How to Read a Pump Curve for Irrigation Systems
Choosing an irrigation pump without reading its performance curve is guesswork. The pump curve removes the guesswork. It maps what the pump actually delivers across a range of flow rates, so you can confirm a match before the pump goes in the ground.
Step One: Establish Your System Requirements
Determine the gallons per minute (GPM) and pounds per square inch (PSI) the system needs to operate. For this example, assume the requirement is 30 GPM at 55 PSI.
Step Two: Convert PSI to Total Dynamic Head
Irrigation pump performance is typically rated in Total Dynamic Head (TDH) rather than PSI. To convert, multiply PSI by 2.31.
55 PSI × 2.31 = 127 feet of TDH
The system requirement is now expressed as 30 GPM at 127 TDH.
Step Three: Plot the Operating Point
On the pump curve chart, the horizontal axis represents GPM and the vertical axis represents TDH. Locate 30 GPM on the horizontal axis and follow it up. Locate 127 TDH on the vertical axis and follow it across. Mark the intersection. This is the system's operating point — where the pump needs to perform.
Step Four: Read the Performance Curve
The performance curve is an arc that runs from upper left to lower right on the chart. It represents the pump's actual output at varying flow rates. The operating point you marked needs to fall on or below the curve.
If the point sits below the curve, the pump can meet the system demand. If the point sits above the curve, the pump cannot — select a different model.
Three Rules to Follow
- The operating point must be on or below the curve. Above the curve means the pump comes up short.
- Choose a point in the left to center portion of the curve. A point on the far-right side of the curve puts the pump at the edge of its range — efficiency drops and wear increases.
- Size for GPM first, then confirm TDH. The curve shows an inverse relationship: as GPM increases, available TDH decreases. Lead with flow, then verify head.
