Watson/Cumulus rain total seems wrong

[souwesterly]

Hurray – we’ve had some rain at last!
Well, more than the typical 1 – 2 mm we’ve been getting (when it’s rained at all) for the past few months.
And from this comes my query.

How accurate is my W-8681 rain gauge? Or my Cumulus display?

My Watson W-8681 rain gauge obviously works well. Despite prolonged inactivity recently, it’s still managed to pick up the odd 0.3 mm of light rain when it’s fallen. It’s set to start each 24 hour period at midnight.

So, today, my Cumulus on-screen display showed 12.5 mm – the wettest day here since 15th January. Well, it would have been….

As a back-up, and through old habit, I also have two funnel rain collectors. The rain collects below a funnel and I pour it into measuring jars. As I’ve done for years and years…..

But each of those containers gave me between 9 and 9.5 mm (only a whisker over, I reckoned, so I called it 9 mm)

‘Strange…..’ I thought!
So I checked with a local wunderground station (Huish College in Taunton) – and their site gave a figure of 9.1 mm for the day.

So why did my Watson gauge give me over 3 mm more rain?

[Siting – they’re all in my garden. The W-8681 is on a 3 metre scaffold pole – rock solid and well clear of any buildings or trees.
One rain gauge is on a flat roofed shed – raised above it to avoid back-splash.
The other is up nearer the house on a 1.5 metre 3” x 3” post and perhaps 2 metres from any obstacle.]

*Perhaps wrong to post it here – please move it to suit, Steve.

[steve]

How accurate is my W-8681 rain gauge?

Not particularly. Although they tend to under-read in most cases, by as much as 25%.

Or my Cumulus display?

Cumulus accurately reports what the station tells it, as far as I can tell. You can check that anyway by comparing it with what the console says.

You can check the operation of your gauge under ideal conditions by calculating the volume of water required for a particular rainfall measurement (based on the cross-sectional area of the collector) and pouring it slowly into the gauge.