Number that wasn’t there

Never trust an average.

Especially when it comes to rivers.

You can have a bunch of 50s in a row: 50, 50, 50, 50, 50, 50 week after week for years on end to the point where it would be a “no-brainer” to describe what you are seeing as a “50.”

But then along comes one 5,000 and all of a sudden every number cruncher in town (and trust me, the hydrology world is crawling with them) is trying to sell you the snake oil of the thing your seeing as being an “800” …

That’s the number you get when you average six 50s and one 5000.

It’s the same story with flows.

What you see most of the time is the median flow, and if you had to describe the river, that’s probably the number you’d hang your hat on.

Another problem with the average is that – in this case – it’s a purely fictional number.

Call the river a “50” if you’re a day-in day-out type hydrologist as I am, or call it a “5000” if you’re the type of hurricane-hunter hydrologist that loves peak flows, but don’t go calling it an “800” …

Because that’s a “number that wasn’t there”…

Unless of course you’re computing annual flow volumes.

In that case, computing the average is a common hydrologic stepping stone for tabulating the annual flow volume – followed by a couple unit conversions – and multiplying by 365.

Hydrology is full of caveats!

That’s the number one rule in hydrology: exceptions are the rules. It’s corollary: averages are fool’s gold. (But it looks so shiny?!)
Photos are from eastern Belgium, the last is looking down the spillway of the Barrage de la Vespre.

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