Opened 3 years ago

Closed 3 years ago

#3052 closed defect (fixed)

Watts to MegaJoules/Day conversion issue

Reported by: cholmes Owned by: grass-dev@…
Priority: normal Milestone: 7.0.5
Component: Imagery Version: svn-releasebranch70
Keywords: i.evapo.mh Cc: ychemin
CPU: Unspecified Platform: All

Description (last modified by neteler)

I think I may have found an error in a few files from the grass-ci repo.

A few files:

https://github.com/GRASS-GIS/grass-ci/blob/f1a15d967611006e920b8dc2f191391e44392019/imagery/i.evapo.mh/mh_samani.c#L14

contain a line like:

ra = ra * (84600.0 * 1000.0);	/*convert W -> MJ/d */

I'm guessing the magic constant 84600 is intended to be the number of seconds in a day, which is actually 86400. Also, Watts * Seconds gives Joules. If you intend to get MegaJoules?, you must divide by 1000, not multiply (but maybe the comment is just wrong, and you wanted milliJoules?).

Change History (7)

comment:1 Changed 3 years ago by martinl

Component: DefaultImagery
Keywords: i.evapo.mh added

comment:2 Changed 3 years ago by ychemin

article is here for file mh_eto.c :

https://www.researchgate.net/profile/Peter_Droogers/publication/226830392_Estimating_Reference_Evapotranspiration_Under_Inaccurate_Data_Conditions/links/0f31753a0483fd97c0000000.pdf

ref to equation 5 page 40.

equation 1 page 36 references the units used: RA-> MJ / m2 / d

MegaJoules? per meter square per day.

So the conversion is indeed wrong in the ticket.

comment:3 Changed 3 years ago by ychemin

Resolution: fixed
Status: newclosed

comment:4 Changed 3 years ago by neteler

For reference: r68564

comment:5 Changed 3 years ago by neteler

Cc: ychemin added
Description: modified (diff)
Resolution: fixed
Status: closedreopened
Version: unspecifiedsvn-releasebranch70

Reopening for the issue: 84600 should be 86400 (?)

comment:6 Changed 3 years ago by neteler

Backported changes for i.eb.* + i.evapo.*: fix for MegaJoules? per meter square per day (backport of r68564, r68572, r68573, r68574, r68579, trac #3052); all sync'ed to trunk

Done in r68580 (releasebranch_7_2) and r68581 (releasebranch_7_0).

@cholmes: thanks for your important hint and please test.

comment:7 Changed 3 years ago by martinl

Resolution: fixed
Status: reopenedclosed

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