Red staff gauge.

Part 1: Clear as mud – Definitions, communication & obfuscation in hydrometry

Have you ever tried to explain the semantic difference between stage, gauge height and water level? Or why the distinction is even needed or useful?

Water level seems obvious enough and I use this term for any still pool (e.g. reservoir) where the height of the water surface is uniform. I use the term stage where the height of a flowing water surface is a function of a control feature – i.e. in a river channel. Whereas water level and stage can both conceptually exist independently of being measured, a gauge height is the height of a water surface relative to a specific measurement location – a gauge height without a gauge is meaningless.

It seems I am overly pedantic. The international standard “Hydrometry – vocabulary and symbols” ISO/FDIS 772:2011 lumps stage, gauge height and liquid level together in one definition (1.79): “elevation of the free surface of a stream, canal, river, lake or reservoir relative to a specified datum” whereas water level is defined as (1.172): “altitude reached by the surface of flowing or still water”. These concepts seems fundamental to the definition of hydrometry which is given as (1.144): “science and practice of the measurement of water including the methods, techniques and instrumentation used”.

I am curious about the ISO distinction between liquid level and water level. I find it interesting that liquid level is measured in terms of elevation whereas water level is measured in terms of altitude. Everyone knows what water is, right? Apparently not, the Dihydrogen Monoxide (DHMO) hoax is a humorous example of the difference between our common understanding of the substance versus a chemically accurate (if deliberately misleading) description.

Achieving a shared understanding of water is fundamental to developing a useful ontological reference of data search terms.  We like to associate the physical substance of water with geographic features that contain water (e.g. aquifers, oceans, water courses, watersheds) that we can base a data search on. If we are going to share our data we should agree on what water is and how to measure it and these definitions, in turn, will help us to define the containers for which some measure of level, volume, mass or flux rate are valuable.

Whereas I think of water level as being valid with respect to a polygon, stage as being valid with respect to a line (i.e. river cross section) and gauge height as being valid at a point location ISO does not agree with me (or is it me who does not agree with ISO?).

Water is a powerful solvent that, in its liquid form, is almost always associated with other substances either in solution or in suspension.

What then, is the threshold concentration that distinguishes between what we would perceive as being water versus something that is merely ‘wet’?

Water phase-shifts readily, if we want a water elevation, it is widely assumed that we are interested in the liquid water elevation but does it matter that water could be ice or vapor? Does the density or velocity vector of the water matter? I live in Vancouver, where we live under a 5 km deep layer of water for part of the year – fortunately the liquid water density above sea level is low enough (barely!) that we can still breathe without developing gills.

Clearly, the art of communication in the business of hydrometry is anything but clear. We are an independent breed involved in a place-based activity. We spend our time in the back country not at international conferences arguing about semantics. Our need for clarity is local, within our region or agency. It didn’t matter that someone on the other side of the world used the same terms differently or different terms for the same thing. That is now all changing.

N.B. I did participate in a peer review of ISO FDIS 772:2011 and even though I submitted a number of detailed comments I now wonder how I could have failed to comment on the terms used for the measurement of the height of water. I would be interested to hear the opinions of the readers of this blog whether ISO got it right or whether this is something I should bring back to the table.

  • Bob Halliday
    Posted at 1:46 pm, August 9, 2012

    Interesting, years ago I chaired the ISO Technical Committee that produced the first trilingual version of ISO 772. Producing the document required balancing customs and practices in various countries, taking into account language/translation issues. One of my little hobby horses at the time was to try to eliminate terms that were self-evident or straight lifts from standard dictionaries. There was also a need to take into account other glossaries of hydrometric terms such as WMO 385.

    To deal with Stu’s question I don’t like the current definition of water level as there is no mention of datum. As it stands, I would simply delete the definition as self-evident or insufficient. (The WMO definition does specify water level with respect to a known datum.)

    To me, stage, rightly or wrongly, always indicates some sense of head and is most correctly used for flowing streams. Interestingly from a river hydraulics perspective relative stages are more significant than absolute stages so a local datum is fine. No need to chat about water levels. I can apply the same sort of logic to gauge heights for streams or lakes. The datum must be known, but again it could be a local datum. Given current DGPS realities I would say that “liquid level” or “water level”, if used, should imply a reference to an absolute datum – geoid centred like WGS85or the US NAVD88 – or Helmut heights like the current Canadian vertical datum CGVD28.


  • John R Gray
    Posted at 8:11 pm, August 15, 2012


    The USGS has several information sources that address the question-at-hand. It might be worthwhile to surf the various sources at:

    The first source is from 1972, or 40 years to consider your query, if you will.

    Langbein and Iseri ( provide the following:

    Gage height. The water-surface elevation referred to some arbitrary gage datum. Gage height is often used interchangeably with the more general term stage although gage height is more appropriate when used with a reading on a gage.

    Stage. The height of a water surface above an established datum plane; also gage height. considers stage the same as gage height (gauge height to you non-USGS folk…) as follows:

    Gage height – See Stage

    Stage – Height of the water surface above an established datum plane, such as in a river above a predetermined point that may (or may not) be at the channel floor.

    I write on behalf of my Office of Surface Water colleagues, I believe, when I thank you for including us in this discussion.

    Best regards,
    John R. Gray

    USGS, Office of Surface Water
    National Sediment Specialist

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