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Tubewell Discharge Calculator

Estimate discharge from a pumped well or tubewell using the trajectory free-fall (jet-flow) method.
Trajectory method for measuring discharge from a horizontal pipe showing horizontal distance X and vertical drop Y
X = horizontal distance  •  Y = vertical drop  •  D = internal pipe diameter

Method: The trajectory free-fall or jet-flow method estimates flow from a full-flowing horizontal circular pipe by measuring the path of the discharged water jet.

Q = 0.0174 × D² × X / √Y

In the equation, Q is discharge in litres per second and D, X and Y are expressed in centimetres. The calculator automatically converts millimetre and inch inputs to centimetres before calculation.

Conditions for use: The pipe should be circular, flowing full, freely discharging, and approximately horizontal at the outlet.

Flow condition: Use the method only when the discharge is reasonably steady and the jet trajectory can be measured clearly.

Accuracy: This is an approximate field method. Under ordinary field conditions, measurement error may approach about ±10%.

Reference basis: U.S. Geological Survey guidance for estimating discharge from a pumped well using the trajectory free-fall or jet-flow method.