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.
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.