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Re: Pro2/Envoy: Strange value for Evapotranspiration
Posted: Sat May 26, 2012 4:19 pm
by wfpost
Re: Pro2/Envoy: Strange value for Evapotranspiration
Posted: Sat May 26, 2012 4:37 pm
by wfpost
or in very detail
http://www.fao.org/docrep/X0490E/x0490e07.htm
The methods for calculating evapotranspiration from meteorological data require various climatological and physical parameters. Some of the data are measured directly in weather stations. Other parameters are related to commonly measured data and can be derived with the help of a direct or empirical relationship. This chapter discusses the source, measurement and computation of all data required for the calculation of the reference evapotranspiration by means of the FAO Penman-Monteith method. Different examples illustrate the various calculation procedures. Appropriate procedures for estimating missing data are also provided.
Meteorological data can be expressed in several units. Conversion factors between various units and standard S. I. units are given in Annex 1. Climatic parameters, calculated by means of the equations presented in this chapter are tabulated and displayed for different meteorological conditions in Annex 2. Only the standardized relationships are presented in this chapter. The background of certain relationships and more information about certain procedures are given in Annex 3. Annexes 4, 5 and 6 list procedures for the statistical analysis, assessment, correction and completion of partial or missing weather data.
Meteorological factors determining ET
Solar radiation
Air temperature
Air humidity
Wind speed
Re: Pro2/Envoy: Strange value for Evapotranspiration
Posted: Sat May 26, 2012 4:58 pm
by wfpost
Re: Pro2/Envoy: Strange value for Evapotranspiration
Posted: Sat May 26, 2012 6:06 pm
by YJB
Hi Boris,
I've mailed you the Davis formula as well as login details for testing purposes.
Ysbrand
Re: Pro2/Envoy: Strange value for Evapotranspiration
Posted: Sat May 26, 2012 7:42 pm
by admin
great, thanks, will give it a try.
Re: Pro2/Envoy: Strange value for Evapotranspiration
Posted: Mon May 28, 2012 7:46 pm
by admin
Ysbrand was so kind to give me access. Looks like I can provide a solution with next update, where daily cumulative reporting of evapotranspiration by Vantage will be handled correctly for all given time resolutions. This will also fix past data, as logging does not change, but just computation of data from it will be corrected.
Re: Pro2/Envoy: Strange value for Evapotranspiration
Posted: Wed Jun 13, 2012 11:30 pm
by Hathor27
Big thanks to Boris and Ysbrand - that's great
Unfortunately there's a new bug when defining graphs on Meteohub Ver 4.9j: If I define a graph for
sol1-et, there will be written a value like
sol1 into the .mg-file instead of
sol1-et. Could you please have a look at this?
Another thing is: I'd like to show
rain1 and
sol1-et(both in unit mm) in the same graph and
same axis (with same scale) - is there a way to do so?
Re: Pro2/Envoy: Strange value for Evapotranspiration
Posted: Thu Jun 14, 2012 1:09 am
by admin
Bug is confirmed, will be fixed with 4.9k.
Re: Pro2/Envoy: Strange value for Evapotranspiration
Posted: Fri Jun 15, 2012 11:43 pm
by Hathor27
I'm sorry, but I found some additional curious behaviour:
If I display ET each hour a day, there are hours without data - you can see this on the followed pictures:
If you have a look at sun, wind and temperature for the same time period, it isn't plausible, why ET has no values between 3pm and 6pm Thursday 14th and between 11am and 6pm Friday 15th
Could you please have a closer look to this, too?
Re: Pro2/Envoy: Strange value for Evapotranspiration
Posted: Sat Jun 16, 2012 8:47 am
by admin
As air humdity is near 100% on 14th 3-6h (caused by rainfall) I find it quite obvious that there is no loss of water into the atmosphere from the ground during that time. At least that is how interpret what evapotranspirationn is about.
Anyway, the numbers are what the vantage reports, it is not a home-brew formular behind it.
Re: Pro2/Envoy: Strange value for Evapotranspiration
Posted: Sun Jun 17, 2012 1:04 pm
by Hathor27
admin wrote:At least that is how interpret what evapotranspirationn is about.
Anyway, the numbers are what the vantage reports, it is not a home-brew formular behind it.
That's the reason I claim this behaviour: My Vantage shows continious values even between 3pm and 6pm Thursday 14th and between 11am and 6pm Friday 15th. Only Meteohub shows
spaces...
...could it be that Meteohub sometimes fetches the value before Vantage has calculated the hourly value?
Re: Pro2/Envoy: Strange value for Evapotranspiration
Posted: Sun Jun 17, 2012 1:59 pm
by YJB
Hi,
Let me try to explain what I see on my setup.
The raw values are showing that MH will only get the update for the previous hour 15 or 16 mins after the hour. This is a raw value which is inch based. I've translated this to mm and show the hourly difference as well:
Code: Select all
Timestamp raw value mm Difference
20120617000028 2 0,051
20120617001500 2 0,051
20120617001602 4 0,102 0,051
20120617011501 4 0,102
20120617011603 6 0,152 0,051
20120617021533 6 0,152
20120617021635 8 0,203 0,051
20120617031457 8 0,203
20120617031559 9 0,229 0,025
20120617041541 9 0,229
20120617041642 10 0,254 0,025
20120617051525 10 0,254
20120617051615 12 0,305 0,051
20120617061529 12 0,305
20120617061603 15 0,381 0,076
20120617071522 15 0,381
20120617071623 19 0,483 0,102
20120617081538 19 0,483
20120617081650 27 0,686 0,203
20120617091535 27 0,686
20120617091636 38 0,965 0,279
20120617101514 38 0,965
20120617101616 46 1,168 0,203
20120617111500 46 1,168
20120617111602 55 1,397 0,229
This matches exactly whith the seqhour1 values:
Code: Select all
seqhour1_sol4_et_mm 0.20_0.28_0.20_0.10_0.08_0.05_0.03_0.03_0.05_0.05_0.05_0.050.00_0.08_0.08_0.08_0.08_0.10_0.15_0.18_0.20_0.23_0.23_0.28_0.30_0.00_
Is also shows that using a 15 min interval (or less) doesn't really make sense, since the value is only updated every hour by Vantage:
Code: Select all
seqmin15_sol4_et_mm 0.20_0.00_0.00_0.00_0.28_0.00_0.00_0.00_0.20_0.00_0.00_0.00_0.10_0.00_0.00_0.00_0.08_0.00_0.00_0.00_0.05_0.00_0.00_0.00_0.03_0.00_0.00_0.00_0.03_0.00_0.00_0.00_0.05_0.00_0.00_0.00_0.05_0.00_0.00_0.00_0.05_0.00_0.00_0.00_0.05_0.00_0.00_0.00_0.00_0.00_0.00_0.00_0.08_0.00_0.00_0.00_0.08_0.00_0.00_0.00_0.08_0.00_0.00_0.00_0.08_0.00_0.00_0.00_0.10_0.00_0.00_0.00_0.15_0.00_0.00_0.00_0.18_0.00_0.00_0.00_0.20_0.00_0.00_0.00_0.23_0.00_0.00_0.00_0.00_
I haven't checked yet when the value is updated on the console.
UPDATE
It turns out that the time on my envoy was lagging 15 mins behind, hence the delay in calculating and posting the data to MeteoHub.
BORIS
To be in sync with the Vantage console / Envoy it turns out that a time difference has some impact since ET is only calculated on top of the hour Would it be possible to sync the Vantage time to MH when doing an initial connect? Maybe a checkbox on the "Weather station page" that says "Synchronize Vantage to MH time" and make sure that MH only forces the sync when MH has synced with NTP within the last day or so?
Thanks,
Ysbrand
Re: Pro2/Envoy: Strange value for Evapotranspiration
Posted: Sun Jun 17, 2012 4:40 pm
by admin
Have you tried 4.9k? I checked the values on a users station and there does not seem to be an error to be corrected by factor 8 or 10. Values are pretty much in snyc which what the console reports (daily values).
You are right, "et_mm" is the total amount of evapotranspiration for the given time resolution. You can plot hourly values by putting res to "hour1" and variables should provide something like "last60m_sol0_et_mm" which gives you the data you are looking for.