Related Resources: fluid flow

Siphon Flow and Discharge Rates

Fluids Engineering and Design

Hydraulics Pneumatics Design and Engineering

Siphon Flow and Discharge Rates

Plastic or aluminum smooth wall siphons and tubes are commonly used to deliver water from a canal or ditch to the furrows. These conduits may also be used to measure the rate of flow. The rate of flow from is used only for siphons and pipes where the flow exits the tube into free air. The following table represent an uncertainty for discharge of about +/-15 percent. Therefore, this method of determining discharge is approximate but could be useful for estimating general usage.


Siphon Flow Rate from Small Pipes Equation and Calculator

Rates of flow through ditch-to-furrow pipes for various heads.

allons Per Minute
Gallons Per Minute
Rate of Flow Through Small Pipe

allons Per Minute
Cubic Ft per Second
Rate of Flow Through Large Pipe


allons Per Minute

The pipe ends should be cut cleanly with no burrs. The tubes and ends should not be dented or deformed. Tight bends should be avoided. The flow past either end of the tube must be slow compared to tube velocity. Tubes can become partially or fully air locked at lower discharges of curves as shown on the above tables. The siphons should be reprimed periodically when operating at the low discharge region. Both submerged ends should be located a distance of 1.5 diameters from channel flow boundaries and water surface. If a vortex forms over the siphon entrance, the entrance should be lowered if possible; otherwise, the vortex should be suppressed. The vortex can be suppressed by rafting a wide board over the intake and hanging additional cross vanes from the board if needed. Bos (1989) gives more details concerning these requirements and provides rating curves in metric units.

Discharge of aluminum or plastic siphon tubes
Discharge of aluminum or plastic siphon tubes at various heads for different tube lengths.


Types of Siphon


Instructions for High-Water Maintenance
Siphon Discharge

Siphon Discharge


United States Department of the Interior
United States Department of Agriculture
Bureau of Reclamation
Water Measurement Manual, 2001

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