Static Head vs Dynamic Head: TDH Calculation Guide
Quick answer
Learn the difference between static head, dynamic head and pumping water level. Calculate TDH correctly and select the right solar water pump.
A practical guide for solar pump installers, distributors and irrigation contractors: what static head and dynamic head really mean, why neither of them is the water level in the well, and how the two are brought together in the Total Dynamic Head (TDH) a pump has to meet.
Choosing the right solar water pump takes more than the depth of the well or the power of the motor. The figure that decides the pump is the Total Dynamic Head (TDH) — the lift plus the losses in the pipework, the fittings and the discharge, taken together rather than as a depth alone.
Many pump buyers and installers mix up four terms that are related but not interchangeable: static head, dynamic head, static water level and dynamic water level. Two of them describe head in a pumping system; two of them describe depth in a well. They are read from different places, they answer different questions, and one cannot be used in place of another.
| Term | What it is | Where it is read |
|---|---|---|
| Static head | Elevation difference between the water surface and the point of delivery, with nothing moving | Two elevations in the system |
| Dynamic head | Resistance the water meets while it flows: friction in pipes, fittings, valves and filters | The pipeline, at the duty flow |
| Static water level | Depth from the wellhead to the water surface with the pump off and the well recovered | In the well, pump stopped |
| Dynamic water level | Depth from the same wellhead to the water surface while the pump runs at the duty flow | In the well, pump running |
Both heads are measured in metres of head. Both water levels are measured in metres below the wellhead. That is exactly where the confusion starts.
What Is Static Head?
Static head is the vertical elevation difference a pump has to overcome, measured between the water surface it draws from and the surface it delivers to, with nothing moving. It is the elevation part of the duty, and it carries no friction loss.
If water has to be lifted from a water level 40 m below ground to an open storage tank 10 m above ground:
Static head = 40 + 10 = 50 m
Static head depends on elevation, not on the length of the pipe. A 200 m horizontal pipeline does not add 200 m of static head. It adds friction loss, which is a different component, and it appears only when water is flowing.
What Is Dynamic Head?
Dynamic head is the part of the duty that exists only while water is moving: the head needed to push water through the pipework against the resistance of the system.
Dynamic head covers the friction losses created by:
- pipes and fittings
- elbows and valves
- filters and other hydraulic components
Friction loss rises with flow rate. At a given flow it depends on the pipe diameter, the pipe length, the pipe material and the fittings in the run. A pipeline may give 8 m of friction head loss at the duty flow the site needs, and that 8 m belongs in the TDH.
A note on wording: some manuals use “dynamic head” for the whole duty, losses included. In this guide it is the flow-related part only, so the friction loss and the elevation lift are never counted twice.
Important: dynamic head is not the same as the dynamic water level in a borehole. One is a resistance inside a pipeline; the other is a depth measured in the well. The next section is the one that matters most for a deep well submersible pump.
Static Water Level vs. Dynamic Water Level
For a deep well submersible pump, two further measurements are read from the well itself. Both are depths measured from the same reference point — the wellhead — to the water surface. Neither of them is a head.
Static Water Level (SWL)
The static water level is the depth from the wellhead to the groundwater surface when the pump is not running and the well has recovered.
Dynamic Water Level (DWL)
The dynamic water level — also called the pumping water level (PWL) — is the depth from the same wellhead to the water surface while the pump is running at a specified flow rate.
When pumping starts, the water level usually drops, because water is being taken out of the well faster than the aquifer refills it. That drop is the drawdown:
Drawdown = Dynamic water level − Static water level
An example well:
| Parameter | Value |
|---|---|
| Total well depth | 150 m |
| Static water level | 30 m |
| Dynamic water level | 50 m |
| Drawdown | 20 m |
| Pump installation depth | 80 m |
The drawing below shows where each of those five figures is measured on the well:
In this example the pump must be sized from the water level during operation — 50 m — and not from the static water level of 30 m.
The installation depth of 80 m matters for submergence and motor cooling. It is not the pumping head, and it does not by itself decide the pump.
How to Calculate Total Dynamic Head (TDH)
Total Dynamic Head is the total head a pump has to develop to deliver the required water flow. For a typical deep well pumping into a storage tank, the practical formula is:
TDH = Pumping water level depth + Discharge elevation + Friction loss + Required outlet pressure head
The pumping water level depth and the discharge elevation are both measured from the same wellhead reference point. Expressed the other way round, the pumping water level depth is the static head plus the drawdown:
TDH = Static head + Drawdown + Friction loss + Required outlet pressure head
For the pressure term, 1 bar is approximately 10.2 m of water.
Practical example: a solar irrigation project
A farmer wants to pump groundwater into an elevated water storage tank. The project conditions are:
| Parameter | Value |
|---|---|
| Well depth | 150 m |
| Static water level | 30 m |
| Dynamic water level | 50 m |
| Pump installation depth | 80 m |
| Tank water level above ground | 15 m |
| Estimated pipe friction loss | 8 m |
| Additional outlet pressure | 0 m |
The TDH is built up from four terms:
Dynamic water level depth 50 m → + discharge elevation 15 m → + friction loss 8 m → + outlet pressure 0 m → TDH 73 m
TDH = 50 + 15 + 8 + 0 = 73 m
The same duty read the other way: a static head of 45 m (30 m + 15 m), a drawdown of 20 m, friction of 8 m and no outlet pressure — 73 m again.
The pump therefore has to deliver the required flow rate at approximately 73 m of total dynamic head, and it is selected by reading its Q-H performance curve at that duty point.
The 8 m friction figure is an assumption for this example. Real losses have to be calculated from the pipeline dimensions and the operating flow — HOBER’s pipe friction loss calculator is the first step, because the same pump delivers less water on a long, small-bore pipeline than on a short, wide one.
Common Mistakes When Calculating TDH
Mistake 1: Using the total well depth as the pumping head
A 150 m deep well does not automatically call for a pump rated at 150 m of head. The operating water level, the discharge elevation, the pipeline losses and the required outlet pressure are what decide the TDH.
Mistake 2: Using the static water level instead of the pumping water level
The water level drops while the pump runs. A TDH calculated from the static water level is too low, and the pump then delivers less water than the site expects, especially in the dry season when the level is already lower.
Mistake 3: Leaving out the pipeline friction
Long runs, small pipe diameters, valves and fittings all add resistance. On high-flow irrigation systems these losses can be a large part of the TDH, and they are the part that changes most when the pipework changes.
Mistake 4: Choosing a pump by its maximum head alone
A pump advertised with a maximum head of 100 m does not necessarily deliver the required flow at 73 m. Maximum head is normally quoted at or near zero flow. Always read the performance curve at the duty point, not at the end of the curve.
Why TDH Matters for Solar Water Pump Systems
A solar pumping system works under changing solar irradiance. The available solar power sets the pump’s operating speed, its flow rate and its operating point on the curve, so a pump sized against the wrong head can under-deliver even when the array has enough nominal capacity.
For reliable solar pumping performance, installers should verify:
- required water flow (m³/h)
- total dynamic head (m)
- the pump Q-H performance curve at that duty point
- pump motor rated power and voltage
- PV array sizing and voltage compatibility
- solar pump inverter compatibility
- well yield, seasonal drawdown and minimum pump submergence
The pump and the solar pump inverter have to be selected as one compatible system. The pump is read from the water and the duty; the drive is read from the pump. Neither is chosen on its own — see deep well solar pumping for the arrangements this applies to.
How HOBER Helps Installers Select the Right Solar Pump Solution
HOBER helps solar installers, water pump distributors and irrigation contractors select solar pumping equipment from the actual requirements of the project rather than from a catalogue page.
Solar pump inverters for existing AC pumps
A compatible existing AC water pump can be driven from solar power without replacing the whole pumping system: the pump stays as it is, and the inverter is what puts the array in front of it. See AC pump solar conversion.
Solar water pumps for new installations
For a new installation the pump type is read from the water source and the duty — deep well, surface, centrifugal or booster — according to the required flow rate, the total dynamic head and the application.
PV sizing and technical support
Our team supports pump matching, PV array recommendations, electrical compatibility, installation guidance and troubleshooting, through installer support and project support.
For accurate pump selection, send the required flow rate, the dynamic water level, the delivery elevation, the pipeline details and the required outlet pressure.
Project step
Need flow data, constant pressure or remote monitoring on a pumping project?
Send the water source, the required flow, the total head and the pump voltage — HOBER answers with a matched configuration.
FAQ: Static Head, Dynamic Head and Total Dynamic Head
Is static head the same as static water level?
No. Static head is an elevation difference inside a pumping system. Static water level is a depth measured in the well when the well is not being pumped.
Is dynamic head the same as dynamic water level?
No. Dynamic head is the flow-related resistance in the pipework. Dynamic water level is the water surface depth measured while the pump is running.
Should TDH use the static or the dynamic water level?
For a deep well submersible pump, use the pumping (dynamic) water level at the design flow rate.
Does a deeper pump installation always mean a higher TDH?
No. The depth at which the pump is installed does not by itself set the static lifting head. The pumping water level and the discharge conditions are the relevant factors.
Does TDH change with the water flow?
Yes. Pipeline friction losses change with flow rate, and the pumping water level can also change as the flow and the drawdown change.
Work Out the TDH, Then Select the Pump
Correct TDH calculation is what makes a solar water pump selection efficient and reliable. Remember:
- Static head = the elevation difference between the water surface and the point of delivery, with nothing moving
- Dynamic head (friction component) = the flow-related resistance of the pipework
- Drawdown = the lift the site loses while the pump runs (dynamic water level − static water level)
- TDH = static head + drawdown + friction losses + required pressure head, for the simplified systems discussed here
For a deep well, take the elevation lift from the dynamic water level, not from the total well depth and not from the static level.
Once the TDH and the required flow are known, the next step is to turn them into a pump model. Enter the water source, the flow and the head in HOBER’s Pump Selection, and send the duty point to our team through contact — including the pump nameplate if an AC pump is already on site, so the inverter that goes in front of it can be confirmed.
HOBER — Solar pumping made simple.
Website: hobertek.com