An irrigation pump needs to deliver the required flow at the required total head. A flow rating or motor power on its own does not tell you whether a pump will operate your irrigation system.
Start with the zone that will operate
List the sprinklers or emitters running at the same time and add their required flows at their specified pressures. Separate zones may have different duty points; adding every emitter on the property can overstate the simultaneous demand.
- Flow: the simultaneous zone demand, with any separately identified flushing or other operating requirement.
- Levels: the lowest operating water level and the elevation of the delivery point.
- Pressure: the required pressure at a clearly identified outlet or zone inlet.
- Losses: suction and delivery pipework, fittings, valves, filters and other equipment at that flow.
- Supply conditions: water temperature and quality, available source yield, suction arrangement and electricity supply.
Use consistent reference points. If the specified zone-inlet pressure already covers the downstream lateral losses, do not add those same losses a second time.
Worked example: 10 L/s at about 48.5 m head
This is a hypothetical water duty calculation. It assumes steady flow, water density 1,000 kg/m³ and gravity 9.81 m/s². The velocity-head difference between the selected reference sections is assumed to be zero.
| Input | Value |
|---|---|
| Flow | 10 L/s = 0.010 m³/s = 36 m³/h |
| Delivery elevation above source | 12 m |
| Delivery minus source pressure | 250 kPa |
| Pipe and fitting losses at 10 L/s | 8 m, assumed |
| Additional equipment losses | 3 m, assumed |
Pressure head = Δp / (ρg) = 250,000 / (1,000 × 9.81) = 25.48 m.
Total head = elevation difference + pressure head + losses
H = 12 + 25.48 + 8 + 3 = 48.48 m
The preliminary duty point is 36 m³/h at 48.5 m head.
Hydraulic power = ρgQH = 4.76 kW. With an assumed pump efficiency of 70% and motor efficiency of 90%, estimated electrical input is 4.76 / (0.70 × 0.90) = 7.55 kW. These efficiencies are example inputs, not guarantees or a motor nameplate recommendation.
Check the operating point on the manufacturer's curve
A candidate pump must meet the system curve at a suitable operating point. Check the actual model, speed and impeller against its permitted operating range, efficiency, power curve and net positive suction head requirements. Repeat this for materially different zones and water levels.
The example does not establish suction performance, cavitation margin, motor selection, surge protection or pipe pressure rating. Confirm those separately using the installation conditions and current manufacturer data. Record filter losses for the operating condition you are designing for.
Common pump-sizing questions
Can I choose a pump by horsepower?
Power is only one check. Two pumps with similar motor ratings can deliver very different combinations of flow and head. Start with the duty point and compare the actual curves.
Is head the same as pressure?
Head expresses energy per unit weight as a length of fluid column. For the water density used here, 100 kPa corresponds to about 10.19 m. Total pump head also accounts for elevation, losses and any velocity-head difference.
What should I send to a pump supplier?
Send the duty points, source and delivery levels, suction layout, water details, available power, pipe route and the basis for your loss estimates. Identify the operating scenarios, rather than sending only a desired flow.
References and next steps
The head and hydraulic-power relationship is described in KSB's pump lexicon: Head. Pipe losses depend on the chosen hydraulic method; see the US EPA EPANET 2.2 User Manual, section 3.1.
Next, work through irrigation pipe sizing and friction loss, or browse Tenselo's pump sizing and selection calculators.