Over-irrigating wastes precious water, energy and money and can lead to leaching or run-off of nutrients that can pollute groundwater, rivers and estuaries.
Under-irrigation results in poor production and high costs per unit of product.
The goal is to achieve efficient water use and time spent planning and designing your irrigation system will be an investment in the future of your crop.
Why irrigate?
Apart from water being essential for plant survival, it is also the mechanism for root systems to access fertilisers and trace elements in the soil.
At the crop-level irrigation can be significant in three key areas:
- Increased production — where seasonal rainfall is inadequate, irrigation delivers the moisture and nutrients the plant needs to develop optimal crop levels.
- Frost control — overhead sprinklers are sometimes used to minimise damage in frost-sensitive crops. The sprinklers can be manually operated or set to start automatically when the temperature falls to a pre-determined level on still nights.
- Heat reduction — evaporation from overhead sprinklers will reduce stress in some crops that might be adversely affected by extreme heat.
Consider soil types
Soil characteristics will influence the type and timing of your irrigation program.
Moisture will drain towards the root zone and plant utilisation and water use efficiency will depend on how long it is held there. The ideal soil will have sufficient organic matter to hold moisture and be coarse enough not to become waterlogged.
The two soil type extremes are coastal sand and heavy clay:
- sandy soil — lacking in organic matter, sandy soil allows the water and nutrients to leach down and out of the root zone. Short irrigations applied more often will keep the root zone wet for longer.
- clay soil — heavy clay soils are fine textured and may not have enough drainage capacity for good root development. If too much water is applied too often, waterlogged soil can result. Longer intervals between irrigations will help with drainage.
Irrigation systems
The irrigation system you select will be determined by the crop you grow, the topography and the efficiency of the system.
The following are common types of irrigation systems:
- Flood irrigation is the least efficient option and is only used on laser-levelled pasture blocks. Flooded furrow irrigation is often used in large scale row crops.
- The centre pivot, travelling and fixed overhead sprinklers, are suited to pasture and annual horticultural crops on flat land.
- Micro sprinklers are a good option in tree crops. Compared to larger sprinklers they are efficient, saving water by only watering the ground under the trees and not the inter-row space. They work at lower pressure and are cheap to run.
- Trickle and drippers are efficient, economical systems used in annual row crops, orchards, vineyards and floriculture. Both are well suited to water efficient domestic and ornamental gardens.
- Subsurface drip irrigation delivers the water directly to the root zone eliminating evaporation loss and is commonly used in annual vegetable and floriculture crops where good quality water is available. Subsurface systems can be expensive when used over a large area and periodical line flushing is required to keep the lines open.
System design
There are a number of points to consider when designing an irrigation system:
- Aim for a balanced system with the pump operating at close to peak efficiency and desirable pressure at reasonable power consumption.
- If the elevation of the irrigated area varies by more than 2m, lay the irrigation main line up and down the slope, with the lateral pipes laid out close to the contour.
- Differences in pipe sizes up and down the slope may help to even out the delivery rate to different laterals.
- Everything else being equal, water flow rate will be reduced as pipe size decreases.
- Alternatively, built-in flow restrictors on each lateral line will allow you to balance the water flow.
- Where possible, sprinkler rows should be placed at right angles to prevailing winds.
- In the south-west of Western Australia most of the wind in the summer irrigation season comes from the east. Therefore laying the rows of sprinklers in a north-south alignment will produce maximum watering uniformity on flat land.
- Sprinkler spacing should be arranged to give maximum uniformity of water application.
- Spacing sprinklers further apart is a false economy.
- Buy the right pump and filter system allowing for any future expansion of the area to be irrigated.
- Check the quantity and quality of water available when designing your irrigation system.
Pipe sizes
There are many factors involved in selecting pipe sizes for specific applications:
- Pump size and power requirements will be influenced by pipe sizes and lengths.
- Small diameter pipes may be cheaper, but will have more internal resistance to water flow (i.e. head loss due to friction) and could require more power resulting in lower system efficiency.
Scheduling when and how much to irrigate
Irrigation scheduling depends on either measuring soil moisture in the root zone with a moisture meter or measuring the average daily evaporation rate. This measurement when combined with a ‘crop factor’ measures the crop’s water harvesting characteristics.
A crop’s water harvesting characteristics can be affected by the crop’s root structure and the size of the soil volume from which the plant roots can extract water.
When scheduling your irrigation, make sure you:
- irrigate only when necessary
- decide how much water to apply and when
- don’t irrigate in windy conditions, as even light winds can upset the distribution efficiency of sprinkler irrigation systems.
Operating the system
The pump operating pressure should be restricted to avoid ‘misting’.
If a large proportion of the water emerges from the sprinklers as a mist, the pump pressure is too high resulting in evaporation losses before the water hits the ground.
Once you have the right system and irrigation schedule in place, you will save water, time and money.