Total Dynamic Head and Flow Rate Calculation
Every deep well pump selection starts with two numbers: how much water you need per hour, and how much head the pump must generate to deliver it. This guide shows how both are calculated, with worked examples for borehole irrigation systems.
Part 1 — The four components of total dynamic head
Well depth is not head
The depth of a borehole tells you how much pipe you need. It does not tell you what the pump must work against. Total dynamic head is the sum of four separate quantities, and three of them have nothing to do with how deep the well is.
Dynamic water level
The vertical distance from ground level to the water surface while the pump is running at the design flow.
This is the single largest component in most deep wells, and the one most often replaced by the static level by mistake.
A pumping test is the only reliable source. If seasonal data exists, size on the worst month.
Discharge height
The vertical rise from ground level to the highest point the water must reach: a storage tank inlet, a pivot connection or an elevated reservoir.
Measured as elevation difference, not as pipe length. A 400 m pipeline across flat ground adds no discharge height, only friction.
Friction loss
The head consumed by the water moving through the rising main, the surface pipeline, bends, valves and fittings.
It grows roughly with the square of the flow velocity, which is why an undersized pipe is expensive forever.
Read it from the pipe manufacturer chart for your exact diameter, material and flow.
Required outlet pressure
The pressure that must remain at the delivery point for the system to work: sprinkler nozzles, a drip system inlet or a pressurised network.
Converted to metres: 1 bar is approximately 10.2 metres of water column.
A tank that is simply filled by gravity needs none; a sprinkler system may need 25–40 m.
Part 2 — Working out the required flow rate
Flow follows the demand, and is capped by the well
For irrigation, the required flow comes from the area, the peak daily water requirement of the crop and the number of hours the system runs each day. The borehole yield then sets an absolute ceiling that no pump can exceed.
Worked example
A 20 hectare field with a peak crop demand of 7 mm per day, irrigated 18 hours a day.
Required flow = 20 × 7 × 10 ÷ 18 = 77.8 m³/h.
Add an allowance for distribution losses before selecting, and check that the well can sustain this rate.
The well is the limit
If the pumping test shows the borehole yields 60 m³/h at an acceptable drawdown, a 78 m³/h pump will simply draw the level down until it runs dry.
The options are then to extend the operating hours, add storage, or drill a second well — not to fit a larger pump.
Round to a real duty point
Pump series are supplied in discrete stage counts. Choose the configuration that sits just above your calculated duty point rather than well beyond it.
Operating far to the right or left of the best efficiency point costs energy and shortens life.
Part 3 — A complete worked example
From a field measurement to a model number
The following example follows a single installation through every step, from the pumping test to a shortlist of FANPO series.
| Step | Input | Value |
|---|---|---|
| 1 | Static water level | 82 m |
| 2 | Dynamic water level at 60 m³/h | 118 m |
| 3 | Discharge height above ground | 12 m |
| 4 | Rising main and surface pipeline | 180 m total |
| 5 | Friction loss from the pipe chart | 9 m |
| 6 | Required pressure at the outlet | 2 bar = 20 m |
| 7 | Total dynamic head | 118 + 12 + 9 + 20 = 159 m |
| 8 | Duty point | 60 m³/h at 159 m |
The pump is selected for 159 m, not for the 82 m static level and not for the well depth.
Which FANPO series fits this duty point?
At 60 m³/h the candidates are the 6013 cast iron (up to 60 m³/h and 420 m) and the 6046 stainless steel (up to 59 m³/h and 372 m).
Both reach well beyond 159 m of head, so the stage count rather than the series limit decides the final configuration.
If the borehole is wider
In an 8″ casing the 8075 or 8420 would run closer to their best efficiency point at this flow, which lowers the energy cost over the life of the installation.
Next step
Send the eight values in the table above through the contact page and the FANPO engineering team will return a stage-by-stage selection with absorbed power and the matching motor.
Part 4 — Pipe sizing and friction loss
One pipe size up is almost always the cheaper decision
Friction loss rises with the square of velocity, so a pipe that is one size too small is paid for every hour the pump runs, for the whole life of the installation.
| Flow velocity | Assessment | Consequence |
|---|---|---|
| Below 1.0 m/s | Oversized pipe | Higher pipe cost, but very low friction loss. Sediment may settle in long horizontal runs. |
| 1.5–2.5 m/s | Normal design range | The usual compromise between pipe cost and pumping energy. |
| Above 3.0 m/s | Undersized pipe | Friction loss climbs steeply, motor power rises and water hammer risk increases. |
These are general design ranges. Always take the actual loss figure from the chart for your pipe material and diameter.
Do not forget the fittings
Bends, non-return valves, gate valves and the well head assembly each add loss. On a short pipeline they can exceed the loss in the straight pipe.
Rising main and surface line are different
The rising main inside the borehole is usually limited by the casing. The surface pipeline is not, so it is often the cheapest place to reduce friction.
Static head does not change with flow
Discharge height stays the same whatever the flow. Only friction loss and drawdown grow as flow increases, which is why the duty point moves as the system changes.
Frequently asked questions
How do I convert bar to metres of head?
One bar is approximately 10.2 metres of water column. A requirement of 3 bar at the outlet adds about 31 metres to the total dynamic head.
Should I add a safety margin to the head?
A modest margin for a falling water level is sensible. A large margin is not: it pushes the pump away from its best efficiency point and wastes energy every hour it runs.
My well depth is 200 m. Do I need a 200 m pump?
No. You need a pump sized for the total dynamic head at your design flow. If the dynamic water level is 110 m and there is little lift and friction above ground, the required head may be far below 200 m.
Does the pump setting depth change the head?
Setting the pump deeper does not increase the head it must produce, because the head is measured from the water level, not from the pump. It does add pipe length and therefore a little friction.
What if the dynamic water level is unknown?
Then the selection is a guess. Commission a pumping test at the intended flow rate; it costs far less than replacing an incorrectly sized pump and motor.
Where do I find head per stage?
On each FANPO product page. The pump selection guide explains how to read the series tables and reach a stage count.