Watts to MegaJoules/Day conversion issue

[cholmes]

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_eto.c#L15
• ​https://github.com/GRASS-GIS/grass-ci/blob/f1a15d967611006e920b8dc2f191391e44392019/imagery/i.evapo.mh/mh_original.c#L14

​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?).

[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.

[neteler]

For reference: r68564

[neteler]

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

[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.

[martinl]

No activity, closing, feel free to reopen if needed.