Why is "Q" used to symbolize stream discharge?

[Richard Marston]

Do you know why the letter "Q" is used to symbolize stream discharge?

 

Does it have something to do with Q = quaque (Latin) in medical usage being the dosage rate for prescripions (e.g., # pills per day)?

I honestly don’t know the answer about where Q comes from.  However, it’s been broadly used not only in river science but also in other sciences such as thermodynamics, fluid dynamics, and for gas and liquid flows.

Q (upper case) is used for volumetric flow rate (L^3/time).  q (lower case) is used for flux rate (L^3/(L^2*time)). [Darcy’s Law is an example of flux rate.)  The distinction between upper and lower case is not always adhered to, but I’ve seen it around for several  decades. 

In thermodynamics Q can be used for heat flux (W/m^2), heat flow (watts), total heat (joules), or heat generation (W/m^3)… at least according to Wiki. 

But this still begs the questions of why the letter "Q" is used for flux.

 

It’s use seems pretty well embedded for transfers of liquids, gases, and energy.  It would be interesting to look at some original early manuscripts (Newton, Bernoulli, Darcy, etc.) to see if they used Q.

Dr. Richard A. Marston

University Distinguished Professor

Co-Editor-in-Chief, Geomorphology

Jefferson Science Fellow, U.S. Dept. of State

 

Department of Geography

Kansas State University

Manhattan, KS 66506-2904

Rmar...@ksu.edu

[Daniel Viviroli]

Richard,

that's an interesting question, and I have also wondered many times about this.

One explanation I heard is that "q / Q" is derived from the French "quantité", meaning "amount, quantity, measure, number". If the symbol was introduced around the 18th century, it would be plausible since France led the world of Science then (e. g. Lavoisier, Laplace, Berthollet).

Best,

Daniel