Dear Doc Hydro: I periodically run across the miners inch as a unit of water measurement, usually associated with measures of water diversion at a headgate. My question is: An inch of what? I can go to my water measurements conversion manual and find that 38 miners inches equals 1 cfs (in Colorado, it varies from state to state), but I surmise that there was some measurement device that was designed so that a depth of flow of one inch produced a flow rate of one miners inch. If so, what was the device?

As you point out, the miners inch does not represent a fixed and definite quantity of water, being measured generally by the arbitrary standard of the various ditch companies. For this reason, a miners inch is 0.020 cfs (1/50th of a cfs) in Idaho, Kansas, Nebraska, South and North Dakota, New Mexico, Utah, Washington, and southern California, while it equals 0.025 cfs (1/40th of a cfs) in northern California, Nevada, Arizona, Oregon, and Montana, while Colorado maintains that a miners inch equals 0.026 cfs (1/38th of a cfs) and our neighbors to the north in British Columbia insist that a miners inch equals 0.028 cfs (1/36th of a cfs). Since the value of a miners inch varies by locality, most western states have established the value by statute.

The miners inch was primarily intended for measuring small quantities of flow and it eventually gave way to the second foot or cubic foot per second (cfs) unit that is commonly used today in the United States to measure flow.

To determine the amount of water, early miners used a miners inch box, a special type of free flowing orifice. A miners inch is the quantity of water which discharges through a square inch of opening under a prescribed head. The number of miners inches is equal to the area of the opening in square inches. One source describes this in more detail as the quantity of water that will escape from an aperture one inch square through a two-inch plank, with a steady flow of water standing six inches above the top of the escape aperature, the quantity so discharged amounting to 2274 cubic feet in twenty-four hours.

This amounts to a flow of about 1.5 cubic feet per minute. Thats 11.25 gallons per minute (1/40th of a cfs) if you are in northern California, Nevada, Arizona, Oregon, or Montana but only 9.0 gallons per minute (1/50th of a cfs) if youre from Idaho, Kansas, Nebraska, South and North Dakota, New Mexico, Utah, Washington, or southern California. Doc Hydro says, lets bring on the metric system and end all of this confusion.