3 Planning New Irrigation Systems
3.1 WHETHER TO IRRIGATE OR NOT
Although irrigation has many benefits, whether to irrigate or not is primarily an economic decision. The benefits of irrigation must increase income enough to cover the costs of purchasing, installing, operating, and maintaining the irrigation system and provide an acceptable return on investment for it to be sustainable.
A well-designed and managed irrigation system increases yield. As yield is one of the major determinants of profit, the impact of irrigation on expected profit and profit variation over a number of seasons needs to be assessed. Assuming that product prices and other input prices don’t change, the increase in yield obtained through irrigation translates into increased profits. To obtain the net increase attributable to irrigation, the capital and operating costs of the irrigation system should be subtracted from the income arising from increased yield. If the costs are greater than the increased income, then perhaps irrigation should not be implemented. If increased income is significantly higher than the additional costs, then irrigation is probably worthwhile.
In New Zealand, irrigation is supplemental to rainfall, and the higher the rainfall, the lower the marginal return for irrigation. The difficulty with rainfall is that in many areas it is unreliable, causing high variability in annual yields and profit. With irrigation, yield variability is reduced significantly. Risk reduction is one of the main benefits of irrigation, as it reduces the risk of low yields.
All things considered, risk, however, does not necessarily decrease with irrigation. Risk may be increased in the long term by the decision to irrigate. Irrigated farms are subject to higher costs, and the investment in irrigation may expose the farm to higher financial risks. In addition, legislation may change affecting the cost of capital, access to water may change, increases in energy costs may exceed the increase in product prices, and so on.
Each case must be considered on its merits. Qualified farm and financial advisors should be consulted to determine the economic feasibility of irrigation.
3.2 DESIGN OF NEW SYSTEMS
An irrigation system must meet farmers’ goals. The design of the system is one of the key determinants of whether that system has the potential to reach those goals.
To irrigate efficiently and sustainably, the irrigation system must be designed to be physically capable of meeting the needs that are identified. There is no point in making a decision to apply a given depth of water if the irrigation system is not physically capable of applying that depth. Likewise, there is no point in applying a given depth of water so inefficiently that the crop is only able to use a small proportion of the applied water.
In designing new systems, there are a number of basic design questions that need to be answered relating to water supply, application depths and rates, layout and equipment selection.
Relating to the water supply and irrigation system capacity, typical questions are:
- How much water is needed?
- What maximum supply rate (system capacity) is required?
- Where is the water going to come from?
- How much water is required per season?
- Is storage necessary?
- What consent conditions are likely to be imposed?
Having dealt with the issues relating to the water supply, the operation and management requirements for the system need to be addressed.
- What range of application depths should the system be able to apply?
- What range of return intervals can be accepted?
- What application rate is acceptable?
Finally, the system needs to be designed and irrigation equipment selected so that the system is capable of meeting the above requirements.
3.2.1 Water Supply/System Capacity
The water supply is the heart of any irrigation system, and is often the controlling factor in irrigation feasibility. It determines the area of crop that can be irrigated effectively, and has a large influence on the profitability of irrigation.
Although a number of community based schemes are in existence, water supplies for irrigation in New Zealand are developed primarily by individual landowners. Most farmers do not have the luxury of a choice of supplies. Often, only one choice exists realistically. This may be from groundwater, streams, rivers, harvested into storage, or part of a wider community based scheme. Other sources include municipal supply, sewage, and industrial wastes.
Locating a water supply may be an easy task if a stream or river is available, or if proven groundwater exists. However, a guaranteed supply is not always available.
River diversions are usually a simple method of obtaining water, and are often the cheapest, but river flows are subject to wide natural variations, and storage may be required to improve reliability of supply. Abstraction from rivers is controlled by legislation, and reliability of supply needs to be assessed. Groundwater is, in many situations, a more dependable source of water than river diversions. It is a form of natural storage that does not suffer from evaporative losses. However, the cost of lifting the water to the surface can be a major constraint to intensive use by individual farmers.
A water supply has an initial cost of development, and an ongoing cost for delivery. The ongoing cost for delivery is normally an energy cost, and conserving water and energy is important in maintaining a sustainable irrigation system.
Generally, surface supplies and groundwater supplies in New Zealand fluctuate significantly during the irrigation season, and are replenished by rainfall. When annual withdrawals from groundwater systems exceeds replenishment, water levels fall. Lower water levels increase pumping costs, and can make abstractions difficult or impossible. Energy requirements for pumping will affect irrigation costs, and profit. Some knowledge, therefore, of water supply reliability is required. Just because the water is easily available today does not mean it will be as freely available tomorrow.
3.2.1.1 Quantity of water required
An approximation of how much water is needed can be calculated as follows:
Flow rate (in m3/h) = 10 × A × d
H
where A = area irrigated in hectares
d = gross daily depth of water required in millimetres
H = number of hours of irrigation per day
An alternative way of calculating the flow rate taking into account the actual gross application depth and the rotation or cycle time is:
Flow rate (in m3/h) = 10 × A × D
H × F
where A = area irrigated in hectares
D = depth of water applied in millimetres
H = number of hours of irrigation per day
F = cycle time or return interval in days
The factors d (or D) and F can be obtained from local irrigation specialists or Regional Councils. The factors A and H can be varied according to farmers’ needs.
Any water requirements for frost protection, temperature control or leaching must also be taken into account.
Irrigation specialists take a number of factors into account when determining gross irrigation requirements. Some of these factors are the amount of water used by the crop (which depends on crop type and climate), effective rainfall, soils, carry-over soil moisture from winter rainfall, risk of not meeting soil moisture deficits and irrigation efficiency. A dependable water supply cannot be based on average requirements, as the supply would meet the needs of the crop only half the time if the average were used. High value crops may justify a water supply that will fully meet the needs of the crop nine years out of ten, while with low valued crops it may not be economical to supply total needs in more than five years out of ten. Each case must be evaluated individually.
3.2.1.2 Water quality
Water quality is important in evaluating a water supply. In general, water quality in New Zealand is very good, but in some areas there are problems.
Impurities carried in solution or in suspension determine water quality. Rainfall and snowmelt picks up impurities and silt as the water flows over the ground surface to streams. Water in streams and rivers pick up additional impurities. Runoff and excess irrigation may pick up nitrates, pesticides and other soluble compounds that could cause the water to be unsuitable for irrigation.
Whether water of a certain quality is acceptable for irrigation depends on climate, soils, crops grown, and depth of water applied. In general, water quality problems are higher in shallow groundwater areas, in streams during low flows, and in the downstream reaches of streams. Stream pollution from industrial wastes, and tide and wind conditions affect the quality of irrigation water. Brackish water is contaminated by acids, salts and organic matter.
Unless you are sure that water quality is satisfactory for irrigation, have the water analysed.
3.2.1.3 Location of supply
Location of the water supply has a major effect on the design layout, cost and operation of an irrigation system. Many irrigation systems have ended up being more expensive, both in terms of capital cost and in operating costs simply because the supply location had been chosen inappropriately. A common mistake is to place a well close to a roadway to give easy access to an electricity supply without considering the hydraulic efficiency of the system.
In many instances, location may be fixed by local conditions and be beyond your control. If there is a choice, the supply should be located at the point that will give the lowest estimated cost of delivering water to each part of the irrigation system.
If the water supply is to be pumped for surface irrigation, it is usually located at the highest point in the field, although in some systems it is cheaper to pump small quantities of water to localised high spots than it is to pump all of the water to the highest point. If the water source is a well, it will be necessary to lift all water to the highest point to supply the surface irrigation system. Again, it may be advantageous to have the well near to, but not at, the highest point to minimise pumping costs.
If the supply is a well, and the irrigation system is pressurised, there is generally much more flexibility in location. Although in some places the location depends on the aquifer, in most cases it is possible to locate the well at the optimum point.
On flat ground, the well should be placed near to the centre of the land to be irrigated. This results in the lowest pipeline cost because shorter runs and smaller pipe diameters can be used. If the property is sloping, the well should be placed towards the high end of the property. However, depending on the degree of slope and length of run, the highest point is not usually the optimum location.
Sometimes, more than one well is required to feed into a single system. Proper location of each well is vital to the cost and the long-term operation of the system. Multiple water supply systems can be hydraulically complex, and experts should be consulted.
The best advice is to have a number of options designed and costed for a range of well positions before the wells are drilled.
Remember, you pay for electrical installation costs once; you pay for design inefficiencies for the life of the system.
3.2.2 Application depths and rates
The three main questions that need to be answered when designing an irrigation system are:
- What depth of water should the system be capable of applying?
- How often should this depth be able to be applied?
- At what rate can the water be applied?
The key requirements of the system are to have the ability to match application depths to soil moisture deficits, to ensure that application rate does not exceed soil infiltration rate, and to apply water as uniformly as possible.
Each of these factors has a significant effect on the capacity and on the efficiency of the system.
3.2.2.1 System capacity (application depth and return interval)
At the design stage, the most important factor is to ensure that the irrigation system has the physical capability to apply the depths of water required to satisfy soil moisture deficits. Physical capability refers to the minimum and maximum depths the irrigation equipment is capable of applying, and the time between irrigation applications. The combination of application depths and the time between irrigation applications determines the overall capability of the irrigation system, and is known as system capacity.
System capacity relates to the maximum depth of water than can be applied by the system in a given time, and is normally specified in mm/day or mm/week, or other similar units. The capacity of the system must be sufficient to irrigate at a level that equates to the risk that the farmer is willing to take. In most circumstances, it is impractical and uneconomic to design systems to meet the worst case conditions that may arise.
Usually, irrigation system capacities are determined in one of three ways.
The first and perhaps the most common method is to use Regional Council water allocation guidelines. A council will allocate a given amount of water for irrigation for specified crop groups, and the irrigation system is designed to utilise this amount of water at peak capacity. Examples of the types of allocations in New Zealand are 5 mm/day, or 35mm/week, or 250 m3/ha/week. Consult your local Regional Council for the figures in your region. Regardless of how the water is allocated, each type can be converted to applying a given depth of water in a specified time.
The second method is to design the system to use the available water, where the capability of the water supply is lower than the council allocations. As an example, assume a well has the capability to be pumped at 150 m3/hr, and that you wish to put in an irrigation system to irrigate 100 hectares. If you pump for 24 hours a day, this equates to 3600 m3/day, which is the capacity to apply 3.6 mm/day over the 100 hectares. This is the same as 25.2 mm/week or 252 m3/ha/week.
The third method is to consider the risk of not meeting the full water demand of the crop during peak ET periods, and select a system capacity according to that risk. If you are not prepared to take any risk for a high value crop, you must use a system capacity that takes into account the maximum ET period of the season, irrigation efficiency, the readily available soil moisture, and the irrigation cycle time, so that you have the capability to keep soil moisture above the stress point at all times. For lower values crops, you may decide to risk not meeting the full demand of the crop during peak ET periods, accepting that at times soil moisture will fall below the stress point and cause a reduction in yield.
For good irrigation management, a range of application depths may be required. The extent to which farmers can vary the depth of water applied will depend on their irrigation systems. Automatically controlled systems normally provide the ability to easily change application depths. Surface irrigation systems and some sprinkler irrigation systems do not provide this flexibility. Although most travelling irrigator systems have a constant flow rate, changing the speed at which the irrigator moves over the paddock will vary the application depth. Some irrigation systems can move at any speed within their range and therefore are very flexible in the depths of water they apply. Other systems have a number of set speeds, and a change in flow rate as well as a change in speed may be required to apply a given depth.
It is extremely difficult to design practical borderdyke irrigation systems to apply less than 80 mm of water. If soil types dictate that less than 80 mm will be required, the use of borderdyke irrigation will result in losses to drainage regardless of how well it is managed. On higher water holding capacity soils, applying 80 mm of water may well be acceptable with borderdyke irrigation.
On light soils that are irrigated with sprinkler irrigation, low application depths may be required for efficient irrigation. Some sprinkler irrigation systems are not capable of applying small depths of water, and this must be considered at the design stage.
For travelling irrigator systems, the ability to stop the irrigator moving and turn it off at the end of a run has some advantages, as changing run speeds, for whatever reason, will change the time required to finish the run. The alternative is to close the system down on a time basis and hope that the irrigator is at the right place at the right time. However, this approach can waste water or fail to fully water the end of a run.
Required application depths depend primarily on soil water holding capacities and crop rooting depths. A range of required application depths should be specified for different soil and crop types so that an irrigation system capable of meeting the needs can be selected.
3.2.2.2 Application rate and infiltration rate
Application rate refers to the rate at which water is applied to the soil by the irrigation system. Infiltration rate refers to the rate at which the soil can absorb water, which changes according to the wetness of the soil. Both application rate and infiltration rate are usually expressed in units of mm/hour.
In sprinkler irrigation systems, application rate is governed by the flow rate and wetted coverage of the sprinklers. It must take into account the overlapping patterns of sprinklers.
Ideally, irrigation systems should be selected so that the average application rate of the system does not exceed the infiltration rate of the soil. Systems that apply water at rates greater than the soil can absorb it cause surface redistribution of the water. Water tends to run from the high spots to the low spots in the field. This means that the high areas receive less water than they should and the lower areas receive more than they should resulting in losses to drainage in the low points and possible loss of production on the high points. Excessive runoff can result in water flowing off the irrigated field and possibly off the farm altogether.
Excessive application rates can therefore be reflected in a number of indicators, particularly daily percentage of irrigation water flowing onto the farm that is stored in the root zone, daily visual assessment of the amount of surface runoff, and a number of the production indicators, such as profit per unit of water used.
Application rates of irrigation systems tend to be fixed according to the type of system and the design parameters. Although it is possible in some cases to reduce the application rates of systems by reducing operating pressure, nozzle sizes, or by changing the type of sprinkler on sprinkler irrigation systems, this is rarely done. It is better to ensure that systems with the correct application rates are selected at the design stage, rather than trying to change the system later.
Average application rates for irrigation systems are relatively easy to measure or calculate. It is more difficult to make an assessment of soil infiltration rates. Guidelines do exist, and soil experts can be consulted. In the absence of expert advice, infiltration rates may be measured using soil infiltrometers, or a visual interpretation based on irrigation performance on similar soils may be used. Be aware that soil infiltration rates change over time. The infiltration rates of well-structured irrigated soils can be considerable higher than on previously unirrigated soils of similar types. A visual measure of surface runoff may well indicate this.
3.2.2.3 Application uniformity
Even with irrigation systems capable of applying the required depth of irrigation at the required rate, there are significant opportunities for inefficiencies through not applying water evenly. Poor application uniformity can be one of the main reasons for surface redistribution and losses to drainage.
The reasons for poor uniformity of sprinkler irrigation systems include:
- Poor or unsuitable sprinkler distribution patterns;
- Incorrect spacing of sprinklers;
- Component manufacturing variations;
- Wrong operating pressures of sprinklers;
- Pressure variations in the system; and
- The effect of wind on the sprinkler patterns.
The effects of poor uniformity may be difficult to detect with any of the indicators proposed. In extreme cases, reductions in yield become apparent in underwatered areas. However, detecting overwatering due to poor uniformity, especially if average application depths and application rates are correct, is extremely difficult.
Physically measuring application depths is one way of doing this. A number of rain gauges placed under sprinkler irrigation systems can give a visual indication of application depths and uniformity. These measurements give results that depend on the conditions occurring at the time, and a series of tests may be required to give an accurate representation of system performance. Measuring application depth and uniformity under surface irrigation systems is more difficult.
Overall, poor application uniformity means that more water is needed per unit of production. Poor uniformity can be detected in the total quantity of water needed in a season as more water is used than would otherwise be necessary to produce a given yield.
As with application rate, the options to physically improve uniformity are limited once a system is designed and installed. Sprinklers may be exchanged for more suitable models, system operation may be changed to reduce pressure variations, sprinkler spacing may be modified, and irrigation may be turned off in windy conditions, but beyond that very little can be changed.
Careful irrigation management can help to reduce losses to drainage. By applying smaller depths of water and not bringing soil moistures back to the full point, the variations in application depth caused by the non-uniformity can be absorbed. Again, there are limits to what can be done. The best solution is to choose the most appropriate system at the time of purchase. The systems that give high application uniformity under ideal (design) conditions will usually outperform poorer systems under adverse conditions.
3.2.2.4 Calculation of application depth and return interval
To calculate the application depth and return interval for design, the following example is given to illustrate one of the methods described above.
Lets assume that you are using Regional Council recommendations for your system capacity, and in your situation you can use 330 m3/ha/week.
You wish to irrigate 84 hectares with one irrigator, and will be operating your system for 23 hours per day at the peak of the season. Each irrigator run will irrigate 6 hectares at a time.
Your soil type is a medium silt loam with a readily available soil moisture content in the root zone of 80 mm. Ideally, you want to allow a margin for rainfall and application uniformity to improve your efficiency, so you would like to design your system to apply about 60 mm in an application.
Pumping rate (m3/hr) = System capacity (m3/ha/week) x Area (ha)
Daily run time (hours/day) x 7
= 330 x 84
23 x 7
= 172
Application depth (mm) = Pumping rate (m3/hr) x Daily run time (hr/day)
Daily area irrigated (ha/day) x 10
= 172 x 23
6 x 10
= 66
Rotation time (days) = Total Area (ha) )
Daily area irrigated (ha/day)
= 84
6
= 14
The gross application is about 66 mm, which is acceptable for a soil with readily available soil moisture of 80 mm. The net application is probably about 75 percent of this, i.e. 50 mm or 3.6 mm/day on a 14-day rotation.
If you were in the situation where your soil was very light and only had a readily available soil moisture of 40 mm in the crop root zone, then applying 66 mm gross would result in a significant percentage of this water draining through the soil profile. In this case, you would need to apply less water by operating your irrigator at higher speed so it applied a smaller depth (perhaps shifting twice daily at the peak of the season). An alternative is to use two irrigators so that each irrigator applied 33 mm on a run. This is possible because each irrigator is watering half the number of paddocks, and therefore needs to apply half the amount of water.
3.2.3 Other Considerations for Irrigation System Selection and Design
3.2.3.1 General layout
In conceptual terms, one of the key factors is to design the farm around the irrigation system, not the irrigation system around the existing internal farm layout. This may mean moving fences, removing shelterbelts or trees, and perhaps changing the position of drains or water races, or putting in new accessways. Irrigation should take priority as it is a long-term investment.
3.2.3.2 Surface runoff
As stated earlier, surface runoff of water can result from applying water at excessive application rates or with very poor application uniformity. As the infiltration rate of soils tends to decrease with the time of irrigation, the likelihood of surface runoff increases with increasing application depths. Surface redistribution or runoff is less likely when small depths are applied.
Water that runs off the area being irrigated is not able to be used by the crop regardless of the crop and soil conditions, and is wasted. The benefit of the water applied is reduced, and production per mm of water applied is reduced.
Losses from micro-sprinkler, mini-sprinkler and drip irrigation due to surface runoff are normally less of a problem, although they can occur in poorly managed systems.
Sprinkler irrigation systems are often affected by wind blowing water from the irrigated area, or irrigators or sprinklers throwing water outside the irrigated area. Wind losses can be reduced by not irrigating in excessively windy conditions, or selecting an irrigation type that is less affected by the wind. Watering rectangular fields with circular application patterns most often causes watering outside of irrigated areas. To reduce or prevent this, control of the pattern is required (by using part circle sprinklers for example), or some areas of the field such as the corners may need to be left unwatered.
Measuring these losses can be difficult. A visual interpretation is all that is possible in many cases.
3.2.3.3 Effects on soil
The breakdown of soil particles at the soil surface is mainly relevant to high-volume sprinkler irrigation. This is caused by the impact of the irrigation water on the soil particles causing either movement of the particles or the breakdown of the soil into smaller particles. The heavier the crop cover, the less likely there will be a problem.
The main issues are water droplet size, intensity, and angle of contact of the water stream with the soil surface, which are functions of the system design, operating pressure and nozzle sizes.
To reduce problems with soil breakdown and movement, it may not be possible to use particular types of irrigation systems in some circumstances. If a problem with soil movement and breakdown is occurring, it is difficult to modify the system to prevent it. Running smaller nozzles at higher pressures may help to reduce droplet sizes, but beyond that, not much can be done.
This illustrates the importance of selecting the right machine for the job at the design stage of the process.
The effects of system design on topsoil depth are minimal. However, some soils are highly susceptible to water erosion. Unless a good crop cover can be established before irrigation, these soils should not be irrigated with flood irrigation.
3.2.3.4 Wastewater irrigation
Wastewater from agricultural enterprises, such as piggeries or dairy sheds, can be a valuable nutrient resource to a farm. There is a considerable fertilising benefit to be gained when applying wastewater to pasture and cropping land. Wastewaters contain nitrogen, phosphorus and potassium. They may also contain other elements such as calcium, magnesium and sulphur as well as other trace elements beneficial to soils, pasture and crops.
Unlike clean water irrigation, wastewater irrigation may need to occur when the soil is not in a moisture deficit situation, and for this reason, some of the design aspects differ from those required for clean water irrigation.
There are a number of factors that should be taken into account when selecting a wastewater application site such as:
- soil types - permeable soils are preferable;
- depth to groundwater - areas with deep water tables are preferable;
- topography - level sites, without humps or hollows which may generate ponding or runoff, are preferable;
- proximity to surface waterways;
- proximity to neighbouring dwellings, roads and other public places should be as far as possible;
- accessibility - the application area is best placed close to where the wastewater is generated. This reduces the capital cost and the operation of the system is easier to monitor and system failure can be quickly identified.
Well-designed spray application systems operate with minimal wastewater spray drift, ensure even application, require minimal manual shifting and are readily expandable.
Pumps should be selected that:
- have duties to match the required flow rate and operating pressure of the system;
- include seals and bearings designed to withstand wastewater conditions;
- are designed with large clearances to minimise blockages;
- are easy to maintain; and
- are constructed from non-corrosive materials.
Normal clean water pumps are not suitable for pumping wastewater. Electric motor driven pumps are preferable.
The delivery pipeline should utilise swept ‘bend’ fittings in preference to sharp ‘elbow’ fittings’. Pipe velocities should not exceed 2 m/s to reduce frictional losses and water hammer, and should not fall below 0.7 m/s to prevent silting of solids.
Hydrants provide for connecting the irrigator to the buried pipeline. Hydrants with ‘T’ section joins are not suitable as suspended solids can settle in the dead section of the mainline causing blockages.
Travelling irrigators or sprinklers especially designed for wastewater application should be used. Rubber sprinkler nozzles are preferable as they expand if a stone or any solid material comes through the delivery line and will not block as easily. To reduce aerosoling effects low-pressure spray nozzles (i.e. 100 kPa to 300 kPa) with large orifices (i.e. between 8 mm and 16 mm) should be used. Travelling irrigators have a number of advantages over stationary and multiple sprinkler systems.
It is possible to use an existing clean water irrigation system if the wastewater is sufficiently diluted.
If the application of animal wastewater is not permitted by a regional rule in the Regional Plan then a discharge permit would be required for the activity. To obtain this information you should contact your Regional Council or Unitary Authority.
To protect groundwater and to minimise nuisances and odour problems, Regional Council regulations tend to restrict the wastewater nitrogen loading onto the land, and give buffer distances from waterways and public places.
3.2.3.5 Production/Unit of Irrigation Energy Input
The energy input into irrigation systems in New Zealand normally refers to electricity required for pumping, although centre-pivot and lateral move irrigators also require an additional energy source for propulsion.
Choosing the right pump for the application is vital to minimising energy use. There are significant differences in the maximum pump efficiencies of different pump models. Also, electric motor efficiencies can vary between models. This means that there can be significant differences in energy use between pumps that provide similar duties.
The lowest cost per cubic metre of water pumped generally occurs when the pump is being operated at a flow equal to or higher than the flow at the maximum efficiency point. Choosing a pump with an operating duty above the maximum efficiency flow will also result in less loss of irrigation system performance as the pump wears.
3.2.3.6 Production/Unit of Labour Used for Irrigation
The labour required to operate irrigation systems varies enormously. Fully automated systems can reduce the labour required for daily operation of the system to a few minutes per day. However, automatically controlled systems cost more, and depending on the system, can have a significant labour requirement for maintenance. The capital cost of automation should be weighed against the labour cost including maintenance to obtain a comparative cost.
3.2.3.7 Reliability of Irrigation Systems
To get the best out of an irrigation system, it is important that it is running to specification. All systems require repairs and maintenance, with some requiring more than others.
As systems age, the money and time spent on repairs and maintenance increases, and may become a significant part of the total running costs of the system. In addition, breakdowns can result in loss of production, particularly if they occur at the peak of the season.
When first purchasing an irrigation system, finding out how reliable the system is, how much maintenance is required, and how many years service can be expected from the system is recommended. Poor water quality due to sand, organic materials, precipitation of solids, and iron in the water can have a significant effect on system life and reliability. It is important that you choose system components appropriate to the quality of water.
Monitoring the number of hours lost per season due to system failure will allow the reliability of the system to be assessed, and provide guidance as to when the system or parts of the system should be replaced.
3.3 DESIGN CHECK/AUDITING
For new systems, an independent design check will help to ensure that the system will perform to specification, and should be carried out before the design is finalised. For existing systems, a design audit determines the capability of the system, and can identify limitations or problems in the system.
Many farmers do not know how well their irrigation systems are performing, how much water they are applying, or even whether their system is designed correctly. It is very difficult to tell by looking at the system. If you can see there is a problem, the situation is usually quite serious.
The cost of correcting mistakes or improving systems is easy at the design stage, but for existing systems can range from minor to extensive. It may not be cost effective to make changes to a poorly designed system, and a range of options may need to be considered. However, unless an irrigation system has the capability to provide acceptable performance, it will always be difficult to operate it in an efficient and sustainable manner.
3.4 INSTALLATION/TESTING/COMMISSIONING
All materials should be installed in accordance with the manufacturer’s instructions.
In particular, attention should be given to pipe laying, installation of thrust blocks, and installation of pumps.
The irrigation system should be properly tested and instructions given on the correct operation and maintenance of all equipment.
3.5 DOCUMENTATION
It is very important that the designer/installer provides written documentation of the design and installation.
Specifications of the system should be clearly stated, and the results of testing and commissioning given.
Operation and maintenance requirements should also be documented.
Contact for Enquiries
MAF Information Services
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PO Box 2526
Wellington, NEW ZEALAND
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