convertRadiationFromWattsToMegaJoules conversion factor of 0.0864
Hi,
I am coding a new parser to read values directly from a weather station. Meanwhile, I found something about the solar radiation units that I would like to confirm.
1) The SOLARRADIATION unit described in RainMachine documentation (https://support.rainmachine.com/hc/enus/articles/228620727HowtointegrateRainMachinewithdifferentweatherforecastservices) is:
SOLARRADIATION = "SOLARRADIATION" #[megaJoules / square meter per hour]
2) The asceDaily formula (https://github.com/sprinkler/rainmachinedeveloperresources/blob/master/formula/formula.py) use the same unit.
fRs = 16.502 [MJ m2 h1]
3) The conversion function RMUtils.convertRadiationFromWattsToMegaJoules is applying a conversion factor of 0.0864.
This conversion factor generates a different unit than expected. as described (http://www.fao.org/3/X0490E/x0490e0i.htm) the conversion 1 W m2 = 0.0864 MJ m2 day1 returns de solar radiation per day and not per hour.
Can you confirm that the conversion factor from watts to megajoules is correct?
From my point of view, the conversion factor should be 0.0036. From the maths we have:
1 W/m2 = 1 J/m2/Sec
1 J/m2/Sec * 3600 Sec/Hour = 3600 Joules/m2/Hour
3600 Joules/m2/Hour / 1,000,000 = 0.0036 MJ/m2/Hour
You could use 0.0036 as a direct multiplier to get from W/m2 to MJ/m2/Hour.
Regards,
Pedro

Hi,
Update to my findings.
 The ASCE formula specification (https://epic.awi.de/id/eprint/42362/1/ascestzdetmain2005.pdf), the equation (16) uses as units [MJ m2 d1].
 I find other Python ASCE Daily formula implementation at http://pydoc.net/RefET/0.1.0/refet.daily/, and it also uses as units for Incoming shortwave solar radiation [MJ m2 day1].
Based on these findings, can you confirm that the SOLARRADIATION unit is [MJ m2 day1], instead of [megaJoules / square meter per hour] as described on rain machine formula and documentation?
If the unit is [MJ m2 day1], then the above 0.0864 conversion factor present at convertRadiationFromWattsToMegaJoules, is correct.
Regards,
Pedro

Yes it's [MJ m2 h1] see here: https://github.com/sprinkler/rainmachinedeveloperresources/blob/master/formula/formula.py
fRs = 16.502 [MJ m2 h1]  measured incoming solar radiation. If == None it is estimated from fKrs and fTMinC/fTMaxC

If there is an error it can probably go unspotted because if ET0 is returned by parser irrigation will use this value. If ET0 is not provided then the formula will try to calculate it using what parser provides. In some cases SOLAR RADIATION is not returned for all stations in CIMIS/FAWN so to really affect irrigation, CIMIS/FAWN will have to not send ET0 and send SOLAR RADIATION and probably this happens with a low chance.

I do understand that for CIMIS and FAWN the error if it exists, do not have a high impact.
However, on my use case, the personal weather station does not report the ETo, but it indicates the Solar Radiation in w/m2.
I want to add the Solar Radiation reported into Rainmachine.If the formula.py is expecting [MJ m2 h1] but if the conversion function convertRadiationFromWattsToMegaJoules returns [MJ m2 day1] it does not seem right.
Please, confirm the convertRadiationFromWattsToMegaJoules returned units.
Please sign in to leave a comment.
Comments
10 comments