Just How Dense Are We? Does Our Choice of Dimension Affect Our Understanding of Water Processes & Policies?

Hydrology Water Processes Policies

We usually report water quantity information as a volumetric rate (e.g. m3/s); we usually report water quality information as a concentration (e.g. mg/l); and we usually report precipitation as a length (e.g. mm). But we don’t have to.

The mass of water is related to its volume by its density which, conveniently, can be assumed to be unity (1). This means that we could just as easily report water information using the dimension of mass.

Would reporting water information in a different dimension change the way that we understand water?

For example, in Vancouver, BC we use 173.4 m3 of treated water per person per year and we get 1.117 m of rainfall per year. What does this information tell you about our water footprint? There is no intuitive linkage between this data and our dependency on our watersheds. However, I could tell you that we consume 173.4 t (1 tonne = 1000 kg) of water per year and we ‘only’ get 1.117 t/m2 of rainfall each year. This puts our place in the watershed into better perspective. You don’t even need a calculator to ‘see’ that — even without accounting for losses — we need more than 150 m2 of watershed area per person.

That simple insight could change the conversation about water management.

It doesn’t need to be a choice of either/or. We already routinely report water data in different dimensions/units for sediment and water quality depending on the end-use of the data.

The average sediment concentration in the Mississippi River at St. Louis is 324 mg/l. Is that a big number or a small number? Most people do not have any context for evaluating sediment concentration data so it doesn’t contribute to a meaningful understanding of the role of the Mississippi River in landform processes. However, if I report that the sediment load of the Mississippi River is 250,000 t/d and I also provide a bit of additional context that a common railway car has a capacity of 100 t, then a new understanding of the Mississippi River emerges from the data.

Similarly, with water quality data we may report that a stream with a flow of 5.0 m3/s has a total nitrogen concentration of 1.5 mg/l. Those values may be meaningful if I am already an expert in the field of water quality. However, for a watershed stewardship NGO trying to understand the role of agriculture in water quality, these data might be a bit intimidating. However, suppose we were to report that the flow of water is 5 t/s of water and the flow of nitrogen is 7.5 g/s. Even a non-expert can intuitively understand that there is proportionately very little nitrogen in the water but the total quantity of nitrogen can quickly add up. For example, if a bag of nitrogen fertilizer at the local feed store weighs 20 kg then this concentration is equivalent to one bag of fertilizer being dumped in the stream every 44 minutes.

The way we traditionally report water data is not wrong.

There are some very good reasons for the conventions that we have widely adopted. However, in the distant past, when these conventions became entrenched the target audience for water data was primarily engineers. Perhaps there is now room to be a bit more creative in reporting water data. Perhaps, with a bit of innovation we could make the information from our data more meaningful for everyone sharing our watersheds.

Do you know of anyone who has experimented with reporting water data in non-conventional units and dimensions? How did that work out?

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5 responses to “Just How Dense Are We? Does Our Choice of Dimension Affect Our Understanding of Water Processes & Policies?”

  1. Probably the most humorous thing that I’ve seen was riding with an irrigation’s district ditch rider and observing his methods of measuring water. I had been assigned to accompany him to learn a little about their operations. On one stop, he had to open a lateral gate off of the main canal. He manually opened the gate to a point then abruptly stopped. No indication that he was looking at a staff or other indicator to estimate the flow. I asked him what the target flow was and he said 25 CFS. “How can you tell that?”, I asked.

    “The Gate top came up to my knee cap” was the reply.

    Seriously, we deal with the measurement of water on a daily basis. We deal in terms of flow in CFS and Acre Feet. Somebody asked me what an acre foot was. Of course, I gave the standard reply. However can any of us envision seeing an acre and then estimating a foot of water depth on it? Same goes with telling someone what a CFS represents.

    I got thinking about that one day and applied it to one of the applications that I wrote. I measure flow in real time from one of our dams. I take measurements on a minute to minute basis. I could see how many CFS had flowed in that minute then converted the value to units of measure that anyone could relate to. I took a screen shot of it. As amusing as it was, I used it as the background on my twitter page. For your amusement https://twitter.com/RiverKey2640

  2. Brilliant…

  3. If you do not use the traditional units the regulatory agencies will not accept the reports. Alternate units are fine to help explain things to “non-experts” or non technical people but not regulatory or expert technicians.

  4. Matthew Mabey May 16, 2016 at 2:48 pm

    Nonsense. Comparing tonnes and tonnes per square meter is no more, or less, intuitive than comparing cubic meters to meters of precipitation. The ratio is exactly the same 173.4/1.117=155.2
    Far more meaningful to the woman on the street to say that we need 155 square meters of watershed (or even better to say an area about the size of a suburban house) for each and every person, and that this would leave no water for fish, wildlife, or babbling brooks. Better still to estimate that to provide our water needs, we need the ecosystem services of about 1 acre of relatively natural land for ever person in Vancouver, B.C. Meanwhile, there are about 23 people per acre living in Vancouver, B.C. So we tell that woman on the street that we need a natural area more than 20 times the size of Vancouver, B.C. to provide the ecosystem services that the City of Vancouver needs. We could quibble over how exact my 1 acre per person is for a Pacific Northwest ecosystem, but the point is that it takes a natural area many times the size of a city to sustainably provide the ecosystem services that the urban denizens need.
    To communicate with the general public takes a lot more than a slight change in the units used.

  5. The human brain needs the smallest, most manageable unit and all communications are audience (and we, as scientists are rarely the audience) dependant so this thread makes sense to me – and is not new. I often communicate sediment loads in kg/m2 (and invite the class to imagine placing that into their fish tank) and water consumption in minutes under the shower. It’s all relative and thanks for the thread

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