Total dynamic head, usually shortened to TDH, is the energy per unit weight of water that a pump must overcome at a stated flow. It is the horizontal axis partner to flow on a pump curve. If TDH is missing, “which pump?” has no defensible answer. This is especially important for solar systems: available power changes through the day, while the water level and pipe system determine the hydraulic load.
What belongs in total dynamic head?
The US Department of Energy’s pumping-system guide separates system head into static/elevation, pressure and dynamic components. For farm planning, use four visible parts: lift from the pumping water level to the discharge reference; additional elevation to the tank or field; pressure required at the outlet; and friction through pipe, fittings, valves, filters and irrigation equipment.
Pressure can be converted to head of water. As a practical approximation, 1 bar is about 10.2 m of water head. If a drip system needs 1 bar at its inlet, that is roughly 10 m of head before allowing for losses. Do not add pressure twice: a tank elevation may already create part of the available pressure.
Static level, pumping level and drawdown
The static water level is measured after the borehole has recovered and no pumping is taking place. When pumping starts, the level usually falls. The difference between static and pumping water level is drawdown. The US Geological Survey explains that pumping creates a cone of depression and that water levels can fall below the intake if withdrawal and recharge conditions do not support the rate.
For TDH, the relevant starting elevation is normally the pumping water level at the intended flow. A test that reports only static level cannot show the operating lift or sustainable yield. Ask for the test rate, duration, stabilized pumping level, recovery observations, borehole diameter, pump-intake setting and water quality result. A seasonal safety allowance may be necessary where dry-season levels are lower, but it should come from local evidence rather than an arbitrary percentage.
Do not confuse pump setting depth with lift. A submersible pump can be installed well below the pumping level to maintain submergence; hydraulic lift still begins at the water surface, subject to the exact system model. The setting matters for cooling, submergence, cable, pipe and protection, but adding the entire installation depth as head can grossly oversize the system.
A field worksheet for TDH
| Component | Example | Evidence to collect |
|---|---|---|
| Pumping water level below pump datum | 24 m | Pumping-test record at design flow |
| Delivery elevation above datum | 6 m | Survey, level or reliable contour |
| Required outlet pressure | 10 m | Irrigation/tank design requirement |
| Pipe and fitting friction | 4 m | Flow, internal diameter, length, fittings |
| Estimated TDH | 44 m | All components at the same flow |
The example is intentionally above the 15 m total-head boundary stated in Futurepump’s FAQ for its SE1 and SF2 surface-pump range. That instantly rules those cited models out for this particular 44 m duty. It does not rule out solar pumping; it points toward an appropriately engineered higher-head pump, often submersible for a borehole.
Use the ShambaPump calculator to see how TDH affects hydraulic and estimated input power. Its efficiency and solar-derating values are screening assumptions, not a substitute for the manufacturer’s curve, controller limits and array design.
Why friction cannot be a fixed percentage
Friction rises strongly with flow and falls when pipe internal diameter increases. A long, narrow pipe can consume more head than the vertical lift. Filters load as they become dirty; valves, bends, non-return valves and emitters add losses. Therefore “add 10%” may be too much for one layout and dangerously little for another.
Build a pipe schedule: material and internal diameter, straight lengths by section, elevation, every major fitting, filter specification, valve and required endpoint pressure. Calculate loss at the design flow for each section. If irrigation is zoned, test the most demanding zone rather than averaging unlike routes. Ask the supplier to show the selected duty point inside the efficient operating region—not at the extreme edge of the curve.
Borehole and abstraction checks in Kenya
Kenya’s Water Resources Authority states that permitting regulates abstractions and that authorization to construct works precedes the water permit. Its listed groundwater requirements include a hydrogeological assessment during application and, later, a borehole completion record and water-quality analysis. WRA also says abstraction with works without authorization is illegal. Current category, fee and process details must be confirmed directly with WRA.
Hydraulic sizing and legal allocation answer different questions. A pump may physically lift more than a permit or sustainable source allows. Include the authorized daily volume and maximum abstraction rate in the control settings. Dry-run protection, flow measurement and a tank float switch can help keep operation inside physical and regulatory boundaries.
Method and limitations
This guide uses a component TDH model suitable for building a quote brief. It does not calculate pipe friction, borehole hydraulics, surge, water hammer, cable sizing or aquifer sustainability. Values must share the same elevation datum and design flow. A qualified designer should review the pumping test and full system curve.
Frequently asked questions
Is borehole depth the same as head?
No. Drilled depth describes the hole. TDH is based on operating water level, destination elevation, required pressure and friction at a specified flow.
Why is a pumping test important?
It connects abstraction rate to drawdown and recovery. USGS notes that well yield depends on the aquifer, well and pump, and that apparent yield can be overstated if tests do not approach stable conditions.