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Step 4.3.1 - What planning issues do I need to consider?

Back to - Step 4.3 - What needs to be considered in the planning, design, cost and management of subsurface drip irrigation (SDI)?

There are many aspects that should be considered during the planning of an SDI system. Planning considerations focus on the following questions.

Step 4.3.1 - What planning issues do I need to consider?
Step 4.3.1.1 - What are the general advantages and disadvantages of SDI systems?
Step 4.3.1.2 - What are the key components of SDI systems?
Step 4.3.1.3 - What shape and area of land can be irrigated?
Step 4.3.1.4 - Can undulating land be irrigated?
Step 4.3.1.5 - What soil types can be irrigated?
Step 4.3.1.6 - What are the water supply issues?
Step 4.3.1.7 - What are the water quality issues?
Step 4.3.1.8 - Can I use saline groundwater for irrigation?
Step 4.3.1.9 - Do I need a drainage system?
Step 4.3.1.10 - What are my catchment obligations?
Step 4.3.1.11 - What are the filtration and flushing requirements of the system?
Step 4.3.1.12 - Can I grow other crops with the same setup?
Step 4.3.2 - What needs to be considered in the design stage?
Step 4.3.3 - What does SDI cost?
Step 4.3.4 - How do I manage the system?
Step 4.3.5 - What system maintenance is required?
Step 4.3.6 - Installation of SDIá


Step 4.3.1.1 - What are the general advantages and disadvantages of SDI systems?

Every irrigation system has advantages and limitations and no irrigation system is appropriate under all environmental conditions. The advantages and limitations of SDI systems are described in Table 1, and Table 3.1 (Step 3).

Table 1: Advantages and disadvantages of SDI systems

Advantages of SDI systems
  • Potentially the most efficient irrigation application system if properly planned, designed, installed and managed.
  • There is potential for increases in yield and quality of the crop.
  • There is no wind disturbance as in sprinkler systems.
  • Irrigation application occurs below the surface, so effects of infiltration such as surface crusting and water ponding are reduced.
  • Subsurface water application reduces water losses due to evaporation and runoff, and can also reduce weed establishment.
  • Allows greater control of irrigation scheduling.
  • Emitters need lower operating pressure than sprinklers.
  • Can be used in irregularly shaped areas and around fixed objects.
  • Land leveling requirement is not critical as in surface irrigation.
  • Labour requirement is relatively less.
  • Fertiliser application efficiency can be higher when applied as fertigation with irrigation water. The growth of harmful bacteria, fungi and other pests and diseases is reduced with a dryer crop microclimate.
  • Provides the option to run the system manually or automatically.
  • Staged installation of the system is possible.
  • Filtration is an essential component of SDI. Filtered water could benefit stock health and milk quality if reticulated to the stock water supply.
Disadvantages of SDI systems
  • High initial capital cost is the main impediment to the adoption of SDI technology with operating costs a further concern.
  • Most of the system is buried, so faults can be difficult to detect, locate and repair.
  • Susceptibility to clogging of emitters due to water impurities, rust or other chemical precipitates, micro-organisms or root penetration.
  • Difficult to detect and repair broken drip tapes and emitters.
  • Risk of salt accumulation at the fringes of the wetting zone.
  • Rodents and insects can damage drip tape.
  • An inappropriate design or material used (e.g main and sub-main, drip tape, emitters, pump, filter etc.) can reduce irrigation application efficiency and yield.
  • Crop establishment can be a problem, particularly on light soils. An alternative means of initiating germination and establishment is recommended.
  • May not be feasible for short duration leased land.
  • Requires a different set of management skills and commitment to a maintenance regime.
  • Pugging by stock can damage drip tapes and lead to water leakage and ponding.
  • Soil compaction can reduce water infiltration in high traffic areas and restrict plant growth.
  • Crop establishment and productivity is influenced by wetted area and wetted volume, which can dictate root development and constrain growth.
  • Relies on appropriate pump and filtration systems.


Step 4.3.1.2 - What are the key components of SDI systems?

The main components of SDI systems are pump, filter, pressure gauge, flow meter, main and sub-main, valves, drip tape and emitters (Figure 1). These components are necessary to properly manage and maintain the system. The descriptions here are a guideline only as the specific site and system conditions for your installation must be considered when designing your system.

Pump

Generally, centrifugal pumps (Figure 2) are used to pump water from surface supply systems or storages. The selection of the right size of the pump is important and depends on the required flow rate and pressure head. The total head required includes pumping lift, friction losses, elevation changes, system pressure and pressure loss across the filter and other components (control valves, flow meters, check valves, main and sub-main supply lines). A large area to be irrigated as a single unit (block) requires higher pump pressure compared to irrigating sub-blocks separately. The pump efficiency is the ratio of hydraulic energy at the discharge of the pump divided by the mechanical energy delivered to the pump. A pump efficiency of 75-85% is desirable. Centrifugal pump with motor
Figure 2. Centrifugal pump with motor

Pump characteristic curves (Figure 3) are provided by manufacturers to show the performance of pumps with respect to pressure (head), flow rate, speed and power. Characteristic curves can be of two types (a) a constant speed with different impeller diameters, or (b) variable speed with constant impeller diameter. In order to know the appropriate performance of the pump you should know the pump duty, which is the required flow rate and pressure of the pump. For example in Figure 3b, pump duty is 9.75 litres per second at 2300 rpm and 60 m head for an efficiency of 70%. Pump suppliers can help you in the selection of the right pump for your system.

(a)
Figure 3. Characteristic curves of centrifugal pump (Yiasoumi, 2003)
(b)
Figure 3. Characteristic curves of centrifugal pump (Yiasoumi, 2003)
Figure 3. Characteristic curves of centrifugal pump (Yiasoumi, 2003)

Pressure gauges

Pressure gauges (Figure 4) are used to monitor the system at various points, typically at the beginning of main line, at the end of the system and at flushing. Small variations across the field may be due to friction losses in the system, but substantial non uniformity of pressure across the field can mean that either the pump is not working properly or that there is leakage or blockage in the system. SDI systems have a relatively low pressure requirement, with a minimum input pressure of 100 kPa and a maximum pressure limited by the wall thickness of the drip tapes.
Figure 4. Liquid filled pressure gauge
Figure 4. Liquid filled pressure gauge

Main and sub-main

The main water supply line (Figure 5) is usually made of PVC and runs from the water supply source (irrigation supply channel, bore or farm dam) through the pump, filter, flow meter and pressure valve to the main control valve. One main line can control different areas or fields (sub-blocks) by using a different control valve for each sub-block.

Sub-mains are also made of PVC pipe and connect the main line to the drip tapes in each field or sub-block. A second sub-main at the end of each field connects the downstream ends of the drip tapes. Sub-mains are the part of the system in which flushing valves, air release valves, secondary flow meters and automation components are installed. The sub-main installation depth should be below the depth of the drip tapes so that soil particles or other impurities can accumulate in the sub-main, otherwise the material will accumulate in the drip tape and clog the emitters. The diameter of the main and sub-main lines depends on the required flow rate, water pressure and friction loss, which in turn depends on the pipe length and diameter, and flow rate. Supply tubes (or connectors) connect the sub-mains to each end of the drip tapes.

Figure 5. Installing a main line for a 2.0 ha SDI system
Figure 5. Installing a main line for a 2.0 ha SDI system
Drip tape and emitters

Drip tape is made of polyethylene. Usually a neoprene grommet, polyethylene barb adapter, clamps and polyethylene connector (60-90 cm long) are used to connect the sub-main to each drip tape (Figure 6). Emitters are located on the drip tape at a regular spacing. There are many types of emitters available in the market, such as vortex, diaphragm, adjustable flow, dripline, pressure compensating and non pressure compensating. Emitters come in a variety of sizes and flow rates, and should match your crop water requirement, the requirements of the site and the hydraulic characteristics of the soil. Low flow rate emitters may not adequately irrigate the crop, while high flow rate emitters may cause water losses in the form of deep drainage and/or runoff. A well designed system will have a uniform flow rate across the field. Where there are large elevation changes in a field or long lateral lengths, pressure compensating emitters or division into sub-blocks should be considered for better performance.

Figure 6. Drip tape connected to sub-main
Figure 6. Drip tape connected to sub-main line

Flow meters

The installation of a flow meter in the main line is necessary for good irrigation management. The flow meter is installed in the main line downstream of the pump to measure the volume of water used per irrigation and for the season (Figure 7a). Flow meters for this purpose usually measure flow in megalitres (ML) or cubic metre (m3). Multiplying m3 by 0.001 will convert flow rate to ML. Usually one flow meter is enough in the main line if the whole system is managed as one block but if the field is managed as sub-blocks then separate flow meters can be used to measure and manage irrigation of sub-areas (Figure 7b).

(a)
Figure 7.  Flow meters installed in the (a) main line and (b) sub-main line
(b)
Figure 7.  Flow meters installed in the (a) main line and (b) sub-main line
Figure 7. Flow meters installed in the (a) main line and (b) sub-main line

Filter

The filter is a critical part of SDI systems and increases the life expectancy of the system. Filters remove physical, chemical and biological impurities (Section 4.3.1.7) and prevent the blockage of the drip tapes and emitters. If filters are not installed, small sand particles can create wear and tear to different parts of the system. Particles greater than 300 Ám in size cannot pass through emitters and will cause clogging, whilst particles greater than 100 Ám can lodge in emitter labyrinth passages. There are many types of filter systems available such as screen, cartridge, media, disk (Figure 8) and centrifugal filters. The selection of the most appropriate filter depends on your water quality and budget. Surface water generally requires a greater degree of filtration than good quality groundwater. Sometimes a combination of filters gives better results than using a single type of filter. Filters will block up as sediment accumulates, so backwash cycles are required to remove accumulated material. Good SDI equipment suppliers can provide guidance on selection of the best filters for your specific requirements.Figure 8. Disk filter installed in SDI system
Figure 8. Disk filter installed in SDI system

Valves

Valves play important role in controlling water pressure and flow rate to optimise system performance and facilitate management. Some of the valves used in SDI systems are described below.

Pressure regulating valve

The purpose of pressure regulating valve (Figure 9) is to maintain the system pressure at the required level. These valves are of two types, adjustable and non-adjustable. Adjustable pressure valves, with a pressure sensor that opens and closes the valve to regulate the pressure, are generally used in SDI systems. Figure 9. Adjustable pressure regulating valve
Figure 9. Adjustable pressure regulating valve

Control valve

Control valves are used to turn water on and off in the system. Depending on the layout, there may be one control valve for the whole field or separate valves for each su-block.

Air release valve

Automatic air release valves (Figure 10) are used to avoid vacuum in the system. These valves prevent soil being sucked into the emitters when the system is shutdown or when the drip tape laterals are drained. They are usually installed in the flushing manifold (sub-main).Figure 10. Air release and flush valve
Figure 10. Air release and flush valve

Backflow prevention valve

Backflow prevention valves are used between the irrigation water source and the pump to avoid contamination of water sources from fertiliser, herbicides, pesticides, acid or chlorination that may have been added to the irrigation system.

Flushing valve

The main purpose of the flushing valves (Figure 11) is to enable flushing of the system in order to remove sediments and other contaminants which can block the system. Flushing flow is directed through the drip tapes to the sub-main and out though the flushing valves. For effective flushing the water velocity in the drip tapes should be higher than normally required for irrigation. The size of the flushing valve required will depend on the area to be flushed. Generally, manually operated flushing valves are used. It is recommended that all automatic or independent control valves be capable of manual operation.

Figure 11. Flushing valvesFigure 11. Flushing valves
Figure 11. Flushing valves

Fertiliser unit

To apply liquid fertiliser with irrigation water (fertigation), a fertiliser unit needs to be attached to the system. It consists of a fertiliser tank (Figure 12a), supply pipe, fertiliser pump (Figure 12b) and control valves. Be aware that SDI irrigation manufacturers may recommend specific fertilisers for their systems, and to avoid disputes it may be prudent to either use products that are recommended or check with the system supplier.
(a)(b)
Figure 12. (a) Fertiliser tankFigure 12. (b) Fertiliser injecting pump
Figure 12. (a) Fertiliser tank and (b) fertiliser injecting pump

Further information
Queensland Department of Environment and Resource Management - Sub-surface drip irrigation: System components (external link)


Step 4.3.1.3 - What shape and area of land can be irrigated?

SDI systems can be implemented fairly flexibly with respect to field shape and size. Indeed, SDI may have particular application on areas of potentially productive land that are too difficult to set up for either surface or centre pivot irrigation because of irregular shape, topographic relief, trees or infrastructure. Limitations on layout are generally related to impacts on system operating pressure uniformity and flow rates, which ideally should not vary by more than about 10 percent. Irregular layouts are likely to be more expensive to design and install because of the requirement for proportionally greater lengths of sub-main lines and more complex plumbing. Long runs of drip lines should be avoided because of friction losses in the drip line, which impact on flow rate uniformity.

Further information
Queensland Department of Environment and Resource Management - Sub-surface drip irrigation: System design (external link)


Step 4.3.1.4 - Can undulating land be irrigated?

Land leveling is not as critical with SDI systems as it is with surface irrigation. However, increased undulation can increase the design complexity so major undulations should be removed and the field levelled (Figure 13) if feasible. Soil ripping is important to improve rooting depth and soil drainage, and cross ripping will improve the lateral movement of water and irrigation uniformity. With substantial variation in topography it may be more economical to divide the field and manage the irrigation of each part separately. Pressure compensating emitters, which are designed to reduce the effect on emitter flow rate of pressure changes in dripper lines, should be used on undulating sites.

Given the investment required to install SDI, you should ensure that the site has good surface drainage to avoid production losses from inundation or waterlogging after heavy rainfall.

(a)(b)
Figure 13. (a) Ripping and (b) levelling of landFigure 13. (a) Ripping and (b) levelling of land
Figure 13. (a) Ripping and (b) levelling of land

Further information
Queensland Department of Environment and Resource Management - Sub-surface drip irrigation: Installation (external link)


Step 4.3.1.5 - What soil types can be irrigated?

For pasture production in northern Victoria, SDI technology works best on duplex soils featuring a strong texture contrast down the soil profile, typically consisting of a relatively well drained or pervious loam or sandy loam topsoil (A-horizon) with a fairly abrupt texture change to a not so well drained or low permeability medium to heavy clay soil (B-horizon). The low permeable B-horizon soil improves the lateral movement of water in the A-horizon. In light textured soils without a clay subsoil, deep drainage can be substantial and much closer drip tape spacing is required to ensure adequate irrigation of the areas between dripper lines and to avoid substantial water losses to deep drainage (Figure 14).

SDI can also be used on heavy cracking clays but there can be problems with soil shrinkage and swelling, associated constriction of tapes and high longitudinal pressures associated with cohesion along the drip line. Information of soil types in the Shepparton region can be found in Section 2.2.1.

Figure 14. Wetting pattern for 0.6 and 1.4 m drip tape spacing after 30 mm water was applied on fine sandyFigure 14. Wetting pattern for 0.6 and 1.4 m drip tape spacing after 30 mm water was applied on fine sandy
Figure 14. Wetting pattern for 0.6 and 1.4 m drip tape spacing after 30 mm water was applied on fine sandy loam in northern Victoria

Further information
For further information see the Information Note - Small Farm: water infiltration and drainage of soils


Step 4.3.1.6 - What are the water supply issues?

Drip systems typically wet a small volume of soil per emitter and the required flow rate for the whole system is generally less than that required for border-check systems. However, drip systems are irrigated more frequently than other irrigation systems, so a continuous water supply is necessary for irrigation management. The total water requirement for the coming season can be estimated from the average of previous years’ evapotranspiration. Because SDI systems need frequent irrigations, you should investigate placing a repeating order with your water authority rather than placing a separate order for each irrigation event.

In the Goulburn Murray Irrigation District of northern Victoria, direct pumping from a supply channel is the simplest way to supply water to your SDI system, however approval is required from Goulburn-Murray Water (G-MW). Discussion with the G-MW is important to ensure that they can supply the required amount of water. An alternative option may be to pump from an on-site storage dam or to use groundwater. However the ground water quality should be tested to ensure the water is fit for irrigation.


Step 4.3.1.7 - What are the water quality issues?

The good quality irrigation water is important for the soil and crop health as well as system longevity. The physical, chemical and biological impurities of irrigation water can affect SDI performance by clogging and damaging the system. A laboratory analysis of source water can provide information on water quality.

Physical impurities

Physical impurities can be suspended soil particles, grass and roots. It is recommended that a representative sample of the water should be tested for turbidity (sediment levels) to gauge the amount of particles in suspension. A test of irrigation supply channel, river and bore water turbidity was carried out as part of a SDI study on pasture at Dookie in northern Victoria (Figure 15). The particle size analysis showed that the water samples to be very different in terms of particulates. The Broken River water showed a significant amount (approximately 75%) of particulates greater than 100 Ám in size. The irrigation supply channel water also showed a significant amount of >100 Ám particles (approximately 40%). The bore water was clearly of better quality in terms of particulates and only contained a small fraction over 100 Ám. The laboratory analysis showed that particulates <100 Ám in size are not problematic to performance and are easily cleared through the dripper. The study concluded that water filtration was required particularly for river and channel water prior to use in SDI systems.

Figure 15. Particle size in groundwater, Broken river and channel water (Dassanayake et al. 2009)
Figure 15. Particle size in groundwater, Broken river and channel water (Dassanayake et al. 2009). Graph courtesy of Melbourne Uni and Gardiner Foundation.

Chemical impurities

Chemical impurities in supply water can also clog SDI systems, and acid treatment is necessary to remove precipitate contaminants such as white calcium carbonate or reddish brown iron oxide. The application of acid solution into the irrigation water can break up scale deposits (Reich et al. 2009). The acid most commonly used in SDI systems is sulfuric acid. Use of acid at higher concentration than recommended can damage SDI systems, while too low concentration may not fully clear blockages.

The use of acids is dangerous and chemical protocols need to be followed for their handling, storage and deployment. It is essential to read and follow instructions and precautionary measures when using acids and other chemicals. For further information contact SDI service providers and equipment suppliers.

Biological impurities

Biological clogging can be from bacteria, algae or other vegetable matter. Injecting a mild chlorine solution can help clean bacterial slime accumulations inside drip tape and emitters. At low concentration (1 to 5 ppm) chlorine can kill bacteria and remove iron oxidation.

The potential for clogging of the system by physical, chemical and biological impurities is given in Table 2.

Table 2. Potential for clogging of SDI by irrigation water (Pitts et al. 1990)
FactorSymbolUnitsMinorModerateSevere
pH< 7.07 - 8> 8.0
BicarbonateHCO3meq/L< 2.0> 2.0> 2.0
IronFemg/L< 0.20.2 - 1.5> 1.5
ManganeseMnmg/L< 0.10.1 - 1.5> 1.5
Hydrogen sulphideH2Smg/L< 0.20.2 - 2.0> 2.0
Total dissolved solidsTDSmg/L< 500500 - 2000> 2000
Suspended solidsmg/L< 5050 - 100> 100
Bacteria countNo./ML< 10, 00010, 000 – 50, 000> 50, 000

Further information
'Section Step' 4.3.1.8
Information Note - Measuring the salinity of water
Information Note - Testing and interpretation of salinity and pH
Queensland Department of Environment and Resource Management - Sub-surface drip irrigation: System maintenance (external link)


Step 4.3.1.8 - Can I use saline groundwater for irrigation?

Salinity in the southern Murray Darling Basin is dominated by sodium chloride (NaCl). Salinity adversely affects soil health, degrades soil structure and reduces soil biological activity. Salinity reduces the capacity of plants to take up water, and in higher concentrations is both toxic to plants and affects the availability of nutrients such as K, Mg, N and P. Table 3 summarises recommended irrigation water salinity concentration for surface irrigation of forage species in the Shepparton Irrigation Region. In the absence of adequate rainfall for leaching there is a risk that salt could rapidly accumulate in the topsoil of SDI fields, so guidelines for surface irrigation need to be used with caution when considering SDI systems. Given the investment required to establish SDI systems and in consequence the need to produce high yielding, high value crops from these systems, it would usually make sense to use poor quality water.

EC units: 1000 EC = 1000 μS/cm = 1 dS/m = 640 ppm = 1 mmhos/cm


Table 3. Recommended electrical conductivity of irrigation water for forage species (DPI, 2006)
SpeciesApplied irrigation water (EC)Comments
Sensitive plants
Clover (white, red, cluster, suckling and subterranean)
Up to 800Suitable for use with all crops. Above 800 EC sensitive plants will suffer some yield loss.
Moderately-sensitive plant
Balansa clover, Persian clover, strawberry clover and lucern
800-1500Sensitive plants have increasingly reduced growth. Moderately-tolerant plants should suffer little yield loss with good management at the lower end of this range. At the upper end, some yield loss occurs.
Moderately-tolerant plants
Berseem clover, sorghum, tall fescue, phalaris, perennial ryegrass, cocksfoot, wheat and paspalum
1500-3000Moderately sensitive plants will suffer increased yield loss. Moderately-tolerant plants should suffer little yield loss with good management at the lower end of the of this range. At the upper end of some yield loss occurs.
Tolerant plants
Tall wheatgrass, puccinellia, bermuda grass, barley (grain)
3000-5000Moderately tolerant plants will suffer increased yield decline. Only tolerant plants should be grown with very good irrigation/soil management.

Towards the top end of this range some yield decline will occur for some of the tolerant plants.

Further information
Information Note - Reusing saline groundwater safely - strategies to reduce risk
Information Note -
Using saline groundwater in the Shepparton Irrigation Region


Step 4.3.1.9 - Do I need a drainage system?

Good surface and subsurface drainage systems provide the means for managing the productivity impacts of surface inundation and high watertables. Unlike surface irrigation systems, which require good surface drainage in order to operate efficiently, SDI systems do not generate runoff. However significant runoff can occur after rainfall. Rainfall is also the primary driver of the shallow groundwater systems of northern Victoria. So good surface drainage with connection to the regional surface drainage system and options for controlling shallow watertables with groundwater pumping are both important factors in managing climate risk.


Step 4.3.1.10 - What are my catchment obligations?

There are a number of off-farm issues that have to be considered before irrigation development can commence. These issues are discussed in detail in Section 2.2.11.


Step 4.3.1.11 - What are the filtration and flushing requirements of the system?

Laboratory analysis of the irrigation water can help in the selection of filters. Better quality water can reduce the capital cost involved for filtration. Despite good filtration, flushing of SDI drip lines is essential. Particles less than 100 Ám can remain in drip lines and can accumulate so these particles need to be purged from drip lines regularly. The frequency of flushing is dictated by the quality of water, the more turbid the water supply the more frequently the lines will need to be flushed. The relatively high water requirements of pastures and fodder crops generally demand more flushing because the particles in water are likely to accumulate faster, so filter backwash cycles are likely to be more frequent for pasture applications compared with horticultural and viticultural applications.

Further information
Queensland Department of Environment and Resource Management - Sub-surface drip irrigation: System maintenance (external link).


Step 4.3.1.12 - Can I grow other crops with the same setup?

A well designed, installed and managed SDI system should last at least a decade, so given the changes that the irrigation sector, and the dairy industry in particular, have seen in the past decade in northern Victoria, it would be prudent to consider potential alternative crops when designing an SDI system. Crop water requirement and rooting depth (Table 4) are key factors that need to be considered when planning an alternative crop on an existing SDI system. The large initial investment cost of SDI systems requires high yield from a high value crop for the system to be viable. Discussion with DPI agronomy staff and SDI service providers and suppliers may help with the development of an adaptable design.

Further information
Information Note - Small Farm: Pasture establishment (PDF - 46 KB)

Table 4. Range of effective rooting depth for different crops (Allen et al. 1998)
CropRooting depth (m)
Broccoli0.4 - 0.6
Cabbage0.5 - 0.8
Cauliflower0.4 - 0.7
Lettuce0.3 - 0.5
Tomato0.7 - 1.5
Cucumber0.7 - 1.2
Potato0.4 - 0.6
Barley, Oats, Spring wheat1.0 - 1.5
Winter wheat1.5 - 1.8
Maize0.8 - 1.7
Millet1.0 - 1.2
Sorghum1.0 - 1.2
Lucerne - for hay1.0 - 2.0
Lucerne for seed1.0 - 3.0
Clover hay, Berseem0.6 - 0.9
Rye grass hay0.6 - 1.0
Grazing pasture0.5 - 1.5
Bermuda1.0 - 1.5
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