Pro2/Envoy: Strange value for Evapotranspiration

[wfpost]

^^^^
http://www.hydroskript.de/html/_index.h ... p0505.html

[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

[wfpost]

whereas this wiki article gives the quickest overview

http://en.wikipedia.org/wiki/Penman_equation
http://en.wikipedia.org/wiki/Penman_equ ... .281993.29

[YJB]

Hi Boris,

I've mailed you the Davis formula as well as login details for testing purposes.

Ysbrand

[admin]

great, thanks, will give it a try.

[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.

[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?

[admin]

Bug is confirmed, will be fixed with 4.9k.

[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?

[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.

[Hathor27]

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?

[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

[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.