How to work out total dynamic head
Total dynamic head is the height the pump has to push water to once everything is counted: how far it lifts, what the pipe costs it, and what pressure has to be left at the far end. It is half of the duty point. The other half is flow, and neither number means anything without the other.
Five parts
Static level, drawdown, discharge lift, pipe friction and outlet pressure. Add all five, then size against the total.
Friction is bought once
Diameter enters the equation to the power of 4.87. Going one pipe size up is usually cheaper than the bigger pump it saves.
The pipe has a ceiling
Head is pressure. Check what the discharge line is rated for before the pump choice is locked in.
The five parts that add up
| 1. Static water level | The distance from the wellhead down to where the water rests with the pump off. Measured, not taken from the drilling report. On an old borehole the level may have moved. |
| 2. Drawdown | How much further the level falls once the pump is running, at the flow you intend to take. The driller's pumping test has this. Without it, sizing is guesswork. |
| 3. Discharge lift | From the wellhead up to the water surface in the tank, or to the highest point in the line. A tank on a 6 m stand adds 6 m, and the level inside it adds more. |
| 4. Friction in the pipe | Everything the water rubs against on the way, riser plus the run to the tank. Worked out below. |
| 5. Pressure needed at the outlet | Nothing if the pipe discharges into an open tank. Roughly 10 to 15 m for drip, 20 to 30 m for sprinklers. Convert from bar by multiplying by 10.2. |
Add all five. That total is what the pump has to overcome, and the flow you want has to be available at that total, not at some easier point on the curve.
Friction, with the numbers filled in
Hazen-Williams is the usual method for water in pipes at ordinary temperatures, and it is simple enough to check by hand.
hf = 10.67 · L · Q1.852 ÷ ( C1.852 · D4.87 )
L = pipe length in m · Q = flow in m³/s · D = internal diameter in m · C = roughness coefficient · hf = head lost to friction, in m
Two things about the exponents are worth noticing. Flow is raised to 1.852, so doubling the flow nearly quadruples the friction. Diameter is raised to 4.87 in the denominator, so a small increase in bore makes a large difference. That second one is where most of the money is.
Roughness coefficient C
| Pipe | C |
|---|---|
| PVC, HDPE, PE — new | 150 |
| PVC or PE — after years in service | 140 |
| Steel or galvanised — new | 130 |
| Galvanised — aged, scaled | 100 |
Use the bore, not the nominal size
D in the equation is the inside diameter. These are the bores of common lay-flat and reinforced hose sizes. Rigid pipe in a heavier pressure class has thicker walls and a smaller bore than the same nominal size in a light class, so check what you are actually buying.
A worked example
A borehole on a farm. Water rests 40 m down. The pumping test shows the level falls another 8 m at the flow wanted. The tank stands 6 m above the wellhead. The pump sits 48 m down and the tank is 120 m away, so the pipe run is 168 m of PVC. The requirement is 8 m³/h into an open tank, so nothing extra is needed at the outlet.
| Static water level | 40.0 m |
| Drawdown | 8.0 m |
| Discharge lift to the tank | 6.0 m |
| Friction, 168 m of 2" PVC at 8 m³/h | 4.0 m |
| Bends, valves and the non-return, allowed at 10% | 0.4 m |
| Pressure at the outlet | 0 m |
| Total dynamic head | 58.4 m |
So the duty point is 8 m³/h at 58.4 m. Round it to 8 m³/h at 60 m and that is the number every supplier should be answering. Note what it is not: it is not 48 m, the pump setting depth, and it is not the 90 m the hole was drilled to.
What the pipe size does to the answer
Same 168 m run, same 8 m³/h, same PVC. Only the bore changes.
| Bore | Friction | Velocity | |
|---|---|---|---|
| 1-1/2" (38 mm) | 16.8 m | 1.96 m/s | Too much. A third of your head goes to friction. |
| 2" (51 mm) | 4.0 m | 1.09 m/s | Workable. |
| 2-1/2" (64 mm) | 1.3 m | 0.69 m/s | Comfortable. |
| 3" (76 mm) | 0.6 m | 0.49 m/s | Friction has stopped mattering. |
Going from 1-1/2" to 3" takes 16 m off the head the pump has to produce. On a 58 m duty that is more than a quarter of the job, and it is bought once with pipe rather than paid for every day in a bigger pump and a bigger array. Keep velocity under roughly 2 m/s; above that, friction climbs fast and so does wear.
And what more flow does
Same 168 m of 2" PVC, flow changing:
| Flow | Friction | Velocity |
|---|---|---|
| 4 m³/h | 1.1 m | 0.54 m/s |
| 8 m³/h | 4.0 m | 1.09 m/s |
| 16 m³/h | 14.5 m | 2.18 m/s |
The pipe has its own ceiling
Head is pressure. Every 10.2 m of head is about 1 bar, and the discharge line carries it. This is the check nobody does: the pump gets chosen properly, then the hose on site cannot hold what the pump produces. Working pressures for PVC lay-flat hose, converted to the head they correspond to:
| Hose grade | Size | Working pressure | Head it can hold |
|---|---|---|---|
| Standard duty | 3/4" – 1-1/2" | 5 bar | about 50 m |
| Standard duty | 2" – 5" | 4 bar | about 40 m |
| Standard duty | 6" – 10" | 3 bar | about 30 m |
| Medium duty | 3/4" – 1-1/2" | 7 bar | about 70 m |
| Medium duty | 2" – 6" | 6 bar | about 60 m |
| Heavy duty | 3/4" – 4" | 10 bar | about 100 m |
| Heavy duty | 6" | 8 bar | about 80 m |
Check this against the example above
A 58 m duty puts close to 6 bar on the line. A standard-duty 2" lay-flat hose is rated 4 bar. That job needs at least the medium-duty grade, and rigid pipe for the riser itself. Lay-flat hose belongs on the surface run, not down the borehole.
Five ways this goes wrong
Using borehole depth as head
The drilled depth tells you how deep the hole is, not how far the water has to be lifted. Water usually rests well above the bottom. Measure from the wellhead down to the water, not to the bottom of the hole.
Leaving out drawdown
The level drops once the pump starts, and keeps dropping until the well reaches equilibrium. A pump sized against the resting level looks fine in the morning and loses flow by midday.
Measuring pipe by its outside size
Friction depends on the bore. A 2" pipe in a heavier pressure class has a smaller bore than a thin-walled one in the same nominal size, and the friction goes up accordingly.
Forgetting the pressure the irrigation needs
Drip needs roughly 1 to 1.5 bar at the inlet, sprinklers 2 to 3 bar. That is another 10 to 30 m of head on top of everything else, and it is the item most often missing from an enquiry.
Ignoring what the pipe can take
Head is pressure. A 60 m pumping head puts about 6 bar on the discharge line. Standard lay-flat hose in the larger sizes is rated for 3 to 4 bar.
Send us the number
The calculator does all of the above from your inputs and writes out the three sentences to send a supplier. Once you have the duty point, send it over and we come back with a pump, its curve with your point marked, and a priced bill of materials.
Total dynamic head FAQ
What is total dynamic head on a solar pump?
Total dynamic head is the height the pump has to push water to once everything is counted: the static water level, the drawdown while pumping, the lift to the tank, the head lost to friction in the pipe, and any pressure that has to be left at the outlet. It is one half of the duty point; flow is the other half.
Is borehole depth the same as head?
No, and treating it as head is the most common sizing error. A 120 m borehole does not mean 120 m of head. Water usually rests well above the bottom of the hole, so the real lift can be far less. It can also be more once drawdown, the lift to the tank, friction and outlet pressure are added.
How do I calculate friction loss in a pump discharge pipe?
Hazen-Williams is the usual method for water at ordinary temperatures: hf = 10.67 × L × Q^1.852 ÷ (C^1.852 × D^4.87), with L in metres, Q in m³/s, D the internal diameter in metres and C the roughness coefficient (150 for new PVC or HDPE, 130 for new steel, 100 for aged galvanised pipe).
Does pipe size really change the pump I need?
Substantially. On a 168 m run carrying 8 m³/h, a 1-1/2" bore loses 16.8 m to friction while a 3" bore loses 0.6 m. That 16 m difference is bought once with pipe rather than paid for every day in a bigger pump and a bigger array.
What pressure can PVC lay-flat hose take?
Standard duty runs 5 bar in the small sizes and 3 to 4 bar from 2" upward, which corresponds to about 30 to 50 m of head. Medium duty reaches 6 to 7 bar and heavy duty 10 bar up to 4". A 60 m pumping head puts close to 6 bar on the line, so it needs at least medium duty, and rigid pipe for the riser.
How much pressure does drip irrigation need at the inlet?
Roughly 1 to 1.5 bar for drip and 2 to 3 bar for sprinklers, which is another 10 to 30 m of head on top of the lift and friction. Multiply bar by 10.2 to convert to metres of head.
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