Topic: CoCoRaHS vs. Davis Vantage Pro 2 Rainfall Measurements

[ocala]

Interesting.   
A straight bucket shape where the water simply drains into the hole instead of being forced into a smaller area via the tapered shape would reduce the volume somewhat.
Right?   

[zackdog]

Yesterday's test of the outer cylinder with 1" of water and the funnel in place resulted in a loss of 0.01".

Mark

[miraculon]

I had another exact match between the CoCoRaHS gauge and the Davis. The common thread has been that these are rather heavy, straight-down rainfall events with little or no wind. (very light wind from the SW)

I do have a problem with a neighbor's tall shrub to the SE, that can have a rain shadow effect, but I have had lower catch on the Davis from other wind directions when it is windy.

I am becoming more convinced that the problem with the Davis rain collector is the tapered shape, rather than calibration or tipping mechanism issues in general. I know that spider webs, dirty "buckets", friction in the pivot, etc. have an effect, but we are talking about an inherent issue here.

Greg H.

Significant rainfall overnight. Total for CoCoRaHS was exactly an inch. (this doesn't happen very often...)

Wind was from the SE when rain was at the heaviest.

CoCoRaHS: 1.00"
VP2: 0.86"
NovaLynx 8" Tipping Bucket: 1.07"

The NovaLynx was 7% high and the VP2 was -14%.

Is this the VP2 cone shape, or the rain shadow? Which predominates? The local Marina VP2 recorded 0.84", which is roof-mounted in the clear of any obstructions.

One recent day with little/no wind not out of SE rain shadow matched CoCoRaHS, last night with SE wind negative discrepancy.

Greg H.

[Andy G]

Last 2 days rain

Davis VP2 2.89"
CoCoRaHS 2.98"

[floodcaster]

I had another exact match between the CoCoRaHS gauge and the Davis. The common thread has been that these are rather heavy, straight-down rainfall events with little or no wind. (very light wind from the SW)

I do have a problem with a neighbor's tall shrub to the SE, that can have a rain shadow effect, but I have had lower catch on the Davis from other wind directions when it is windy.

I am becoming more convinced that the problem with the Davis rain collector is the tapered shape, rather than calibration or tipping mechanism issues in general. I know that spider webs, dirty "buckets", friction in the pivot, etc. have an effect, but we are talking about an inherent issue here.

Greg H.

Significant rainfall overnight. Total for CoCoRaHS was exactly an inch. (this doesn't happen very often...)

Wind was from the SE when rain was at the heaviest.

CoCoRaHS: 1.00"
VP2: 0.86"
NovaLynx 8" Tipping Bucket: 1.07"

The NovaLynx was 7% high and the VP2 was -14%.

Is this the VP2 cone shape, or the rain shadow? Which predominates? The local Marina VP2 recorded 0.84", which is roof-mounted in the clear of any obstructions.

One recent day with little/no wind not out of SE rain shadow matched CoCoRaHS, last night with SE wind negative discrepancy.

Greg H.

My Davis during heavy events is typically 15 to as high as 20% low compared to the manual CoCoRaHS gauge. Less so during nice steady rain with minimal wind. I'm surprised to see that your NovaLynx was higher, albeit slightly. I may have you confused with another forum member but at one time were you making comparisons with a RainWise TB? I might try adding one in the future.

[miraculon]

I had another exact match between the CoCoRaHS gauge and the Davis. The common thread has been that these are rather heavy, straight-down rainfall events with little or no wind. (very light wind from the SW)

I do have a problem with a neighbor's tall shrub to the SE, that can have a rain shadow effect, but I have had lower catch on the Davis from other wind directions when it is windy.

I am becoming more convinced that the problem with the Davis rain collector is the tapered shape, rather than calibration or tipping mechanism issues in general. I know that spider webs, dirty "buckets", friction in the pivot, etc. have an effect, but we are talking about an inherent issue here.

Greg H.

Significant rainfall overnight. Total for CoCoRaHS was exactly an inch. (this doesn't happen very often...)

Wind was from the SE when rain was at the heaviest.

CoCoRaHS: 1.00"
VP2: 0.86"
NovaLynx 8" Tipping Bucket: 1.07"

The NovaLynx was 7% high and the VP2 was -14%.

Is this the VP2 cone shape, or the rain shadow? Which predominates? The local Marina VP2 recorded 0.84", which is roof-mounted in the clear of any obstructions.

One recent day with little/no wind not out of SE rain shadow matched CoCoRaHS, last night with SE wind negative discrepancy.

Greg H.

My Davis during heavy events is typically 15 to as high as 20% low compared to the manual CoCoRaHS gauge. Less so during nice steady rain with minimal wind. I'm surprised to see that your NovaLynx was higher, albeit slightly. I may have you confused with another forum member but at one time were you making comparisons with a RainWise TB? I might try adding one in the future.

The Rainwise was probably ValentineWeather, I would guess.

There are two calibration targets for the NovaLynx gauge. Assuming a 946mL amount of water (measured on the approved CoCoRaHS Escali scale), these targets are either 1.15" or 1.19". I used a #72 gas orifice on my homebrew calibrator and got a consistent 1.19" over three runs. With the 1/16" orifice, I got 1.09" which is low against the 1.15" target.

Since this is so rate dependent, and I do get agreement within 0.01" of the CoCoRaHS gauge from time to time, I don't plan on changing the calibration again. I think that there are so many variables with rain rate, wind, geometry, etc. that it is impossible to get an exact match anyways.

Greg H.

[floodcaster]

Great info. Thanks much.

I think that there are so many variables with rain rate, wind, geometry, etc. that it is impossible to get an exact match anyways.

Absolutely, especially under varying rainfall rates.

[miraculon]

I had another match between CoCoRaHS and the VP2 rain gauge during yesterdays brief but intense rain. (0.26")
Once again this was straight-down heavy rain that only lasted about 10 minutes.
Winds were stronger (22 mph) and from SW to NW direction.
I am now suspecting that my neighbors shrub is the main culprit, with wind to the SE. I have a small lot and I don't think that she is willing to remove the tall shrub, so I guess that I have to live with the situation.
By the way, the rain amounts for the local Marina (2,000 feet away) and my NovaLynx were 0.28" and 0.29" respectively.
At this time, I see no need to mess with the stop screws on the VP2.

Greg H.

[ggsteve]

I have two Rainwise buckets.  Way too little rain for any meaningful analysis but it is all I have to go on in this year of drought.  This morning a front blew through.  The Rainwise Rainew self-contained rain gauge and the Cocorahs gauge right next to it and at the same height both registered .06" of rain.  I am concerned that the Rainwise Mk III bucket that I just moved from the peak of the roof to the eve only recorded .04"  I hope that trend doesn't continue.

[Bluefudge]

I had to do another comparison of five different rain gauges (my other post is on page 4 i believe) after we had a day and some of heavy rain and little to no wind.

The five to compare:
VP2: rooftop
Vue: fencepost (8 feet above ground)
CoCoRaHS: lower but near Vue
Torrent: lower but also near Vue
Acurite 5 in 1: only 4 feet above grass

The rainfall amounts:
VP2: 70.4 mm (2.77 in.)
Vue: 69.1 mm (2.72 in.)
CoCo: 71.0 mm (2.80 in.)
Torrent: 80.0 mm (3.15 in.)
Acurite: 79.8 mm (3.14 in.)

A number of other tests always show the Torrent and Acurite show higher amounts during big rain events.  The VP2 rooftop gauge is very close to the fence mounted Vue and CoCoRaHS which makes me happy of course.  However when we had a heavy rain storm and very windy conditions last month the VP2 showed significantly less rain then the Vue and CoCoRaHS, as expected I suppose. 

[WheatonRon]

I had to do another comparison of five different rain gauges (my other post is on page 4 i believe) after we had a day and some of heavy rain and little to no wind.

The five to compare:
VP2: rooftop
Vue: fencepost (8 feet above ground)
CoCoRaHS: lower but near Vue
Torrent: lower but also near Vue
Acurite 5 in 1: only 4 feet above grass

The rainfall amounts:
VP2: 70.4 mm (2.77 in.)
Vue: 69.1 mm (2.72 in.)
CoCo: 71.0 mm (2.80 in.)
Torrent: 80.0 mm (3.15 in.)
Acurite: 79.8 mm (3.14 in.)

A number of other tests always show the Torrent and Acurite show higher amounts during big rain events.  The VP2 rooftop gauge is very close to the fence mounted Vue and CoCoRaHS which makes me happy of course.  However when we had a heavy rain storm and very windy conditions last month the VP2 showed significantly less rain then the Vue and CoCoRaHS, as expected I suppose.

Other than your Acurite and Torrent tools, I would view these results as excellent! The differences between the CoCo gauge and the Davis products are statistically insignificant in my view. And yes, during heavy rainfalls, the tipper in the VP2 will always read less than the gold standard—the CoCo gauge—it can’t keep up with the volume of rain. Nature of the beast I suppose.