planting
  • prevent timely crop harvesting
  • cause plant stress and reduce yields

The main purpose of installing subsurface drainage is to benefit crop production and in most settings there will be little or no beneficial impact on water quality.

What do I need to know about it: 

Considerations

  • Without controlled drainage or incorporation of treatment practices (such as saturated buffers, bioreactors and P-filters) subsurface drainage is likely to have a negative impact on water quality.
  • Is there an old clay tile drainage system already in my field?
  • Are there suitable edge-of-field outlets, such as an agricultural ditch, for my subsurface drainage system?
  • Does the edge-of-field collection system have sufficient capacity to discharge subsurface drainage from my field?
  • What if any approvals do I need from local, county, state and federal agencies?
  • Are there utility and road-right-of-ways that need to be considered?

Costs

Costs are site-specific and depend on the size of the field, the spacing of the pipes, the size of the pipes and the type of pipe

  • For example, a 100 acre field with 4 inch diameter corrugated plastic pipe spaced 50 ft apart will require more than 87,000 ft of pipe which might cost about $1/ft installed. Once larger diameter mains, connectors and outlet pipes are added to the system a cost of $1000+/acre is probable for this system

Yield increases and timely planting and harvesting benefits will within a few years usually return a profit on the capital investment in subsurface drainage.

How does it work: 
  • Most of the water in soil pores is held in the pores by suction forces and cannot be removed by gravity flow.
  • Following a wetting event (rainfall, snow melt, irrigation, or a rising water table) a small amount of the water in soil pores might be above a threshold called field capacity. Some of that amount can drain, through large pores, into a subsurface drain due to gravity flow.
  • The maximum amount of water that can be removed by a subsurface drainage system is the drainable porosity times the depth of the soil layer being drained. Typically, the drainable porosity is a few percent of the soil profile volume.

Click here for Subsurface Drainage scientific literature

Design: 

Design

  • Drains are typically designed to lower the water table at least 0.5 ft within 24 hours.
  • Consult your drainage contractor for design assistance.  
  • Typical, sizes, widths and depths of subsurface drainage are shown in Table A.
  • Size of area drained for different pipe sizes and pipe grades are shown in Table B.

Table A. Typical drain spacing and depths for parallel drains (ASABE EP260.5 Standard)

 

Soil Type

Permeability

Drain Spacing (ft)

Drain Depth (ft)

Fair Drainage 1/4 inch/day

Good Drainage 3/8 inch/day

Excellent Drainage 1/2 inch/day

Clay Loam

Very Low

70

50

35

3.0 - 3.5

Silty Clay Loam

Low

95

65

45

3.3 - 3.8

Silt Loam

Moderately Low

130

90

60

3.5 - 4.0

Loam

Moderately

200

140

95

3.8 - 4.3

Sandy Loam

Moderately High

300

210

150

4.0 - 4.5

 

Table B. Area drained for different pipe grades and different drainage conditions

 

Pipe Diameter (inches)

Area Drained (acres) for Different Pipe Grades (ft/ft) and Different Drainage Conditions

0.001

0.05

0.1

Fair

Good

Excellent

Fair

Good

Excellent

Fair

Good

Excellent

3

1.4

1.2

1.1

3.1

2.7

2.4

4.4

3.8

3.4

4

3.0

2.6

2.3

6.8

5.8

5.2

9.5

8.1

7.3

5

5.5

4.7

4.2

12

10

9.4

17

15

13

6

8.8

7.6

6.8

20

17

15

28

24

22

8

18

15

14

40

35

31

57

49

44

10

31

26

24

69

59

53

96

82

74

12

47

41

36

105

90

81

148

127

114

15

79

68

61

176

151

136

250

215

193

18

128

110

99

285

245

220

403

346

311

 

NRCS Standard 606: Subsurface Drain