Sustainability is on everyone’s lips today as we sift through the climate change debate and strive for ever decreasing impact of our daily lives on our environment.
Of particular interest in the irrigation industry is the issue of irrigation efficiency, principally to preserve water resources. However, to supply those water resources almost always involves pumping water, and pumping system efficiency cannot be separated from irrigation system efficiency. They are inextricably entwined.
Sustainability is on everyone’s lips today as we sift through the climate change debate and strive for ever decreasing impact of our daily lives on our environment.
Of particular interest in the irrigation industry is the issue of irrigation efficiency, principally to preserve water resources. However, to supply those water resources almost always involves pumping water, and pumping system efficiency cannot be separated from irrigation system efficiency. They are inextricably entwined.
But what exactly is the relationship between pumping efficiency and irrigation efficiency.
Firstly. Let’s define them both:
Irrigation system efficiency can be defined as:
The ratio of water beneficially used by crops to the total water diverted or released for irrigation.
This is commonly measured as Distribution Uniformity (DU).
For every unit of DU deficiency, more water needs to be applied, as defined by the Scheduling Co-efficient (SC), defined as the reciprocal of DU. Ie, SC = 1/DU.
DU’s and hence SC’s can be measured on site with a catch can test.
Hereinafter, Irrigation System Efficiency will be referred to as “Sustainable Irrigation”.
Pumping system efficiency is defined by Pumping Specific Energy:
“Pumping specific energy is a measure of the energy a pump uses to move a specific volume of fluid”.
It can be expressed as:
kWh/Megalitre = 2.73 x TDH (metres) / (pump ɳ x motor ɳ x drive ɳ)
where:
TDH is the total head required to be delivered by the pump to overcome Static lift and System resistance. This is the major variable in sustainable pumping, especially in the design stage.
Hereinafter, Pumping System Efficiency will be referred to as Sustainable Pumping.
Hydraulic Modelling
To demonstrate the relationship between sustainable pumping and sustainable irrigation, let’s examine an hydraulic model of a turf/landscape irrigation system.
It is represented below as a 20m3/hr pump, pumping via 100 metres of 80mm uPVC PN9 pipe to 6 x Hunter I25 rotors, #20 nozzle, spaced at 17m.
A filter has been omitted for clarity and consistency of pumped head.
Photo 1: A 20 m3/hr pump supplies 6 x sprinklers in a typical irrigation system.
The I25 sprinkler performance with #20 nozzle is defined by Hunter Distribution data as having the following DU’s:
Graph 1: Data obtained from Hunter Pro-file Distribution Analysis Software
Note the presence of the 50mm PGV block control valve and 65/80NB Pressure Sustaining Valve (PSV).
Lets examine the relationship of pumped head, pipe friction loss, measured as C Value and pipe water velocity.
Let’s also assume there’s no lift or elevation, so TDH = 53m.
Graph 2: Low pumped head, 53m, with new pipe C=150, water velocity 1.5 m/s.
This would be the typical industry designed irrigation system.
The red circles indicate the first and last of the six sprinklers.
The residual pressure at the end sprinkler is 403 kPa. The DU at this pressure, as seen from the purple vertical line on the right graph is 83%. The Scheduling Coefficient (SC) is 1/0.83 = 1.20
That’s not an unusual SC for any irrigation system.
Head loss from first to last sprinkler – CID Guideline
The Certified Irrigation Designer (Irrigation Association, USA) course states that the maximum recommended head loss from first to last sprinkler is 10%.
In our model, head loss range for the new pipe design is 45 to 41m, = nom 10%. (refer to red circles in previous graph)
This design parameter is widely accepted.
Reduction in Pipeline Efficiency
Graph 3: below indicates the reduction in Hazen and Williams C Values from new pipe, nominally 0.003mm wall roughness, nominally C=150, to 0.2mm wall roughness, nominally C=120 for a mid range pipe diameter such as 65mm.
For a 65mm pipe, with k=0.2mm, the C Value reduction from C=150 to C=120 is a 20% reduction.
This resulting pipe performance reduction is very common due to contaminated water but the symptoms are often confused with “worn out sprinklers”.
Graph 4: After the pipe condition has deteriorated to C=120, here’s what the hydraulic gradient looks like for C=120, v= 1.5 m/s at 53m pump head:
In addition, the pressure difference between first and last sprinkler is now 16%, outside CID guidelines of 10%.
Improve sustainable pumping, what happens to sustainable irrigation?
In our last article on Sustainable pumping meets the objectives of the European taxonomy | Irrigazette, we showed that sustainable pumping was significantly improved by reducing water velocity, with optimised water velocity for smaller diameter pipes <100mm ideally at ave 1.0m/s.
This is physically achievable by nominally raising each sector pipe by one size up. So, a 50mm pipe goes to 65mm, an 65mm pipe goes to 80mm, etc.
Reduce pipe water velocity to 1.0 m/s
If the pipework was designed with 1.0 m/s nominal water velocity, a considerable improvement in residual head is experienced.
Graph 5: New pipe system, with C=150, V=1.0 m/s
Graph 6: Pipework deteriorated to C=120, v=1.0 m/s at 53m pump head.
Raising pump set point to 60m
It is clear from the above that commencing with a low pump set point to minimise pumping energy requirements has resulted in unfavourable DU’s, therefore a high SC.
If we were to start off with a higher pump set point of say 60m head, instead of 53m, the DU’s would be nearer the top of the DU curve and therefore improve SC’s at both new pipe and k=0.2mm pipe performance.
Graph 7: Consider firstly new pipe at 60m head, V=1.0 m/s
Graph 8: Next, Consider c=120 pipe at 60m head, V=1.0 m/
So, raising the pump set point from 53m to 60m raised pumping energy by 13%, but at the same time, only reduced the SC by 3%.
In this case, there is a trade-off between the pumped head (sustainable pumping) and DU’s (sustainable irrigation), but clearly in favour of a lower pump set point with low 1.0 m/s velocity.
However, lowering water velocity to 1.0 m/s means we can have both low pump set point and low SC’s. ie, we can have our cake and eat it too!
Sustainable Pumping vs Sustainable Irrigation: What’s the Nexus?
It is clear from this example that maintaining low water velocities (around 1.0 m/s) in landscape or golf irrigation systems has a profound advantage for sustainable pumping (minimising pumped head) and significant advantages for sustainable irrigation by minimising Scheduling Coefficients, thereby minimising excess water requirements.
It is also clear that designing pipe systems to C=150 quickly turns unfavourable as the water quality takes its inevitable toll on pipeline efficiency.
Ultimately, in cases where water is sourced from rivers, dams and recycled water, and has the potential to develop biofilm, its desirable to use C=120 as a pipeline design parameter. Whilst this will appear to initially result in an “over-pressurised system”, Variable Frequency Drives will adjust the delivery pressure over the life of the system, ensuring that the irrigation is delivered exactly what pressure is needed over the life of the system.
What about sediment buildup?
Some operators maintain that low water velocities such as 1.0 m/s result in build up of sediment in pipes.
Sediment typically found in dirty water will settle out below 0.2 m/s. In fact, in almost all cases, settlement does not occur during irrigation, but after irrigation ceases.
The best way to handle loose sediment build up is to flush pipes before each irrigation, to allow sediment that settles between irrigations to be eliminated.
Flushing does not “clean” pipes.
Flushing will not remove biofilm but will only remove loose sediment provided that it’s done on a regular basis.
Irrigation systems utilising potential dirty water should consider automatically activated flushing valves in the design stage, programmed to run before each irrigation.
However, to clean pipes properly requires pipeline pigging but that’s a subject for another article!
Rob Welke, Author
Certified Irrigation Agronomist
Managing Director of WATER PUMPING INSTITUTE
Adelaide, South Australia
June 2025
Acknowledgement: Assistance has been received in preparation of this article, and gratefully acknowledged, from Alan Michelsen, Independent Consultant and Certified Irrigation Designer (Landscape and Golf), Glenelg Design, 35 yrs experience, Adelaide, South Australia.
Training courses : WATER PUMPING INSTITUTE’s business model includes training courses to empower irrigation professionals to improve irrigation designs and modify existing designs to improve their sustainable pumping and sustainable irrigation outcomes.
Refer to web site: https://www.waterpumping.institute/sustainable-pumping-for-irrigation-europe
The software used for this article will be available free issued at our Sustainable Pumping training courses.
