Parshall Flume & Flow from One Reading
Measures canal flow
Enter the throat width and the head reading to get the discharge in cubic feet per second, litres per second and cubic metres per hour — the standard, self-cleaning way to gauge a canal or field channel.
Read a Parshall flume
Next: read the head only at the Ha point upstream of the throat and look up 39.4 L/s; keep the flume running free (no downstream submergence) or the rating no longer holds.
Free-flow empirical rating Q = 4·W·H^(1.522·W^0.026); valid only within the flume's head range and when not submerged.
Parshall flume — key facts
- Equation (US)
- Q ≈ 4·W·H^(1.522·W^0.026)
- W
- throat width (ft)
- H
- head (ft)
- Reading
- single head at the throat
- Self-cleaning
- sediment passes through
- Best for
- canals & field channels
- Flow regime
- free (unsubmerged)
- Privacy
- Runs in your browser; nothing uploaded
One head reading, the whole flow
The Parshall flume is one of the most trusted devices for measuring open-channel flow, and its appeal is its simplicity: water is squeezed through a shaped throat that forces critical flow, so the discharge depends on a single head reading at the throat. Read the depth, and the standard equation Q ≈ 4·W·H^(1.522·W^0.026) turns it straight into a flow rate. Because the flume is self-cleaning, sediment and debris pass through instead of silting up behind it, which makes it accurate and low-maintenance on real canals and field channels.
This tool computes the discharge in cubic feet per second, litres per second and cubic metres per hour from your throat width and head reading. Use it to gauge how much water a channel is delivering, to share irrigation supply fairly, or to check a pump-and-channel system. Pair it with the Weir Flow, Channel Flow and Float-Method Stream Flow tools to measure flow whatever structure your channel has.
One reading is enough
A single head gives the discharge directly.
Self-cleaning
Sediment passes through, not behind it.
Canal-ready accuracy
Trusted for canals and field channels.
Every unit
Flow in cfs, L/s and m³/h at once.
Frequently Asked Questions
What is a Parshall flume?+
A Parshall flume is a specially shaped open channel — a converging inlet, a narrow throat and a diverging outlet — that forces water through a critical-flow section. That lets you read the discharge from a single head measurement at the throat, with no separate downstream gauge. It's a standard device for measuring flow in canals and field channels.
How is the flow rate calculated?+
For a free-flowing Parshall flume in US units the discharge is Q ≈ 4·W·H^(1.522·W^0.026), where W is the throat width in feet and H is the head in feet, giving Q in cubic feet per second. The exponent shifts slightly with throat width, which the formula captures, and the tool then converts the result to L/s and m³/h.
What is the throat width?+
The throat is the narrowest section of the flume, and its width sets the flume's size and capacity. Standard Parshall flumes come in fixed throat widths — from a few inches for small field channels up to several feet for large canals. Enter the width of your installed flume so the discharge equation uses the right coefficient.
Where do I measure the head?+
Read the head (the water depth above the flume floor) at the standard upstream gauging point in the converging section, not in the throat itself. The flume's geometry references the discharge equation to that point, so a consistent, correctly located reading is what makes the flow figure accurate.
Why use a flume instead of a weir?+
A Parshall flume is self-cleaning: its smooth, converging shape lets sediment and debris pass through rather than collecting behind it as they do at a weir. It also works with a small head loss, so it suits flat field channels and silty canals where a weir would silt up or back water up too far.
Is this for free or submerged flow?+
The standard equation here is for free flow, where the downstream level is low enough that it doesn't back up into the throat. Most field installations run free. If the channel downstream is high enough to submerge the flume, a separate submergence correction is needed and the simple equation over-reads the flow.
What units does the tool report?+
It reports discharge in cubic feet per second (cfs), litres per second (L/s) and cubic metres per hour (m³/h), plus the head used. That covers both imperial and metric workflows so you can match it to your irrigation records or pump and channel-design figures.
How accurate is a Parshall flume?+
A well-built, correctly installed and unsubmerged Parshall flume is accurate to within a few percent — among the best of the simple field methods. Accuracy depends on the flume matching the standard dimensions, a level installation, and reading the head at the proper point with a still, undisturbed water surface.
Can I use it for canals and field channels alike?+
Yes — Parshall flumes are made in a wide range of throat widths to suit everything from small on-farm field channels to large supply canals. Pick the throat width that matches your channel's flow range, install it level, and a single head reading gives you a reliable discharge any time.