Measuring discharge with “rating curves” in Antarctica

The temperatures have been rising lately. Warmer weather and intense solar radiation combine to melt glacial ice and snow patches. The melt water then runs down-gradient forming streams! The stream team monitors these streams to account for how much water is flowing, where it is going, and when it is going there.  A major responsibility of the Stream Team is measuring stream discharge at many sites throughout the Dry Valleys.

A stream’s discharge is the volume of water passing a cross section of stream per unit time. Discharge is always expressed as volume/time (e.g. litres per second, cubic feet per second, cubic meters per second, etc.). Instead of a river, let’s think about your kitchen faucet. If you turn the faucet on and allow water to pour into a measuring cup for 30 seconds, then turn the faucet off, you’d be left with some volume of water (you can measure this, of course!). To calculate the discharge of your faucet, simply divide the volume of water collected by the time it took to collect the water (30 seconds).

When we want to calculate the discharge of a stream or a river, we would likely need a very large measuring cup to collect all of the water coming downstream for any period of time! So, we use a different approach. Turns out, we can also calculate discharge by multiplying the velocity of stream water by the cross-sectional area of the stream channel. Velocity is always expressed as length per time and area is always expressed as length squared. Multiplying velocity by area, we get volume per time (e.g. litres per second, cubic feet per second, cubic meters per second, etc.). Simply a different path to the same result – a discharge measurement!

My friends and I have been graciously trusted with fancy and expensive instruments, which can accurately measure the velocity of flowing stream water, as well as the cross-sectional area of the stream channel. So, whenever we have the chance to visit a stream, we can easily measure discharge. But, we cant be everywhere all the time. So the dilemma arises – How can we know the discharge of many streams in the Dry Valleys all the time?

Turns out, my friends and I have also been graciously trusted with fancy devices that measure the height of water flowing over top of them. We call these devices “stage loggers,” because they can continuously measure the stage (i.e. height) of water, and store the measurements on a small computer chip (data logger). We can deploy a stage logger in a stream and then walk away, knowing that we’ll return to a wealth of data, telling us the height of water in the stream over time. So, lets recap a bit. We are able to measure the discharge of the stream sometimes, but we can measure the stage of a stream all the time.

We use something called “rating curves,” to relate stream stage to stream discharge. “Rating curves,” are mathematical relationships (discharge = f(stage)) between stage and discharge, which allow us to estimate a stream’s discharge, given a steam’s stage. To establish these relationships, we maintain and build control structures. Control structures are built to back-up flowing water behind the structure. This enables us to get very accurate stage measurements at very low flows and very high flows, alike. Below is a photo of our team, working on a control structure located on the Onyx River. Usually these are constructed out of sand bags and rocks.

Stream Team works to re-construct a dilapidated control structure

That’s the brief basics of measuring discharge with rating-curves!



p.s science is the bees knees, dude.

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