Parshall Flume & Flow from One Reading

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.

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.

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.

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.

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.

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.

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.

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.

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.