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.