System capacity is a critical design component of any centre pivot or lateral move (CPLM) irrigation system. It is a measure of how quickly water can be applied to a paddock. It depends upon the flow rate of the water supply and the area of crop irrigated in the cropping season. Having an appropriate system capacity can be the difference between producing a high or low performing crop, or crop failure.
This article outlines key terms and considerations required to determine what system capacity is right for your situation. Further information can be found in the Agriculture Victoria Centre Pivot and Lateral Move System Capacity Factsheet.
What is System Capacity?
System capacity is the maximum depth of water that can be applied to the irrigated area in 24 hours. It is the main benchmark that the pumps, pipes and sprinkler design is based on for a CPLM system and can be calculated as follows:
Example: a 400 m long centre pivot irrigates 50 ha. With a pump flow rate of 6 ML/day, it has a system capacity of:
What is Managed System Capacity?
The managed system capacity is the volume of water actually applied during an irrigation cycle divided by the area irrigated. The managed system capacity is less than the designed system capacity because of system losses, and management practices that reduce the time that the machine is actually operating (e.g. maintenance, time of application).
By considering factors such as crop water requirement, application efficiency (AE) and the available operating hours of the system (discussed below), managed system capacity can be calculated, and the required flow rate can be applied to the system design. Understanding and applying managed system capacity can help ensure your irrigation system capacity is able to effectively meet peak crop requirements.
The depth of water that is applied in an irrigation pass depends on the system capacity of the CPLM and the speed setting of the machine.
Application Efficiency (AE)
Application efficiency is the proportion of the water applied that the crop can use. For calculating managed system capacity 90 % is a conservative AE to assume for peak crop water requirement conditions.
The pumping utilisation ratio (PUR) is the proportion of time that the machine needs to operate to supply the peak crop water requirement. The recommended maximum PUR of 0.85 is equivalent to operating for 6 days in 7, or 20 hours in 24.
Example: During the peak of the growing season, the pump averages 6 days use out of 7 (with PUR of 0.85). The centre pivot’s estimated application efficiency is 0.90 (90%). The designed system capacity of the system is 14mm/day.
Therefore, the managed system capacity is:
14 mm x 0.9 x 8.5 = 10.7 mm/day
Peak Water Requirement
This is the highest rate of crop water use, usually expressed in mm/day, and managed system capacities must be greater than it. The peak water requirement is crop and location specific. A ‘crop coefficient’ (Kc) is used to express a crop’s relative water use, more information on calculating location and crop specific water can be found in the CPLM System Capacity Factsheet.
Peak crop water requirements can be factored into the managed system capacity and calculated as follows:
Overall, the System Capacity of your CPLM should be sufficient to meet the crop water requirements for extreme ET0 events. A CPLM with a marginal system capacity requires a high level of management to avoid productivity loss, and major crop losses could occur with a breakdown or power outage. A conservative (high) system capacity allows more flexibility of operation and more security.
Content sources and further information
Contact Agriculture Victoria irrigation extension officers:
Echuca: 03 5482 1922
Kerang: 03 5473 0180
Maffra: 03 5147 0800
Mildura/Irymple: 03 5051 4500
Tatura: 03 5833 5222