Ralph Parshall, a Colorado State University professor, started work on measuring flow in 1915. Six years later he filed for a patent on his "Parshall Flume," a measuring device based on the Venturi effect.

Parshall's invention, simple to calibrate, self-cleaning, with wide flow ranges, revolutionized water management around the world. Suddenly it was possible to enforce water laws. Flow in ditches and creeks could be measured accurately. Water in ditches could be divided equitably between various users.

Today, the Parshall flume is still the most widely used measuring device throughout the world--and along Boulder's ditches.

The Parshall flume uses a simple depth measurement of water flowing through a fixed known constricted throat to calculate flow.

The narrow throat of the flume produces a differential head which is proportional to flow rate. The relationship between depth and flow is exponential. For each throat width there exists a simple exponential equation of the form: Q = kH^n where Q is the flow in cubic feet per second, H is the depth in feet and k and n are constants that depend on the size of the flume.

Right, clockwise from the top:

Ralph Parshall's diagram of his flume.  The Improved Venturi Flume, Fig 20 p 44, Courtesy of  Colorado State University, Water Resources Archive

Ralph Parshall taking flume measurements, 1946.  Courtesy of  Colorado State University, Water Resources Archive

Letter to Dry Creek Davidson Ditch, 1937  Boulder Water Commissioner Thomas Platt required Boulder ditches to install Parshall flumes in the 1930's..  Courtesy of Carnegie Branch Library for Local HIstory, Boulder CO