Border-check or sprinkler irrigation for perennial pastures

In many situations, border-check irrigation is an efficient method for irrigating pastures. If poorly managed or used on inappropriate soils, border-check systems can use too much water.

This can contribute to:

• development of shallow water tables
• nutrient losses to river systems
• salinisation problems.

In areas where border-check irrigation may be inappropriate, conversion from a border-check to a centre pivot system is one option for reducing water use and alleviating the environmental impacts of irrigation.

Amount of water needed for perennial pasture

The amount of irrigation water required by a perennial pasture in a season (the irrigation requirement) is equal to the pasture water requirement minus effective rainfall. The average irrigation requirement varies across northern Victoria, increasing towards the north and west (Table 1).

Table 1 — Twenty year average annual irrigation requirement for perennial pasture, assuming 80 per cent effective rainfall.

Additionally, effective rainfall and irrigation requirement can also vary considerably from year to year.

The daily amount of irrigation water required by perennial pasture is also important in selecting and designing an irrigation system.

At Tatura, the highest average daily irrigation requirement occurs in January (Table 2). The daily irrigation requirement will increase towards the north and west of Tatura.

To determine the centre pivot system capacity (depth of water applied to the whole field each day), this daily requirement is adjusted to allow for system inefficiencies and above average daily crop water demand (Table 2). This system capacity assumes that the system will operate 24 hours a day 7 days a week.

Finally, to allow for breakdowns and flexibility in operations (to limit irrigation to 5 days a week or to use off-peak power) the system capacity can be increased further.

Table 2 shows centre pivot system capacity requirements at Tatura for a system operating 5 days a week and for a system only operating during off-peak electricity rate times (88 hours/week out of a total of 168 hours/week).

Table 2 – At Tatura – average daily crop water use in January and centre pivot system capacity for a system operating 24 hrs/day 7 days a week, 24 hrs/day 5 days/week and in off-peak electricity hours only.

When purchasing a centre pivot system, clarify with a designer how you should/want to operate the system and get a system with appropriate capacity for your requirements. Of course the capital cost of the system increases as system capacity increases.

Centre pivot compared to border-check irrigation

Irrigation Efficiency (IE) is a measure of the proportion of applied water that goes to the crop root zone.

IE is 100% when all applied water is captured by the plant roots and is lower than 100% when run-off and deep percolation losses occur.

Irrigation system design and management can greatly affect IE. As soil types become lighter, border-check system IE often decreases as deep percolation losses become greater.

In appropriate circumstances border-check IE can approach that of efficient pressurised systems.

Management, topography and soil variability impact on the performance of border-check systems to a greater extent than on sprinkler systems. This results in more numerous situations where high IEs cannot be obtained using border-check irrigation.

The improvement in IE associated with conversion from border-check to centre pivot is likely to be greatest where existing border-check systems cannot capture and reuse surface run-off, or where soil types are permeable and deep drainage losses are high.

It is important to note that well managed border-check irrigation can potentially achieve better IE than poorly managed sprinkler irrigation.

There are no hard and fast rules on what levels of water savings you can achieve by converting to a centre pivot system.

Check how efficient your irrigation is

You can estimate the level of water saving you may achieve on farm by calculating the IE on your farm. Most simply, IE can be approximated by:

IE =

• irrigation requirement (ML/ha)
• applied irrigation water (ML/ha)

Please note that this equation only holds for farms that supply more water than the irrigation requirement.

Irrigation requirement varies from year to year so it would be best if you can average your irrigation water use over a number of years.

Little scope for water savings exists where high IEs are already being achieved. Large water savings are possible if your current IE is low.

A local trial comparing sprinkler to border-check irrigation

A 2-year field experiment comparing water use and pasture production under border-check and sprinkler irrigation was conducted at Tatura. The economic implications of adopting sprinkler irrigation were also appraised. The soil type at the experimental site was Lemnos loam.

Lemnos loam is a duplex soil characterised by brown to greyish brown loam to clay loam extending to 0.12 metres. This top horizon is underlain by a 0.30 to 0.3m layer of reddish brown medium to heavy clay, turning to brown or yellowish brown medium clay from 0.45 to 0.60 metres. The final infiltration rate of Lemnos loam is typically less than 2mm/hr.

Recorded water use

Border-check irrigation achieved irrigation efficiencies of 75%, while sprinkler irrigation achieved efficiencies of 90%.

In both years of the experiment the sprinkler system used 2 ML/Ha less irrigation water than the border-check system. It was assumed that all run-off from the border-check system was reused.

The border-check system used more water because it had greater deep drainage  and evaporation losses.

Deep drainage losses are likely to be greatest when the water-table is deep (3 or more metres below the surface) and so does not restrict water flow to depth, or contribute to pasture water use.

The experimental site was located near a groundwater pump and did not have a shallow water-table. These conditions meant that the water savings in the experiment were most likely towards the upper limit of what could be achieved on a farm with Lemnos loam soil.

Well managed sprinkler systems should be able to closely match water application with crop water requirement. With border-check irrigation, a larger variability in IE, both within and between farms, can be expected.

The potential water savings derived from replacing bordercheck with sprinkler irrigation need to be evaluated on a case-by-case basis.

Pasture production and composition

In the first year of the experiment sprinkler and border-check system pasture production were equal.

Sprinkler irrigation produced 2 tonnes of dry matter/ha more pasture than border-check irrigation in the second year of the field experiment.

The most likely reason for the increase in pasture production under sprinkler irrigation was the more frequent but lighter application of water, which alleviated the waterlogging and water stress problems associated with border-check irrigation. The choice of irrigation system had no consistent impact on pasture composition.

It is impractical (or not economical) to apply the high management inputs used under experimental conditions to working farms. The experiment did show though that in some circumstances there is potential to 'produce more with less water' using sprinkler irrigation.

Irrigation costs and returns

The information in this section is based on 2003 costs and returns on capital.

Sprinkler irrigation

Sprinkler irrigation is expensive.

Typical costs of purchase and installation of a centre pivot system will be between $2500 and $4000 per hectare.

Capital costs include:

• site survey and design
• purchase and installation of pump station
• mainline and sprinkler unit
• construction of new laneways and fences
• earthworks to fill in channels, flatten check banks and make some adjustments to grades to provide adequate drainage
• electricity connection.

Costs increase towards $4000 as the capacity of the system increases.

For example if:

• electricity has to be run to the site
• dam for water storage is required
• greater amounts of laneways, fencing or earthworks are required.

Centre pivot system

Centre pivot capital costs can compare favourably with new border-check layouts if substantial earthworks are required for the border-check system.

Operating costs of centre pivot systems (pumping, maintenance, labour and water) are of the order of $600/ha, compared to about $480/ha for border-check systems.

Conversion from border-check to centre pivot irrigation is expensive.

Conversion can be economically viable for a dairy farm if water savings can be achieved and used to expand the area of irrigated pastures on the property. Conversion is less economic when there is no potential to expand the irrigated area on the property.

Sprinkler versus centre pivot

Where conversion to centre pivot can achieve water savings greater than 3ML/ha, or if the cost of water increases substantially in the future, sprinkler irrigation may become economic on farms without any available land for expansion.

Conversion also becomes more favourable under conditions of reduced water availability.

Deep drainage considerations

The cost associated with the loss of irrigation water through deep drainage will depend on a number of factors.

If a groundwater pump can be installed to recycle deep drainage for farm reuse, little water is actually lost from the production system.

As the cost of groundwater pumping is relatively small, conversion to sprinkler irrigation to minimise deep drainage losses may not achieve any substantial water savings in such areas and might not be economically feasible. Groundwater pumping and farm reuse is limited to areas with pumpable, low salinity aquifers and large areas of land do not fall into this category.

The cost of deep-drainage losses is likely to be greater where groundwater pumping and farm reuse is not possible (areas with saline groundwater and no pumpable aquifers). Under these conditions, conversion to sprinkler irrigation may be a cost-effective option to minimise deep drainage losses to the groundwater system.

Conclusions

This research project clearly showed that there is scope to grow more pasture from less water by using sprinkler rather than border-check irrigation.

Economic analysis showed that conversion to sprinkler irrigation can be an economical proposition under some conditions.

The benefits of converting will depend on individual farm conditions.

A process to help you decide between systems

Large investments like sprinkler systems require careful consideration of options before implementation. When considering conversion from border-check to centre pivot, a number of issues will have to be addressed.

Five steps in the process of evaluating a change in irrigation system on your farm are:

1. Development goals — clearly establish the goals you want to achieve as a result of the conversion.
2. Assess local conditions — collect physical (soils, water availability, land shape, topography) and institutional information  whole farm planning process, regulatory information such as tree removal) to allow assessment of the benefits of conversion.
3. Pre-selection — evaluate data to assess the likelihood of achieving your development goals.
4. Feasibility design and economics — get detailed designs and costings from qualified irrigation designers. Evaluate the financial implications to your farm.

Based on these steps you can make your final decision.

In these four steps there are a number of key issues to address:

Development goals

Do you want to save water? If water efficiency is your main aim, check your current IE. Where does your farm sit on the IE diagram? If you are on the high end of the border-check bar, conversion to sprinkler might not save you much water.

Current and future land use — do you plan to only grow perennial pasture or do you want your irrigation system to be flexible enough to grow fodder crops as well? Sprinkler allows greater flexibility in water application and is well suited to cropping.

Local conditions

Soils and layout — are most of the soils on your farm heavier than Lemnos loam? Is your current border-check layout efficient? Do you have a reuse system? If the answers to these three questions is 'Yes', conversion to sprinkler might not be a high priority for your farm.

Water availability and available land for expansion — are you short of water and/or do you have 'spare' noncommandable land available? In that case conversion to sprinkler might be a viable option.

Water-table and subsurface drainage — is your water-table less than 1m from the surface for much of the year and you can't find a pumpable aquifer under your property? Conversion to sprinkler could be an option to lower your water-table.

Native vegetation remnants — large sprinkler systems require extensive areas of treeless land. Permits are required to clear remnant vegetation. In some situations it might not be feasible to install a centre pivot system.

Pre-selection

At this decision point you use all the information available to decide if you will proceed with your development project.

Any decision should include the possibility of upgrading a current border-check system.

This will definitely be less expensive than changing systems. If saving water is a goal then watering quickly by increasing the capacity of channel and bay structures or clearing weeds from channels may help. A reuse system or groundwater pump will also help reduce water lost from the farm. If reducing labour is a goal then an automatic irrigation system will be worth investigating.

Design and economics

Irrigation management

Irrigation management with centre pivot systems is very different to border-check system management. A steep learning curve will be encountered in the first season of operation.

As a general guide for pasture irrigation in summer in northern Victoria, two irrigations should be scheduled each week — with each applying about 25 to 30mm of water. Depending on system capacity it will take 2 or 3 days to apply water to the whole area during each irrigation.

Management is heavily dependent on factors such as system design and soil and crop type. It is vital that management issues be clarified with designers before a centre pivot system is purchased.

Labour requirements

Well designed and maintained sprinkler systems can provide trouble free operation and can be fully automated to minimise labour requirements. Labour inputs of about 2 hrs/ha/yr for centre pivot systems compare favourably with inputs in the order of 4 hrs/ha/yr for border-check. Automated border-check systems can reduce the labour input considerably, but will still require greater labour input than a centre pivot system.

Maintenance costs

Maintenance costs of irrigation systems have been estimated in literature on average as approximately 2% and 4% of capital outlay for border-check and centre pivot systems. An average capital outlay of $3000/ha for centre-pivot would thus result in annual maintenance costs of $120/ha. This cost will be slightly lower when the system is new, and increase over time.

Although centre pivot systems are reliable, like any machine they require systematic routine maintenance. Required maintenance procedures should be clarified with designers prior to equipment purchase.

Energy use

For electric centre pivot systems average annual pumping costs are about $150/ha (based on April 2003 costs and considering a mix of peak and off-peak rates).

Installation costs

The typical cost of purchase and installation will be between $2500 and $4000 per hectare. This cost can compare favourably with the cost of a new border-check layout development.

How centre pivots operate

Centre pivot irrigators use low-pressure nozzles to irrigate circular areas. Mini pivot systems are available that water areas from 0.4 to 20Ha. Large units can be up to 800 metres long with 18 or 20 towers, irrigating approximately 200Ha.

Centre pivot systems can be powered by either electricity or diesel fuel.

Water supply comes from one fixed centre hydrant.

The circular shape of the irrigation area results in non-commandable corners of square paddocks. End-guns help to capture some of this area, but at least 10 per cent of the area of a square paddock will remain unirrigated.

Centre pivot systems have an average lifespan of 15 to 20 years.

Other irrigation systems

Linear move systems are likely to be the most suitable for use on large areas in northern Victoria. They have higher labour input requirements than centre pivot systems.

Irrigation application of these systems is similar to centre pivot, but the boom moves laterally along the paddock, allowing full command of rectangular-shaped areas.

The system requires water supply along the full length of the field, usually a channel. Often these systems are diesel-powered to allow for mobility of the power source.

Other commercially available irrigation systems include:

• travelling irrigators
• 'van den Bosch'
• K-line systems
• fixed 'pipe and riser' impact-sprinkler systems
• subsurface drip systems.

If centre pivot and linear move systems do not suit your needs you should talk to local suppliers and designers about the applicability of these systems.