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APSIMInitiative / ApsimX

ApsimX is the next generation of APSIM
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photoperiod function does not return the correct daylength #5150

Closed junqi108 closed 4 years ago

junqi108 commented 4 years ago

@hut104 @HamishBrownPFR I was trying to use the daylength to trigger the endoDormancy of the grapevine. I found the photoperiod function does not return the correct daylength. for example, around 21 March, daylength should be around 12. however, the photoperiod function returns 13. image

zur003 commented 4 years ago

"Daylength" is a fuzzier concept than one might expect. Note the following comments from the MathUtilities,DayLength function:

/// Civil twilight ends after sunset or begins before sunrise when the solar depression angle is 6deg;. e.g SunAngle = -6deg; /// Nautical twilight : 12deg; /// Astronomical twilight : 18deg;

I take this to mean that "daylength" includes a bit of time at dawn and dusk when the sun is actually below the horizon.

junqi108 commented 4 years ago

yeah, adding the twilight could make the daylength a bit longer. I wonder whether this corresponds to the photoperiod that used in plant phenology.

zur003 commented 4 years ago

@junqi108 - Note that the MathUtilities.DayLength function does allow the solar depression angle to be specified. I'm not sure what photoperiod function you are using, but if you get results like that with a depression angle of 0, then I would agree that something is wrong.

junqi108 commented 4 years ago

sorry, I think I am wrong. I tested with a depression angle of 0, and it produces good result.