Total Dynamic Head & Size the Pump to the Real Head

Total Dynamic Head is the total head, in metres of water, that a pump must produce to move your design flow from the source to the point of use. The tool adds four parts: TDH = static lift + friction loss + operating pressure + delivery head. Together they are the head the pump works against, and with the flow they fix the duty point you use to select a pump from a manufacturer's curve.

Static lift is the vertical distance from the water surface up to the pump or the field datum — the height the water has to be raised against gravity. Delivery head is any additional elevation the water must reach at the far end, such as a tank on a stand or a field on higher ground. The tool keeps them as separate inputs so you can account for both the source depth and the destination height clearly.

Drip and sprinkler emitters only work at their rated pressure — a sprinkler might need 25 m of head (about 2.5 bar) at the nozzle, and a dripper around 10 m. That pressure has to be present on top of all the lift and friction, so it is added into the TDH. Flood and furrow irrigation need almost no operating pressure, so for those systems this term is near zero.

Friction loss is the head the water loses dragging along the suction and delivery pipes and through fittings, and it grows with flow and shrinks with pipe diameter. You can compute it with the Hazen-Williams equation for the pipe run and add an allowance for fittings, then enter the total here. Because friction rises steeply with flow, an undersized pipe can make the friction term dominate the whole TDH.

Take the TDH together with the design flow rate and read across a pump's performance curve to find a pump whose curve passes through, or just above, that flow-and-head point at good efficiency. Then check the motor power needed at that duty and confirm the NPSH available exceeds the pump's NPSH required. The TDH is the head axis of that selection — get it wrong and the pump is over- or under-sized.

If the real head is higher than you sized for, the pump rides back up its curve and delivers less flow than you need — sprinklers throw short, drip lines starve at the far end, and the field is under-watered. Over-estimating wastes money on an oversized pump and motor and can push too much pressure into the system. Adding the four terms carefully, with a realistic friction figure, avoids both errors.

TDH is the head your system demands; the pump's rated head is what a given pump can supply. The goal is to choose a pump whose head at your flow matches the system's TDH at that flow — the intersection of the system curve and the pump curve, called the duty point. So they should be equal at the operating flow, but they are different concepts: one describes the field, the other the machine.

Yes — on a well or pond where the water level falls during pumping or across a dry season, the static lift grows, so the TDH rises and the pump delivers less. For a falling source, size the pump for the head at the lowest expected water level, not the full level, or the flow will fade just when demand is highest. Re-run the calculation at the drawn-down level to be safe.