Just in case

[water01]

Just in case some of you have been as thick as me and expected Build 3149 to automatically figure out I had a solar enabled weather station and calculate the Evapotranspiration there is a setting!!

Station Settings / Common Options / Evapotranspiration

There was me sat there thinking that this is taking longer than the hour Mark mentioned, so I took a look around and discovered the setting!!

I have got so used to Mark doing things automatically it didn't occur to me there would be a setting!!

[mcrossley]

It is almost impossible to determine ahead of time if some stations have a solar sensor.

[water01]

I should have realised that Mark, but thicko here didn't realise as it wasn't mentioned in the release notes or the updates.txt that meant that there had to be a new option!!

Just thought I ought to mention it as I fell into the trap!!

[mcrossley]

I've added a note to the announcement, thanks.

[water01]

And by the way Mark these are the readings at 16.00 today in case it helps you evaluate the "experiment".

Solar Radiation 77 W/m² UV Radiation 0.0
Sunshine today 0.6 hrs Evapotranspiration today 0.021 mm

[mcrossley]

Because the calculation is done over each hour, you need the min/max for temp & hum, mean solar radiation, mean wind speed, latitude, longitude, and altitude.

In the UK I'd expect a daily total of somewhere between 0.3mm and 1.5mm at this time of year.

[water01]

Happy to get those except not sure there is a webtag for mean (average) solar Radiation?

Think all the others are possible as far as I can see.

[mcrossley]

I'd just keep an eye on your daily totals (in the day file) and see if they look sensible. You can look at my historic values from my VP2 before I switched to a WLL, they give indicative numbers for the UK here: https://weather.wilmslowastro.com/datasummary.php

[Phil23]

Obviously my VP/2 has the Console reporting Evapotranspiration.

Enabling it on the Ecowitt....

Will be interesting to see the comparison.

Edit:- I don't see any page that displays it in the interface.

Would it be logical to add it to the Today/Yesterday pages as the hourly to date figure?

Phil.

[Matt.j5b]

Great to see this addition, as I was rather disappointed that the WLL won't calculate ET from the solar sensor on a secondary transmitter!

To overcome that I have used SQL to calculate that (and evaporation) but is rather approx. and rough at the moment and needed more work, so it will be interesting to see how they compare.

With the first day of this after sundown I had a total of 6.16 mm compared to 7.1 mm with my calculation - so seems to be in the ballpark.

[water01]

Mark, could it be that my ET reading is inches rather than millimetres as that would make more sense when comparing our readings?

[mcrossley]

The ET value is in the same units as your rainfall - for obvious comparisons.

[water01]

OK so that is inches so that explains my readings.

[Matt.j5b]

I have done some comparisons of my previous calculation of ET compared to the formulas that have been used in CMX using my station data for the year so far. Since this was added to CMX, the daily totals that I calculated for this comparison are quite similar to daily totals recorded by CMX since this was added to CMX.

Code: Select all

Month	ET_CMX	ET_bom1	ET_bom2	*a	*b	*c
-----------------------------------------------------------
Jan	123.45	164.5	156.8	141.2	123.46	141.11
Feb	101.62	149.3	129.9	117.1	101.6	117.7
Mr	89.51	122.6	106.9	102.7	89.47	104.24
Apr	84.34	105.4	87.4	94.6	84.41	95.81
May	75.79	86	70	85.5	75.75	85.58
Jun	63	72.4	53.4	70.6	62.9	85.58
Jul	71.6	81.2	66.5	81.5	71.69	79.97
Aug	92.82	106.2	68.9	101.9	92.75	102.54
Sep	112.58	135.3	88.9	125	112.46	122.97
Oct	55.69	66.1	52.2	60.4	55.71	60.42
------------------------------------------------------------
Total	870.4	1089	880.9	980.5	870.2	995.92

ET_Bom1: As a comparsion to a nearby BoM Station - Archerfield Airport that get fairly similar weather but is more exposed.

ET_Bom2: As a comparsion to a nearby BoM Station - Brisbane that generally doesn't get as hot (closer to the coast) but not as exposed as Archerfield.

*a: ET that I have previously have calculated based on the Davis formulas.

*b: Looking at the FAO document for calculating in hourly time steps it suggests to use hourly averages. I notice when I use the direct average instead of (min+max)/2 for the station parameters and averaging solar radiation for clear sky radiation, I get similar results however.

*c: Changing Stefan-Boltzman constant from 4.903e-09 to 2.043e-10 I get higher values but is consistent with my previous calculation. I noticed in the FAO document that the 2.043e-10 constant should be used in hourly totals if I am reading this correctly.

[mcrossley]

Interesting, thanks.

I had coded the Stefan-Boltzman constant to be 4.903e-09, but as you point out that is a per day value. Since we are calculating by the hour it makes sense to divide that by 24.
However I cannot see a reference to doing that in the FAO document?