1. Subsurface Drip Irrigation (SDI)
Michael Kizer
OSU Extension Irrigation Specialist
Biosystems & Agricultural Engineering

2. Subsurface Drip Irrigation Advantages
High water application efficiency
Uniform water application
Lower pressure & power requirements
Adaptable to any field shape
No dry corners
Adaptable to automation

3. Subsurface Drip Irrigation Disadvantages
High initial cost
Water filtration required
Complex maintenance requirements
Flushing, Chlorination, Acid injection
Susceptible to gopher damage
Salt leaching limitations

4. Subsurface Drip-Center Pivot Comparison (¼-Section Field; ET = 0
Subsurface Drip-Center Pivot Comparison (¼-Section Field; ET = 0.25 in/day)
Subsurface Drip
Center Pivot
Area Irrigated
160 acres
125 acres
Initial Cost
$ /acre
$ /acre
Irrigation Efficiency
90-95%
70-85%
Water Requirement
gpm/acre
gpm/acre
Operating Pressure
10-20 psi
25-35 psi
Energy Requirement
(250-ft lift, ¼ mile supply line)
36 hp-hr/ac-in
48 hp-hr/ac-in

5. Schematic of Subsurface Drip Irrigation (SDI) System
Pump Station
Backflow
Prevention
Device
Flowmeter
Chemical
Injection
System
Air & Vacuum
Release Valve X
Pressure Gage
Flush Valve
Dripline
Laterals
Zones
1 and 2
Submain
Flushline
Filtration System x
Zone Valve
Diagram courtesy of Kansas State University

6. Netafim Typhoon® Drip Irrigation Tubing
(Clear Demo Tubing)
16-mm diameter, seamless, 13-mil thick extruded PE tubing
Emitter outlet
Turbulent flow PVC emitter welded inside tubing

7. Netafim Typhoon® Drip Irrigation Tubing
Flap over emitter outlet:
- prevents root intrusion
- prevents blockage by mineral scale

8. T-Systems® Drip Irrigation Tubing
Water Inlets to Emitter
Emitter Outlet

9. Typical Drip Tubing Installation for Row Crops
Non Wheel- Track Row
12 – 14 in
Drip Tubing
Wetting Pattern
60 in
60-inch dripline spacing is satisfactory on silt loam & clay loam soils

10. Wetting Pattern of a Subsurface Drip Lateral
Photo Courtesy of Kansas State University

11. Wider dripline spacings may not work.
Photo Courtesy of Kansas State University

12. Plowing in drip tubing

13. Trenching across the drip tubing ends for PVC manifold installation

14. Drip tubing end after being sheared by the trencher

15. Typical Drip Tubing Connection to Submain Stainless Steel Wire Tie
Tee in PVC Submain Line w/ 3/4” FPT Opening
Stainless Steel Wire Tie
5/8” PE Supply Tube
3/4” FPT - 5/8” PE Barb Adapter
5/8” Drip Tubing
[An identical connection will attach the far end of the drip tubing to the flushing manifold line.]

16. Typical Drip Tubing Connection to Submain
(1 ½ -inch Submains and Larger)
Supply Submain or Flushing Manifold
Stainless Steel Wire Twist Tie
Neoprene Grommet Inserted in 21/32” hole in manifold
5/8” Polyethylene Supply Tubing
5/8” Drip Irrigation Tubing
Polyethylene Barb Adapter Inserted in Grommet

17. Components for Drip Lateral-Submain Connection
Stainless Steel Wire Twist Tie
21/32” Hole in Submain
Neoprene Grommet
5/8” Polyethylene Supply Tube (Usually 2-3 ft long)
Polyethylene Barb Adapter
5/8” Drip Irrigation Tubing

18. Flush Risers on Distal End of Research Plots
Air Vent to Release Trapped Air from Laterals
Ball Valve for Manual Flushing of Drip Laterals

19. Drip Emitter Discharge
Q = kHx
where:
Q = emitter discharge, gph
k = emitter discharge coefficient, gph/ft of head
H = emitter operating head, ft
x = emitter discharge exponent
(Emitters with x<0.4 are considered pressure compensating, ie discharge fluctuates less with varying pressure variations.)

20. Discharge and Pressure Variation
Pressure Variation (Pmin/Pave)
Discharge Variation (qmin / qave)
X = 0.2
X = 0.5
95%
99%
97%
90%
98%
85%
92%
80%
96%
89%
75%
94%
87%
70%
93%
84%

21. Drip Irrigation and Elevation Changes
2.31 ft of elevation change = 1 psi pressure change
A 10-ft elevation change (4.3 psi) in a 35-psi sprinkler system is a 12% pressure variation, resulting in a 6% change in sprinkler discharge
A 10-foot elevation change in a 10-psi drip system is a 43% pressure variation, resulting in a 17% change in emitter discharge (emitter x=0.44)

22. Elevation Effect on Drip Emitter Pressure and Discharge
Higher Elevation
Lower Operating Pressure
Less Discharge
Lower Elevation
Higher Operating Pressure
More Discharge
2.31 ft = 1 psi
Pressure compensated emitters are needed on undulating fields

23. Emission Uniformity EU = F [Cv (qmin/qave)] where:
EU = Emission Uniformity, (0-100%)
Cv = Coefficient of manufacturing variation of emitters, (0-1.0)
qmin = Minimum emitter discharge along the lateral, (gph)
qave = Average emitter discharge along the lateral, (gph)
EU  80% for drip tubing systems in field crops is acceptable.

24. Coefficient of Variability, Cv
Cv is a measure of variability in the manufacture of emitters
In low flow emitters a very small variation in dimensions can have a huge effect of discharge
The Cv of major manufacturer’s equipment is published (manufacturer’s tests and industry group tests)

25. Performance of Various Dripline Products
Data from Center for Irrigation Technology (CIT)
Product X CV
Chapin (18 gph/100 ft, 12 in)
Roberts (15 mil, 24 gph/100 ft, 12 in)
Netafim Typhoon (25 mil, 18 in)
Rainbird Rain Tape (8 in)
Netafim Ram (0.4 gph, hard hose)
GAIA (3/8 in ID, porous hose)
Slide Courtesy of Kansas State University

26. System Emission Uniformity (Recommended Values from ASAE EP405)
Emitter Type
Spacing (ft)
Topography
Slope (%)
EU Range (%)
Point Source: Perennial crop
> 13
Uniform
< 2
90 to 95
Steep/Undulating
> 2
85 to 90
Point source: Perennial or Semipermanent crop
< 13
>2
80 to 90
Line Source: Perennial or Annual crop
All
70 to 85

27. Maximum Lateral Length
Lmax = maximum lateral length, ft
Dlateral = lateral line diameter, in
Semitter = emitter spacing on lateral, ft
Hinlet = lateral pressure at inlet from manifold, psi
qemitter = average emitter flow rate, gph
xemitter = emitter discharge exponent, unitless
EU = required emission uniformity for the lateral, %

28. Maximum Lateral Length 5/8-in and 7/8-in diameter T-Tape TSX Level Slope : 8 psi Inlet Pressure
Emitter Spacing
Emitter Discharge
EU = 90%
EU = 85%
D=5/8 in
D=7/8 in
24 in
0.34 gph
672 ft
1203 ft
850 ft
1521 ft
0.50 gph
519 ft
929 ft
657 ft
1175 ft
36 in
0.5 gph
1.0 gph
427 ft
765 ft
541 ft
967 ft
48 in
807 ft
1444 ft
1022 ft
1827 ft

29. System Design Guidelines for Uniformity
Run lateral lines along contour or slightly downslope
Arrange irrigation zones so that, as nearly as possible, all emitters in the zone are at same the elevation
Balance flow requirements of all zones if possible
Use pressure compensating emitters on undulating lateral runs
Divide water flow as soon as possible to minimize pressure loss and required pipe sizes

30. Filtration
Filtration removes solid contaminants (suspended solids, precipitates, organic particles) from the water supply
Filtration should be the last treatment process before the water goes to the irrigation system
Match filter system to the irrigation system size, emitter characteristics, and the water contaminant load

31. There are many different types of filtration systems.
                         
The type is dictated by the water source and also by emitter size.

32. Filtration Requirements for Drip Emitters
Filter openings should be 1/7th – 1/10th the size of the emitter orifice
0.020-inch orifice

33. Sand Media Filter
For water with heavy load of organic (algae) or inorganic ( silt, clay) contaminants.
To back-wash properly, the upward flow of water must be high enough to “float” the top portion of the filter sand.

34. Sand Media Filter Sizing
Contaminant Level
Concentration
(ppm)
Filter Loading Rate
(gpm/ft2)
Light
0 – 10
Medium
10 – 100
20 – 25
Heavy
15 – 20

35. Sand Filter Maximum Flow Rate (gpm per tank)
Loading Rate
(gpm/ft2)
Tank Diameter (inches) 18 24 30 36 48 15 27 47 74
106
189 20 35 63 98
141
251 25 44 79
123
177
314 53 94
147
212
377

36. Sand Media Types and Sizes
Sand Media Number
Effective Size
(mm)
Uniformity Coefficient
Media Type
Filtration Level (mesh)
# 8
1.50
1.47
Crushed Granite
# 11
0.78
1.54
# 16
0.66
1.51
Crushed Silica
# 20
0.46
1.42

37. Minimum Backwash Flow Rate (gpm per sand media tank)
Media Type
Tank Diameter (inches) 18 24 30 36 48
# 8 51 91
141
201
360
# 11 26 74
105
188
# 16 32 57 89
126
225
# 20

38. Backwashing
Using two or more small filter units allows the use of filtered water from one or more units to backwash other filter units individually.
Filtration Mode
Backwashing Mode

39. Grooved Disc Filters For moderately-to-very dirty water.
A series of grooved, plastic discs held together by spring pressure removes particles.
Spring pressure on the discs can be relieved for back-washing.

40. Screen Filters
For water with light load of inorganic contaminants. A plastic or metal screen removes particles.

41. Chlorination
Chlorine is a strong oxidizing agent that prevents water contaminants from fouling microirrigation systems.
Biological growths (bacterial slime, algae)
Growth prevented
Dissolved minerals (iron, manganese, etc.)
Oxidized so oxides can be filtered out

42. Chlorination Modes
Continuous
Intermittent
Superchlorination

43. Continuous Chlorination
Used when water treatment is the goal (iron or manganese precipitation)
Concentration: 1 – 2 ppm
Injection Time: Continuous

44. Intermittent Chlorination
Used to prevent or kill biological growths (algae or bacterial slime)
Concentration: ppm
Injection Time: minutes
Frequency: Depends on severity of the problem

45. Superchlorination
Used to dissolve organic buildup blocking emitters (algae or bacterial slime)
Concentration: ppm
Injection Time: Until all lines are filled; Shut system down; Leave standing 24 hours; Flush system
Frequency: As needed for remediation

46. Chlorine Injection Rate
IR = Q x C x S
where:
IR = chlorine injection rate, gph
Q = irrigation flow rate, gpm
C = chlorine concentration required, ppm
S = strength of chlorine source, %

47. Example IR = Q x C = 500 gpm x 20 ppm = 0.92 gph 167 x S 167 x 65%
An irrigator wants to inject 20 ppm free chlorine into his 500 gpm irrigation system using calcium hypochlorite (65% Cl2) as the chlorine source. What is the required injection rate?
IR = Q x C = 500 gpm x 20 ppm = gph x S x 65%

48. Chlorination and pH
The water pH determines if free Cl2 becomes hypochlorite (OCl-), or hychlorous acid (HOCl) which kills organisms times more effectively.

49. Chlorine Sources Calcium hypochlorite (65-70% Cl2)
Granular “swimming pool” chlorine
Calcium precipitation problems in hard water
Sodium hypochlorite (5-15% Cl2)
Liquid household bleach
Preferred source for hard water
Chlorine gas (100% Cl2)
Poison gas
Very large systems & trained technicians only

50. Acid Injection Prevents mineral precipitation
Dissolves scale build-up on emitters (Reduce pH to 4.0 for minutes)
Can provide fertility
N-phuric acid (nitrogen)
phosphoric acid (phosphorous)

51. Venturi Chemical Injector
throttling valve
Bypass venturi injection device for injection of liquid chlorine, liquid fertilizer or acid.
Cutaway of a venturi injector cross-section.
chemical suction port

52. Hydraulic Powered Chemical Injector
drive water exhaust port
drive water inlet & filter
chemical solution injection port
chemical solution intake

53. Chemical Injection Pump
positive displacement piston pump

54. SDI System Maintenance
Lateral flushing schedule (sediment)
Chlorine injection schedule (biological growths)
Acid injection schedule (chemical precipitates & scaling)

55. SDI System Longevity
In 1990 Sundance Farms (Maricopa, AZ) was irrigating Pima cotton with a SDI system (Reed Bi-Wall®) that had been in continuous use for 18 years.
Kansas State University currently has SDI research plots that have been using the same equipment for 13 years.

56. Percent of First Season Flow Rate
K-State SDI Research Site, Colby, Kansas
23 separate zones represented
110
105
Percent of First Season Flow Rate
100 95 90 1 2 3 4 5 6 7 8 9 10 11 12 13
Years

57. Salt Movement Under Irrigation with Saline Water
Subsurface Drip
Sprinkler/Flood
Salt accumulation leached downward by successive water applications
Salt accumulation leached radially outward from drip tubing

58. Salt Accumulation and SDI
Rate of accumulation is slowed because of more efficient water application
Good quality irrigation water (TDS<500 mg/l) and reasonable annual rainfall (16 in.) makes the problem minimal in OK Panhandle
If salt has accumulated on soil surface, operate system during light rains to prevent leaching into irrigated portion of root zone

59. PREC Subsurface Drip Irrigation System
Funded by USDA-CSREES grant
Primary goal is to evaluate SDI for application of swine lagoon effluent
Filtration requirements
Odor release
Nutrient utilization
Salt accumulation in soil
Long-term performance of system
Secondary goals to evaluate water use efficiency of system

60. Small research plots during supply line installation

61. PREC Subsurface Drip Irrigation Plot Map

62. Small Subsurface Drip Irrigation Plots Replicated Agronomic Studies
The sixteen, 3-plot blocks (Plots 1-48) are separated on N-S by 15-ft alleys, and on E-W by 30-ft turn-row area.
50 ft
Plot 1: gph 12 inch spacing on the lateral
15 ft
Plot 2: gph 12 inch spacing on the lateral
45 ft
Plot 3: gph 12 inch spacing on the lateral

63. Large Subsurface Drip Irrigation Plots
Field-Scale System Hydraulic Studies N
600 ft
Plot 49: gph 24 inch spacing on the lateral
60 ft
Plot 50: gph 24 inch spacing on the lateral
240 ft
Plot 51: gph 18 inch spacing on the lateral
Plot 52: gph 18 inch spacing on the lateral

64. SDI Water Application Rates (inches/hour) (60-inch tubing spacing)
0.16 gph
0.043
0.034
0.026
0.21 gph
0.056
0.045
0.33 gph
0.088
0.071
0.053
0.53 gph
0.142
0.113
0.085
Emitter Spacing
Emitter Discharge

65. SDI Water Application Rates (inches/hour) (30-inch tubing spacing)
0.16 gph
0.086
0.068
0.052
0.21 gph
0.112
0.090
0.33 gph
0.176
0.142
0.106
0.53 gph
0.284
0.226
0.170
Emitter Spacing
Emitter Discharge

66. Go ‘Pokes!