Citizen Science - Rainfall Measurement

 Citizen science projects are generally about collecting data, lots of data, often in a form suitable for analysis by more experienced researchers. Extra data are beneficial whether for the study of bird migration, the documentation of Super Nova explosions, digitizing handwritten meteorological observations, or counting the number of polar bears in the Arctic.

The weather data from the Davis Weather Station in our back garden is uploaded to Weatherlink.com and made publicly available. Just click the cross (+) in the top right corner of any craftycabbage.com page to get the current weather conditions in our garden.

In the near future, I hope to be able to automatically upload our weather data to WOW (the UK Met Office's Weather on Web platform) so it is available to weather and climate researchers.

However, you do not need an expensive weather station to be able to contribute. Everyone can put out a rainfall collector and monitor daily or weekly precipitation. Most homemade rainfall gauges are based on a clear plastic bottle modified with stuff readily found in most homes. See here, here, here, and here for examples. And there are plenty of YouTube videos as well.

A disadvantage of this type of rain gauge is its inaccuracy for small amounts of rain because the collection area is limited to the diameter of the bottle. Either reducing the diameter of the bottom collector or increasing the diameter of the upper collecting funnel would help improve accuracy for lower precipitation events. This, however, removes the simplicity of the original design and requires some additional adjustments to the raw data to obtain the correct rainfall values.

Knowing rainfall levels and patterns is important for one of the citizen science projects I'm currently involved with: FOUW river water testing. River Wye levels in Hereford are dependent, to a large extent, on rainfall in the Welsh Hills where the source of the Wye is. It can take one to two days for rain in Wales to percolate its way down to Hereford. On the other hand, the water levels in the tributaries we test are governed primarily by run-off from nearby agricultural land, gardens and paved surfaces (paths and roads). So knowing the local rainfall data are important in assessing the impact of different types of run-off.

One of our citizen science colleagues, who lives near the testing sites, has built his own rain gauge using a plastic bottle as described above. However, rather than measure the height of the collected rainfall, Alan weighs the bottle daily using standard kitchen scales (to the nearest gram). This provides good accuracy for even small quantities of collected rain but requires additional calculations to convert the mass of water to rainfall in millimetres (mm) - e.g. an accurate value for the rainfall collection area (aperture) is required.

Another option is to buy a readymade rain gauge that has already been calibrated. There is plenty of choice for under £10. I asked for this one as a Christmas present ...

Rain Gauge

Now you are probably asking why do I need a cheap basic rain gauge when I already have a high-tech calibrated gauge as part of my Davis weather station? So, I shall tell you why.  When I compare my 'state-of-the-art' weather station rainfall values with those from Alan's homemade rain gauge, Alan's values are, pretty much all the time, higher. For example, one day I recorded 11.8 mm and Alan recorded 19.2 mm; the following day we both recorded 0.8 mm. Alan and I live about one mile apart - Alan south of the River Wye whereas we are north of the river.

There could be a number of reasons for the discrepancies: (i) my reading is wrong, (ii) Alan's reading is wrong, (iii) both readings are wrong, (iv) on average, it rains more at Alan's location south of the river, (v) the location of one or both of the rain gauges is non-optimum.

After careful thought, my inclination was to go with (v) - non-optimum positioning of my rainfall collector. We live in a built-up urban environment making it almost impossible to place the rain collector in an ideal position. Instructions for the correct positioning of the rain gauge vary depending on which website you visit. Generally, the rain gauge should be out in the open, off the ground but sheltered from the wind. See here, here, here, and here for example instructions.

My Davis rain gauge is situated about 2 metres above ground to minimise the rain shadow effect of the surrounding buildings. It is attached to a pole (on the north side of the rain collector) that extends above the rain collector and is not ideal - however, the prevailing wind direction is from the south west so it hopefully will have only a relatively minor influence on rainfall values.🤞 

In order to test this hypothesis (i.e. the Davis rain gauge is an accurate measure of rainfall at this location), I will compare its values with those from my simple rain gauge. This will take some time as I will need to place the secondary gauge in various positions around the garden and compare results using both methods.

With the rain gauge placed in open ground in the kitchen garden ...

Site #1

... the measurements so far are reasonably close (within ~10%) but there is a clear trend for the Davis system to underestimate precipitation:

	Davis Weather Station	Simple Rain Gauge
Rainfall during 2 - 3 January	2.8	3.0
Rainfall during 2 - 6 January	5.6	6.0
Rainfall during 2 - 7 January	10.2	11.5

Note: the cover of the rain gauge retains some water - an issue only for light rain. One user of this type of rain gauge suggested leaving the lid off - something I might consider at a later date. 

Finally, I compared rainfall data for January (up to 10 pm on the 6th January) recorded by Davis Weather Stations within a 5 mile radius of our weather station. Values ranged from 3.6 mm to 11.6 mm with a mean value of 7.5 mm: our Davis weather station recorded a value of 6.8 mm. This year, precipitation has been in the form of local showers so some geographical variability is to be expected.